seismic imaging of deep low velocity zone beneath the dead sea basin and transform fault implications for strain localization and crustal rigidity 1 new seismic observations from the dead sea basin dsb a large pull apart basin along the dead sea transform dst plate boundary show a low velocity zoneextending to a depth of 18 km under the basin the lower crust and moho are not perturbed these observations are incompatible with the current view of mid crustal strength at low temperatures andwith support ofthebasin snegative load byarigid elastic plate strain softening in the middle crust is invoked to explain the isostatic compensation and the rapid subsidence of the basin during the pleistocene whether the deformation isinfluencedbythepresenceoffluidsandbyalonghistoryof seismic activity on the dst and what the exact softening mechanism is remain openquestions the uplift surrounding the dstalso appears to be an upper crustal phenomenon but its relationship to a mid crustal strength minimum is less clear the shear deformation associated with the transform plate boundary motion appears on the other hand to cut

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

comments on nd sr isotopic compositions of lower crustal xenoliths evidence for the origin of mid tertiary felsic volcanics in mexico by j ruiz p j patchett and r j arculus

measurement and parameterization of albedo variations at haut glacier d arolla switzerland spatial and temporal variations of surface albedo on haut glacier d arolla switzerland during the 1993 and 1994 ablation seasons are described correlation and regression analyses are used to explain the albedo variations in terms of independent meteorological and surface property variables parameterizations are developed which allow estimation of albedo variation in surface energy balance models snow albedo is best estimated from accumulated daily maximum temperatures since snowfall on deep snow 0 5 cm w e depth a logarithmic function is used while on shallow snow 0 5 cm w e depth an exponential function is used to enable the albedo to decay to the underlying ice or debris albedo the transition from deep to shallow snow is calculated as a function of decreasing snow depth combined r 2 0 65 this new parameterization performs better than earlier schemes because accumulated daily maximum temperatures since snowfall correlate strongly with snow grain size and impurity concentration the main physical controls on snow albedo ice albedo may be parameterized by its relationship to elevation r 2 0 28 but this approach results in only a small improvement over the assumption of a constant mean ice albedo

the arabian continental alkali basalt province part i evolution of harrat rahat kingdom of saudi arabia the harrat rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from yemen in the south through saudi arabia and jordan to syria in the north the continental intraplate volcanism which produced the arabian lava fields harrats was contemporaneous with opening of the red sea collision of the arabian and eurasian plates and uplift of the afro arabian dome harrat rabat has evolved over the past 10 m y it contains two prominent lateritic disconformities which separate three stratigraphic units the shawahit 10 2 5 ma hammah 2 5 1 7 ma and madinah 1 7 recent basalts comprising 68 19 and 13 of the total harrat volume respectively the shawahit basalt is composed of coarse grained dictytaxitic olivine transitional basalt otb and minor 10 alkali olivine basalt aob which erupted through scoria cones and shield volcanoes in contrast the hammah basalt is dominated by equal volumes of fine grained intergranular to intersertal aob and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes sparse domes and flows of mugearite and benmoreite first appear in the hammah basalt the madinah basalt spans the full compositional range of harrat rahat 103 analyzed samples comprise otb 8 aob 47 hawaiite 32 mugearite 4 benmoreite 8 and trachyte 2 here scoria cones dominate whereas shield volcanoes are sparse and occur only in the lower madinah basalt domes and associated pyroclastic deposits are common the final events of harrat volcanism include 11 post neolithic eruptions 4 500 yr old and two historical eruptions in 641 and 1256 a d arabian harrat magmatism may have been initiated by partial melting of garnet perido tite in the asthenosphere at a depth 100 km accumulation of this partial melt at the crust mantle boundary 37 44 km may have resulted in miocene uplift of the afro arabian dome harrat rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of shawahit otb lava chemical and petrographic data suggest that these otb magmas ascended rapidly with little crustal residence time the data support a model of open system fractionation for aob and hawaiite lavas of the younger hammah and madinah basalts in this model some of the potential shawahit otb was trapped in crustal chambers during magma ascent crystal fractionation in these chambers was accompanied by their periodic replenishment by rising otb magma this resulted in the mixing of magma types and the development of aob and hawaiite magmas which are anomalously enriched in highly incompatible elements the mugearite benmoreite and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment

improvement of lake ice thickness retrieval from modis satellite data using a thermodynamic model observations of ice thickness are limited in high latitude regions at a time when they are increasingly being requested by operational ice centers this study aims to improve the retrieval of lake ice thickness using data from the moderate resolution imaging spectroradiometer modis on board nasa s aqua p m and terra a m satellites the accuracy of ice thickness retrievals based on modis lake ice surface temperature list is investigated using a commonly used heat balance equation and the retrieved ice thicknesses are compared to in situ measurements from the canadian ice service the accuracy of ice thickness estimates is improved when using snow depth from the 1 d thermodynamic lake ice model canadian lake ice model climo rather than an empirical relationship between snow depth and ice thickness utilized in the recent investigations taking into account all data over the study period 2002 2014 the mean bias error and the root mean square error are reduced from 0 42 to 0 07 m and 0 58 to 0 17 m respectively with the novel approach proposed herein however this approach is limited to ice thickness estimations of less than ca 1 7 m

diamictic sediments within high arctic lake sediment cores evidence for lake ice rafting along the lateral glacial margin sediment cores from six small lake basins in the canadian high arctic reveal a gravel rich 30 by weight to gravel poor 2 diamict facies underlying massive post glacial clayey silt ten other lakes contain a second diamict facies within what are interpreted to be glaciolacustrine sedimentary assemblages the sedimentology clast fabrics and fossil remains diatoms ostracodes and chironomid head capsules within both diamict facies suggest that these deposits are not tills clast fabrics yielded low s1 0 41 0 57 and high s3 0 09 0 22 eigenvalues placing them within the range of ice rafted diamictons and glacigenic sediment flows the high percentage of clast dip angles 45 15 61 random clast azimuth and lower diamict contacts conformable to underlying current bedded sediment favours an origin as a rain out or settling deposit samples of the matrix and scrapings of clasts from the diamicts revealed a diatom assemblage dominated by littoral and planktonic forms such as are found in the littoral regions of the lakes today this contrasts sharply with the assemblages within the overlying clayey silt in which benthic forms predominate clasts are thus interpreted to have been rafted from the littoral areas of the lake the process proposed to explain this is rafting by the lake ice cover in a glacial marginal environment early season meltwater impounded along the lateral margin of retreating cold based glaciers would buoyantly lift the lake ice cover and any adfrozen lake sediment higher lake levels and increased areal extent of seasonal freeze on between the lake ice cover and the lake bed would allow the redeposition of littoral sediments to the benthic regions through greater lateral shifting of the ice cover as it broke up incision by meltwater streams into the lateral glacial margins would later isolate the lake allowing seasonal warming of lake water enough to support the growth and maturation of the ostracode and chironomid species found as fossils within the diamicts

nd and sr isotopic compositions of lavas from the northern mariana and southern volcano arcs implications for the origin of island arc melts nd and sr isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern mariana and southern volcano arcs values of end range from 2 4 to 9 5 whereas 87sr 86sr ranges from 0 70319 to 0 70392 these vary systematically between and sometimes within arc segments the nd and sr isotopic compositions fall in the field of ocean island basalt oib and extend along the mantle array lavas from the volcano arc mariana central island province and the southern part of the northern seamount province have end to 10 and 87sr 86sr 0 7032 to 0 7039 these are often slightly displaced toward higher 87sr 86sr at similar end in contrast those lavas from the northern part of the mariana northern seamount province as far north as iwo jima show oib isotopic characteristics with end and 87sr 86sr 0 7035 to 0 7039 plots of 87sr 86sr and end versus ba la and la yb n support a model in which melts from the mariana and volcano arcs are derived by mixing of oib type mantle or melts therefrom and a metasomatized morb type mantle or melts therefrom an alternate interpretation is that anomalous trends on the plots of nd and sr isotopic composition versus incompatible element ratios found in some s nsp lavas suggest that the addition of a sedimentary component may be locally superimposed on the two component mixing of mantle end members

