a numerical study on convergence of alongshore flows over the texas louisiana shelf 1 a hydrodynamic model is used to investigate convergent alongshore flows over the texas louisiana shelf characterized by down coast flows over the northern shelf encountering up coast flows over the southern shelf the model s ability to reproduce realistic current salinity and surface elevation fields is demonstrated through positive model skill scores when comparing model simulations to observational data the convergent flows are explored on both weather band and seasonal time scales for weather band scales this study focuses on wintertime convergent events the model predicted locations of convergent flows are supported by current measurements we find that the formation of convergent flows is primarily caused by along coast variation in the alongshore component of wind forcing which in turn is due to the curvature of the texas louisiana coastline in general the alongshore currents are well correlated with alongshore winds however the convergence points of currents and winds are not colocated but rather convergence points in ocean currents typically occur down coast of convergence points in the wind this offset is demonstrated to be mainly caused by buoyancy forcing that can drive the convergence location in the currents farther down coast no specific temporal pattern is found for the weather band convergence locations whereas at seasonal time scales the monthly mean convergence exhibits a prominent seasonal pattern with up coast migration of convergence locations in spring and summer and down coast migration in fall and winter

near encounter with asteroid 253 mathilde overview abstract on june 27 1997 the near spacecraft carried out the first ever encounter with a c type asteroid flying by 253 mathilde at a distance of 1212 km we summarize findings derived from 330 images obtained by near s msi camera which cover about 60 of the surface of the asteroid the highest resolution achieved was about 160 m pixel mathilde is a low reflectance object geometric albedo 0 047 with principal diameters of 66 48 44 km the mean radius of 26 4 1 3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data mathilde s surface morphology is dominated by large craters at least four of which have diameters comparable to the radius of mathilde the two largest ishikari and karoo have diameters of 29 3 and 33 4 km respectively no evidence of layering is exposed in the crater walls but suggestions of downslope movement are present the surface density of craters in the diameter range from 0 5 to 5 km is close to equilibrium saturation a situation in which as many craters are being destroyed as are being produced observed depth to diameter ratios for craters in this size range are close to those observed on the lunar surface a disruption lifetime of about 4 billion years has been estimated for mathilde based on the mass determination obtained from doppler tracking d k yeomans et al 1997 science 278 2106 2109 and the volume derived from msi images the average density of mathilde is remarkably low 1 3 0 3 g cm 3 a value consistent with a rubble pile structure for the interior assuming that mathilde s rock type is similar to that found in cm meteorites the porosity of the interior must be some 50 shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by mathilde s underdense interior a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events except for the lower geometric albedo 0 047 0 005 the photometric properties of mathilde are closely similar to those of phobos the surface is extremely homogeneous in terms of both color or albedo specifically no color or albedo variations associated with craters have been identified

surface of young jupiter family comet 81p wild 2 view from the stardust spacecraft images taken by the stardust mission during its flyby of 81p wild 2 show the comet to be a 5 kilometer oblate body covered with remarkable topographic features including unusual circular features that appear to be impact craters the presence of high angle slopes shows that the surface is cohesive and self supporting the comet does not appear to be a rubble pile and its rounded shape is not directly consistent with the comet being a fragment of a larger body the surface is active and yet it retains ancient terrain wild 2 appears to be in the early stages of its degradation phase as a small volatile rich body in the inner solar system

seasonal upper shelf circulation along the central western gulf of mexico a preferential upcoast flow reinforced by the recurrent arrival of loop current eddies

investigating δ 18 o of turbo sarmaticus l 1758 as an indicator of sea surface temperatures abstract the large marine gastropod turbo sarmaticus was and still is an important food item for coastal foragers along the south coast of south africa opercula of this species are well preserved in both holocene and pleistocene archaeological sites offering the opportunity to reconstruct past sea surface temperatures from 18o of shell carbonate over much of the last glacial cycle we have analysed 18o and 13c in 16 opercula and one shell of modern animals collected alive from an area with good sst records in order to explore how reliably this species records water temperature through the year and to investigate possible vital effects that may influence the isotope record we find that t sarmaticus deposits aragonite in both operculum and shell in oxygen isotopic equilibrium with seawater operculum 18o values of this species reliably reflect water temperatures between c 14 and 24 c although they do not always capture temperature extremes especially in winter likely due to slower growth under stressful growth conditions these thresholds probably alter according to factors such as variations in nutrient supply the averaged maximum and minimum values across the set of sixteen serially sampled opercula very closely approximate the temperatures of the warmest and coolest months and therefore provide a good measure of the seasonal temperature range and of mean annual sst 18o of edge increments can to some extent be used to indicate season of collection within the broad categories of warmer cooler seasons

west florida shelf response to upwelling favorable wind forcing kinematics the barotropic responses of the west florida continental shelf to idealized upwelling favorable alongshore and offshore wind stresses are studied using the three dimensional time dependent primitive equation princeton ocean model pom when forced with uniform winds the shelf circulation evolves quickly to a quasi steady state a southeastward alongshore wind lowers sea level along the coast and drives a southeastward coastal jet with a relatively weak northwestward return flow farther offshore a southwestward offshore wind lowers sea level along the west florida coast and raises sea level along the panhandle coast two independent circulation gyres are set up in association with these regionally different sea level distributions an anticyclonic gyre off the west florida coast consisting of a southeastward coastal jet and a broad return flow over the middle shelf and a cyclonic gyre off the panhandle coast consisting of a strong northwestward coastal jet and an adjacent narrow southeastward undercurrent these gyres are separated by the big bend region in both cases alongshore and offshore wind forcing the circulations are fully three dimensional with opposing surface and bottom boundary layer flows accounting for the across shelf transports the shapes of the coastline and the isobaths are important determinants of the shelf wide responses several locally maximum upwelling regions are identified for geometrical reasons these include the panhandle coast south of apalachicola bay the west florida coast south of tampa bay along the florida keys and at the shelf break where topographic rossby waves are evident

spatial scale analysis of hydrographic data over the texas louisiana continental shelf on the basis of hydrographic data collected by the texas louisiana shelf circulation and transport processes study latex a and on earlier cruises we examined the energetic scales of spatial variability over the texas louisiana continental shelf shelf scale spatial reference fields were sought to represent the general distributions of circulation and water properties over the shelf at the time of the observations various methods were explored for determining such reference fields of potential temperature salinity and geopotential anomaly at the sea surface relative to 70 db spatial reference fields obtained from mean may fields and from polynomials fitted to individual may cruise data were compared on the basis of those comparisons quadratics were selected to fit property distributions from individual cruises and so to yield reference fields smaller scale anomaly fields were obtained by removing the reference fields from the observed distributions calculation of correlation versus separation distance based on these anomaly fields then allowed estimation of spatial scales of anomaly fields for cross shelf and along shelf transects the zero crossing scale and the gaussian decay scale are shown to be essentially the same and the zero crossing scale is used the principal results for the anomaly scales are 1 cross shelf scales over the western shelf are shorter order 15 km than those in the eastern and central regions order 20 km 2 along shelf spatial scales are of the order of 35 km 3 there is no significant difference in cross shelf scales at the surface middepth and bottom and 4 along shelf scales are essentially the same over the western and eastern regions of the shelf over the midshelf 50 m isobath and along the shelf break 200 m isobath and at different depths along the 200 m isobath the same spatial scales are found when using data with spatial resolution of 1 10 km cross shelf and 10 20 along shelf to obtain the anomaly fields so the data resolution used is adequate to represent the scales the variances of the observed shelf wide salinity temperature and geopotential anomaly are greater cross shelf than along shelf the variance of the cross shelf anomaly fields is around 10 of the shelf wide fields that of the along shelf anomaly fields is about 35 of that in the shelf wide fields the analysis of scales when grouped by season did not show persuasive evidence of seasonal variation

