seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

evolution of the deep atlantic water masses since the last glacial maximum based on a transient run of ncar ccsm3 during the last deglaciation from approximately 21 to 11 thousand years ago the high latitudes of the atlantic ocean underwent major changes besides the continuous warming the polar and subpolar ocean surface received a large amount of meltwater from the retracting ice sheets these changes in temperature and salinity affected deep waters such as the antarctic bottom water aabw and the north atlantic deep water nadw which are formed in the southern ocean and in the northern north atlantic respectively in this study we present the evolution of the physical properties and distribution of the aabw and the nadw since the last glacial maximum using the results of a transient simulation with ncar ccsm3 in this particular model scenario with a schematic freshwater forcing we find that modern nadw with its characteristic salinity maximum at depth was absent in the beginning of the deglaciation while its intermediate version glacial north atlantic intermediate water gnaiw was being formed gnaiw was a cold and relatively fresh water mass that dominated intermediate depths between 60 and 20 n at this time most of the deep and abyssal atlantic basin was dominated by aabw within the onset of the bolling allerod period at nearly 15 thousand years ago ka gnaiw expanded southwards when the simulated meridional overturning circulation overshoots the transition between gnaiw and nadw ocurred after that when aabw was fresh enough to allow nadw to sink deeper in the water column when the nadw appears 11 ka aabw retracts and is constrained to lie near the bottom

the mechanisms of sea ice melt pond formation and evolution 1 a series of observations were made on melting first year landfast arctic sea ice near barrow alaska to explore the seasonal evolution of melt pond coverage observations of pond coverage albedo and ice properties are combined with terrestrial lidar measurements of surface topography and meltwater balance to quantitatively identify the timing and role of mechanisms driving pond coverage the formation of interposed fresh ice is found to eliminate meltwater percolation through early pond formation and allow widespread ponding well above sea level pond drainage to sea level occurs principally by horizontal meltwater transport over the ice surface to macroscopic flaws freeboard loss caused by buoyancy decline as the ice thins controls pond growth late in the melt season after percolation begins the majority of the macroscopic flaws that drain melt ponds to sea level are observed to develop from brine drainage channels within the ice a simple thermodynamic model of meltwater percolation illustrates that fresh meltwater inflow causes pores in the ice to either shrink and freeze shut or enlarge based on initial size and ice temperature this threshold behavior of pore diameter controls both the blockage of smaller pores with interposed ice and the enlargement of larger brine drainage channels to allow meltwater drainage the results identify links between the temporal evolution of pond coverage and ice temperature salinity and thickness providing new opportunities to realistically parameterize ponds and summer ice albedo within sea ice models

oxygen isotope fractionation during the freezing of sea water the dependence of oxygen isotope fractionation on ice growth rate during the freezing of sea water is investigated based on laboratory experiments and field observations in mcmurdo sound antarctica the laboratory experiments were performed in a tank filled with sea water with sea ice grown under calm conditions at various room temperatures ranging from 58c to 208c in mcmurdo sound the ice growth rate was monitored using thermistor probes for first year landfast ice that grew to 2 m in thickness combining these datasets allows for the first time examination of fractionation at a wide range of growth rates from 0 8 10 7 to 9 3 10 7 ms 1 in the analysis a stagnant boundary layer model is parameterized using these two independent datasets as a result the optimum values of equilibrium pure ice fractionation factor and boundary layer thickness are estimated it is suggested that a regime shift may occur at a growth rate of 2 0 10 7 ms 1 a case study on sea ice in the sea of okhotsk where the growth rate is modeled by coupling the thermodynamic properties of the sea ice with meteorological data demonstrates the utility of the fitted models

