hot water drilling and bore hole closure in cold ice drilling bore holes in deep cold ice masses by hot water methods and maintaining these holes with sufficient diameter to allow down hole experimentation poses a major obstacle to the investigation of conditions beneath ice sheets and ice streams closure of the water filled holes by refreezing is the dominant difficulty in this paper we describe calculations of heat transfer from the drilling system to the ice and the subsequent time dependent motion of the phase boundary defining the bore hole wall results are presented with the view of optimizing the bore hole radius at depth for a fixed drill performance and a variable rate of drilling calculation of melting refreezing rates at the bore hole wall requires the use of a one dimensional time dependent numerical heat flow model with a distorting mesh which follows the changing hole size the delay of hole closure is discussed with a view to keeping holes open long enough to allow instruments to be lowered to the glacier bed while realizing that drilling system performance may be marginal because of logisitical and or expenditure constraints the relative merits of drilling a large hole which is very time consuming with a small drill and the use of water soluble antifreezes which have a history of creating plugs of ice slush are discussed a method of creating a stable hole filled with antifreeze in which ice slush does not occur is described the recent application of these theoretical ideas to the planning and implementation of successful hot water drilling programs in antarctica and greenland is also presented

borehole imagery of meteoric and marine ice layers in the amery ice shelf east antarctica a real time video camera probe was deployed in a hot water drilled borehole through the amery ice shelf east antarctica where a total ice thickness of 480 m included at least 200 m of basal marine ice down looking and side looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below but the transition was neither microscopically sharp nor flat indicating the uneven nature at centimetre scale of the ice shelf base upstream where the marine ice first started to accrete marine ice features were imaged including platelet structures cell inclusions entrained particles and the interface with sea water at the base the cells are assumed to be entrained sea water and were present throughout the lower 100 150m of the marine ice column becoming larger and more prevalent as the lower surface was approached until near the base they became channels large enough that the camera field of view could not contain them platelets in the marine ice at depth appeared to be as large as 1 2 cm in diameter particles were visible in the borehole meltwater probably marine and mineral particles liberated by the drill but their distribution varied with depth

stable isotope analysis of the cretaceous sulfur cycle we report the sulfur and oxygen isotope composition of sulfate 34sso4 and 18oso4 respectively in coexisting barite and carbonate associated sulfate cas which we use to explore temporal variability in the marine sulfur cycle through the middle cretaceous the 34sso4 of marine barite tracks previously reported sulfur isotope data from the tropical pacific the 18oso4 of marine barite exhibits more rapid and larger isotopic excursions than the 34sso4 of marine barite these excursions temporally coincide with ocean anoxic events oaes neither the 34sso4 nor the 18oso4 measured in marine barite resembles the 34sso4 or the 18oso4 measured in coexisting cas culling our data set for elemental parameters suggestive of carbonate recrystallization low sr and high mn sr improves our record of 18oso4 in cas in the cretaceous this suggests that the cas proxy can be impacted by carbonate recrystallization in some marine sediments a box model is used to explore the response of the 34sso4 and 18oso4 to different perturbations in the marine biogeochemical sulfur cycle we conclude that the 34sso4 in the middle cretaceous is likely responding to a change in the isotopic composition of pyrite being buried coupled possibly with a change in riverine input on the other hand the 18oso4 is likely responding to rapid changes in the reoxidation pathway of sulfide which we suggest may be due to anoxic versus euxinic conditions during different oaes

modeling the formation and deposition of frazil ice beneath filchner ronne ice shelf large areas of the filchner ronne ice shelf are underlain by layers of marine ice which form when supercooled seawater circulating beneath the ice shelf freezes the freezing process initially produces a suspension of disc shaped frazil ice crystals and these are subs quently deposited onto the ice shelf base in areas where the flow of water is slack enough this has been modeled assuming that the freezing takes place within buoyant plumes of ice shelf water ascending the ice shelf base from source regions near the grounding lines of the major inlet glaciers the deposition of the majority of the suspended frazil ice is found to occur in spatially discrete bursts where peak rates of accumulation at the ice shelf base exceed 1 m yr 1 of solid ice there is a good correlation between the location of the zones of crystal deposition and the position of the upstream limits of the marine ice layers the high rates of localized accumulation account for the rapid buildup observed in the layer thickness which then gradually declines as the marine ice is carried downstream with the flow of the ice shelf model results also suggest an origin for the ice platelets observed at depth in the water column near the filchner ice shelf

fine scale variability of isopycnal salinity in the california current system this paper examines the finescale structure and seasonal fluctuations of the isopycnal salinity of the california current system from 2007 to 2013 using temperature and salinity profiles obtained from a series of underwater glider surveys the seasonal mean distributions of the spectral power of the isopycnal salinity gradient averaged over submesoscale 12 30 km and mesoscale 30 60 km ranges along three survey lines off monterey bay point conception and dana point were obtained from 298 transects the mesoscale and submesoscale variance increased as coastal upwelling caused the isopycnal salinity gradient to steepen areas of elevated variance were clearly observed around the salinity front during the summer then spread offshore through the fall and winter the high finescale variances were observed typically above 25 8 kg m 3 and decreased with depth to a minimum at around 26 3 kg m 3 the mean spectral slope of the isopycnal salinity gradient with respect to wavenumber was 0 19 0 27 over the horizontal scale of 12 60 km and 31 to 35 of the spectra had significantly positive slopes in contrast the spectral slope over 12 30 km was mostly flat with mean values of 0 025 0 32 an increase in submesoscale variability accompanying the steepening of the spectral slope was often observed in inshore areas e g off monterey bay in winter where a sharp front developed between the california current and the california under current and the lower layers of the southern california bight where vigorous interaction between a synoptic current and bottom topography is to be expected

on sampling the ocean using underwater gliders surface to 1000 m and back took 5 6 h and covered 5 3 km resulting in a horizontal speed of 0 26 m s 1 seasoar undulated between the surface and 400 m completing a cycle in 11 min while covering 2 6 km for a speed of 3 9 m s 1 adjacent profiles of temperature and salinity are compared between the two platforms to prove that each is accurate spray and seasoar data are compared through sections isopycnal spatial series and wave number spectra the relative slowness of the glider results in the projection of high frequency oceanic variability such as internal waves onto spatial structure the projection is caused by doppler smearing because of finite speed and aliasing due to discrete sampling the projected variability is apparent in properties measured on depth surfaces or in isopycnal depth no projected variability is seen in observations of properties on constant density surfaces because internal waves are intrinsically filtered wave number spectra suggest that projected variability affects properties at constant depth at wavelengths shorter than 30 km these results imply that isobaric quantities like geostrophic shear are valid at wavelengths longer than 30 km while isopycnal quantities like spice may be analyzed for scales as small as a glider measures

ocean circulation beneath filchner ronne ice shelf from three dimensional model results a high resolution three dimensional ocean circulation model is applied to the cavity beneath filchner ronne ice shelf fris the model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands the surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses in the former case leading to freezing while melting at the ice shelf base results in the latter case the pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath fris an anticyclonic circulation around the korff and henry ice rises with melting west of korff ice rise and freezing on the eastern side and north of henry ice rise is the main source for an ice pumping mechanism that produces the observed large marine ice body in the central filchner ronne ice shelf the estimates for net melting for realistic conditions at the open ocean boundary are 40 50 km 3 yr 1 indicating that ice shelf ocean interaction is an important contribution to the mass balance of the ice shelf