lithospheric composition and thermal structure of the arabian shield in jordan abstract in this paper a unique set of samples from the uppermost crust down to the lithospheric mantle of jordan is analyzed for composition and petrophysical properties density thermal conductivity radiogenic heat production these data covering a vertical section of almost 65 km are used in conjunction with surface heat flow to generate a detailed and comprehensive lithospheric thermal model that reflects the conditions of the arabian shield as prior to the post oligocene onset of lithosphere thinning and voluminous basaltic volcanism the pre miocene model geotherms based on conductive surface heat flows of 55 and 60 mw m 2 a meet the range of lithosphere asthenosphere boundary depths of 110 160 km known from seismology b conform to results of thermomechanical models on the origin of the dead sea basin that started in miocene time and c are consistent with typical xenolith derived geotherms for terranes of similar age and lithospheric thickness moho temperatures at depths between 35 and 40 km of the as in pre miocene times were most likely in the order of 530 650 c with mantle heat flows averaging between 24 and 29 mw m 2 results contradict former views of the late proterozoic early cambrian stabilized as being an anomalously cold terrane a cold thermal structure inferred from previously measured low surface heat flows generally 45 mw m 2 is inconsistent with the thickness composition and petrophysical properties of the stable lithosphere of the shield

a gps based three dimensional lightning mapping system initial observations in central new mexico a gps based system has been developed that accurately locates the sources of vhf radiation from lightning discharges in three spatial dimensions and time the observations are found to reflect the basic charge structure of electrified storms observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown recently identified as the source of trans ionospheric pulse pairs tipps observed by satellites from space the bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges the latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm peak vhf radiation from the energetic discharges is observed to be typically 30 db stronger than that from other lightning processes and to correspond to source power in excess of 100 kw over a 6 mhz bandwidth centered at 63 mhz

crustal accretion in the pan african nd and sr isotope evidence from the arabian shield abstract nd and sr isotope determinations on late precambrian to early palaeozoic igneous and sedimentary rocks from the arabian shield are used to investigate the proportion of reworked older crust and the rate at which new crust was generated during the pan african event eight rb sr whole rock isochrons on igneous suites yield ages in the range 770 590 ma and initial 87 sr 86 sr ratios of 0 7038 0 7023 these data confirm that magmatism in this area was largely restricted to the period 850 550 ma and the initial ratios are sufficiently low to preclude significant contributions from a long lived upper crustal source the initial 143 nd 144 nd ratios of a variety of lithologies including several samples of possible basement are all higher than the contemporaneous values for chur e nd 1 6 to 6 9 suggesting that many were derived directly from the upper mantle and that any inferred crustal source regions for the remainder could not have separated from likely lree depleted mantle reservoirs before 1200 ma the arabian shield therefore provides an example of rapid crustal growth during the late proterozoic and contrasts with the damara intracratonic belt of namibia where nd and sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period

coincident observations of lightning by the forte photodiode detector the new mexico tech lightning mapping array and the nldn during steps 1 opportunities for observation of lightning simultaneously from space and ground are relatively rare one such opportunity for ground truth occurred during the steps field program on 25 june 2000 the forte satellite passed over a storm that was also observed by a vhf lightning mapping array lma of the 190 flashes mapped during the 2 minute 45 second pass 26 had associated satellite optical data and 3 of these were coincident with the only nldn ground strike locations in the storm during that time the maximum height horizontal extent and duration of flashes detected optically tended to be greater than for those flashes not optically detected results in this paper a companion paper and other published papers suggest that at least under some circumstances cg flashes are relatively more likely than intra cloud ic flashes to be observed by an optical sensor in orbit

island arc character and late precambrian age of volcanics at wadi shwas hijaz saudi arabia geochemical and sr and nd isotopic evidence abstract a summary description of two volcanic sequences named the surgah and the shwas formation situated in the wadi shwas area at long 42 0 lat 20 0 in the arabian shield is given major and trace element concentrations including ree abundances are presented as well as sr and some nd isotope results one sequence the surgah formation shows a tholeiitic differentiation trend from basaltic to rhyolitic rocks whereas the shwas formation has calc alkaline affinities both sequences have low concentrations of incompatible elements notably rb nb zr and ti the coexistence of a tholeiitic and a calc alkaline suite along with these low concentrations is strongly reminiscent of some present day island arc environments an rb sr errorchron age of 721 55ma was obtained on the surgah formation with high initial87sr 86sr ratios around 0 705 the rare earth abundances are overall very low and the patterns show a relative depletion of the light ree in the least evolved rocks indicating a relatively light ree depleted mantle source a time integrated light ree depleted source is also indicated by the fairly radiogenic143nd 144nd ratios end values around 8 the nd model age of source depletion tchurnd is around 2 2 ga

the mantle beneath the red sea margin xenoliths from western saudi arabia abstract xenoliths from alkali basalts in western saudi arabia provide the opportunity to study the composition and rheology of the mantle beneath the red sea rift margins characteristics of mantle xenolith suites from each of three localities in western saudi arabia can be related to locality position relative to the rift axis and to crustal thickness and heat flow at each locality mantle xenoliths from harrat al birk nearest the rift axis are dominantly websterites spinel plagioclase amphibole olivine garnet clinopyroxenite and two pyroxene gabbro olivine peridotite xenoliths are rare garnet clinopyroxenites contain zoned clinopyroxene with fe al rich rims and reaction rims on garnet formed by breakdown of garnet to orthopyroxene clinopyroxene spinel plagioclase zoning and reaction rims are interpreted as forming under conditions of increasing temperature thermobarometry on harrat al birk garnet bearing xenoliths yield high temperatures 1015 1040 c at about 12 kbar the abundance of plagioclase bearing assemblages may be related to a relatively shallow upper mantle which extends up into stability fields for plagioclase bearing pyroxenite and peridotite harrat al kishb and harrat hutaymah are farther from the red sea axis on the flanks of the rift the mantle xenolith suites of al kishb and hutaymah are similar consisting of abundant spinel peridotite and spinel pyroxenite xenoliths and minor garnet pyroxenites plagioclase bearing xenoliths are extremely rare the harrat hutaymah suite includes wehrlite and amphibole bearing peridotite lithologies not found at al kishb variation of peridotite composition may reflect varying degrees of partial melt extraction igneous textures of some pyroxenite xenoliths and structural relationships in composite peridotite pyroxenite nodules suggest that pyroxenites formed by crystallization of magmas within mantle veins abundant pyroxenites and fragments of amphibole veins reflect the activity of magmas and hydrous fluids within the mantle thermobarometry of al kishb and hutaymah garnet bearing nodules yield temperatures of 1000 1050 c at pressures of about 13 5 16 0 kbar mineral zoning and exsolution features indicate decreasing temperature conditions in the mantle at the flanks of the red sea rift