snow depth and ice thickness measurements from the beaufort and chukchi seas collected during the amsr ice03 campaign in march 2003 a field validation campaign was conducted on the sea ice near barrow ak the goal of this campaign was to produce an extensive dataset of sea ice thickness and snow properties depth and stratigraphy against which remote sensing products collected by aircraft and satellite could be compared chief among these were products from the polarimetric scanning radiometer psr flown aboard a nasa p 3b aircraft and the aqua advanced microwave scanning radiometer for the earth observing system amsr e the data were collected in four field areas three on the coastal sea ice near barrow ak and the fourth out on the open ice pack 175 km northeast of barrow the snow depth ranged from 9 4 20 8 cm in coastal areas n 9881 for three areas with the thinnest snow on ice that had formed late in the winter out in the main pack ice the snow was 20 6 cm deep n 1906 the ice in all four areas ranged from 138 219 cm thick n 1952 with the lower value again where the ice had formed late in the winter snow layer and grain characteristics observed in 118 snow pits indicated that 44 of observed snow layers were depth hoar 46 were wind slab snow and ice measurements were keyed to photomosaics produced from low altitude vertical aerial photographs using these and a distinctive three way relationship between ice roughness snow surface characteristics and snow depth strip maps of snow depth each about 2 km wide were produced bracketing the traverse lines these maps contain an unprecedented level of snow depth detail against which to compare remote sensing products the maps are used in other papers in this special issue to examine the retrieval of snow properties from the psr and amsr e sensors

quantitative analysis of trace oh in garnet and pyroxenes to calibrate infrared ir spectroscopy for quantitative analysis of trace structural oh in specific minerals we have determined concentrations of h in pure separates of mantle derived pyrope garnet 56 6 ppm h 2o by weight augite 268 8 ppm h 2o and enstatite 217 11 ppm h 2o by manometry after heating the samples and extracting h 2 gas under vacuum ir spectroscopy confirmed the presence of intrinsic oh in these samples prior to extraction and indicated between 86 and 100 removal of h during the extraction procedure the integral specific absorption coefficients of 1 39 0 14 l 7 09 0 32 and 15 6 0 94 ppm h 2o cm 2 for pyrope augite and enstatite respectively allow precise spectroscopic determination of the oh content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million uncertainties in accuracy depend on mineral composition and characteristics of the oh absorption spectrum and are estimated to range between 10 and 50

effects of al content on water partitioning between orthopyroxene and olivine implications for lithosphere asthenosphere boundary abstract to investigate the partitioning coefficient of water between orthopyroxene opx and olivine ol d opx ol under low water concentrations 3 387 wt ppm similar to the earth s mantle conditions high pressure experiments have been conducted at pressures of 1 5 6 gpa and a temperature of 1573 k the experiments were performed with kawai type multi anvil and piston cylinder apparatus by using starting materials of natural ol and synthetic opx with various al contents the water contents were obtained with a vacuum type fourier transform infrared spectrometer ft ir6100 irt5000 ir spectra of ol and al bearing opx in this study are similar to those obtained by high pressure experiments bai and kohlstedt 1993 and natural rocks grant et al 2007 respectively it is believed that broad bands in ir spectra of natural opx are due to effect of crystal distortion by large al substitution on the contrary ir spectra of al free opx are not consistent with those reported by rauch and keppler 2002 likely because of the large difference of water fugacity d al free opx ol is 1 at all pressure conditions or decreases with increasing pressure however the water contents of al bearing opx are significantly larger than those of ol at the same conditions in addition the effect of al concentration in opx on d opx ol becomes larger with increasing pressure the high al content in opx significantly increases d opx ol and the trend increases with increasing pressure d opx ol drops sharply at the pressure at which the al concentration of opx becomes nearly 0 in the earth s mantle conditions this conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no al the dramatic change of d opx ol may explain the lithosphere asthenosphere boundary beneath oceans and continents

circulation on the north central chukchi sea shelf abstract mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central chukchi shelf previous studies indicated that pacific waters crossed the chukchi shelf through herald valley in the west and barrow canyon in the east we find a third branch through the central channel onto the outer shelf the central channel transport varies seasonally in phase with bering strait transport and is 0 2 sv on average although some of this might include water entrained from the outflow through herald valley a portion of the central channel outflow moves eastward and converges with the alaskan coastal current at the head of barrow canyon the remainder appears to continue northeastward over the central outer shelf toward the shelfbreak joined by outflow from herald valley the mean flow opposes the prevailing winds and is primarily forced by the sea level slope between the pacific and arctic oceans current variations are mainly wind forced but baroclinic forcing associated with upstream dense water formation in coastal polynyas might occasionally be important winter water mass modification depends crucially on the fall and winter winds which control seasonal ice development an extensive fall ice cover delays cooling limits new ice formation and results in little salinization in such years bering shelf waters cross the chukchi shelf with little modification in contrast extensive open water in fall leads to early and rapid cooling and if accompanied by vigorous ice production within coastal polynyas results in the production of high salinity 33 shelf waters such interannual variability likely affects slope processes and the transport of pacific waters into the arctic ocean interior