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

simulations of ice ocean dynamics in the weddell sea 2 interannual variability 1985 1993 investigations of sea ice ocean interaction on the continental shelf in the southwestern weddell sea reveal a strong correlation between fluctuations of atmospheric forcing and the variability of sea ice formation anomalies of meridional wind stress in the inner weddell sea are consistent with the phase of the antarctic circumpolar wave acw positive anomalies of northward wind stress cause an increase of sea ice export in the same and of sea ice formation in the following year leading to an increased production of high salinity shelf water driven by a varying zonal density distribution over the continental shelf the circulation in the filchner ronne ice shelf cavity fluctuates between two modes each of which features a characteristic distribution of basal freezing and melting regions thus signals of interannual atmospheric variability propagate into the deep ocean and the sub ice shelf cavities

variability in sulfur isotope records of phanerozoic seawater sulfate the s of seawater sulfate reflects processes operating at the nexus of sulfur carbon and oxygen cycles however knowledge of past seawater sulfate s values must be derived from proxy materials that are impacted differently by depositional and post depositional processes we produced new timeseries estimates for the s value of seawater sulfate by combining 6710 published data from three sedimentary archives marine barite evaporites and carbonate associated sulfate with updated age constraints on the deposits robust features in multiple records capture temporal trends in the s value of seawater and its interplay with other phanerozoic geochemical and stratigraphic trends however high frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints the amount of noise quantified from the variograms of each record increases with age for all s proxies indicating that post depositional processes obscure detailed knowledge of seawater sulfate s s value deeper in time

improving the geoid combining altimetry and mean dynamic topography in the california coastal ocean satellite gravity mapping missions altimeters and other platforms have allowed the earth s geoid to be mapped over the ocean to a horizontal resolution of approximately 100 km with an uncertainty of less than 10 cm at finer resolution this uncertainty increases to greater than 10 cm achieving greater accuracy requires accurate estimates of the dynamic ocean topography dot in this study two dot estimates for the california current system with uncertainties less than 10 cm are used to solve for a geoid correction field the derived field increases the consistency between the dots and along track altimetric observations suggesting it is a useful correction to the gravitational field the correction is large compared to the dynamic ocean topography with a magnitude of 15 cm and significant structure especially near the coast the results are evidence that modern high resolution dynamic ocean topography products can be used to improve estimates of the geoid

poleward flows in the southern california current system glider observations and numerical simulation 1 three years of continuous spray glider observations in the southern california current system ccs are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern ccs gliders provide upper ocean observations with good across shore and temporal resolution along two across shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state persistent poleward flows are observed in three areas within 100 km of the coast at point conception within the southern california bight scb and offshore of the scb and the santa rosa ridge srr poleward transport by the flows within the scb and offshore of the srr exceeds the poleward transport off point conception suggesting that the poleward flows are not continuous over the 225 km between observation lines the numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching point conception the poleward current offshore of the srr is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast this westward propagation is tied to westward propagating density anomalies originating in the scb during the spring summer upwelling season when wind stress curl is most strongly positive the across shore wave number frequency and phase speed of the westward propagation and the lack of across shore transport of salinity along isopycnals are consistent with first mode baroclinic rossby dynamics

characterizing the semidiurnal internal tide off tasmania using glider data the spatial structure of the semidiurnal internal tide in the vicinity of tasmania is characterized using temperature and salinity data from seaglider and slocum glider deployments wavelet analysis of isopycnal displacements measured by the gliders was used to isolate the semidiurnal internal tide with a solid signal observed both to the east and to the south of tasmania the signal south of tasmania was attributed to local forcing while that to the east of tasmania was found to have propagated from the south east to the north west a result which supports previous studies indicating the presence of an internal tidal beam originating over the macquarie ridge south of new zealand displacement amplitudes were observed to be amplified in the vicinity of the continental slope with the incoming tidal beam shown to be both reflected and scattered on the continental slope and shelf and energy transferred to higher modes

radar detectability of a subsurface ocean on europa abstract a spacecraft in orbit around jupiter s moon europa can use ice penetrating radar to probe for a possible liquid water ocean beneath europa s surface ice and to characterize other important subsurface structure consideration of available constraints on the properties of europa s ice possible subsurface temperature gradients and possible impurities in the ice places an upper limit of about 10 km on the depth to which an ocean might be detectable with an orbiting radar

transgressions rethinking beringian glaciation abstract the purpose of this investigation is to encourage a fresh look at pleistocene beringia heretofore flooding of bering strait has been cited as the only barrier to migration with marine sea transgressions being a sea gate that closed off migration during glacial interstadials and interglaciations however the possibility exists that glacial advances were also barriers with marine ice transgressions being an ice gate that closed off migration during glacial stadials and glacial maxima this possibility proceeds from the marine ice transgression hypothesis mith which states that marine ice sheets form on the broad arctic continental shelf of northern hemisphere continents when sea ice thickens grounds and domes in shallow water and then transgresses landward as continental ice sheets and seaward as floating ice shelves hughes 1987 landward transgression is onto coastal lowlands during pleistocene glaciations a marine ice sheeet extending from spitsbergen to greenland may have transgressed the circumpolar continental landmass at its lowest and narrowest gap central beringia and calved into the pacific ocean four models of beringian glaciation are presented based on the distinction between marine glaciation and highland glaciation central beringia was glaciated only in highlands in the traditional model hopkins et al 1982 was also glaciated by a self sustaining ice shelf floating over the deep ocean basins of the bering sea in the model by grosswald and vozovik 1984 was glaciated by a marine ice sheet that covered highlands the continental shelf and supplied the ice shelf in a model for maximum pleistocene glaciation and was glaciated by a marine ice sheet in the chukchi sea that merged with highland glaciers transgressed the continental shelf of the western bering sea and calved into the southern bering sea along the edge of the continental shelf in a model for the last glaciation field tests are suggested to assess the viability of these four models the first model is already established for highland glaciation in alaska but less established in siberia the last model should be the easiest to evaluate for marine glaciation the last model limits human migration across the beringian land bridge to brief intervals between stadials and interstadials of the last glaciation cycle when both the ice gate and the sea gate were opened to human migration this model can influence the sea change now underway among quaternary scientists studying peopling of the americas based on the archaeological linguistic and ethnic diversity among native american populations

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

glider observations of the north equatorial current in the western tropical pacific the north equatorial current nec of the pacific ocean advects subtropical subpolar and tropical water masses repeat underwater glider observations of the nec from june 2009 to january 2014 along 134 3 e provide absolute zonal geostrophic velocity transport and water mass structure at length scales of 10 1000 km the nec is strongest near the surface and persistent eastward undercurrents are identified deeper than potential density surface 26 kg m 3 at 9 6 n and 13 1 n mean transport from the surface to 27 3 kg m 3 and 8 5 n 16 5 n is 37 6 sv 106 m3 s 1 with a standard deviation of 15 6 sv the transport variability is greatest deeper than 26 kg m 3 due to undercurrent variability wavelet analysis at scales of 10 80 km reveals extrema of fine scale salinity variance along isopycnals spice variance high spice variance is found in the north pacific tropical water nptw and the north pacific intermediate water npiw with a spice variance minimum between water masses at 25 5 kg m 3 a horizontal cox number ch relates salinity variance at fine scales 10 80 km to that at greater length scales 120 200 km as a function of density ch is nearly vertically uniform indicating that the stirring of mean salinity gradients enhances fine scale salinity variance nptw with an estimated horizontal eddy diffusivity of order 104 m s 2 is a useful tracer for the region and may be used to relate the fine scale salinity variance to an eddy diffusivity