a critical assessment of subglacial mega floods a case study of glacial sediments and landforms in south central alberta canada the existing subglacial megaflood explanation of glacial sediment deposition and landform production in south central alberta relies upon four sets of observations and associated interpretations 1 a subglacial genesis for all glacilacustrine sediments lying stratigraphically below a local melt out till or ice stagnation facies 2 a melt out genesis for the ice stagnation facies 3 a syn depositional status for the glacitectonic disturbance visible in most exposures due to periodic ice bed re coupling and 4 a subglacial megaflood origin for a truncation surface and clast lag that occurs at the top of the sedimentary exposures and the carving of hummocky terrain and flutes in the area we provide alternative interpretations of the sediment landform assemblages of the area and demonstrate that alternative simpler interpretations of the evidence can be proposed specifically we propose 1 a proglacial rather than subglacial origin of glacilacustrine sediments 2 a glacilacustrine origin for the melt out till and its subsequent glacitectonic disturbance by minor glacier readvances 3 postdepositional rather than syn depositional glacitectonic disturbance due to a significant advance by glacier ice after the accumulation of a thick sequence of glacilacustrine sediments 4 that some linear chains of hummocks are the morphological expression of glacitectonic folding and thrusting 5 that the use of the truncation surfaces to propose a subglacial fluvial origin for hummocky moraine is flawed logic because they do not constitute the tops of hummocks but rather occur on the interfluves that separate the numerous small coulees incised into the valley sides during postglacial time truncation surfaces and clast lags occur locally on the beds of spillway tracts and can be explained as the products of erosion by subaerial spillway water the mega flood explanation of flutings and hummocky moraine in the region is also questioned because these landforms record a regional ice flow imprint of mis aligned and cross cutting flow sets terminating at series of inset and locally cross cutting and lobate transverse ridges moraines these landform associations preclude a subglacial flood origin for the flutings and demonstrate that the transverse ridges are more appropriately interpreted as recessional push moraines the similarity of the glacial landforms of this region to the active temperate glacial landsystem produced by historical glacier recession in modern glacierized catchments where sheetfloods have not been influential in subglacial landform production allows us to apply a modern analogue to landscape interpretation that does not include subglacial sheetflooding

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

surface roughness and bulk heat transfer on a glacier comparison with eddy correlation more than 100 simultaneously measured profiles of wind speed and temperature were analyzed to determine the aerodynamic and thermal roughness lengths of glacier ice the results proved to be extremely sensitive to measurement height which is difficult to define precisely on the hummocky terrain of a glacier therefore aerodynamic roughness lengths were estimated from the microtopography of the surface yielding 2 46 mm for ice and 5 5 mm for snow despite the indeterminate nature of the profile analysis it did support the view that the thermal roughness length must decrease as the roughness reynolds number increases a relationship which has been modelled in the literature good agreement between bulk transfer estimates and eddy correlation measurements of fhe sensible heat flux density on melting ice and snow was obtained by combining the aerodynamic roughness length with a suitable model for thermal roughness length

petrology and geochemistry of granulite xenoliths from central hoggar algeria implications for the lower crust granulitic xenoliths enclosed in neogene alkali basalts from the atakor and adrar n addjer volcanic massifs in central hoggar southern algeria include mafic metaigneous and felsic metasedimentary rocks the main mineral phases of metaigneous xenoliths are plagioclase and pyroxenes whereas quartz feldspars sillimanite garnet and sapphirine occur in metasedimentary granulites the metaigneous xenoliths represent a magmatic sequence of cumulitic and non cumulitic rocks which resembles gabbroic bodies associated with anorthosites and has calc alkaline affinities similar types and distributions of granulitic xenoliths are widespread and are probably characteristic of the lower crust in general suggesting its homogeneity on a large scale but heterogencity on a local scale

reply to comments on nd sr isotopic compositions of lower crustal xenoliths evidence for the origin of mid tertiary felsic volcanics in mexico by k l cameron and j v robinson cameron and robinson question the meaning of model nd ages obtained by ruiz et al 1988a for xenoliths from many localities on the easternmost edge of the sierra madre occidental smo this is a legitimate concern because the age of the xenoliths has direct bearing on the genesis of the voluminous silicic ignimbrites of the smo ruiz et al 1988b the source of these volcanics is the underlying and most important issue cameron and co workers have to be congratulated for their pioneering work on the volcanics of the smo besides the careful field work and k ar dating by f w mcdowell s group at texas summarized in mcdowell and clabaugh 1979 and p e damon at arizona i e damon et al 1981 very little petrochemical data existed for this large volcanic province until cameron started working on the problem of the origin of the silicic volcanics in the late 70 s after detailed field and petrologic studies in the batopilas area bagby 1979 cameron et al 1980 lanphere et al 1980 cameron and hanson 1982 cameron and co workers concluded that most of the smo volcanic felsic in chihuahua was produced by crystal fractionation of mantle derived basalt with little or no crustal assimilation contamination or any other crustal interaction in a more recconnaissance type study of volcanic rocks from various places of the sierra madre occidental in northern mexico see figure 1 of cameron and cameron 1985 it was concluded that the parental mantle material needed to produce the silicic volcanic rocks of the smo had sr and nd isotopic values near chur as cameron and co workers admitted their studies were hampered by a complete lack of knowledge of the mexican lower crust the suggestion was however that the lower crust and upper mantle would be sufficiently different in their isotopic and some chemical compositions and therefore that any interaction between mantle derived and the lower crustal materials would be evident in the final chemical and isotopic compositions of the

east antarctic sea ice albedo thickness distribution and snow cover characteristics of springtime sea ice off east antarctica were investigated during a cruise of the australian national antarctic research expedition in october through december 1988 the fractional coverage of the ocean surface the ice thickness and the snow cover thickness for each of several ice types were estimated hourly for the region near the ship these observations were carried out continuously during the 4 weeks the ship was in the ice thin and young ice types were prevalent throughout the region and the observations show a systematic increase in the total area weighted pack ice thickness including open water area from only 0 2 m within 50 km of the ice edge to 0 45 m close to the coast ice thickness averaged over the ice covered region only is also relatively thin ranging from 0 35 m near the ice edge to 0 65 m in the interior these values are probably typical of average winter thickness for the area the average snow cover thickness on the ice increased from 0 05 m near the ice edge to 0 15 m in the interior average ice concentration increased from less than 6 10 near the ice edge to 8 10 in the interior the ship observed concentrations were in good agreement with concentrations derived from passive microwave satellite imagery except in some regions of high concentration in these regions the satellite derived concentrations were consistently lower than those estimated from the ship possibly because of the inability of the satellite sensors to discriminate the appreciable percentage of very thin ice observed within the total area spectral albedo was measured for nilas young grey ice grey white ice snow covered ice and open water at wavelengths from 420 to 1000 nm allwave albedo was computed by using the spectral measurements together with estimates of near infrared albedo and modeled spectral solar flux area averaged albedos for the east antarctic sea ice zone in spring were derived from representative allwave albedos together with the hourly observations of ice types these area averaged surface albedos increased from about 0 35 at the ice edge to about 0 5 at 350 km from the edge remaining at 0 5 to the coast of antarctica the low average albedo is in part due to the large fraction of open water within the pack but extensive fractions of almost snow free thin ice also play an important role

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

lake ice ablation rates from permanently ice covered antarctic lakes in the mcmurdo dry valleys of antarctica three large permanently ice covered closed basin lakes exist along the floor of taylor valley lake ice ablation loss of ice mass is calculated as the sum of sublimation and surface melt and is the driver of ice cover turnover in these systems in taylor valley both manual and automated lake ice ablation rates have been calculated from 2001 to 2011 results indicate relatively consistent winter ablation of 0 07 0 21 m 0 2 0 7 mm w e d 1 summer ablation of lake ice is more variable and ranges from 0 25 to 1 62 m 5 31 mm w e d 1 over an average 51 day period previous to this study ablation rates have been cited as 0 35 m a 1 in the dry valleys from sublimation modeling based on meteorological variables we show that this value has significantly underestimated mean ablation and ice cover turnover on the taylor valley lakes

granulite xenoliths from western saudi arabia the lower crust of the late precambrian arbian nubian shield mafic and intermediate granulite xenoliths collected from cenozoic alkali basalts provide samples of the lower crust in western saudi arabia the xenoliths are metaigneous two pyroxene and garnet granulites mineral and whole rock compositions are inconsistent with origin from red sea rift related basalts and are compatible with origin from island arc calc alkaline and low potassium tholeiitic basalts most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid initial87sr 86sr ratios are less than 0 7032 except for two samples at 0 7049 the sm nd data yield tdm model ages of 0 64 to 1 02 ga similar to typical arabian nubian shield upper continental crust the isotopic data indicate that the granulites formed from mantle derived magmas with little or no contamination by older continent crust calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of gettings et al 1986 based on geophysical data from western saudi arabia estimation of mean lower crustal composition using the granulite xenoliths and the gettings et al 1986 crustal model suggests a remarkably homogeneous mafic lower crust and an andesite or basaltic andesite bulk composition for pan african juvenile continental crust