circulation over the continental shelf in the northern gulf of mexico abstract more than 750 surface drifters were deployed over the northern continental shelf in the gulf of mexico gulf from 1993 through 1998 drifters were mostly air deployed on a schedule giving continuous data over much of the texas louisiana and west florida shelves for an entire year the drifter data are analyzed to give descriptive statistical and dynamical views of the northern gulf circulation on synoptic to seasonal scales circulation on both the texas louisiana and west florida shelves is characterized by coherent along shore and across shore modes which explain more than 50 of the variance a pronounced seasonal cycle exists on the texas louisiana shelf but not the west florida shelf both shelves display synoptic signals due to local wind forcing strong connectivity between the shelves occurs during isolated period of strong winds associated with the passage of tropical storms eddies impinging upon the shelf are responsible for moving drifters cross shore offshore flows generally occur prior to reversals in the along shore direction regions with near zero mean velocities but large total energy exist drifters leave these areas and move thorough much of the gulf and into the atlantic within 90 days despite negligible annual mean eulerian velocities

current and hydrographic variability in the northwestern gulf of mexico from july 1980 to february 1981 ten current meters on three moorings were deployed in the 200 to 700 m depth range over the continental slope in the northwestern gulf of mexico the currents were characterized by energetic fluctuations with time scales of a week to several months westward drifting loop current anticyclones provided the principal driving mechanism for the fluctuations longshore current speeds at the 200 m depth occasionally exceeded 70 cm s and were persistently 50 cm s during a 2 month period in the fall except during a hurricane the currents were only marginally coupled with the winds measured at brownsville texas tidal motions accounted for 1 of the current variance

extending the radiocarbon calibration beyond 26 000 years before present using fossil corals fossil coral is an excellent archive to extend the radiocarbon 14c calibration beyond tree ring records however published coral data older than 26 000 years before present are too disparate for practical calibration we propose an explanation for these discrepancies trace amounts of secondary calcite and organic matter in fossil corals shift the 14c ages toward younger ages in a series of acid leaching experiments to simulate the standard cleaning procedure prior to 14c analysis we measure up to a 300 enrichment of diagenetic calcite in corals due to the solubility differences between calcite and aragonite in model calculations we show the 14c age offsets produced by acid leaching could be hundreds to thousands of years when typical amounts of calcite are present in samples we demonstrate the necessity and our ability to detect 0 2 calcite in aragonite by x ray diffraction and apply 0 2 calcite as our a priori criterion for our coral samples the estimated age offsets of samples with 0 2 calcite fall within the reported analytical uncertainty of 14c dating in addition we present 14c results from an extended hydrogen peroxide h2o2 pretreatment for coral samples to remove organic matter our data show that treated samples yield consistent or older 14c dates compared to non treated samples these rigorous but necessary screening and cleaning techniques provide precise reproducible and accurate radiocarbon calibration data

effects of micromilling on δ18o in biogenic aragonite 1 extraction of small aliquots g mg from aragonite samples such as corals sclerosponges bivalves and otoliths is required for bulk oxygen isotope analyses however aragonite is a metastable polymorph of calcium carbonate that undergoes transformation to calcite which is the more thermodynamically stable polymorph of caco3 this polymorph transformation occurs via heating and or stress and such conditions may occur during drilling during this polymorph transition exchange with atmospheric or organic c and o is possible therefore extracting aliquots during drilling without modification or contamination via isotope exchange is imperative for accurate isotopic analysis using a micromill aliquots were taken from aragonite cod otoliths that had been grown in the laboratory under stable temperature conditions x ray diffraction analysis showed that although 6 of the sample converts to calcite during drilling there is no significant effect on 18o

surface geomorphology of jupiter family comets a geologic process perspective abstract recent spacecraft encounters with jupiter family comets have revealed markedly diverse surface morphologies wild 2 is dominated by steep walled and flat floored depressions tempel 1 is relatively smooth and exhibits evidence for flows and layering while hartley 2 is bi lobed with knobby terrain at its ends and a much smoother terrain in its middle this diversity of surface morphologies has been interpreted as an evolutionary sequence belton m j s 2010 icarus 210 881 897 where jupiter family comets evolve from wild 2 morphology then to tempel 1 morphology and finally to hartley 2 morphology we propose instead that the diversity of surface morphology reflects geologic processing with diverse outcomes in addition to impact cratering we consider surface modification driven by the cometary activity which is responsible for gas and dust production we consider eolian erosion that may be driven by the outflow of cometary vapor making use of information from wind tunnel experiments and in situ studies of eolian erosion on mars we adopt the model of van der waals cohesion recently proposed by scheeres et al scheeres d hartzell c sanchez p swift m 2010 icarus 210 968 984 and find that the average co 2 vapor outflow flux at hartley 2 of 0 95 10 17 cm 2 s 1 implies wind speeds sufficient to mobilize particles of 10 cm size even close to the icy reservoirs where the vapor is evolved below the surface we suggest that particles are mobilized and entrained in flows within sub surface outflow channels emerging to be readily lifted into the coma and fragmenting in the process although water production from hartley 2 is greater most of it is evolved in the coma from icy particles and does not contribute to eolian erosion of the nucleus on the other hand the much lower vapor outflow flux at tempel 1 of 4 10 14 cm 2 s 1 is insufficient in the present model to mobilize particles but is consistent with generating repeated fluidization episodes the wild 2 case is intermediate with average outflow flux 7 10 16 cm 2 s 1 which would be in our model sufficient to support eolian erosion the steep walled flat floor depressions on wild 2 may have originated as impact features but have been subsequently modified by eolian erosion causing slope recession we do not have definitive evidence to discriminate between an evolutionary sequence and diverse geologic outcomes but we suggest future in situ observations at comets that may do so

does crater saturation equilibrium occur in the solar system abstract similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system this is hypothesized to result from a steady state saturation equilibrium being approached or achieved by cratering processes this hypothesis conflicts with some recent interpretations however it accounts for 1 a similarity in maximum relative crater density below certain theoretically predicted values on all heavily cratered surfaces 2 a leveling off at this same relative density among 100 m scale secondary craters in populations on lunar maria and other sparsely cratered lunar surfaces 3 the approximate uniformity of maximum relative densities on saturn satellites in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors the lunar frontside upland crater population sometimes described as a well preserved production function useful for interpreting other planetary surfaces is found not to be a production function it was modified by intercrater plains formed at least partly by early upland basaltic lava flooding recently confirmed spectrophotometrically consistent with this counts in pure uplands those lacking intercrater plains match the proposed saturation equilibrium density variations among large d 64 km crater populations are found but these may involve several hypothesized mechanisms that rapidly obliterate large craters especially on icy surfaces recent models in which different populations of interplanetary bodies hit different planets need further appraisal