decline in arctic sea ice thickness from submarine and icesat records 1958 2008 1 the decline of sea ice thickness in the arctic ocean from icesat 2003 2008 is placed in the context of estimates from 42 years of submarine records 1958 2000 described by rothrock et al 1999 2008 while the earlier 1999 work provides a longer historical record of the regional changes the latter offers a more refined analysis over a sizable portion of the arctic ocean supported by a much stronger and richer data set within the data release area dra of declassified submarine sonar measurements covering 38 of the arctic ocean the overall mean winter thickness of 3 64 m in 1980 can be compared to a 1 89 m mean during the last winter of the icesat record an astonishing decrease of 1 75 m in thickness between 1975 and 2000 the steepest rate of decrease is 0 08 m yr in 1990 compared to a slightly higher winter summer rate of 0 10 0 20 m yr in the five year icesat record 2003 2008 prior to 1997 ice extent in the dra was 90 during the summer minimum this can be contrasted to the gradual decrease in the early 2000s followed by an abrupt drop to 55 during the record setting minimum in 2007 this combined analysis shows a long term trend of sea ice thinning over submarine and icesat records that span five decades

ocean circulation and fronts as related to ice melt back in the chukchi sea the work reported herein was supported in part by the arctic submarine laboratory naval ocean systems center san diego california under project order nos 000007 and 00164

electrical conductivity of enstatite as a function of water content implications for the electrical structure in the upper mantle abstract the electrical conductivity of ca free aluminous enstatite with various water contents has been determined at a pressure of 3 gpa in a kawai type multi anvil apparatus impedance spectroscopy was performed for both hydrogen doped and undoped samples in a frequency range from 0 1 hz to 1 mhz to examine the effect of water on conductivity two conduction mechanisms were identified for hydrogen undoped samples at temperature of 1000 1723 k and for hydrogen doped samples at relatively lower temperature range of 500 900 k to minimize dehydration of samples for the hydrogen undoped samples the activation enthalpy is around 1 9 ev at the higher temperatures range 1300 k suggesting that the dominant charge transfer mechanism is fe 2 fe 3 hopping small polaron conduction for the hydrogen doped samples measured below 900 k the activation enthalpy decreases from 1 11 to 0 70 ev and the conductivity values systematically increase with increasing water content suggesting that proton conduction is the dominant conduction mechanism taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account all electrical conductivity data were fitted to the formula 0 h exp h h kt 0 p c w exp h p 0 c w 1 3 kt where 0 is pre exponential factor c w is the water content in weight percent h is the activation enthalpy h p 0 is the activation enthalpy for proton conduction at very low water concentration is the geometrical factor k is the boltzmann constant t is absolution temperature and subscripts h and p represent hopping and proton conductions respectively using the present results a laboratory based conductivity depth profile in the earth s upper mantle has been constructed as a function of water content comparison of our model with the currently available geophysical observations beneath the eastern pacific rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine

interannual variability of summer sea ice thickness in the siberian and central arctic under different atmospheric circulation regimes extensive drill hole and electromagnetic induction measurements of sea ice thickness in the siberian and central arctic seas in the summers of 1993 1995 and 1996 reveal significant interannual variability in the laptev sea minimum and maximum modal first year ice thicknesses amounted to 1 25 and 1 85 m in 1995 and 1996 respectively ice thickness correlates with ice extent which reached a record minimum in august 1995 and was well above average in 1996 these differences are explained by the strength and location of a summer cyclonic atmospheric circulation pattern affecting both ice advection and surface melt from drifting buoys deployed in 1995 and satellite radar backscatter data first and second year ice regimes are delineated differences in first year ice backscatter coefficients between 1993 1995 and 1996 are explained by differences in level ice surface roughness the lagrangian evolution of ice thickness between 1995 and 1996 is studied while the shape of the thickness distribution does not change significantly the mean modal ice thickness of the ice field increases from 1 80 m 1 25 m in 1995 to 2 86 m 2 25 m in 1996 the thickness distribution of second year ice in 1996 closely agrees with that of level multiyear ice downstream in the transpolar drift obtained in 1991 in 1996 mean level ice thickness increases at 0 23 and 0 16 m deg 1 with latitude in the kara and laptev sea sectors of the arctic ocean respectively