some aspects of ice phenology on ponds in central alaska usa ice phenology freeze up break up and duration was monitored for five seasons between autumn 2001 and spring 2006 at 11 small shallow ponds in the boreal forest of central alaska usa the sequence in which freeze up fu day of 100 ice cover and break up bu day of zero ice cover occurred at the 11 ponds showed moderately high to very high coherence each season this was probably due to fu and bu being poorly correlated with pond morphometry area depth bu is strongly correlated with april mean air temperature a 18c change in mean april air temperature is equivalent to a 1 86 day change in bu fu and air temperature relationships are inconclusive primarily because post fu warm intervals in two autumns cause an anticorrelation between mean september air temperature and fu mean ice duration varies between 205 and 225 days and is strongly correlated with maximum ice thickness through its effect on bu a 10 mm change in maximum ice thickness will cause a 0 6 day change in ice duration maximum ice thickness and ice composition snow ice congelation ice also have a strong influence on break up when all data from all ponds and all years are considered the predictability of fu and bu sequence the minor role of morphometry in fu and bu the strong role of april mean air temperature in bu and the role of maximum ice thickness in duration suggests that these ponds would be good sites for continued long term observation of phenology and the influence of weather climate variation and change and for freeze up break up process studies particularly the role of ice composition and albedo

geochemistry and sr and nd isotopic study on rare metal bearing granitic rocks central eastern desert egypt abstract three post orogenic granitic plutons exposed in the egyptian basement complex namely homr akarem homrit mukbid and nuweibi were selected for geochemical and nd sr isotopic studies based on petrographic and geochemical considerations the granitic rocks are divided into medium grained pink granites and albite rich granites both types exhibit peraluminous character with many chemical similarities to the high y nb a subtype granites owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity their compositions resemble within plate magma originated in attenuated crustal lithosphere whole rock rb sr isochron ages of the medium grained pink granites from homr akarem and homrit mukbid plutons are 541 4 ma and 527 26 ma respectively the nuweibi albite granite nag represents the youngest granite phase and yields a rb sr age of 490 27 ma the medium grained pink granites show a moderate and narrow range of initial 87 sr 86 sr ratio 0 7064 0 7081 and negative t nd values 1 31 to 6 68 the nag has an initial 87 sr 86 sr 0 7071 and t nd 3 26 to 4 72 comparable to those of the medium grained pink granites the sm nd data yield model ages t dm of 0 96 to 1 70 ga similar nd model ages 1 2 1 8 ga have been reported for other post orogenic granites from southwest egypt and northern sudan the nd and sr isotopic compositions of the studied mineralized granites suggest the involvement of old lree enriched continental crust of pre pan african age and lower crust or mantle derived magmas the nd and sr isotopic variations can be explained by a mixing hypothesis

a comparison of simulated and measured lake ice thickness using a shallow water ice profiler in northern regions where observational data is sparse lake ice models are ideal tools as they can provide valuable information on ice cover regimes the canadian lake ice model was used to simulate ice cover for a lake near churchill manitoba canada throughout the 2008 2009 and 2009 2010 ice covered seasons to validate and improve the model results in situ measurements of the ice cover through both seasons were obtained using an upward looking sonar device shallow water ice profiler swip installed on the bottom of the lake the swip identified the ice on off dates as well as collected ice thickness measurements in addition a digital camera was installed on shore to capture images of the ice cover through the seasons and field measurements were obtained of snow depth on the ice and both the thickness of snow ice if present and total ice cover altering the amounts of snow cover on the ice surface to represent potential snow redistribution affected simulated freeze up dates by a maximum of 22 days and break up dates by a maximum of 12 days highlighting the importance of accurately representing the snowpack for lake ice modelling the late season ice thickness tended to be under estimated by the simulations with break up occurring too early however the evolution of the ice cover was simulated to fall between the range of the full snow and no snow scenario with the thickness being dependant on the amount of snow cover on the ice surface copyright 2011 john wiley sons ltd

thermal structure and water ice heat transfer in a shallow ice covered thermokarst lake in central qinghai tibet plateau abstract a large number of lakes and ponds are unevenly distributed over the qinghai tibet plateau qtp little is known about their ice processes and thermal regimes seasonal ice mass balance thermal regime and stratification of under ice water were investigated in a shallow thermokarst lake in central qtp using in situ observations and global reanalysis data congelation ice grew to 60 70 cm depth while continuous surface sublimation caused a total ice loss of over 30 cm the bulk lake temperature below ice remained above 3 0 c through the ice freezing period and rose gradually up to 7 9 c during the melting period the vertical thermal structure of under ice water consisted of a stable strongly stratified interfacial layer il and an underlying convective layer reaching the lake bottom a warm layer existed just beneath the il since the middle equilibrium period and increased in thickness and temperature there was seasonal variation of il depth and temperature gradient in response to the ice thermodynamics and atmospheric conditions the calculated daily water ice heat flux fw was of 10 w m 2 magnitude seasonal variation of fw manifested both diffusive and convective heat transport to the ice water interface this study suggests that strong penetration of solar radiative flux is the dominant contributor to high fw which results in a relatively thin ice compared with other equivalent high latitude climate

lake ice growth and decay in central alaska usa observations and computer simulations compared abstract the canadian lake ice model climo a one dimensional thermodynamic model withunsteady heat conduction and penetrating solar radiation is used to simulate ice growth and decay onshallow ponds in and near fairbanks central alaska usa simulations are compared with observationsof ice thickness and composition snow ice congelation ice freeze up break up and duration simulations run using us national weather service meteorological data as input variables do not agreewellwith ice thickness measurements the simulations improve significantly when the model is run withmore representative locally measured data forair temperature and depth of snowon the ice the causesof some discrepancies between simulations and observations are discussed and some suggestions formodel improvements are made 1 introduction lake ice growth and decay includes freeze up in theautumn a long period of growth and thickening in winter a short period of ice melting and thinning and finally break up and the complete disappearance of the ice cover inspring the sensitivity of lake ice growth and decay particularly freeze up break up and thus duration some times referred to as ice phenology to climate variabilityandchange has been demonstrated by observation on theground and from space e g palecki and barry 1986 robertson and others 1992 wynne and others 1998 magnuson and others 2000 the sensitivity of icephenology and thickness to climate variability and changehas also been investigated using computer models e g liston and hall 1995b vavrus and others 1996 stefan andfang 1997 zhang and jeffries 2000 an important aspectof computer modelling of lake ice indeed of any modellingexercise is the evaluation of model performance by com parison of simulated results with recent observations andmeasurements heron and woo 1994 liston and hall 1995a fang and others 1996 walsh and others 1998 me nard and others 2002 duguay and others 2003 using the canadian lake ice model climo me nard andothers 2002 duguay and others 2003 morris and others 2005 simulate the effects of the mid 1970s alaska climateshift on lake ice phenology thickness and composition congelation ice snow ice in central alaska usa and thesensitivity of those variables to possible future changes in themagnitude and timing of air temperature and precipitation inthe region prior to performing the climate sensitivity study we assessed the performance of climo by simulating icephenology thickness and composition in ice seasons 2001 02 and 2002 03 and comparing the results with observationsand measurements made at ponds in the vicinity of pokerflat research range pfrr and in fairbanks central alaska this paper describes the model performance assessment which is based on a more comprehensive dataset than wasavailable to duguay and others 2003 for pfrr ice season1999 2000 this assessment includes freeze up and break up data obtained every 1 2days at 11 ponds and weeklydata for ice thickness and the depth and density of the snowon the ice during the winter