contributions of an ancient evaporitic type reservoir to subglacial lake vostok chemistry abstract we present here the first comprehensive study of the chemical composition of accretion ice from lake vostok ion chromatographic analyses were performed on samples obtained along the deeper part of the vostok ice core samples were taken from 3350 down to 3611 m depth both in glacier ice and subglacial lake ice the total ionic contents of two accretion ice layers a few meters thick and centered around 3540 and 3590 m depth are several times lower than those of glacier ice very low concentrations were also observed in the deeper part of accretion ice below 3609 m depth elsewhere the total ionic content is variable but remains 5 to 50 times higher than in glacier ice whatever its total ionic content the ionic composition of accretion ice is significantly different from what is observed in glacier ice it is dominated by sodium chloride homogeneously distributed throughout the ice lattice as well as calcium and magnesium sulfate likely located in solid inclusions or to a lesser extent at grain boundaries chemical considerations combined with additional studies of sulfur and oxygen isotopes in sulfate and iron measurements strongly suggest that glacier water recycling and bedrock hydrolysis do not play a prominent role in providing impurities to accretion ice it is more likely that nacl rich water carrying fine sulfate salt particles is sporadically incorporated in the ice accreting in a shallow bay upstream from vostok the origin of such salty water which should also contribute to lake salinity is discussed

optical televiewer based identification and characterization of material facies associated with an antarctic ice shelf rift we have drilled 13 boreholes within and around a through cutting rift on the unofficially named roi baudouin ice shelf east antarctica logging by optical televiewer optv combined with core inspection has resulted in the identification and characterization of several material facies outside the rift optv imaged annual layering indicates 150 years of accumulation over the 66 m length of one of the boreholes luminosity analysis of this image also reveals the presence of numerous dark melt layers as well as a systematic decrease in background luminosity interpreted in terms of a progressive increase in light transmission during firnification we identify four material facies within the rift snow granular ice marine ice and unconsolidated platelets we interpret the granular ice facies as snow that has been saturated by percolating sea water and the underlying marine ice as compacted buoyant platelets that have adhered to the rift base core sections reveal the presence of tubular channels within the marine ice indicating that it is macroporous and permeable to sea water the lower boundary of this facies merges into a mushy layer of unconsolidated platelets that were successfully imaged by optv revealing irregular sub horizontal layering similar to that reported previously on the basis of directional borehole video

trapped diurnal internal tides propagating semidiurnal internal tides and mixing estimates in the california current system from sustained glider observations 2006 2012 abstract from 2006 2012 along 3 repeated cross shore transects california cooperative oceanic fisheries investigations lines 66 7 80 and 90 in the california current system 33 609 shear and 39 737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52 000 profiles elevated diffusivity estimates and energetic diurnal d1 and semidiurnal d2 internal tides are found a within 100 km of the coast on lines 66 7 and 80 and b over the santa rosa cortes ridge srcr in the southern california bight scb on line 90 while finding elevated mixing near topography and associated with internal tides is not novel the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge both d1 and d2 internal tides are energy sources for mixing at these latitudes the d1 internal tide is subinertial on line 90 d1 and d2 tides are equally energetic over the srcr the main site of elevated mixing within the scb numerous sources of internal tides at the rough topography in the scb produce standing and or partially standing waves on lines 66 7 and 80 the dominant energy source below about 100 m for mixing is the d1 internal tide which has an energy density of the d2 internal tide on line 80 estimated diffusivity estimated dissipation and d1 energy density peak in summer the d1 energy density shows an increasing trend from 2006 to 2012 its amplitude and phase are mostly consistent with topographically trapped d1 internal tides traveling with the topography on their right the observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode 1 rossby radius of 20 30 km despite the variable mesoscale it is remarkable that coherent internal tidal phase is found

the origin of green icebergs in antarctica a comparison of samples from a translucent green iceberg with a core from the ronne ice shelf revealed an excellent agreement in isotopic composition crystal structure and incorporated sediment particles marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs it most likely results from ice pump processes which produce large amounts of ice platelets in the water column beneath ice shelves these subsequently accumulate and become compacted into bubble free desalinated ice iceberg and drift buoy trajectories indicate that green icebergs observed in the weddell sea originate from the amery ice shelf rather than from the ronne ice shelf although the latter ice shelf is also a potential source

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

a model for calcium magnesium and sulfate in seawater over phanerozoic time a theoretical model has been constructed for calculating the concentrations of dissolved calcium magnesium and sulfate in seawater over the past 550 million years phanerozoic time it is demonstrated that the weathering of ca carbonates and silicates by carbonic acid can be generally ignored in the modeling a previous model of the carbon cycle is used to guide input to the oceans of mg from carbonate and silicate weathering by h 2 co 3 similarly a previous model of the sulfur cycle is used to guide input fluxes to the ocean by calcium sulfate weathering and the weathering of ca and mg carbonates and silicates by sulfuric acid the sulfuric acid is derived from the oxidation of pyrite during weathering plus the oxidation of sulfur gases derived from the metamorphic volcanic decomposition of pyrite and caso 4 calcium is removed from seawater as sedimentary carbonates and sulfates so 4 is removed as sedimentary pyrite and caso 4 and mg is removed by exchange for ca via basalt seawater reaction and or dolomitization plus minor burial in sediments with pyrite as anoxic dolomite by adjusting starting conditions modeling results can be found that agree well with results of studies of salt fluid inclusions for variations of ca and so 4 this is especially true of the timing when ca was more abundant in seawater than so 4 and vice versa general agreement was also found with fluid inclusion results for changes in the mg ca ratio thereby distinguishing calcite seas from aragonite seas because of this overall agreement the modeling offers a new perspective on why oceanic ca mg and so 4 have varied over phanerozoic time

seasonal evolution of upper ocean horizontal structure and the remnant mixed layer 1 we discuss the seasonal evolution of upper ocean thermohaline structure at small horizontal scales the upper 350 m of a 1000 km long section in the subtropical north pacific was observed in winter spring and summer with 3 14 km horizontal resolution four vertical regions had distinct density and salinity structure the mixed layer remnant mixed layer high stratification layer and permanent thermocline the remnant mixed layer consists of water from the winter mixed layer left over after restratification the remnant mixed layer was most similar to the mixed layer in winter and spring and most similar to the high stratification layer below in summer the high stratification layer had elevated stratification that varied seasonally the permanent thermocline varied little seasonally and was horizontally and vertically uniform in comparison in all seasons density ratios showed that mixed layer s differences tended to compensate in density with the strongest tendency toward compensation in winter density ratios were temperature dominated in the remnant mixed layer consistent with salt fingering salinity anomalies were largest at the surface and decayed with depth in all seasons spectra of isopycnal depth and s anomalies along isopycnals are compared between the three seasons and four vertical layers isopycnal depth variance at 30 46 km wavelengths decreased from winter to spring to summer by a factor of 2 10 in stratified regions by treating salinity anomalies as a tracer the effective isopycnal diffusivity in the remnant mixed layer was estimated to be 1 4 m2 s 1 over 30 46 km wavelengths