the shape topography and geology of tempel 1 from deep impact observations deep impact images of the nucleus of comet tempel 1 reveal pervasive layering possible impact craters flows with smooth upper surfaces and erosional stripping of material there are at least 3 layers 50 200 m thick that appear to extend deep into the nucleus and several layers 1 20 m thick that parallel the surface and are being eroded laterally circular depressions show geographical variation in their forms and suggest differences in erosion rates or style over scales 1 km the stratigraphic arrangement of these features suggests that the comet experienced substantial periods of little erosion smooth surfaces trending downslope suggest some form of eruption of materials from this highly porous object the deep impact images show that the nucleus of tempel 1 cannot be modeled simply as either an onion layer or rubble pile structure

the morphology and surface processes of comet 19 p borrelly the flyby of the nucleus of the comet 19p borrelly by the deep space 1 spacecraft produced the best views to date of the surface of these interesting objects it transformed borrelly from an astronomical object shrouded in coma of gas and dust into a geological object with complex surface processes and a rich history of erosion and landform evolution based on analysis of the highest resolution images stereo images photometry and albedo we have mapped four major morphological units and four terrain features the morphological units are named dark spots mottled terrain mesas and smooth terrain the features are named ridges troughs pits and hills in strong contrast to asteroids unambiguous impact craters were not observed on borrelly s surface because of the relatively short period of this comet surface erosion by volatile sublimation is in geologic terms a very active process the formation and the morphologies of units and features appear to be driven by differential rates of sublimation erosion erosional rates across the comet are probably controlled by solar energy input and the location of the subsolar point during perihelion differences in energy input may produce different varieties of sublimation erosional landforms the terrains on borrelly suggest that solar energy input could map directly into erosional processes and landforms

reprint of unexpected isotopic variability in biogenic aragonite a user issue or proxy problem abstract the present study seeks to investigate sources of isotopic variability in the commonly used paleoclimate archive the marine bivalve arctica islandica with an emphasis on the potential of human induced variability arising from sampling techniques stable carbon 13ccarbonate and oxygen 18ocarbonate isotopes were analyzed for split intra sample and replicate intra and inter shell samples taken from a group of laboratory reared individuals a natural population from northern norway and a natural population from the gulf of maine usa compared to analytical uncertainty of 0 17 and 0 30 for 13c and 18o respectively among the natural populations the mean difference between shell splits and shell replicates ranged from 0 12 and 0 33 for 13c and 18o respectively our data suggest that heterogeneity of the carbonate material i e large range of isotopic composition within one sample due to seasonal environmental variability may contribute to unexpected variability more than human induced error from sampling imprecision when collecting whole annual increments furthermore 13c from juvenile shells were highly variable 2 standard deviation 0 65 approximately four times more variable than analytical precision high precision among 18o measurements of the laboratory reared shells confirm the presumption that shells reliably and consistently precipitate in isotopic equilibrium with ambient seawater monte carlo simulations of measurements from this population allowed characterization of improvements in uncertainty at increasing levels of replication substantial reduction in uncertainty occurs when increasing from two to three shells however replication using a total of four shells further decreased uncertainty to within the 99 confidence level published studies sometimes compensate for uncertainties by replicating records over multiple individuals or multiple transects within the one individual oftentimes however isotope records are constructed from single individuals or transects and therefore fail to provide thorough estimates of proxy error our findings suggest that replication of carbon and oxygen isotope measurements of contemporaneously produced aragonite is necessary in order to reduce proxy derived noise furthermore population specific estimates of uncertainty related to natural variability among individuals should be investigated in order to provide more realistic representations of proxy noise when reporting isotope time series

decadal cyclicity of regional mid holocene precipitation evidence from dominican coral proxies 1 the stable carbon 13c and oxygen 18o isotopic compositions of four holocene specimens of montastraea sp and siderastrea sp coral skeletons from the dominican republic were analyzed to examine decadal scale fluctuations in regional precipitation the specimens range in length from 47 to 197 years individually and range in age from 7 2 to 5 2 kyr oscillations in the 13c and 18o of coral skeletons from the holocene enriquillo valley occur with a seemingly consistent frequency of 15 20 years for one coral 6 ka this decadal to multidecadal mode of variability is statistically significant the decadal to multidecadal mode of stable isotopic variability may represent cyclic fluctuations in mid holocene migration of the intertropical convergence zone itcz or in the hydrogeology of the region as governed by fluctuations in the passage of tropical storms and hurricanes regional temperature is believed to act as only a secondary control on the 18o values of these corals however itcz migration and or oscillations in storm frequency at this site may ultimately reflect decadal scale patterns in greater tropical north atlantic sea surface temperatures the length and resolution of climate variability represented in this study indicate a persistent decadal to multidecadal mode of atlantic climate fluctuation throughout the mid holocene that is indistinguishable from observed historical climate

isotopic behavior during the aragonite calcite transition implications for sample preparation and proxy interpretation abstract at the earth s surface aragonite a polymorph of calcium carbonate is thermodynamically unstable and readily converts to calcite a process accelerated by heating and mechanical stress this study has investigated the behavior of the 13 c 12 c 18 o 16 o ratios and the clumped isotope temperature proxy 47 during this mineral transition aragonite samples were partially or completely converted to calcite in an oven normal atmospheric composition and pressure or in a sealed tube in the presence of a vacuum or pure co 2 of varying isotopic compositions other samples were converted from aragonite to calcite using a drill these experiments demonstrate a relationship between the 13 c and 18 o values and percent transition from aragonite to calcite the 47 value of the aragonite changed independently from the mineral transition at temperatures as low as 125 c and was accompanied by a 0 5 decrease in the 18 o value this change occurred irrespective of the presence of co 2 suggesting that water trapped in the mineral may facilitate some of this alteration results show that sampling techniques such as drilling produce sufficient heat to convert aragonite to calcite and can considerably alter 47 even small degrees of conversion 10 can increase clumped isotope derived paleotemperature estimates by nearly 10 c

stable isotope composition alteration produced by the aragonite to calcite transformation in speleothems and implications for paleoclimate reconstructions abstract aragonite a mineralogical constituent of speleothems in cave environments is unstable and susceptible to inversion to calcite a diagenetic process that involves changes in the mineralogy texture and geochemistry of speleothems however the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated in this study two aragonite stalagmites sn3 and sn15 from the shennong cave southeast china were found partially inverted to calcite as determined by x ray diffraction xrd and scanning electron microscopy sem analyses and thin section inspections under microscope the fiber relics and textural ghosts of aragonite preserved in coarse and equant mosaic calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite the stable isotope compositions 13 c and 18 o given in per mil versus vpdb standard of primary aragonite and secondary calcite were analyzed and compared along both growth layers and growth axes the results show that along growth layers differences of 13 c values between aragonite and calcite are negligible 0 1 0 2 whereas differences of 18 o values between aragonite and calcite are significant 0 63 0 87 comparisons along growth axes show similar results i e differences of 13 c values are negligible 0 06 0 22 whereas differences of 18 o values are significant 0 85 0 29 most likely the aragonite in sn3 and sn15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces by a dissolution reprecipitation process occurring in trapped pore water in the case of the inversion of aragonite to calcite in speleothems such as that observed in sn3 and sn15 the 13 c values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite although the 18 o values might be cross calibrated to those of primary aragonite if the aragonite calcite fractionation offset is known e g 0 85 0 29 in this study however the 18 o values of secondary calcite should be used with caution in such reconstructions as the 18 o offset value is not consistently invariable