rapid disappearance of perennial ice on canada s most northern lake field records aerial photographs and satellite imagery show that the perennial ice cover on ward hunt lake at canada s northern coast experienced rapid contraction and thinning after at least 50 years of relative stability on all dates of sampling from 1953 to 2007 3 5 to 4 3 m of perennial ice covered 65 85 of the lake surface in summer the ice cover thinned from 2008 onward and the lake became ice free in 2011 an event followed by 26 days of open water conditions in 2012 this rapid ice loss corresponded to a significant increase in melting degree days mdd from a mean sd of 80 4 36 5 mdd 1996 2007 to 136 2 16 4 mdd 2008 2012 the shallow bathymetry combined with heat advection by warm inflows caused feedback effects that accelerated the ice decay these observations show how changes across a critical threshold can result in the rapid disappearance of thick perennial ice

a pan african age for the hp ht granulite gneisses of zabargad island implications for the early stages of the red sea rifting abstract up to now the age of granulite gneisses intruded by the zabargad mantle diapir has been an unsolved problem these gneisses may represent either a part of the adjacent continental crust primarily differentiated during the pan african orogeny or new crust composed of miocene clastic sediments deposited in a developing rift crosscut by a diabase dike swarm and gabbroic intrusions and finally metamorphosed and deformed by the mantle diapir previous geochronological results obtained on zabargad island and al lith and tihama asir complexes saudi arabia suggest an early miocene age of emplacement for the zabargad mantle diapir during the early opening of the red sea rift in contrast sm nd and rb sr internal isochrons yield pan african dates for felsic and basic granulites collected 500 600 m from the contact zone with the peridotites devoid of evidence for retrograde metamorphic minerals from a felsic granulite provide well defined rb sr and sm nd dates of 655 8 and 699 34 ma for the hp ht metamorphic event 10 kbar 850 c the thermal event related to the diapir emplacement is not recorded in the sm nd and rb sr systems of the studied gneisses in contrast the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the rb sr isotopic system of the mafic granulite the initial 143 nd 144 nd ratio of the felsic granulite is higher than the contemporaneous value for chur and is in agreement with other nd isotopic data for samples of upper crust from the arabian shield this result suggests that source rocks of the felsic granulite were derived at 1 0 to 1 2 ga from either an average morb type mantle or a local 2 2 ga lree depleted mantle zabargad gneisses represent a part of the disrupted lower continental crust of the pan african afro arabian shield during early stages of the red sea rifting in the miocene these precambrian granulites were intruded and dragged upwards by a rising peridotite diapir

numerical simulation of permafrost thermal regime and talik development under shallow thaw lakes on the alaskan arctic coastal plain 1 thaw lakes are one of the most obvious manifestations of the hydrological system at work in the tundra regions of the alaskan arctic coastal plain but the extent of the role of thaw lakes in arctic land atmosphere interactions and feedback has yet to be fully understood this study uses a two dimensional heat transfer model with phase change under a cylindrical coordinate system to simulate the long term influence of shallow thaw lakes on the thermal regime of permafrost and talik development on the alaskan arctic coastal plain on the basis of previous studies of permafrost and thaw lakes at barrow alaska a series of simulation cases was conducted using different combinations of long term mean lake bottom temperature and lake depth the simulated results indicate that shallow thaw lakes are a significant heat source to permafrost and talik for a thaw lake with a long term mean lake bottom temperature of greater than 0 0 c a talik forms under the thaw lake the maximum talik thicknesses vertical distance from the ground surface to the permafrost surface are 28 0 43 0 and 53 2 m 3000 years after the formation of a shallow thaw lake with long term mean lake bottom temperatures of 1 0 2 0 and 3 0 c respectively talik development rate is very high in the first several years after a thaw lake formation and decreases gradually with time no talik forms below a thaw lake with a long term mean lake bottom temperature equal to or lower than 0 0 c but the temperature of permafrost below the thaw lake increases with time three thousand years after the formation of a thaw lake with a long term mean lake bottom temperature of greater than or equal to 2 0 c ground temperature increases of more than 0 5 c occur as far as 300 m from the lake shore and as deep as about 400 m below the ground surface it is concluded that variation of long term mean lake bottom temperature has a significant influence on permafrost thermal regime and talik development continued monitoring for thaw lake bottom temperature and ground temperature under shallow thaw lakes is needed to further improve the simulation

spectral reflectances of snow and fresh water ice from 340 through 1100 nm interpretation des mesures effectuees au spectroradiometre a balayage determination des plages de reflectance de la neige fraiche ou peu transformee des inclusions de glace de la neige fondante regelee des crepes du brash et slush

a comparison of the energy balance and bulk aerodynamic approaches for estimating glacier melt description de 2 modeles d ablation de glacier sur la base d observations meteorologiques par utilisation d une approche aerodynamique et d un bilan energetique on teste les modeles avec les donnees sur le glacier ivory

the petrogenesis of a type magmas from the amram massif southern israel the 550 530ma alkaline magmatic suite of the amram massif southern israel was emplaced during the transition from an orogenic to an intra plate tectonic setting in the northeastern arabian nubian shield ans the suite ranges from 45 6t o 788 wt sio2 and consists of rhyolites alkali quartz syenites quartz syenites monzonites and co magmatic mafic to felsic alkaline dikes these rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content the felsic members of the suite display a type characteristics and are genetically linked through fractionation to the more mafic ones moderately positive initial end values a2 05 low initial 87 sr 86 sr values 07036 2 high mgo and fe2o3 concentrations 410 895 and 100 125 wt respectively and relatively flat rare earth element patterns la yb na 64 09 in the amram mafic dikes 456 495 wt sio2 suggest their derivation from the sub continental lithospheric mantle above the garnet stability zone the melts program was used to quantitatively model the chemical evolution of the suite extensive anhydrous fractionation 490 of plagioclase alkali feldspar clinopyroxene olivine and minor timagnetite and apatite from parental mafic magmas represented by the amram mafic dikes produced the rhyolitic compositions as well as the intermediate members of the suite this suggests the presence of a large unexposed body of cumulate rocks at depth as well as fusion of a large source region equivalent to an 5km layer in the lithospheric mantle regarded as a representative example for similar a type outcrops in this region this petrogenetic model further suggests that neoproterozoic early cambrian a type magmatism in the northeastern ans represents a significant post orogenic addition of mantlederived material to the juvenile crust this magmatic episode was of a similar magnitude to that of the cenozoic extensionrelated alkaline volcanism of the arabian plate

sedimentation during marine isotope stage 3 at the eastern margins of the glacial lake humber basin england the stratigraphic sequence at north cave on the eastern margins of the lake humber basin records the deposition of a fluvioperiglacial fan lfs 1 4 with early sedimentation lf1 dating to marine isotope stage mis 3 optically stimulated luminescence date range 41 8 38 6 ka and 14c dates 41 6 49 ka bp three phases of permafrost and ice wedge development during mis 3 are evident and indicate possible fan abandonment and hence periods of reduced nival runoff involution structures dated to 11 1 ka with large boulders and fine grained sorted circles in lf4b are interpreted as periglacially cryoturbated littoral deposits with boulders derived from anchor ice initially deposited at the margins of lake humber up to an altitude of 8 m od during mis 2 the style and age of fluvioperiglacial fan deposition at north cave is compatible with several mid devensian sites around britain characterized by significant nival melt and run off from steeply incised valleys in permafrozen cuesta landscapes this phase of fluvioperiglacial fan aggradation to near or below 0 m od is recorded around the glacial lakes humber and fenland basins and indicates that no glacial lakes existed at that time