thin and thinner sea ice mass balance measurements during sheba 1 as part of a large interdisciplinary study of the surface heat budget of the arctic ocean sheba we installed more than 135 ice thickness gauges to determine the sea ice mass balance while installing these gauges during the fall of 1997 we found that much of the multiyear ice cover was only 1 m thick considerably thinner than expected over the course of the yearlong field experiment we monitored the mass balance for a wide variety of ice types including first year ice ponded ice unponded ice multiyear ice hummocks new ridges and old ridges initial ice thicknesses for these sites ranged from 0 3 to 8 m and snow depths varied from a few centimeters to more than a meter however for all of their differences and variety these thickness gauges sites shared a common trait at every site there was a net thinning of the ice during the sheba year the thin ice found in october 1997 was even thinner in october 1998 the annual cycle of ice thickness was also similar at all sites there was a steady increase in thickness through the winter that gradually tapered off in the spring this was followed by a steep drop off in thickness during summer melt and another tapering in late summer and early fall as freeze up began maximum surface melting was in july while bottom ablation peaked in august combining results from the sites we found an average winter growth of 0 51 m and a summer melt of 1 26 m which consisted of 0 64 m of surface melt and 0 62 m of bottom melt there was a weak trend for thicker ice to have less winter growth and greater net loss for the year however ice growth was also impacted by the snow depth considerable variability was observed between sites in both accretion and ablation the total accretion during the 9 month growth season ranged from zero for thick ridged ice to more than a meter for young ice ponds tended to have a large amount of surface melting while ridges had considerable bottom ablation

arctic sea ice melt in 2008 and the role of solar heating there has been a marked decline in the summer extent of arctic sea ice over the past few decades data from autonomous ice mass balance buoys can enhance our understanding of this decline these buoys monitor changes in snow deposition and ablation ice growth and ice surface and bottom melt results from the summer of 2008 showed considerable large scale spatial variability in the amount of surface and bottom melt small amounts of melting were observed north of greenland while melting in the southern beaufort sea was quite large comparison of net solar heat input to the ice and heat required for surface ablation showed only modest correlation however there was a strong correlation between solar heat input to the ocean and bottom melting as the ice concentration in the beaufort sea region decreased there was an increase in solar heat to the ocean and an increase in bottom melting

synoptic airborne thickness surveys reveal state of arctic sea ice cover 1 while summer arctic sea ice extent has decreased over the past three decades it is subject to large interannual and regional variations methodological challenges in measuring ice thickness continue to hamper our understanding of the response of the ice thickness distribution to recent change limiting the ability to forecast sea ice change over the next decade we present results from a 2400 km long pan arctic airborne electromagnetic em ice thickness survey in april 2009 the first ever large scale em thickness dataset obtained by fixed wing aircraft over key regions of old ice in the arctic ocean between svalbard and alaska the data provide detailed insight into ice thickness distributions characteristic for the different regions comparison with previous em surveys shows that modal thicknesses of old ice had changed little since 2007 and remained within the expected range of natural variability