objectively mapped stream function fields on the texas louisiana shelf based on 32 months of moored current meter data on the basis of current measurements observed at 31 moorings during april 1992 through november 1994 as part of the texas louisiana shelf circulation and transport processes study we describe the horizontally nondivergent part of the near surface shelf wide scale low frequency circulation and its variability on the texas louisiana continental shelf by means of objectively fitted stream function fields the 32 month mean velocity stream function patterns generally support the cochrane and kelly 1986 circulation schema for the shelf circulation in which an elongated cyclonic gyre exists during the nonsummer period september through may but with flow in the summer june to august directed upcoast toward the mississippi delta over the entire shelf there exists significant interannual variability of the stream function fields especially in the transition period from spring to summer to fall and along the shelf break strong upcoast downcoast currents dominate at the western shelf break associated with the presence of energetic anticyclones cyclones the dominant pattern of stream function variability deduced by empirical orthogonal function eof analysis of 32 individual monthly mean patterns contains about 89 of the variance and shows a simple shelf wide along shelf pattern of circulation the monthly evolution in amplitude of this dominant eof pattern clearly mimics the monthly evolution of the along shelf component of the wind with squared correlation of 0 83 this provides strong evidence that the seasonal variation of the shelf circulation is wind forced in support of cochrane and kelly the second and residual eofs show more complex features near the shelf break that are associated with loop current eddies known to impact the continental slope and shelf break

unexpected isotopic variability in biogenic aragonite a user issue or proxy problem abstract the present study seeks to investigate sources of isotopic variability in the commonly used paleoclimate archive the marine bivalve arctica islandica with an emphasis on the potential of human induced variability arising from sampling techniques stable carbon 13ccarbonate and oxygen 18ocarbonate isotopes were analyzed for split intra sample and replicate intra and inter shell samples taken from a group of laboratory reared individuals a natural population from northern norway and a natural population from the gulf of maine usa compared to analytical uncertainty of 0 17 and 0 30 for 13c and 18o respectively among the natural populations the mean difference between shell splits and shell replicates ranged from 0 12 and 0 33 for 13c and 18o respectively our data suggest that heterogeneity of the carbonate material i e large range of isotopic composition within one sample due to seasonal environmental variability may contribute to unexpected variability more than human induced error from sampling imprecision when collecting whole annual increments furthermore 13c from juvenile shells were highly variable 2 standard deviation 0 65 approximately four times more variable than analytical precision high precision among 18o measurements of the laboratory reared shells confirm the presumption that shells reliably and consistently precipitate in isotopic equilibrium with ambient seawater monte carlo simulations of measurements from this population allowed characterization of improvements in uncertainty at increasing levels of replication substantial reduction in uncertainty occurs when increasing from two to three shells however replication using a total of four shells further decreased uncertainty to within the 99 confidence level published studies sometimes compensate for uncertainties by replicating records over multiple individuals or multiple transects within the one individual oftentimes however isotope records are constructed from single individuals or transects and therefore fail to provide thorough estimates of proxy error our findings suggest that replication of carbon and oxygen isotope measurements of contemporaneously produced aragonite is necessary in order to reduce proxy derived noise furthermore population specific estimates of uncertainty related to natural variability among individuals should be investigated in order to provide more realistic representations of proxy noise when reporting isotope time series

deep impact excavating comet tempel 1 deep impact collided with comet tempel 1 excavating a crater controlled by gravity the comet s outer layer is composed of 1 to 100 micrometer fine particles with negligible strength 1000 kelvins a large increase in organic material occurred during and after the event with smaller changes in carbon dioxide relative to water on approach the spacecraft observed frequent natural outbursts a mean radius of 3 0 0 1 kilometers smooth and rough terrain scarps and impact craters a thermal map indicates a surface in equilibrium with sunlight

short term environmental change in a jurassic lagoon deduced from geochemical trends in aragonite bivalve shells the bathonian age inverbrora member of the brora coal formation northern scotland was deposited in a predominantly fresh water coastal lagoon setting however two prominent shell beds within the mostly argillaceous succession have been interpreted as resulting from temporary inundation of marine water and establishment of brackish conditions the ensuing lagoon hydrology and paleoenvironment were investigated by measuring stable isotopic 18o and 13c and trace element mg sr mn ba variations within five exceptionally well preserved aragonite shells of the bivalve isognomon murchisoni transects were sampled across the visible growth banding with a resolution of 0 25 0 15 mm for stable isotopes and 0 05 mm for trace elements oxygen and carbon isotope data define a broadly covariant trend 18o between 0 4 and 4 8 peedee belemnite pdb 13c between 2 2 and 0 7 pdb that supports a variably brackish water setting with slight seasonal evaporation absolute trace element values vary from shell to shell but show consistent patterns of covariation profiles of mg ca across shell layers display multiple sharp episodic minima that are coincident with peaks in sr ca and ba ca this is interpreted as documenting inundation of the restricted brackish water lagoon by relatively cooler seawater leading to a temporary increase in metabolic activity of the bivalves less frequent minima in mg ca sr ca 18o and 13c record fresh water influx and dramatic salinity reduction in the lagoon bivalve metabolic efficiency was decreased and in some cases breaks in shell growth occurred this study shows that in hydrodynamically complex settings trace element variation in bivalve shells can provide a detailed record of short term environmental change

persistent lagrangian transport patterns in the northwestern gulf of mexico abstractpersistent lagrangian transport patterns at the ocean surface are revealed from climatological lagrangian coherent structures clcss computed from daily climatological surface current velo

assessing the reproducibility and reliability of estuarine bivalve shells saxidomus giganteus for sea surface temperature reconstruction implications for paleoclimate studies studies using oxygen isotopes y 18 o of mollusk shells to determine paleotemperature need to assume water y 18 o values which could severely influence calculated temperatures we analyzed aragonitic shells of the butter clam saxidomus giganteus deshayes 1839 to determine the reproducibility of the isotopic signal between individuals and to assess how precisely temperature could be calculated given known salinity and temperature furthermore carbon isotopes are also investigated as an environmental proxy the abundance of well preserved s giganteus shells in archeological and geological deposits in northwestern north america makes them a particularly suitable species for paleoclimate studies seasonally resolved stable oxygen isotope profiles in three s giganteus shells collected from the same site in puget sound washington usa were well correlated 0 77br 2 b0 87 although there were differences up to 0 58x in high resolution y 18 o profiles of the three shells the difference between the average y 18 o of each shell was less than half of this 0 19x and half of what has been reported for between colony coral variability y 13 c profiles on the other hand were more complex with shell y 13 c being about 2 5x lower than expected equilibrium values however this roughly conforms to the idea that about 10 of the shell carbon originates from metabolic co2 both y 18 o and y 13 c indicate that s giganteus do not grow during periods of reduced salinity despite the excellent reproducibility of y 18 o between shells and the fact that salinity effects were duly considered calculated temperature still differed from instrumental temperature applying different salinity y 18 o water relationships to average shell y 18 o and considering salinity from the shell collection site and a nearby offshore station resulted in calculated average water temperatures ranging from 1 7 to 6 4 8c warmer than measured although we could not determine if s giganteus precipitate their shells in isotopic equilibrium we believe that the difficulty in predicting temperature arose from not being able to accurately determine