an energy balance model of lake ice evolution a physically based mathematical model of the coupled lake lake ice snow and atmosphere system is developed for studying terrestrial atmospheric interactions in high elevation and high latitude regions the ability to model lake ice freeze up break up total ice thickness and ice type offers the potential to describe the effects of climate change in these regions model output is validated against lake ice observations made during the winter of 1992 93 in glacier national park montana u s a the model is driven with observed daily atmospheric forcing of precipitation wind speed and air temperature in addition to simulating complete energy balance components over the annual cycle model output includes ice freeze up and break up dates and the end of season clear ice snow ice and total ice depths for two nearby lakes in glacier national park each in a different topographic setting modeled ice features are found to agree closely with the lake ice observations model simulations illustrate the key role that the wind component of the local climatic regime plays on the growth and decay of lake ice the wind speed affects both the surface temperature and the accumulation of snow on the lake ice surface higher snow accumulations on the lake ice depress the ice surface below the water line causing the snow to become saturated and leading to the formation of snow ice deposits in regions having higher wind speeds significantly less snow accumulates on the lake ice surface thus limiting snow ice formation the ice produced by these two different mechanisms has distinctly different optical and radiative properties and affects the monitoring of frozen lakes using remote sensing techniques

ice processes and surface ablation in a shallow thermokarst lake in the central qinghai tibetan plateau the qinghai tibetan plateau qtp is characterized by a cold climate and a large number of lakes the long ice season necessitates study of the widespread ice covers in the region an unprecedented multidisciplinary field campaign was conducted on lake ice processes in the central qtp during the period 2019 13 the study lake generally froze up in late october or early november and broke up in mid or late april with a maximum ice thickness of 50 70 cm the mass balances at both ice surface and bottom were measured continuously significant ice surface sublimation ablation was detected and accounted for up to 40 of the whole ice thickness over the ice season a simple heat transfer model was developed for the surface ice loss the calculated values were in good agreement with the observations they also indicated that atmospheric conditions including low air humidity and prevailing strong winds are the primary drivers of the ice surface sublimation

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

moist convection as an energy source for the large scale motions in jupiter s atmosphere jupiter s dominant large scale weather patterns dimensions 10 000 km are zonal jets and long lived ovals the jets have been flowing east and west at constant speeds of up to 180 m s 1 for over 100 years these jets receive energy from small scale eddies which pump eastward momentum into the eastward jets and westward momentum into the westward jets this momentum transfer was predicted by numerical models before it was observed on jupiter the large ovals roll between the jets in an anticyclonic direction clockwise in the northern hemisphere and counterclockwise in the southern hemisphere where they regularly assimilate small anticyclonic eddies but from where the eddies receive their energy has been an open question here we argue that the eddies which ultimately drive both the jets and the ovals receive their energy from moist convection this hypothesis is consistent with observations of jovian lightning which is an indicator of moist convection it also explains the anticyclonic rotation and poleward drift of the eddies and suggests patterns of upwelling and downwelling that resemble the patterns of large scale axisymmetric overturning in the earth s atmosphere

genesis of silicic magmas during tertiary continental rifting in yemen abstract the magmatism in yemen associated with oligo miocene continental rifting in the red sea and the gulf of aden comprises a bimodal mafic felsic suite represented by flows dykes and plutons this paper reports the chemical and nd sr isotopic compositions of the felsic rocks trachytes syenites rhyolites and granites associated with the yemen trap series and deals with the controversy in regard to the crustal or mantle origin of a type magmas generated in tensional contexts the overall age of the studied rocks ranges from 26 5 to 16 ma the major and trace element compositions of the felsic rocks can partly be explained by fractional crystallization from some of the basic magmas this process was more extensive in the case of the rhyolites and some of the granites than that of the trachytes and the syenite in addition the trachytes differ from the rhyolites by featuring a negative nb anomaly that suggests contamination by continental crust the trachytes have much lower 143 nd 144 nd ratios and higher 87 sr 86 sr ratios than the associated basalts whereas the rhyolites have much higher 87 sr 86 sr ratios but 143 nd 144 nd ratios identical to those of the basalts the plutons span the whole compositional range of isotopic compositions displayed by the trachytes and the rhyolites the isotopic compositions of the felsic rocks do not support their genesis by direct melting of continental crust rather these rocks were generated in an open magma system that evolved by a combination of fractional crystallization and crustal contamination mixing and afc processes involving heterogeneous crust are both required to explain the trachytic and rhyolitic compositions where various amounts of crystal fractionation and crustal contamination were involved the genetic differences between the trachytes and the rhyolites are related to their respective ages the highest degree of contamination observed in the trachytes occurred contemporaneously with the emplacement of the traps and could have been related to a high thermal gradient in the crust due to massive magma transfer at that time this stage presumably corresponds to the impingement of the afar plume head against the lithosphere the less contaminated and more fractionated rhyolites which formed later during continental rifting are consistent with a lower thermal gradient

petrology of a gneiss amphibolite lower crustal unit from zabargad island red sea abstract a metamorphic unit consisting of amphibolites interlayered with felsic gneisses is exposed on zabargad island in tectonic contact with mantle derived peridotites the amphibolites contain relicts of igneous pyroxenites and gabbros which have recrystallized into mafic granulites the chemistry of the gabbros pyroxenites primary assemblage al augite al orthopyroxene kaersutitic pargasite plagioclase suggests that the gabbro pyroxenites are remnants of a basic layered complex which crystallized at pressures not lower than 9 10 kbar and probably not higher than 12 13 kbar the felsic gneisses were originally made of quartz plagioclase orthopyroxene garnet clinopyroxene magnesiohastingsite an assemblage characteristic of high pressures 5 12 kbar garnet granulite facies the high p tonalitic trondhjemitic composition of some of the gneisses suggests they were originally part of the lower continental crust an interpretation supported by the presence of characteristic zircon crystals with ovoidal morphology the association of granulitic gneisses with high p gabbroic rocks suggests that the lower continental crust was intruded by basaltic magmas the pyroxenite gabbro complex underwent deformation when still hot followed by recrystallization at p of about 5 6 kbar during this event breakdown of the al pyroxenes under hydrous conditions resulted in granulitic al poor pyroxenes pargasitic hornblende plagioclase assemblages both the mafic and sialic granulites subsequently underwent further decompression under increasing fluid partial pressure and oxygen fugacity which resulted in widespread amphibolization of the rocks in the presence of relatively high t chlorine rich metasomatic fluids the resulting assemblage mg hornblende plagioclase ilmeno hematite sphene quartz suggests t of 600 700 c p tot o 2 finally rising fluid partial pressure leads to breakdown of mg hornblende and crystallization of metasomatic phases such as alkali richer alumina poorer mg hornblende actinolite cl biotite cl scapolite and cl apatite the zabargad metamorphic unit probably results from underplating of the lower continental crust by basic magma intrusions before and during the early stages of rifting in the central red sea subsequently the lower crustal gabbro gneiss complex was subjected to a number of retrometamorphic events under decreasing p t conditions probably related to progressive uplift during rifting and crustal thinning the last low p metasomatic event is probably related to the final stages of the emplacement of zabargad island

modeling present and future ice covers in two antarctic lakes antarctic lakes with perennial ice covers provide the opportunity to investigate in lake processes without direct atmospheric interaction and to study their ice cover sensitivity to climate conditions in this study a numerical model driven by radiative atmospheric and turbulent heat fluxes from the water body beneath the ice cover was implemented to investigate the impact of climate change on the ice covers from two antarctic lakes west lobe of lake bonney wlb and crooked lake model results agreed well with measured ice thicknesses of both lakes wlb rmse 0 11 m over 16 years of data crooked lake rmse 0 07 m over 1 year of data and had acceptable results with measured ablation data at wlb rmse 0 28 m over 6 years the differences between measured and modeled ablation occurred because the model does not consider interannual variability of the ice optical properties and seasonal changes of the lake s thermal structure results indicate that projected summer air temperatures will increase the ice cover annual melting in wlb by 2050 but that the ice cover will remain perennial through the end of this century contrarily at crooked lake the ice cover becomes ephemeral most likely due to the increase in air temperatures