indirect measurements of the mass balance of summer arctic sea ice with an electromagnetic induction technique abstract in the framework of the surface heat budget of the arctic ocean sheba study indirect non invasive icemass balance measurements were carried out at a drifting station in the northern chukchi sea between mayand august of 1998 ice thickness was derived from electromagnetic induction em measurements ofapparent conductivity along 13 profiles 60 to 900 m long as shown through sensitivity studies with a 1 d model the apparent conductivity data from individual points can be inverted to yield estimates of icethickness and ablation with an accuracy of approximately 0 05 m for 2 m thick level ice ablation ratesranged between 8 and 18 mm d 1 with total ablation amounting to roughly 0 9 to 1 2 m measurements ofthickness and melt rates along different profiles in undeformed multi year ice corresponded closely indicating that the sampling statistics are adequate the roughness of undeformed ice has been found toincrease during the summer due to deepening of melt ponds and enhanced bottom melt ice under melt pondswas disproportionately thinner most likely a result of thicker snow cover reducing winter accretion

circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum september 2007 marked a record minimum in sea ice extent while there have been many studies published recently describing the minimum and its causes little is known about how the ice thickness has changed in the run up to and following the summer of 2007 using satellite radar altimetry data covering the arctic ocean up to 81 5 degrees north we show that the average winter sea ice thickness anomaly after the melt season of 2007 was 0 26 m below the 2002 2003 to 2007 2008 average more strikingly the western arctic anomaly was 0 49 m below the six year mean in the winter of 2007 2008 these results show no evidence of short term preconditioning through ice thinning between 2002 and 2007 but show that after the record minimum ice extent in 2007 the average ice thickness was reduced particularly in the western arctic citation giles k a s w laxon and a l ridout 2008 circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum geophys res lett 35 l22502 doi 10 1029 2008gl035710

physical and morphological properties of sea ice in the chukchi and beaufort seas during the 2010 and 2011 nasa icescape missions abstract physical and morphological properties of sea ice in the chukchi and western beaufort seas were measured during the 2010 and 2011 june july icescape impacts of climate on the eco systems and chemistry of the arctic pacific environment missions aboard the uscgc healy observations of ice conditions including ice thicknesses types and concentrations of primary secondary and tertiary categories were reported at 2 h intervals while the ship was in transit using the antarctic sea ice processes and climate aspect protocol on ice surveys of ice thickness melt pond depth and ice properties including profiles of internal temperature salinity and isotopic composition were conducted at 21 total ice stations 12 in 2010 and 9 in 2011 comparison to historical ship based observations confirms a multi decadal transition from a multiyear to thinner first year dominated seasonal sea ice pack in the region with much earlier ice retreat than in past decades the ice encountered was predominantly 98 first year with un deformed ice thicknesses ranging from 0 73 to 1 2 m pond coverage was extensive averaging 29 in 2010 and 19 in 2011 resulting in considerable light absorption in the ice and transmission of light to the ocean enhanced melting near the ice edge is consistent with transport of pacific origin heat and or ice albedo feedbacks sediment entrainment was visible in 7 5 and 10 9 of the ice in 2010 and 2011 respectively overall the results indicate a regime shift in the characteristics of the ice cover has occurred in the region over recent decades with substantive implications for thermodynamic forcing light availability in the upper ocean and biological and biogeochemical processes in the ice and water column beneath the results presented have applications for the interpretation of optical and biological measurements in the chukchi sea and serve as record of ice conditions for assessing long term change

on the origin and evolution of sea ice anomalies in the beaufort chukchi sea the origin and space time evolution of beaufort chukchi sea ice anomalies are studied using data and a recently developed dynamic thermodynamic sea ice model first the relative importance of anomalies of river runoff atmospheric temperature and wind in creating anomalous sea ice conditions in the beaufort chukchi sea is investigated the results indicate that wind anomalies are the dominant factor responsible for creating interannual variability in the beaufort chukchi sea ice cover temperature anomalies appear to play a major role for longer time scale fluctuations whereas the effects of runoff anomalies are small the sea ice model is then used to track the position of a positive sea ice anomaly as it is transported by the beaufort gyre toward the transpolar drift stream and then exported out of the arctic basin into the greenland sea via fram strait the model integration shows that sea ice anomalies originating in the western beaufort sea can survive a few seasonal cycles as they propogate through the arctic basin and can account for a notable amount of anomalous ice export into the greenland sea these anomalies however represent a small contribution to the fresh water budget in this area when compared with sea ice fluctuations generated by interannually varying local winds