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

marine ice in larsen ice shelf 1 it is argued that larsen ice shelf contains marine ice formed by oceanic freezing and other mechanisms missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing visible imagery suggests that marine ice currently stabilizes larsen c ice shelf and implicates failure of marine flow bands in the 2002 larsen b ice shelf collapse ocean modeling indicates that any regime change towards the incursion of warmer modified weddell deep water into the larsen c cavity could curtail basal freezing and its stabilizing influence

a neogene seawater sulfur isotope age curve from calcareous pelagic microfossils abstract until now our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites owing to the sporadic occurrence of evaporites through time the secular sulfur isotope age curve contains many gaps with little or no data in order to fill in some of these gaps particularly the neogene we have analyzed the sulfur isotopic composition of carbonate associated sulfate in carbonate tests of planktonic foraminifera other investigators have shown that sulfate may occur in biogenic calcites either lattice bound as micro fluid inclusions in adsorbed phases or as protein polysaccharides whatever the origin the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived as shown by results on recent calcareous foraminifera and macrofossils using this approach for study of miocene to recent pelagic marine sediments supplemented by new data for miocene marine evaporites from the gulf of suez we have found that the 34 s of seawater has decreased about 2 5 over the past 25 m y and that most of the decrease has occurred over the past 5 m y parallelling a decrease in the 13 c of dissolved oceanic bicarbonate from the same interval sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m y alternatively an increase in weathering rates over the past 5 m y would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species in either case the net result would be a decrease in atmospheric p o 2

statistical modeling of a former arctic ocean ice shelf complex using antarctic analogies 1 geophysical mapping and coring of the central arctic ocean seafloor provide evidence for repeated occurrences of ice sheet ice shelf complexes during previous glacial periods several ridges and bathymetric highs shallower than present water depths of 1000m show signs of erosion from deep drafting armadas of icebergs which originated from thick outlet glaciers and ice shelves mapped glacigenic landforms and dates of cored sediments suggest that the largest ice shelf complex was confined to the amerasian sector of the arctic ocean during marine isotope stage mis 6 however the spatial extent of ice shelves can not be well reconstructed from occasional groundings on bathymetric highs therefore we apply a statistical approach to provide independent support for an extensive mis 6 ice shelf complex which previously was inferred only from interpretation of geophysical and geological data specifically we assess whether this ice shelf complex comprises a likely source of the deep draft icebergs responsible for the mapped scour marks the statistical modeling is based on exploiting relations between contemporary antarctic ice shelves and their local physical environments and the assumption that arctic ocean mis6 ice shelves scale similarly analyzing ice thickness data along the calving front of contemporary ice shelves a peak over threshold method is applied to determine sources of deep drafting icebergs in the arctic ocean mis6 ice shelf complex this approach is novel to modeling arctic paleoglacial configurations predicted extreme calving front drafts match observed deep draft iceberg scours if the ice shelf complex is sufficiently large

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

constraints on the late ediacaran sulfur cycle from carbonate associated sulfate we report new sulfur isotope compositions 34s in carbonate associated sulfate cas and pyrite from the lower nama group namibia 550 to 35 is highly unusual in earth history and in the terminal ediacaran is best explained by a high pyrite burial flux pyrite in the nama group is close in isotopic composition to coeval sulfate but the sulfur isotope fractionation between sulfate and pyrite varies widely among different studied basins suggesting highly heterogeneous redox and depositional conditions

how iceberg calving and grounding change the circulation and hydrography in the filchner ice shelf ocean system the formation of bottom water in the southern weddell sea is strongly influenced by the flow of ice shelf water isw out of the filchner ronne ice shelf cavity the breakout of three giant icebergs in 1986 and their grounding on the shallow berkner bank modified the circulation and water mass formation in the filchner trough and the adjacent sea areas hydrographic measurements along the filchner ice shelf front carried out with rv polarstern in 1995 show significant changes in the water mass characteristics and flow patterns in the filchner trough in comparison to measurements from the early 1980s changes in the trough will affect the flow over the sill to the deep weddell abyssal plain we combine a three dimensional ocean circulation model with conductivity temperature depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the filchner ice shelf we assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties results indicate variations of the flow pattern in the filchner trough and on berkner bank where high salinity shelf water the feedstock for isw is produced the calving and grounding impacts illustrate the sensitivity of the ice shelf ocean system to perturbations in local bathymetric settings

ocean interactions with the base of amery ice shelf antarctica using a two dimensional ocean thermohaline circulation model we varied the cavity shape beneath amery ice shelf in an attempt to reproduce the 150 m thick marine ice layer observed at the gi ice core site most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and g1 but produce only minor accumulation at the ice core site and closer to the ice front changes in the seafloor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line to submarine sills and to discontinuities in ice thickness model results showed temperature salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater modeled outflow characteristics at the ice front are in general agreement with oceanographic data from prydz bay a freshwater flux across the grounding line derived from melting beneath the grounded ice sheet would have to be anomalously large to produce the basal marine ice layer and account for the ice shelf water outflow we concur with morgan s inference that the g 1 core may have been taken in a basal crevasse filled with marine ice this ice is formed from water cooled by ocean ice shelf interactions along the interior ice shelf base

ice thickness and sea depth derived from reflection seismic measurements on the central part of filchner ronne ice shelf antarctica in january 1986 reflection seismic measurements were made in the central part of filchner ronne ice shelf north of henry and korff ice rises by a german oversnow traverse with the help of a newly developed ice streamer two people were able to obtain seismic records nearly every 5km over a distance of 150km within 3d processing of digitally recorded seismograms yielded profiles of ice thickness and sea depth the ice thickness varies from 400 to 500 m and is in good agreement with the thickness of 465 m at a drill hole the reflection seismic results are comparable with those obtained from aerogeophysical measurements

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

sulfur isotope ratios in co occurring barite and carbonate from eocene sediments a comparison study abstract in order to investigate the utility of bulk carbonate as a recorder of seawater sulfate sulfur isotope ratios 34ssw co existing pelagic barite and bulk carbonate in eocene sediments from the equatorial pacific ocean iodp exp 320 321 sites u1331 to u1333 were analyzed for their sulfur isotope ratios 34s the 34s from both minerals showed parallel fluctuation throughout the eocene with carbonate associated sulfate cas values about 0 8 heavier than those of barite a similar offset was observed in cas 34s obtained using species specific cleaned planktonic foraminifers in a previous study the consistent results from two distinct minerals suggest that the original 34ssw can be derived from bulk cas analysis if post deposition carbonate recrystallization is minimal the combined data set showed a 5 increase of 34ssw during the eocene from 53 ma to 36 ma with the majority of the shift within 7 myr between 53 and 46 ma this change is more gradual than previously reported the timing of this 34ssw shift coincides with extensive pyrite burial in the arctic ocean supporting the hypothesis that the 5 increase in eocene 34ssw has been caused by 34s enriched water outflow from the arctic ocean

measurements of ice shelf water beneath the front of the ross ice shelf using gliders measurements made by an underwater glider deployed near the ross ice shelf were used to identify the presence of ice shelf water isw which is defined as seawater with its potential temperature lower than its surface freezing point temperature properties logged by the glider included in situ temperature electrical conductivity pressure gps location at surfacings and time for most of the first 30 recorded dives of its deployment evidence suggests the glider was prevented from surfacing due to being under the ice shelf for dives under the ice shelf farthest from the ice shelf front isw layers of varying thicknesses and depth locations were observed between 2 m thick centred at 231 m depth to 93 m thick centred at 360 m for dives under the ice shelf close to the ice shelf front either no isw was observed or isw layers were centred at shallower depths 116 127 m thicker isw layers e g up to 250 m thickness centred at 421 m were observed for some glider dives in open water in front of the ross ice shelf no in situ supercooling water colder than the pressure dependent freezing point temperature was observed

first access to the ocean beneath ekstromisen antarctica by means of hot water drilling a hot water drill developed during the past 2 years at alfred wegener institute was used to penetrate the ekstrom ice shelf several times near the georg von neumayer station the drilling operation was successful and the initial large diameter 35 cm allowed easy access to the ocean one hole was used to install an ultrasonic echo sounder which recorded the ablation at the ice shelf bottom continuously another hole was used for emplacement of a thermistor string throughout the 237 m thick ice shelf for ice temperature measurements several ctd profiles in the 175 m deepwater column and the analysis of water samples provided valuable data for the understanding of ice shelf ocean interactions