composition age and origin of two proterozoic diorite tonalite complexes in the arabian shield abstract large amounts of diorite tonalite magma were intruded into the island arc successions of the southern arabian shield between ca 900 and 700 ma ago major oxide trace element rare earth ree and isotopic data are presented for two plutons exemplifying older and younger members of this plutonic phase the thurrat pluton which was emplaced into virtually unmetamorphosed volcanics of sequence b has yielded a 10 point rb sr isochron indicating emplacement 744 22 ma ago and an initial 87 sr 86 sr ratio of 0 70281 it consists of gabbros diorites quartz diorites tonalites and low al 2 o 3 trondhjemites which are compositionally primitive with depleted lithophile element contents and flat ree patterns with a negative eu anomaly in the trondhjemites the magma was most probably mantle derived and analogies with other trondhjemitic plutons suggest that it was probably emplaced in an island arc setting the bidah pluton which was emplaced into a compositionally very immature succession of metamorphosed volcanics and volcaniclastics of sequence c has yielded a nine point near isochron mswd 2 86 indicating an age of 901 37ma and an initial 87 sr 86 sr ratio of 0 70246 this date is accepted as the age of emplacement the rocks are compositionally primitive gabbros diorites quartz diorites tonalites and granodiorites with depleted lithophile element contents and flat ree patterns with a negative eu anomaly in the more siliceous components the country rock volcanics were formed in an immature island arc environment and the composition of the bidah pluton is suggestive of a mainly mantle derived magma emplaced into that arc the data for these two plutons therefore provide further evidence that most of the rock material added to the arabian shield between 900 and 700 ma ago was derived from the mantle

pan african late precambrian tectonic terrains and the reconstruction of the arabian nubian shield the recognition of precambrian ophiolite suites and their dismembered remnants in association with intraoceanic island arc volcanic and plutonic terrains across much of the arabian nubian shield of eastern egypt sudan ethiopia yemen western saudi arabia and sinai has been used by many authors to support the hypothesis of crustal accretion during late proterozoic time 950 550 ma reassembly of the various fragments provides a mosaic of proterozoic microplates in a regular pattern in which at least five oceanic terrains bounded by the remains of ophiolite belts lie between remobilized continental plates to east and west

radarsat backscatter characteristics of ice growing on shallow sub arctic lakes churchill manitoba canada results from an investigation on the evaluation of radarsat c hh imagery for monitoring ice growth and decay and related processes of shallow sub arctic tundra and forest lakes in northern manitoba canada are presented field observations on the structural and stratigraphic characteristics of snow and ice from four lake sites are used in support of the interpretation of changes in synthetic aperture radar backscatter intensity as a function of time and incidence angle 20 49 results show that bubble inclusions most of which are tubular and oriented in the direction of growth strongly influence backscatter intensity from floating ice in radarsat standard beam mode imagery it is shown that radar return can vary considerably as a function of incidence angle differences of as much as 6 5 db were observed for the same ice cover when observed at steeper 20 35 compared with shallower 35 49 incidence angles during the early stages of ice growth and or when the ice volume contains a small amount of tubular bubbles backscatter intensity from the floating ice measured at shallower incidence angles 35 49 is similar to that observed from the grounded ice at any incidence angle 17 to 11 db a strong decrease in backscatter was observed at all sites during spring thaw and was explained by the microwave signal being absorbed by the wet snow cover and by specular reflection from the standing water ponds on the lake ice surface with its multiple beam mode configurations radarsat offers an improved temporal coverage over ers 1 2 thus making it possible to determine more precisely freeze up and break up dates and timing of bottom freezing from shallow arctic and sub arctic lakes copyright 2002 john wiley sons ltd

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

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

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

machine classification of freshwater ice types from landsat i digital data using ice albedos as training sets abstract a method has been used to convert ground measured freshwater ice albedos to landsat 1 digital numbers indirectly corrected for atmospheric attenuation and path radiance the method has been tested by using the digital numbers as training sets to machine classify landsat 1 digital ice cover data from a portion of northern green bay imaged on 13 february 1975 results showed that the conversion algorithm produced digital numbers that when used as training sets classified approximately 50 of the test area differences between the classified test area and a previous machine classification of the scene as well as the percentage of unclassified area caused by lack of data for some surface types points to the need for a more comprehensive well documented library of signatures representing great lakes ice types

timescales of magmatic differentiation from alkali basalt to trachyte within the harrat rahat volcanic field kingdom of saudi arabia a fundamental goal of igneous petrology is to quantify the duration of time required to produce evolved magmas following influx of basalt into the crust however in many cases complex field relations and or the presence of a long lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas therefore precluding calculation of meaningful timescales here we present field relations geochemistry 40ar 39ar ages and 36cl ages for volcanic rocks from the harrat rahat volcanic field saudi arabia these data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptives from 150 to 17 ka the following eruptive sequence occurred four times 1 alkali basalt 2 hawaiite mugearite or benmoreite and 3 trachyte we interpret each eruptive sequence to result from injection of basalt into the crust and its subsequent differentiation and eruption of progressively evolved magmas we use the interval time between successive eruptions within a given sequence to calculate the duration of time required to produce trachyte from alkali basalt differentiation from alkali basalt to intermediate compositions hawaiite mugearite benmoreite took 2 kyr on average differentiation from intermediate compositions to trachyte took a maximum of 6 6 3 5 to 22 5 1 6 kyr thus the total duration of differentiation was 9 to 25 kyr timescales presented here are insensitive to processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple repeated differentiation sequence

spectral albedos of sea ice and incident solar irradiance in the southern beaufort sea spectral albedos and incident spectral irradiances have been measured over the wavelength range 400 to 2400 nm on the sea ice near the naval arctic research laboratory narl at pt barrow alaska the observation interval extended from mid may when the ice was still relatively cold until mid june when summer melting was fully established the temporal dependence of albedo for the available surface types was obtained over this time interval showing a general decrease from snow and snow covered ice to blue ice and melt ponds data were also obtained for glacier ice on the athabasca glacier for melting lake ice and for certain other nonice surfaces in the vicinity of narl snow and ice albedos are characteristically highest at visible wavelengths decreasing strongly in the infrared because of the increase in absorption by ice and water local maxima in the spectra correspond to minima in the ice and water absorption variations in albedo are due primarily to differences in the vapor bubble density crystal structure and free water content of the upper layers of the ice incident spectral shortwave radiation was measured as a function of cloudiness and the optical thickness of arctic clouds is significantly less than the thickest clouds at lower latitudes the decrease of the infrared component relatiave to the visible portion of the irradiance with increasing cloud cover is determined this can give rise to an increase in wavelength integrated albedos of as much as 15 using the present data a graphical method is outlined by which visible near infrared satellite imagery can be used to distinguish among melt ponds open leads and other spring and summer sea ice surface types

mafic crustal xenoliths and the origin of the lower continental crust abstract the la nb ratio 30 ppm at a mg number of 40 in oceanic plateau basalts and morb provide a useful index to distinguish them from arc basalts morb can generally be distinguished from plateau basalts by their lower nb contents

comparison of ground based 3 dimensional lightning mapping observations with satellite based lis observations in oklahoma 3 dimensional lightning mapping observations obtained in central oklahoma by the new mexico tech lightning mapping array lma have been compared with optical observations of the discharges from space obtained by nasa s lightning imaging sensor lis excellent spatial and temporal correlations were obtained between the two sets of observations all lightning discharges seen by lis were mapped by the lma most of the detected optical events were associated with lightning channels that extended into the upper part of the storm cloud to ground discharges that were confined to mid and lower altitudes were less well detected than intracloud discharges and tended to be detected late in the discharge intracloud discharges were readily detected by lis as soon as they extended into the upper part of the storm and often extensively illuminated the cloud at the end or part way through the discharge the extensive illumination was impulsive in nature and was also seen at the end of some cloud to ground discharges