a nested grid ocean model with application to the simulation of meanders and eddies in the norwegian coastal current oceanic flow phenomena vary in such large ranges of time and spatial scales that even with the fastest and largest computer to date one cannot at reasonable costs compute large scale circulation and at the same time resolve mesoscale features like fronts and or eddies yet these features are dynamically important and their inclusion may determine how well we can model the mean flow including the deep ocean circulation a two way interactive nested grid primitive equation model is developed here for coastal ocean applications notable features of the model are 1 nesting can be specified on any subregion of a coarse grid large domain and there can be more than one nest if required 2 the nested region can be hot started from earlier calculation results of the coarse grid region that is the code automatically by a change of an input flag generates topography wind forcing climatology currents density and other prognostic variables in the nest and steps forward in time 3 a time splitting integration with small timesteps in the nest and large timesteps in the coarse grid domain is used and 4 nested variables are driven by coarse grid solutions around the nest s boundaries where a flow relaxation scheme may be used and at the same time drive the coarse grid evolution through its averaged action in the overlapped region to demonstrate its robustness the model is applied in a february march 1988 real time simulation of meanders and eddies in the norwegian coastal current initialized from a 585 days quasi equilibrium calculation the simulation includes meteorological forcings inflows outflows across the open boundaries inflow of the north atlantic warm water in particular tides coastal and baltic discharges and wintertime hydrography for depths 500 m from march 20 to 31 the development of a meander between the froya and the halten banks is simulated the timing and location of the meander agree well with observed hydrography

coastal sea level response to frontal passages on the louisiana texas shelf sea level variations resulting from wind forcing along the louisiana texas shelf are examined on 5 month winter records local weather patterns during the winter are dominated by cold front passages the associated surface wind field is well organized predominantly in the cross shelf direction regional sea level response however shows considerable variability it is mainly a response to alongshore wind at galveston but cross shelf wind at eugene island this spatially nonuniform response appears to be due to the different water depth over the inner shelf which is shallower off eugene island and favors direct cross shelf wind setup a friction model is applied to each region and the results compare well with observations

oxygen isotope fractionation in marine aragonite of coralline sponges oxygen isotope values of the extant caribbean coralline sponge ceratoporella nicholsoni are compared with published temperatures and 18o of water calculated from salinities the measured values from aragonitic sponge skeletons have a mean offset of 1 0 0 1 from calculated calcite equilibrium values aragonite calcite 1 0010 this is in good agreement with published values from synthetic aragonite they further agree with published near equilibrium oxygen isotope values of temperate and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges these results and the mode of skeleton formation of ceratoporella nicholsoni suggest that these sponges precipitate aragonite close to isotopic equilibrium the temperature dependence of oxygen isotopic fractionation between the aragonite of ceratoporella nicholsoni and water is only roughly constrained by the available data due to the narrow temperature range of the caribbean reef sites however as the data suggest oxygen isotopic equilibrium we can calculate a well constrained temperature equation combining temperate and cold water equilibrium values from molluscs and foraminifera with our sponge data full size image 1 k and full size image 1 k

the annual cycle of the western boundary current in the gulf of mexico it has been known for decades that the mean circulation in the western gulf of mexico is a large anticyclone this flow is driven both by wind stress curl and detached rings from the loop current this paper is an attempt to examine their relative importance by focusing on the annual cycle of the observed flow a new compilation of ship drift data shows that the flow along the western boundary is strongest in july and weakest in october it appears physically plausible that the annual variation of this western boundary current is driven by the annual variation in wind curl augmented by ekman pumping over the western gulf because the maximum in wind curl is so close to the western boundary the phase delay between the annual cycles of curl and current is only approximately 1 month the magnitudes of the curl and resulting current seem physically plausible the response is baroclinic and penetrates to 1000 m the bottom of the main thermocline the change in velocity at the sea surface is consistent with the cross stream pressure gradient as determined by the changes in sea level at the coast and dynamic height changes offshore the annual cycle is relatively large at the annual minimum the flow is only approximately 25 of the maximum the data suggest that this flow could be driven by wind curl but the uncertainty is high consequently the contribution of loop current rings turns out to be relatively small the reason for this is that when rings reach the western wall the steep bottom topography intersects the sloping fluid contours of the rings the rings decay slowly by normal deepwater processes until they reach the edge of the continental shelf and then they decay more rapidly by loss of fluid once the rings have reached the western boundary the decay time scale is found to be only 70 days

eddy energy and shelf interactions in the gulf of mexico sea surface height anomaly data from satellite are continuously available for the entire gulf of mexico surface current velocities derived from these remotely sensed data are compared with surface velocities from drifting buoys the comparison shows that satellite altimetry does an excellent job resolving gulf eddies over the shelf rise depths between 200 and 2000 m if the proper length scale is used correlations between altimeter and drifter derived velocities are statistically significant r 0 5 when the surface slope is computed over 125 km indicating that remotely sensed sea surface height anomaly data can be used to aid the understanding of circulation over the shelf rise velocity variance over the shelf rise from the altimetry data shows regions of pronounced eddy energy south of the mississippi outflow south of the texas louisiana shelf and in the northwest and northeast corners of the gulf these are the same locations where surface drifters are most likely to cross the shelf rise suggesting gulf eddies promote cross shore flows this is clearly exemplified with both warm and cold eddies finally the contribution of gulf eddies and wind stress to changes in the mean circulation are compared results indicate that the eddy generated vorticity flux to the mean flow is greater than the contribution from the surface wind stress curl especially in the region of the loop current and along the shelf rise base in the western gulf future modeling efforts must not neglect the role of eddies in driving gulf circulation over the shelf rise

diagnostic and prognostic numerical circulation studies of the south atlantic bight some of the results from a series of diagnostic and prognostic numerical simulations of the circulation in the south atlantic bight sab are described the numerical model developed for the study is a three dimensional primitive equation time dependent coordinate model with an imbedded turbulent closure submodel which should yield realistic ekman surface and bottom layers an implicit numerical scheme in the vertical direction and a mode splitting technique in time are adopted for computational efficiency a significant portion of the paper is concerned with realistic specification of initial conditions for temperature and salinity surface forcing and lateral open boundary conditions the latter are determined by a simple diagnostic geostrophic and ekman dynamics model which provides dynamically consistent temperature salinity and velocity boundary conditions it appears from an exaination of the numerical simulations that the full model yields results that share many features in common with our general understanding of the circulation of the south atlantic bight the region includes shallow shelf waters as well as deeper water dominated by the gulf stream data for synoptic skill assessment however are not available