thermohaline structure in the california current system observations and modeling of spice variance 1 upper ocean thermohaline structure in the california current system is investigated using sustained observations from autonomous underwater gliders and a numerical state estimate both observations and the state estimate show layers distinguished by the temperature and salinity variability along isopycnals i e spice variance mesoscale and submesoscale spice variance is largest in the remnant mixed layer decreases to a minimum below the pycnocline near 26 3 kg m 3 and then increases again near 26 6 kg m 3 layers of high low meso and submesoscale spice variance are found on isopycnals where large scale spice gradients are large small consistent with stirring of large scale gradients to produce smaller scale thermohaline structure passive tracer adjoint calculations in the state estimate are used to investigate possible mechanisms for the formation of the layers of spice variance layers of high spice variance are found to have distinct origins and to be associated with named water masses high spice variance water in the remnant mixed layer has northerly origin and is identified as pacific subarctic water while the water in the deeper high spice variance layer has southerly origin and is identified as equatorial pacific water the layer of low spice variance near 26 3 kg m 3 lies between the named water masses and does not have a clear origin both effective horizontal diffusivity h and effective diapycnal diffusivity v are elevated relative to the diffusion coefficients set in the numerical simulation but changes in h and v with depth are not sufficient to explain the observed layering of thermohaline structure

roles of marine ice rheology and fracture in the flow and stability of the brunt stancomb wills ice shelf marine ice sometimes as part of an ice melange significantly affects ice shelf flow and ice fracture the highly heterogeneous structure of the brunt stancomb wills ice shelf bsw system in the east weddell sea offers a rare setting for uncovering the difference in rheology between meteoric and marine ice here we use data assimilation to infer the rheology of the brunt stancomb wills ice shelf by an inverse control method that combines interferometric synthetic aperture radar measurements with numerical modeling we then apply the inferred rheology to support the hypothesis attributing the observed 1970s ice shelf flow acceleration to a change in the stiffness of the ice melange area connecting brunt proper with stancomb wills and to examine the consequences of frontal rift propagation we conclude that while the brunt stancomb wills system is currently not susceptible to extreme fragmentation similar to that of the larsen b ice shelf in 2002 our inverse and forward modeling results emphasize its vulnerability to destabilization by relatively rapid changes in the ice melange properties resulting from the interaction of its marine ice component with ocean water or by the further propagation of a frontal rift copyright 2009 by the american geophysical union

kamb ice stream flow history and surge potential a basal zone tens of meters thick of debris laden ice was observed in kamb ice stream west antarctica using a video camera lowered into boreholes made by hot water drilling the debris content varies sometimes abruptly forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction in most parts the concentration of debris is low a few percent by weight with particles often mud clots dispersed in a matrix of clear ice the nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of kamb ice stream including processes leading up to the termination of its streaming behavior and possible reactivation

sulfur cycling in the aftermath of a 635 ma snowball glaciation evidence for a syn glacial sulfidic deep ocean we have analyzed 34 s sulfate and pyrite 18 ocarbonate and 13 ccarbonate and major and trace elemental concentrations including extractable fe and mn phases in four sections of the maieberg formation the cap carbonate sequence to the marinoan glaciation in northern namibia 34 ssulfate profiles and other geochemical characteristics in the basal transgressive cap dolostone keilberg member are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations in the overlying interval that consists of rhythmites deposited during the transgressive high stand large geochemical disparities occur between sections that we interpret to have been deposited in open ocean versus restricted settings in the former a large negative shift in 34 ssulfate of 20 above the cap dolostone accompanies a change in mineralogy from dolomite to limestone a sharp decrease in 18 o and 13 c and a positive spike in fe and mn concentrations in the latter dolomite persists above the cap dolostone 34 ssulfate increases abruptly 18 o and 13 c are invariant and a spike in mn and fe concentrations is present but subtle these contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold euxinic deep water and a cap of warm oxic brackish water flooded the continental shelf during the post glacial transgression the geochemical di e rences between open ocean and restricted sections diminishes upsection presumably recording the gradual mixing and homogenization of the waters along the otavi platform

drilling to the beds of the greenland and antarctic ice sheets a review we give a review of drilling to the beds of the greenland and antarctic ice sheets with a history of just a century we use a chronological rather than a geographical ordering to emphasize the way drilling has developed the review is divided into two parts core drilling and hole only drilling for access to the beds or the deep interior of the ice sheets this is an overview not a comprehensive report on each project while we have endeavoured to include all projects that fit our selection criteria we have not provided all the technical details

submesoscale water mass spectra in the sargasso sea submesoscale stirring contributes to the cascade of tracer variance from large to small scales multiple nested surveys in the summer sargasso sea with tow yo and autonomous platforms captured submesoscale water mass variability in the seasonal pycnocline at 20 60 m depths to filter out internal waves that dominate dynamic signals on these scales spectra for salinity anomalies on isopycnals were formed salinitygradient spectra are approximately flat with slopes of 20 2 6 0 2 over horizontal wavelengths of 0 03 10km while the two to three realizations presented here might be biased more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1km a review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out i quasigeostrophic eddy stirring ii atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening iii a downscale vortical mode cascade and iv horizontal diffusion because of shear dispersion of diapycnal mixing internal wave horizontal strain appears to be able to explain horizontal wavenumbers of 0 1 7 cycles per kilometer cpkm but not the highest resolved wavenumbers 7 30cpkm submesoscale subduction cannot be ruled out at these depths though previous observations observe a flat spectrum well below subduction depths so this seems unlikely primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum the last need not be limited to mode one interior or surface rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves

surface structure and stability of the larsen c ice shelf antarctic peninsula a structural glaciological description and analysis of surface morphological features of the larsen c ice shelf antarctic peninsula is derived from satellite images spanning the period 1963 2007 the data are evaluated in two time ranges a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003 04 austral summer data and an image series since 2003 showing recent evolution of the shelf we map the ice shelf edge rift swarms crevasses and crevasse traces and linear longitudinal structures called flow stripes or streak lines the latter are observed to be continuous over distances of up to 200 km from the grounding line to the ice shelf edge with little evidence of changes in pattern over that distance integrated velocity measurements along a flowline indicate that the shelf has been stable for 560 years in the mid shelf area linear longitudinal features may be grouped into 12 units each related to one or a small group of outlet feeder glaciers to the shelf we observe that the boundaries between these flow units often mark rift terminations the boundary zones originate upstream at capes islands or other suture areas between outlet glaciers in agreement with previous work our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice we thus suggest that suture zones within the larsen c ice shelf and perhaps within ice shelves more generally may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening

ca isotopes in carbonate sediment and pore fluid from odp site 807a the ca2 aq calcite equilibrium fractionation factor and calcite recrystallization rates in pleistocene sediments abstract the calcium isotopic compositions 44ca of 30 high purity nannofossil ooze and chalk and 7 pore fluid samples from odp site 807a ontong java plateau are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor s f between calcite and dissolved ca2 and the rates of post depositional recrystallization in deep sea carbonate ooze the value of s f at equilibrium in the marine sedimentary section is 1 0000 0 0001 which is significantly different from the value 0 9987 0 0002 found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean we hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for ca isotope fractionation during calcite precipitation we describe a steady state model that offers a unified framework for explaining ca isotope fractionation across the observed precipitation rate range of 14 orders of magnitude the model attributes ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface this model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation the ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly deposited carbonate ooze is 30 40 myr and decreases with age to about 2 myr in 2 3 million year old sediment the recrystallization rates determined from ca isotopes for pleistocene sediments are higher than those previously inferred from pore fluid sr concentration and are consistent with rates derived for late pleistocene siliciclastic sediments using uranium isotopes combining our results for the equilibrium fractionation factor and recrystallization rates we evaluate the effect of diagenesis on the ca isotopic composition of marine carbonates at site 807a since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation accordingly diagenesis produces a maximum shift in 44ca of 0 15 for site 807a sediments but will have a larger impact where sedimentation rates are low seawater circulates through the sediment pile or there are prolonged depositional hiatuses