surface albedo measurements over sea ice in the baltic sea during the spring snowmelt period the snow ice albedo was studied during a 4 week field experiment over first year sea ice in the gulf of bothnia baltic sea in spring 2004 observations were made on radiative fluxes cloud cover wind air temperature and humidity as well as snow ice temperature thickness density and grain size the albedo variation during the observation period was large the daily mean albedo ranged from 0 79 over a new snow cover to 0 30 over bare melting ice the evolution of the albedo was related to the surface properties but existing parameterizations based on arctic data did not explain the observations well the snow thickness was found to be the most critical factor affecting the albedo a new parameterization was derived for the albedo dependence on snow thickness to be applied over the baltic sea in spring when periods of melting and freezing alternate but the ice is still relatively thick about 0 6 m the diurnal cycle of solar radiation was large and the snow ice metamorphism due to the melting during daylight and refreezing during the night caused a diurnal albedo cycle with a maximum in the early morning and a minimum in the afternoon with an albedo difference up to 0 14 between the two

lake heimtjonna at dovre mid norway reveals remarkable late glacial and holocene sedimentary environments and the early establishment of spruce picea abies alder alnus cf incana and alpine plants with present centric distributions abstract two and a half meters of sediments from lake heimtjonna in the scandes mountains reveal changes in the vegetation climate and sediment environment since deglaciation the lake was deglaciated in the late glacial lg perhaps as early as 16 18 ka cal bp after deglaciation the sediment environment at the heimtjonna coring point was extraordinary and challenging to interpret the genesis of the 1 7 m basal unsorted sediments including large stones is discussed concluding that the layer was rafted by lake ice after the lg deglaciation the younger dryas yd established a semi perennial lake ice that stopped the deposition of the ice rafted stone rich sediments in the early holocene the unstratified and well sorted clayey silt shows fluvial origin in late holocene strong flood activity including the major flood disaster stor ofsen in ad 1789 caused a sediment hiatus at the coring point from ca 9 5 ka cal bp to ca 250 a cal bp the lg interstadial warming initiated the succession from pioneer plants on mineral soils towards local dwarf shrub heath july temperatures reached at least 7 8 oc in the lg and early holocene papaver radicatum artemisia norvegica and campanula cf uniflora today occurring at dovre and with centric scandinavian distributions locally established all have previously been connected to weichselian survival on nunatak refugia continuous pollen curves of spruce picea abies support the much debated lg and early holocene presence of spruce in the scandes mountains for the first time it is shown that both grey alder alnus incana and armeria occurred in the scandes in the lg and early holocene due to dating errors lg ages is exclusively based on stratigraphical correlations and considerations the lg chronology must be thoroughly tested by future studies providing reliable ams 14c dates of terrestrial fossils

snowmelt runoff processes in a headwater lake and its catchment subarctic canadian shield during the snowmelt season surface runoff into northern canadian shield lakes is governed by the distribution of snow and its ablation storage in catchment slopes and valleys and flow delivery from uplands bottomlands and from upper lakes for the lake studied time lags exist between the commencement of snowmelt and the arrival of runoff to the lake with evaporation from the uplands and bottomlands consuming some of the meltwater produced inflow from an upper lake is delayed with all these inputs the lake becomes flooded first forming a moat followed by an expansion of the open water areas which are subject to evaporation loss lake outflow begins when water level rises above the outlet threshold for the snowmelt season outflow constitutes 40 of total melt and rainfall the lake itself is a major storage that continues to maintain outflow until the lake level drops below the threshold in early summer after which flow ceases snowmelt is the main period when runoff is generated from the headwater catchment to replenish storage depleted in the previous summer and to produce flow along the shield valley that contains a chain of lakes copyright 2006 john wiley sons ltd

bimodal tholeiitic and mildly alkalic basalts from bhir area central deccan volcanic province india geochemistry and petrogenesis abstract bimodal tholeiitic and mildly alkalic basalts occur near bhir in the central part of deccan volcanic province dvp major and trace element concentrations show that of the ten flows nine are tholeiitic and one is an alkalic basalt the bhir basalts have a wide range of chemical composition geochemical variations in the stratigraphic section define three distinct phases of evolution zones 1 to 3 crystal fractionation of plagioclase clinopyroxene olivine and fe ti oxide expanded the compositional range low mg s 39 55 low concentrations of ni and cr and high zr suggest the evolved nature of the bhir basalts fractionation modeling suggests about 42 fractional crystallization in spite of the dominant role of fractional crystallization in the evolution of bhir basalts some other processes must be sought to explain the chemical variations crustal contamination magma mixing and degree of partial melting are suggested to explain the observed chemical variations resorption reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing samples of flows one and four suspected of being contaminated all have enriched sio2 and lile k rb and ba contents and depletion in ti and p believed to be due to granitic crustal contamination as compared to tholeiitic basalts the alkalic basalts are characterized by low sio2 and high tio2 na2o k2o and p2o5 alkalic basalts are richer in lile rb and ba hfse nb zr and y and ree than the tholeiitic basalts the alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the dvp based on major trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of oib was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and or inhomogeneities in the mantle source

nd isotopic characterization of metamorphic rocks in the coast mountains alaskan and canadian cordillera ancient crust bounded by juvenile terranes nd isotopic data are reported for 52 samples from the crustal region between the alexander wrangellia terrane and the stikine terrane of the alaskan and canadian cordillera this region is composed of the gravina belt a jurassic cretaceous assemblage of volcanic and clastic sedimentary rocks the taku terrane a terrane of probable early permian to late triassic age and four assemblages of metamorphic rocks that occur to the west of and within the coast mountains batholith the gravina belt has end t values that range from 1 1 to 8 3 similar to values of the underlying alexander terrane and consistent with the interpretation that it is a juvenile belt that formed in a back arc or intra arc basin within the alexander terrane mid cretaceous plutons that were emplaced into the gravina belt have end t values of 4 4 to 5 7 and were probably produced by mantle derived melts that incorporated some alexander terrane crust the taku terrane has end 0 values that range from 5 5 to 3 3 with corresponding depleted mantle model tdm ages of 440 to 1430 ma a mid cretaceous pluton intruding the taku terrane has an end t value of 5 1 a value indistinguishable from those determined for cretaceous plutons intruding the gravina belt metamorphic rocks east of and structurally overlying the taku terrane are divided into the tracy arm assemblage end 0 26 to 0 tdm 800 2450 ma the endicott arm assemblage end 0 10 to 1 3 tdm 950 1500 ma the port houghton assemblage end 0 9 4 to 1 1 tdm 550 1500 ma and the ruth assemblage end 0 9 4 to 2 0 tdm 650 1300 ma these isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from precambrian continental crust the metamorphic rocks from these assemblages are lithologically very similar to rocks of the yukon tanana ytt terrane of eastern alaska and yukon territory and have such similar u pb detrital zircon ages and nd isotopic compositions to ytt rocks that they are considered part of that terrane possible tectonic scenarios that can explain the present geometry of the ytt with respect to the alexander wrangellia and stikine terranes include 1 the ytt is the upturned stratigraphic basement of the stikine terrane 2 part of the ytt was structurally emplaced beside the stikine terrane in a transpressive tectonic regime 3 the stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the ytt before the final accretion of the alexander wrangellia terrane

theory of the optical properties of lake ice a radiative transfer model is developed to illustrate the processes which determine the spectral albedo and transmission of lake ice the calculated spectral albedo is dominated by specular reflection from the ice surface in the near infrared whereas multiple scattering by bubbles below the surface dominates the visible albedo adding a snow cover to lake ice will normally increase the visible albedo but may reduce the albedo in some regions of the near infrared if the sun is low by reducing the specular reflection in a preliminary test of the model spectral albedo was measured on the natural ice cover of a frozen lake the measurements are explained using the radiative transfer model applied to the air bubble size distribution measured in the same ice the uncertainty in measurement of bubble size distribution leads to uncertainty in the theoretical albedo which is much larger than the error due to approximations used in derivation of the model