fundamentally distinct outcomes of asteroid collisional evolution itokawa and eros 1 the outcomes of asteroid collisional evolution are presently unclear are most asteroids larger than 1 km size gravitational aggregates reaccreted from fragments of a parent body that was collisionally disrupted while much smaller asteroids are collisional shards that were never completely disrupted the 16 km mean diameter s type asteroid 433 eros visited by the near mission has surface geology consistent with being a fractured shard the hayabusa spacecraft visited an s asteroid smaller than 1 km namely 25143 itokawa here we report the first comparative analyses of itokawa and eros geology itokawa lacks a global lineament fabric and its blocks craters and regolith are inconsistent with formation and evolution as a fractured shard unlike eros itokawa is not a scaled down eros but formed by a distinct process of catastrophic disruption and reaccumulation

low frequency circulation on the texas louisiana continental shelf for the texas louisiana coast west of 92 5 w long series of data from near freeport texas together with shorter series from other locations show strong response of coastal current to wind stress in agreement with coastal jet concepts we infer from coastal winds scattered current measurements and distributions of sea surface salinity and geopotential that a cyclonic gyre elongated along the shelf is the dominant feature of the prevailing shelf circulation the inshore limb of the gyre is the coastal jet driven by wind with a west or southward downcoast component which prevails along much of the coast except in july august because the coast is concave the shoreward prevailing wind results in a convergence of coastal currents which marks the downcoast extent of the gyre corresponding to the convergence is a seaward flow which forms the southwest limb of the gyre a prevailing countercurrent north or eastward flow along the shelf break includes the outer limb of the gyre the eastern shoreward flowing limb of the gyre corresponds to divergence along the coast centered near 92 5 w the convergence at the western or southern end of the gyre migrates seasonally with the direction of the prevailing wind reaching south of the rio grande mouth in fall and the cameron offing in july the gyre is normally absent in july but reappears in august september when a downcoast wind component develops

the role of thermal stress fatigue in the breakdown of rock in cold regions abstract many geomorphological studies have ignored the role of thermal stress fatigue engineers have long maintained that thermal stress is an effective process physicists and the ceramics industry have provided a theoretical foundation to explain the nature and mode of operation of thermal breakdown recent geomorphological studies have shown the importance of thermal stress in cold regions by means of high frequency rock temperature data which can identify rates of temperature change in excess of 2 c min 1 the occurrence of weathered material in cold but dry regions can be explained by thermal stress shock differential radiation receipts on slopes of varying orientation and gradient can exert a very strong effect

the structure of the regolith on 67p churyumov gerasimenko from rolis descent imaging the structure of the upper layer of a comet is a product of its surface activity the rosetta lander imaging system rolis on board philae acquired close range images of the agilkia site during its descent onto comet 67p churyumov gerasimenko these images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters at the highest resolution of 1 centimeter per pixel the surface appears granular with no apparent deposits of unresolved sand sized particles the thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters the presence of aeolian like features resembling wind tails hints at regolith mobilization and erosion processes modeling suggests that abrasion driven by airfall induced particle splashing is responsible for the observed formations

a crater and its ejecta an interpretation of deep impact we apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the deep impact event an important question is whether the cratering event was gravity or strength dominated the answer gives important clues about the properties of the surface material of tempel 1 gravity scaling was assumed in pre event calculations and has been asserted in initial studies of the mission results because the gravity field of tempel 1 is extremely weak a gravity dominated event necessarily implies a surface with essentially zero strength the conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution we address that feature here and conclude that even strength dominated craters would result in a plume that appeared to remain attached to the surface we then calculate the plume characteristics from scaling laws for a variety of material types and for gravity and strength dominated cases we find that no model of cratering alone can match the reported observation of plume mass and brightness history instead comet like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few hour time frame with such mechanisms and to within the large uncertainties either gravity or strength craters can provide the levels of estimated observed mass thus the observations are unlikely to answer the questions about the mechanical nature of the tempel 1 surface

eddy and wind forced shelf circulation cochrane and kelly 1986 proposed a cyclonic gyre as the large scale mean circulation on the louisiana texas latex shelf produced by a convergence of coastal currents in the west and a divergence in the east while currents near the coast are presumably wind and buoyancy driven the origin of the eastward flow on the outer shelf and shelf break which forms the seaward limb of the gyre as well as the near shore convergence and divergence are not well understood a numerical model is used to show that the western convergence and shelf break current are driven by collision and stalling of westward propagating loop current eddies in the northwest gulf of mexico and the divergence in the east is caused by shoreward intrusion in the mississippi canyon the western convergence and shelf break current are modulated by the wind curl strongest in summer and weakest in winter on the shelf westward transport is comparable to that observed 0 15 sv 1 sv 106 m3 s 1 only when the westward wind stress is significant 0 3 dyn cm 1 a peak transport of 0 21 to 0 25 sv occurs in autumn of which 0 1 sv is due to wind 0 07 sv is due to river buoyancy and 0 04 to 0 08 sv is due to eddies without the mean westward wind buoyant waters from the mississippi do not spread onto the latex shelf

rapid mud accumulation on the central texas shelf linked to climate change and sea level rise abstract the holocene texas mud blanket tmb is a large 300 km3 containing 5 1011 t of sediment depocenter on the central texas shelf and represents a stratigraphic anomaly in that it is the only significant transgressive highstand depocenter in the northern gulf of mexico other than the mississippi river delta given its size and remoteness from large rivers the origin of the tmb has remained problematic our results demonstrate that the tmb accumulated mainly in the late holocene as a result of increased sediment supply from rivers and an increase in delivery of mississippi river sediments to the region by marine currents approximately 3000 km of high resolution 2d seismic data was used to construct detailed isopach maps of the tmb a robust radiocarbon age model derived from long sediment cores is used to measure its accumulation through time mineralogical xrd results provide information about temporal variability in the source of sediments to the tmb and indicate that sediments came mainly from the colorado brazos and mississippi rivers with the rio grande and smaller rivers of texas having been minor contributors the second largest source is from transgressive erosion of the shelf the tmb accumulated in a shelf depression that exists between the falling stage colorado and brazos deltas to the north and the rio grande delta to the south initial inundation of the outer shelf occurred 17 ka when terrestrial and lagoonal sediments filled the deepest parts of the depocenter between 17 and 9 ka was a time of rapid eustatic rise and low sedimentation var volumetric accumulation rate 0 4 km3 ka mar mass accumulation rate 0 7 106 t y at 9 ka sediment accumulation dramatically increased to 41 km3 ka 69 106 t y this sediment was mainly generated by ravinement of shelf strata including the ancestral brazos and colorado deltas between 7 5 and 4 ka texas was experiencing maximum temperature and minimum precipitation for the holocene the climatic optimum this was a time of minimum sedimentation in the tmb this was followed by a period of remarkable growth during the late holocene 3 5 ka to present when the tmb accumulated 172 km3 total volume or 2 86 1011 t of sediment accounting for 57 of its volume mineralogical results plus the sheer volume of sediment deposited during this time calls for a corresponding increase in sediment delivery to shelf from the brazos and colorado rivers and an increase in the delivery of sediment from the mississippi river by the late holocene the brazos and colorado rivers had filled their lower valleys with sediment thus eliminating onshore accommodation and increasing sediment delivery to the gulf the dramatic increase in sediment delivery from the mississippi river to the tmb during the late holocene is best explained by an increase in southeasterly winds which drive westward flowing marine currents in the northwestern gulf