linkages among runoff dissolved organic carbon and the stable oxygen isotope composition of seawater and other water mass indicators in the arctic ocean 1 flow weighted dissolved organic carbon doc concentrations and 18o values were determined from major arctic rivers specifically the ob yenisey lena kolyma mackenzie and yukon during 2003 2004 these data were considered in conjunction with marine data for doc 18o values nutrients salinity and fluorometric indicators of doc obtained during sampling at the shelf basin boundary of the chukchi and beaufort seas on the basis of these data freshwater in the sampled marine waters is likely derived from regional sources such as the mackenzie the bering strait inflow and possibly eastern siberian rivers including the kolyma or the lena but not rivers farther west in the eurasian arctic freshwater from melted sea ice is insignificant over annual cycles although melted sea ice was a locally dominant freshwater component following summer sea ice retreat in 2002 doc concentrations were correlated with the runoff fraction with an apparent meteoric water doc concentration of 174 1 m this is lower than the flow weighted concentrations measured at river mouths of the five largest arctic rivers 358 to 917 m indicating removal of doc during transport through estuaries shelves and in the deep basin flow weighted doc concentrations in the two largest north american arctic rivers the yukon 625 m and the mackenzie 358 m are lower than in the three largest eurasian arctic rivers the ob 825 m the yenesey 858 m and the lena 917 m a fluorometer responding to chromophoric dissolved organic matter cdom was not correlated with doc concentrations in pacific influenced surface waters unlike previous observations in the atlantic layer nutrient distributions concentrations and derived ratios suggest the cdom fluorometer may be responding to the release of chromophoric materials from shelf sediments shipboard incubations of undisturbed sediment cores indicate that sediments on the bering and chukchi sea shelves are a net source of doc to the arctic ocean

desalination processes of sea ice revisited 1 we reexamine five processes that have been suggested to be important for the loss of salt from sea ice these processes are the initial fractionation of salt at the ice ocean interface brine diffusion brine expulsion gravity drainage and flushing with surface meltwater we present results from analytical and numerical studies as well as from laboratory and field experiments that show that among these processes only gravity drainage and flushing contribute to any measurable net loss of salt we show that during ice growth the salinity field is continuous across the ice ocean interface hence there is no immediate segregation of salt at the advancing front

physical properties of summer sea ice in the fram strait the physical properties of sea ice in the fram strait region of the greenland sea were examined during june and july 1984 in conjunction with the marginal ice zone experiment field program most of the ice sampled within fram strait during this period was multiyear thicknesses and other properties indicated that none of the multiyear ice was older than 4 to 5 years snow cover on the multiyear ice averaged 29 cm while that on first year ice averaged only 8 cm deep this difference may be related to enhanced sublimation of the snow on the thinner first year ice the salinity profiles of first year ice clearly show the effects of ongoing brine drainage in that profiles from cores drilled later in the experiment are substantially less saline than earlier cores thin section examinations of crystal structure indicate that about 75 of the ice consisted of congelation ice with typically columnar type crystal structure the remaining 25 consisted of granular ice with only a few occurrences of snow ice the granular ice consisted primarily of frazil found in small amounts at the top of floes but mainly observed in multiyear ridges the horizontally oriented crystal c axes showed various degrees of alignment ranging from no alignment to strong alignments in which the alignment direction changed with depth implying a change in floe orientation with respect to the ocean current at the ice water interface during ice growth evidence of crystal retexturing was observed in the upper meter of nearly every multiyear core this retexturing consisting of grain boundary smoothing and nearly complete obliteration of the ice platelet brine layer substructure is attributed to summer warming