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

along isopycnal variability of spice in the north pacific two hydrographic surveys in the gulf of alaska and the north pacific subtropical gyre are presented both surveys are roughly perpendicular to lateral temperature gradients and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline isopycnal displacements and horizontal velocities are dominated by internal waves spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms the spectra of spice anomaly gradients along near surface isopycnals roughly follow power laws of kx0 6 0 2 variance spectra power laws of kx 1 4 0 2 and in most cases the spectra become redder at depth the near surface spectra are possibly consistent with the predictions of quasi geostrophic turbulence theory when surface buoyancy effects are accounted for but the spectra at depth are inconsistent with any quasi geostrophic theory probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients symptomatic of fronts vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately kx 1 2 lateral decorrelation length scales are 20 40 km close to the baroclinic rossby radius lateral stirring occurs over large scales with average lateral displacements of about 200 km in the upper 75 m decreasing to 100 km at greater depths the depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important or that there are unconsidered depth dependent stirring mechanisms

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

the spatial distribution and annual cycle of upper ocean thermohaline structure 1 observations of the spatial distribution and persistence of thermohaline structure are presented and show how advection and diffusion affect a passive tracer more than two years of underwater glider observations in the central subtropical north pacific showed thermohaline variability over horizontal scales from 5 to 1300 km thermohaline fluctuations along isopycnals spice fluctuations were elevated in layers throughout the water column with the largest fluctuations near the surface and subtropical frontal regions fluctuations were uncorrelated between the layers but stirred by the same velocity field spice variance had local extrema in the vertical because of differences in source water properties and the influence of neighboring water masses spice variance spanned about three orders of magnitude along deeper isopycnals with larger variance where different water masses met and where velocity and vorticity variance were elevated horizontal wave number spectra of spice had slopes of 2 everywhere in the upper 1000 m submesoscale spice fluctuations had slopes in physical space near the ratio of the coriolis parameter to the buoyancy frequency f n consistent with predictions of quasi geostrophic theory in the mixed layer thermohaline structure had a significant annual cycle with smaller interannual differences thermohaline fluctuations left behind during restratification and isolated from the mixed layer decayed with time because of diffusion along isopycnals horizontal diffusivity estimates in the remnant mixed layer were 0 4 m2 s 1 at 15 28 km wavelengths and 0 9 m2 s 1 at 35 45 km wavelengths

subthermocline eddies over the washington continental slope as observed by seagliders 2003 09 abstractin the california current system subthermocline lenslike anticyclonic eddies generated within the california undercurrent cu are one mechanism for lateral transport of the warm saline waters of the cu garfield et al established the name cuddies for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of cu water this study presents observations of subthermocline eddies collected from a time series of seaglider surveys in the northern california current system gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5 5 yr close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones water properties at the core of anticyclonic eddies were similar to those in the core of the cu over the continental slope these anticyclones are examples of cuddies anticyclonic cyclonic eddies had average radii of 20 4 20 6 km peak azimuthal current speeds of 0 25 0 23

the flow of the undercurrent over the continental borderland off southern california 1 iniroduction 2 background the bottom topography off southern california has been the cc a slow had equatornard flow is the eastern limb of termed a continental borderland to distinguish it from the ordinary the large scale anticyclonic gyre of the temperate north pacific shelf slope morphology found along most continental margins ocean the cc carries relatively cool low salinity waters of shepard and emuy 19411 islands shallow banks basins and subarctic origin to lower latitudes on average the core of the cc troughs form a complex bathymetric region extending from the has velocities of 0 10 to 0 15 m s and traces a path roughly coast to 200 lon offshore the southern california coastline has a parallel to the california baja california coastline localized concave curvature between the headland at point conception to measurements of flow can exceed 0 5 m s davis 19851 an the u s mexican border as a whole this region is referred to as instantaneous realization of the cc includes a tangle of meanders the california bight the geographic bathymetric dguration eddies and narrow jetlike flows and counterllows the cc lies at of the bight exm a pronounced influence upon the local circula varying distances off the coast and at the latitude of the tion patterns and heme upon the distributions of physical e u s mexican border there is a strong shoreward component to sverdrup and fleming 19411 and biological quantities e g the flow in most seasons especially summa the flow splits one allen 1945 owen 19801 during 1985 we conducted an oceano branch turns and flows northward into the california bight graphic survey off southern california the sampling grid was becoming the eastern limb of a cyclonic eddy centered about the designed to reveal the mumce of the geographic bathymetric offshore banks seasonally fall and winter a weak inshore features upon the circulation of the california current system countercurrent ic develops at the surface along various portion6 ccs especially the flow of the california undercurrent cu of the coastline reid et d 19581 at depth the cu cdes the survey grid also includes station lines mss the core of the waters of subtropical origin poleward along the continental slope california current cc and transects an offshore dipole eddy these waters are relatively warm and saline and have low mm simpson and lym this issue the station grid of this study trations of dissolved oxygen the cu also demonstrates a welldiffers considerably from that used by the california cooperative defined seasonality and at times opposes the overlying dace oceanic fisheries investigations calcofi surveys in both its flow maps and sections of the long term seasonal means of relaspatial resolution and depth of sampling the objectives of thic tive geostrophic velocity and distributions of ocean characteristics report are 1 to examine the spatial continuity of the cu through for the ccs are available in calcofi atlas 30 lym et d 19821 the complex bathymeny of the bight 2 to trace the poleward for a comprehensive review of the long term seasonal large scale paths and offshore spread of waters of southern origin transported description of the ccs the readex is referred to recent papas by by the cu 3 to contrast this subsurface flow with the surface hickey 1979 chelton 19841 and lynn andslnpsm 19871 and flow and 4 to compare these results with the long term calcofi to important references of pioneering work given therein mean conditions

orbital tuning of a lower cretaceous composite record maiolica formation central italy 1 a high resolution pelagic bulk carbonate stable isotope record from a central tethyan lower cretaceous composite section is presented three well exposed sedimentary sequences chiaserna monte acuto bosso and gorgo a cerbara sections central italy cropping out throughout the maiolica formation were correlated by a detailed magnetostratigraphy lithostratigraphy and calcareous plankton biostratigraphy in order to reconstruct a continuous composite record from the middle berriasian to the lower aptian the integrated stratigraphy of the three sequences provided an accurate time framework for the new high resolution c isotope curve which is presented in this study the composite 13c signal recorded in the depth domain was analyzed by combined lomb scargle periodogram and weighted wavelet z transform wwz weighted wavelet amplitudes wwa foster wavelet spectral methodologies both appropriate for unevenly sampled curves these tools allowed us to unravel the main frequencies modulating the record and their hypothetical shift in depth respectively the long term 400 000 and 2 400 000 years eccentricity cycles were consistently recorded throughout all the composite record once band pass filtered in these two periodicity bands and compared to the lithologic pattern cycles identified throughout the composite sequence the 13c signal was used as a valuable proxy record for a reliable construction of an orbital tuning of the early cretaceous an estimated age for all the different stratigraphic events recognized throughout the composite record was reported in particular the reestimated ages of the paleomagnetic chrons documented in the upper part of the record show differences with those reported by gradstein et al 2004 up to 2 m y the valanginian carbon shift present in the middle part of the composite sequence was estimated to be 2 3 m y long the good agreement between the estimated age of the base of this positive carbon isotope excursion 136 34 m y and the predominant volcanic phase associated to the parana etendeka large igneous province intrusion confirmed a possible cause effect link between the two events