surface expressions of structural features on eros abstract near shoemaker imaging of the asteroid eros has revealed a variety of surface forms that probably reflect underlying structures they range from global to local in scale the most prominent positive topographic feature is a long ridge rahe dorsum which extends over 18 km and has the morphology of a thrust fault two large ancient trough systems are also present grooves are ubiquitous commonly occur in suborthogonal sets and have linear or scalloped outlines the grooves on eros are similar in morphology and scale to those on other small bodies such as gaspra ida and phobos and are likely to have a similar mode of origin namely drainage of regolith into underlying fractures models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters many craters have been subjected to structural control resulting in squared outlines we find no evidence that specific groove sets are associated with individual impact events however evidence exists that preexisting structures have been reactivated by later impact activity the large scale of rahe dorsum and many grooves on eros s surface suggests that the asteroid is a largely coherent but fractured body

eros shape topography and slope processes stereogrammetric measurement of the shape of eros using images obtained by near s multispectral imager provides a survey of the major topographic features and slope processes on this asteroid this curved asteroid has radii ranging from 3 1 to 17 7 km and a volume of 2535 20 km3 the center of figure is within 52 m of the center of mass provided by the navigation team this minimal difference suggests that there are only modest variations in density or porosity within the asteroid three large depressions 10 8 and 5 3 km across represent different stages of degradation of large impact craters slopes on horizontal scales of 300 m are nearly all less than 35 although locally scarps are much steeper the area distribution of slopes is similar to those on ida phobos and deimos regions that have slopes greater than 25 have distinct brighter markings and have fewer large ejecta blocks than do flatter areas the albedo patterns that suggest downslope transport of regolith have sharper boundaries than those on phobos deimos and gaspra the morphology of the albedo patterns their lack of discrete sources and their concentration on steeper slopes suggest transport mechanisms different from those on the previously well observed small bodies perhaps due to a reduced relative effectiveness of impact gardening on eros regolith is also transported in talus cones and in connected sinuous paths extending as much as 2 km with some evident as relatively darker material talus material in at least one area is a discrete superposed unit a feature not resolved on other small bodies flat floored craters that apparently contain ponded material also suggest discrete units that are not well mixed by impacts

boulders and ponds on the asteroid 433 eros abstract there are 300 features on the asteroid 433 eros that morphologically resemble ponds flat floored and sharply embaying the bounding depression in which they sit because boulders on eros are apparently eroding in place and because ponds with associated boulders tend to be larger than ponds without blocks we propose that ponds form from thermally disaggregated and seismically flattened boulder material under the assumption that repeated day night cycling causes material fatigue that leads to erosion of the boulders results from a simple boulder emplacement thermal erosion model with boulders emplaced in a few discrete events i e large impacts match well the observed size distribution under this scenario the subtle color differences of ponds somewhat bluer than the rest of the surface might be due to some combination of less space weathered material and density stratification of silicate rich chondrules and more metal rich matrix from a disaggregated boulder volume estimates of ponds derived from near laser rangefinder profiles are consistent with what can be supplied by boulders ponds are also observed to be concentrated in regions of low slope and high elevation which suggests the presence of a less mobile regolith and thus a contrast in the resistance to seismic shaking between the pond material and the material that makes up the bounding depression future tests include shake table experiments and temperature cycling fatigue of ordinary chondrites to test the thermal erosion mechanism

ponded deposits on asteroid 433 eros in late january 2001 the near shoemaker spacecraft performed low altitude passes over the surface of 433 eros coordinated observations of the asteroid surface were obtained at submeter resolution by the near laser rangefinder and the multispectral imager this paper presents three independent coordinated observations of a 90 m pond adjacent to a granular debris flow including the highest resolution altimetric measurements of ponded deposits on eros the ponded deposits appear to have been emplaced by fluid like motion of dry asteroidal regolith a simple model of seismic agitation from impacts is developed to account for pond formation on eros the model predicts that ponds should form readily on eros but not on the moon where ponds are not observed the model also suggests that the absence of observable ponds in the largest craters of eros as well as on phobos and deimos may be related to regolith depth

assessing influences on speleothem dead carbon variability over the holocene implications for speleothem based radiocarbon calibration abstract recently it has been shown that u th dated speleothems may provide a valuable archive of atmospheric radiocarbon 14c but the reliability of these records is dependent upon the stability of the dead carbon proportion dcp derived from the soil and bedrock in order to assess climatic influences on speleothem dcp we have investigated dcp variability over the holocene interval where atmospheric 14c is well known based on dendrochronologically dated tree rings by conducting 14c measurements on a u th dated stalagmite hs4 from heshang cave hubei province china 30 27 n 110 25 e 294 m spanning 0 5 9 6 ka we investigated climatic controls on dcp and found that dcp in hs4 has an average value over the holocene of 10 3 1 5 with an average age offset from atmospheric radiocarbon of 875 130 years and displays a response to both precipitation increases and decreases hs4 dcp increases during the wetter mid holocene interval 5 5 7 1 ka likely reflecting a shift to more closed system dissolution in response to increased soil moisture dcp decreases during the 8 2 ka event a time period of dry conditions at heshang cave though the lower amplitude of this shift indicates that dcp may be less sensitive to dry events speleothems are potentially valuable archives of atmospheric radiocarbon especially in older portions of the 14c calibration curve where knowledge of atmospheric 14c is limited however minor climatic influences on dcp could introduce uncertainties of several hundred years to calibrated ages

the morphological diversity of comet 67p churyumov gerasimenko images of comet 67p churyumov gerasimenko acquired by the osiris optical spectroscopic and infrared remote imaging system imaging system onboard the european space agency s rosetta spacecraft at scales of better than 0 8 meter per pixel show a wide variety of different structures and textures the data show the importance of airfall surface dust transport mass wasting and insolation weathering for cometary surface evolution and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material

seasonal circulation on the western shelf of the gulf of mexico using a high resolution numerical model 1 the seasonal circulation on the western shelf of the gulf of mexico is studied using a high resolution numerical simulation historical hydrographic data sea level data and satellite images three regions are distinguished the tamaulipas veracruz tave shelf the louisiana texas latex shelf and the western campeche bank on the tave shelf there is a swift reversal of the along shelf current downcoast from september to march and upcoast from may to august when there is upwelling due to offshore ekman transport circulation on the western campeche bank is upcoast throughout the year the latex shelf has a cyclonic circulation except during summer months when the flow is eastward during spring summer the upcoast current on the tave shelf reaches the southern texas shelf where it encounters a downcoast coastal current favoring offshore transports in the fall winter the downcoast current reaches the southern bay of campeche where it meets an opposing along shelf current generating seasonal offshore transports during fall and winter cool low salinity water from the mississippi and atchafalaya rivers is advected westward along the latex shelf onto the tave shelf developing along shelf fronts and temperature inversions commonly observed over the outer shelf and shelf break the main forcing over the western shelf of the gulf is the along coast wind stress component the existence of the cross shelf transports in the confluence regions is supported by chlorophyll a data up to 80 of the seasonal sea level variability is explained by the along shelf currents and the low frequency variability of the atmospheric sea level pressure