tracer studies of pathways and rates of meltwater transport through arctic summer sea ice 1 at the surface heat budget of the arctic ocean sheba program s field site in the northern chukchi sea snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998 pathways and rates of meltwater transport were derived from tracer studies h218o 7be and release of fluorescent dyes complemented by in situ sea ice permeability measurements it was shown that the balance between meltwater supply at the surface averaging between 3 5 and 10 5 mm d 1 and ice permeability between 10 9 m2 determines the retention and pooling of meltwater which in turn controls ice albedo we found that the seasonal evolution of first year and multiyear ice permeability and surface morphology determine four distinct stages of melt at the start of the ablation season stage 1 ponding is widespread and lateral melt flow dominates several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones significant formation of underwater ice composed between 50 of meteoric water formed at these drainage sites complete removal of snow cover increase in ice permeability and reductions in hydraulic gradients driving fluid flow mark stage 2 concurrent with a reduction in pond coverage and albedo during stage 3 maximum permeabilities were measured with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first year and thin multiyear ice enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration concurrent with increases in pond salinities and 7be advective heat flows of several tens of watts per square meter were derived promoting widening of ponds and increases in pond coverage stage 4 corresponds to freeze up roughly 40 of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under ice ponds with a total net retention of 15 based on this work areas of improvement for fully prognostic simulations of ice albedo are identified calling for parameterizations of sea ice permeability and the integration of ice topography and refined ablation schemes into atmosphere ice ocean models

the effect of possible taylor columns on the summer ice retreat in the chukchi sea the chukchi sea topography consists of a broad flat 50 m deep plain with two prominent shoals herald and hanna which have horizontal dimensions of about 100 km and rise to depths of about 30 m herald shoal in particular is a prominent isolated seamount which because of the warm water flux in summer through bering strait has a warm 0 1 m s 1 flow incident upon it examination of active and passive microwave imagery of the region for 1992 1994 shows that as the general ice cover recedes the ice remains preferentially over herald shoal for 3 4 weeks after the surrounding ice has melted a scale analysis suggests that the cause of the ice persistence is the formation of a taylor column over the shoal which traps cold water and ice above it a similar trapping of ice occurs over hanna shoal and on its eastern slope but the response is complicated by a very different topography and the northward flow down barrow canyon

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

the temperature dependence of water solubility in enstatite the solubility of water in pure enstatite was measured on samples synthesized under water saturated conditions at 15 kbar and temperatures ranging from 700 to 1 100 c polarized ftir measurements on millimetre sized clear crystals showed that water solubility increases strongly with temperature from 101 ppm by weight at 700 c to 269 ppm by weight at 1 100 c the position and shape of the infrared bands hardly changes with temperature with one notable exception a band close to 3 380 cm 1 is present in samples synthesized between 700 and 1 000 c while this band is absent from samples synthesized at 1 100 c this effect appears to be very reproducible and points towards a slight change in the crystal structure of enstatite between 1 000 and 1 100 c at 15 kbar the water solubility data of this study as well as those of rauch and keppler contrib mineral petrol 143 525 536 2002 can be reproduced by the equation k a f text h 2 text o exp delta h 1 text bar rt exp delta v text solid p rt where k is water solubility f text h 2 text o is water fugacity a is 0 01354 ppm bar vsolid 12 1 cm3 mol is the volume change of enstatite during incorporation of water and h1 bar 4 563 j mol is the reaction enthalpy at 1 bar this equation predicts the following behaviour of water solubility in enstatite as a function of pressure and temperature 1 water solubility increases with pressure up to a maximum around 80 kbar 2 water solubility decreases with temperature at 1 bar and 3 water solubility increases with temperature between 10 and 100 kbar if the observed temperature dependence for enstatite were representative for other upper mantle minerals as well it would have the following implications 1 lateral temperature gradients in the upper mantle could cause major variations in water contents at the same depth in particular hot mantle plumes may scavenge water from the surrounding shallow upper mantle 2 the scavenging of water by hot plumes could be a major factor in increasing the mobility of plumes 3 the predicted temperature dependence of water solubility at the base of the upper mantle may allow plumes to bypass the transition zone water filter postulated by bercovici and karato nature 425 39 44 2003

drifting arctic sea ice archives changes in ocean surface conditions 18o profiles in drifting arctic sea ice are coupled with back trajectories of ice drift and an ice growth model to reconstruct the surface hydrography of the arctic ocean interior the results compare well with 18o values obtained by traditional oceanographic methods and known water mass distributions analysis of the stable isotopic composition of sea ice floes sampled at strategic and relatively accessible locations e g fram strait could aid in mapping spatial and temporal variations in arctic ocean surface waters