depositional and diagenetic constraints on the abundance and spatial variability of carbonate associated sulfate abstract marine carbonate rocks are composed in varying abundance of cement micrite abiotic grains and fossils which can provide information about the physical and chemical environments in which they formed geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic including petrographic information available resulting in potentially faulty reconstructions of biogeochemical and environmental conditions these concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies here we have combined x ray absorption near edge structure xanes spectroscopy and x ray fluorescence xrf imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in upper ordovician carbonates from anticosti island canada and early silurian carbonates from gotland sweden across multiple depositional facies lithofacies and fossil communities between anticosti island and gotland are similar which allows for comparison of changes in the dominant s species and their abundance in separate basins associated with variations in glacio eustatic sea level sulfide abundance is greatest in mudstone wackestone and packstone facies where interstitial micrite hosts abundant pyrite sulfate abundance as carbonate associated sulfate cas varies within individual fossil fragments as well as within the same fossil phylum and is particularly high in unaltered brachiopods in contrast sulfate abundance is generally very low in micrite near the detection limit and generally arises in situ from sulfide that has been oxidized as opposed to true cas in different cement fabrics sulfate abundance is greatest in drusy pore filling cements organic sulfur compounds are also detected and although low in abundance are mostly found within micrite the detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization this approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates as well as the effect of depositional setting and diagenesis on cas incorporation re mobilization and ultimate abundance in sedimentary carbonates additionally this work has implications for the cas isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous cas sections throughout earth history

observations of amplified roughness from crystal accretion in the sub ice ocean boundary layer ice crystal accretion on the underside of sea ice and ice shelves a signature of pressure induced supercooling has the potential to alter the energy balance in the ocean boundary layer through enhanced hydrodynamic roughness here we present estimates of crystal driven ocean boundary layer roughness in supercooled water beneath sea ice adjacent to the mcmurdo ross ice shelf data were collected from four sites in mcmurdo sound antarctica between 2007 and 2015 and represent a range of ice shelf affected conditions the results show that drag of the rough ice underside in the presence of platelets is 6 30 times larger than typical levels homogeneously applied in ice ocean interaction models the crystal enhanced drag promotes increased entrainment into the boundary layer from the upper ocean which has the potential to affect ice shelf evolution and sea ice growth through enhanced turbulent exchange of heat and momentum

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

meteoric and marine ice crystal orientation fabrics from the amery ice shelf east antarctica the northwestern sector of the amery ice shelf east antarctica has a layered structure due to the presence of both meteoric ice and a marine ice layer resulting from sub shelf freezing processes crystal orientation fabric and grain size data are presented for ice cores obtained from two boreholes 70 km apart on approximately the same flowline multiple maxima crystal orientation fabrics and large mean grain sizes in the meteoric ice are indicative of stress relaxation and subsequent grain growth in ice that has flowed into the amery ice shelf strongly anisotropic single maximum crystal orientation fabrics and rectangular textures near the base of the 200 m thick marine ice layer suggest accretion occurs by the accumulation of frazil ice platelets crystal orientation fabrics in older marine ice exhibit vertical large circle girdle patterns influenced by the complex stress configurations that exist towards the margins of the ice shelf post accumulation grain growth and fabric development in the marine ice layer are restricted by a high concentration of brine and insoluble particulate inclusions differences in the meteoric and marine ice crystallography are indicative of the contrasting rheological properties of these layers which must be considered in relation to large scale ice shelf dynamics

textural characteristics and impurity content of meteoric and marine ice in the ronne ice shelf antarctica the texture and physical properties of an ice core recovered to 215 m depth from the ronne ice shelf antarctica have been studied with regard to formation and transformation of the ice at a depth of 152 8 m a sharp discontinuity marks the transition between meteoric ice accumulated from above and marine ice accreted from below as testified by electrolytical conductivity and stable isotope measurements as well as geophysical field surveys automated image analysis of thin sections indicates that the decrease in grain boundary density and the increase in grain cross sectional area with depth is commensurate with though not necessarily caused bythermodynamically driven grain growth down to 120m depth corresponding to a vertical strain of roughly 65 as computed with a simple temperature history particle path model the observed increase of grain boundary density i e a decrease of grain size with age in the marine ice is in part explained by the thermal history of this layer sediment inclusions at the top of the marine ice layer affect the observed grain boundary density profile by inhibiting grain growth and dynamic recrystallization this may allow some conclusions on the role of temperature particulate inclusions stress and strain rate in controlling the grain size evolution of deforming ice supplementing earlier laboratory experiments conducted at much shorter time scales salinities 0 026 parts per thousand brine volumes 0 09 0 2 parts per thousand and solid salt concentrations have been computed from electrolytical conductivity measurements mean of 51 0 x 10 6 s cm 1 for the marine ice an assessment of salt incorporation and desalination rates shows that these low salinities can at present only be explained by a unique densification mechanism of under water ice crystals at the base of the ice shelf

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

underwater gliders reveal rapid arrival of el nino effects off california s coast 1 the 2009 2010 el nino marked the first occurrence of this climate phenomenon since the initiation of sustained autonomous glider surveillance in the california current system ccs spray glider observations reveal the subsurface effects of el nino in the ccs with spatial and temporal resolutions that could not have been obtained practically with any other observational method glider observations show that upper ocean waters in the ccs were unusually warm and isopycnals were abnormally deep during the el nino event but indicate no anomalous water masses in the region observed oceanic anomalies in the ccs are nearly in phase with an equatorial el nino index and local anomalies of atmospheric forcing these observations point toward an atmospheric teleconnection as an important mechanism for the 2009 2010 el nino s remote effect on the midlatitude ccs

massive ancient sea ice strata and preserved physical structural characteristics in the ward hunt ice shelf two sea ice layers one measured as 9 m thick the other at least 12 m thick and estimated to be 24 5 m thick have been located by ice core drilling in the west ward hunt ice shelf to examine the preservation of physical structu ral characteristics over long time intervals the crystal structure and brine volumes in the sea ice which possibly dates back to about 3000 bp have been studied the structural characteristics are immediately recognizable as those of undeformed congelation sea ice accreted by stefan growth brine volumes in the ancient sea ice are higher than those in modern multi year ice at the same temperature the preservation of brine over a time span of hundreds to thousands of years is attributed to an absence of surface meltwater to effect brine flushing and the very slow even negligible action of gravity drainage brine pocket migration and brine expulsion the congelation structures indicate that sea ice can grow by the stefan accretion mechanism to thicknesses exceeding the equilibrium thickness 2 5 5 m of most undeformed multi year ice the observed physical structu ral characteristics of the ward hunt sea ice strongly suggest that many of the properties attained by sea ice are permanent and not affected by slow acting physical processes

observations of sound speed fluctuations in the western philippine sea in the spring of 2009 as an aid to understanding long range acoustic propagation in the philippine sea statistical and phenomenological descriptions of sound speed variations were developed two moorings of oceanographic sensors located in the western philippine sea in the spring of 2009 were used to track constant potential density surfaces isopycnals and constant potential temperature surfaces isotherms in the depth range 120 2000 m the vertical displacements of these surfaces are used to estimate sound speed fluctuations from internal waves while temperature salinity variability along isopycnals are used to estimate sound speed fluctuations from intrusive structure often termed spice frequency spectra and vertical covariance functions are used to describe the space time scales of the displacements and spiciness internal wave contributions from diurnal and semi diurnal internal tides and the diffuse internal wave field related to the garrett munk gm spectrum are found to dominate the sound speed variability spi

zonal evolution of alaskan stream structure and transport quantified with argo data the alaskan stream as flows west southwestward along the south side of alaska and the aleutian island arc a western boundary current at the northern edge of the north pacific subpolar gyre the argo float array has improved sampling of the gulf of alaska allowing quantification of the as s zonal evolution from 140 w to 175 w geostrophic alongshore transport of the as in the upper 1000 dbar referenced to an assumed level of no motion at 1000 dbar shows little change from east to west however alongshore absolute geostrophic transports in the top 2000 dbar obtained by combining mean absolute 1000 dbar velocities from float displacements with the geostrophic velocity fields generally increase to the west we estimate full depth transports by fitting a barotropic and the first two baroclinic modes calculated from a climatology to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities from these fits from 2000 dbar to the seafloor flowing west from its formation region at 140 w 145 w the full depth as becomes stronger more barotropic and also narrower once it reaches 160 w with along shore transports increasing from 16 4 4 9 sv 1 sv 106 m3 s 1 at 140 w to 32 6 5 2 sv at 175 w mean concentrations of relatively warm salty oxygen poor and nutrient rich pacific equatorial water pew in the as decrease from 17 8 0 3 to 8 5 0 5 between 140 w and 175 w however the volume transport of pew by the as exhibits little change over the pew density range between these longitudes

microstructure and electrical properties of marine ice and its relationship to meteoric ice and sea ice a 215 m ice core has penetrated the central part of the ronne ice shelf 76 59 s 52 16 w the core consisted of meteoric ice above 152 8 m depth below this the ice was bubble free and of seawater origin the salinity of the layer below 152 8 m depth is less than 0 05 ppt very much lower than typical sea ice the ice represents an unusual source of ice intermediate in salinity and some physical properties between meteoric ice and sea ice the structure of the ice from four different depths where salinities vary by a factor of 4 has been investigated using a scanning electron microscope sem the fabric and grain structures are unusual and seem to depend on the impurity concentration in the ice chemical impurity localization has also been investigated the dielectric properties of the ice show that the ice has a transitional behavior from the linear dependence of conductivity on chloride concentration found in meteoric ice to the less predictable sea ice behavior the behavior may be interpreted in terms of the structure of the ice the higher salinity samples show convoluted grain boundaries small grain size and brine inclusions there are indications that the brine inclusions were liquid at temperatures between 10 c and 30 c by contrast the lower salinity samples possess larger grain sizes and show no evidence of brine inclusions all samples show brine concentrated at triple grain junctions and also along two grain boundaries in higher salinity samples the dielectric properties of the lower salinity samples are well described by a jaccard mechanism with l defects created in proportion to salinity that also describes the behavior of meteoric ice higher salinity samples exhibit lower conductivity than would be seen if l defects continued to be created in proportion to salinity the dielectric and structural data are consistent with a solubility limit of about 300 m for cl in the ice lattice

alternative ice shelf equilibria determined by ocean environment 1 dynamic and thermodynamic regimes of ice shelves experiencing weak 1 m year 1 to strong 10 m year 1 basal melting in cold bottom temperature close to the in situ freezing point and warm oceans bottom temperature more than half of a degree warmer than the in situ freezing point are investigated using a 1 d coupled ice ocean model complemented with a newly derived analytic expression for the steady state temperature distribution in ice shelves this expression suggests the existence of a basal thermal boundary layer with thickness inversely proportional to the basal melt rate model simulations show that ice shelves afloat in warm ocean waters have significantly colder internal ice temperatures than those that float in cold waters our results indicate that in steady states the mass balance of ice shelves experiencing strong and weak melting is controlled by different processes in ice shelves with strong melting it is a balance between ice advection and basal melting and in ice shelves with weak melting it is a balance between ice advection and deformation sensitivity simulations show that ice shelves in cold and warm oceans respond differently to increase of the ocean heat content ice shelves in cold waters are more sensitive to warming of the ocean bottom waters while ice shelves in warm waters are more sensitive to shallowing of the depth of the thermocline

numerical flow simulation at local parts of filchner ronne ice shelf antarctica the munster radio echo sounding res work in the central part of filchner ronne ice shelf has revealed new information different signatures are discernible in the airborne res data local models in the area between henry ice rise and berkner island are used to interpret these signatures in terms of thermal and dynamic parameters from our airborne measurements the shapes of the meteoric and marine ice layers were prepared as input data for dynamic model calculations a general increase in the temperature dependent flow parameter is compared with a spacially variable flow parameter that is derived from model calculations concerning freezing and melting processes velocity and strain fields were calculated by a finite difference method on a local data base between henry ice rise and berkner island

perspectives for development of ice core drilling technology a discussion abstract more than 170 years ago louis agassiz one of the creators of glacial theory made his firstattempt to drill into the bed of unteraargletscher swiss alps since that time various systems forthermal and mechanical drilling have been designed especially for boring into ice and someconventional drill rigs been adopted for ice coring although contemporary ice drilling knowledge andtechniques are now familiar there remain many problems to be solved by advanced moderntechnology specific challenges related to improving old drilling methods and developing new emergingtechnologies include 1 identification of depth limitation of dry drilling 2 improvement of casing 3 searching for the new environmentally friendly low temperature drilling fluids 4 reliableelimination of sticking drills 5 improvement of core quality in the brittle zone 6 additional coresampling from borehole walls after the core has been drilled 7 obtaining oriented core 8 designingautomation drilling systems 9 developing rapid access drills possible ways of solving these problemsare presented below keywords

borehole evidence for a thick layer of basal ice in the central ronne ice shelf the ronne ice shelf floating between berkner island and the antarctic peninsula is the biggest ice shelf in antarctica it drains an estimated 1 2 106km2 of the antarctic ice sheet much of it resting on bedrock below sea level consequently the balance and dynamics of this ice shelf is of importance to antarctic glaciology especially with regard to the integration of this part of the cryo sphere into the global processes that control the climate of the earth extensive radio echo sounding res by robin and others revealed reflections in the central part of the ronne ice shelf at the relatively shallow depth of 100 200 m below surface1 the interpretation of these echoes which varied in strength was ambiguous and the possibility of internal reflecting horizons was thoroughly discussed but after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice it was decided to base a thickness map of the ronne ice shelf on these res echoes1 4 we now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped and that the shallow res reflections therefore do come from internal horizons

significant marine ice accumulation in the ablation zone beneath an antarctic ice shelf high resolution crystallographic salinity and isotopic analyses of a 45 m ice core reveal the presence of a thick layer of marine ice near the grounding line of the nansen ice shelf antarctica the anomalous formation of marine ice in a zone assumed to be the site of active basal melting leads us to propose the hypothesis of large basal crevasses as a favorable environment for important marine ice accretion this hitherto unexplored possibility is supported by the overall field configuration and by the discrepancy in some ice properties between this core and the marine ice sections of previous drilling projects these findings could have important implications for the general stability of ice shelves and their disintegration processes the specific properties of this core reveal that marine ice is post genetically deformed