viscous relaxation on comets three jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters large scale topography generally appears to be softened on these comets although sharp topography is preserved at small scales we find that viscous relaxation of water ice may explain these observations given reasonable assumptions about ice grain size and temperatures attained in the interiors we suggest that both the shapes and the cratering records of jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the kuiper belt

fluidization and multiphase transport of particulate cometary material as an explanation of the smooth terrains and repetitive outbursts on 9p tempel 1 article history the deep impact mission discovered repetitive outbursts on comet 9p tempel 1 and the presence of several smooth terrains on its surface we present new measurements of the extent of the smooth terrains the slopes along their centerlines and the areas of their likely source regions our analysis of these features indicates that they are 0 3 m in size should become fully relaxed during the emplacement time explaining the smooth texture seen in the images in contrast the repetitive outbursts require a gas laden reservoir to have formed in the vicinity of the phase change boundary well below their preferred location we visualize the outbursts to be the result of either spouting or bubble transport to the surface where the release of gas is diurnally modulated by either thermal stresses or h2o sublimation back pressure the source region for the i2 smooth terrain is found to coincide with an h2o ice rich area and we propose that the process of elutriation i e the separation of different classes of particulates depending on their drag properties occurs in the fluidized material as it flows up to and through the surface in this way the material becomes enhanced in h2o crystals relative to siliceous and carbonaceous particulates

calibration of stable oxygen isotope signatures in galapagos corals a 2 year 1993 1994 study was conducted in the galapagos islands ecuador to determine the relationship between 18o in skeletal carbonate and sea surface temperature sst in three species of reef building corals pavona clavus pavona gigantea and porites lobata coral samples were grown at 3 10 and 3 m depth at bartolome island champion iisland and urvina bay isabela island respectively hourly measurements of sst and sea surface salinity sss were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth in addition surface waters were sampled periodically bimonthly to monthly at each site to determine variation in 18o seawater results indicate the mean annual ssts were similar between sites varying from 22 9 c at champion to 23 8 c at urvina bay comparisons of monthly sst averages between instrumental and remote sensing satellite 1 1 grid data show a high correspondence r2 ranging from 0 84 to 0 94 indicating that remote sensing data are useful for interpreting the 18o record in corals when instrumental data are lacking here 18ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of sst in galapagos in general higher resolution coral sampling year resolved more of the monthly variation in sst up to 97 at a sampling resolution of 1 4 samples per millimeter of linear skeletal growth comparisions of the 18ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking sst in addition comparisons between sites were highly concordant with some differences reflecting local variation in sst seasonal patterns however were essentially the same over the entire region thus we conclude that the 18ocoral signal from coral skeletons in galapagos can be used to interpret regional changes in sst variation

fall winter current reversals on the texas louisiana continental shelf abstract fall winter recurrence of current reversal from westward to eastward is identified on the texas louisiana continental shelf using the current meter texas louisiana shelf physical oceanography program latex a and near surface drifting buoy surface current and lagrangian drift program sculp 1 observations in 1993 and 1994 reversal events roughly satisfy the poisson distribution with one current reversal nearly every 12 days synoptic winds seem responsible for the current reversal events other processes such as offshore eddies shed from the loop current and river runoff are less important to change alongshore flow direction at synoptic scales a statistical model is established to predict the synoptic current reversal using the surface wind observations

near resonant ocean response to sea breeze on a stratified continental shelf abstract the spatial structure and temporal characteristics of sea breeze and the associated coastal ocean response in the northwest gulf of mexico are investigated using moored instruments hydrographic stations and wind measurements near the study area of 30 n motions in the diurnal inertial band dib may be significantly enhanced by a near resonant condition between local inertial and diurnal forcing frequencies wavelet analysis is used to quantify the results results indicate that diurnal sea breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation the maximum dib oceanic response occurs in june when there is a shallow mixed layer strong stratification and an approximately 10 day period of continuous sea breeze forcing dib current variance decreases in july and august as the consequence of the deepening of the mixed layer and a more variable phase relationship between the wind and current river discharge varies interannually and can s

recycling sediments between source and sink during a eustatic cycle systems of late quaternary northwestern gulf of mexico basin abstract the northwestern gulf of mexico basin is an ideal natural laboratory to study and understand source to sink systems an extensive grid of high resolution seismic data hundreds of sediment cores and borings and a robust chronostratigraphic framework were used to examine the evolution of late quaternary depositional systems of the northwestern gulf of mexico throughout the last eustatic cycle 125 ka to present the study area includes fluvial systems with a wide range of drainage basin sizes climate settings and water and sediment discharges detailed paleogeographic reconstructions are used to derive volumetric estimates of sediment fluxes volume accumulation rates the results show that the response of rivers to sea level rise and fall varied across the region larger rivers including the former mississippi western louisiana presumably the ancestral red river brazos colorado and rio grande rivers constructed deltas that advanced across the shelf in step wise fashion during marine isotope stages mis 5 2 sediment delivery to these deltas increased during the overall sea level fall due to increases in drainage basin area and erosion of sediment on the inner shelf where subsidence is minimal and transport of that sediment to the more rapidly subsiding outer shelf the sediment supply from the brazos river to its delta increased at least 3 fold and the supply of the colorado river increased at least 6 fold by the late stages of sea level fall through the lowstand repeated filling and purging of fluvial valleys from 119 22 ka contributed to the episodic growth of falling stage deltas during the mis 2 lowstand 22 17 ka the mississippi river abandoned its falling stage fluvial deltaic complex on the western louisiana shelf and drained to the mississippi canyon likewise the western louisiana delta was abandoned presumably due to merger of the red river with the mississippi river terminating growth of the western louisiana delta the brazos river abandoned its mis 3 shelf margin delta to merge with the trinity sabine and calcasieu rivers and together these rivers nourished a lowstand delta and slope fan complex the colorado and rio grande rivers behaved more as point sources of sediment to thick lowstand delta fan complexes lowstand incised valleys exhibit variable morphologies that mainly reflect differences in onshore and offshore relief and the time intervals these valleys were occupied they are deeper and wider than falling stage channel belts and are associated with a shelf wide surface of erosion sequence boundary during the early mis 1 17 ka to 10 ka sea level rise the offshore incised valleys of the calcasieu sabine trinity brazos colorado and rio grande rivers were filled with sediment the offshore valleys of smaller rivers of central texas would not be filled until the late holocene mainly by highstand mud the lower onshore portions of east texas incised valleys were filled with sediment mainly during the holocene with rates of aggradation in the larger brazos and colorado valleys being in step with sea level rise smaller rivers filled their valleys with back stepping fluvial estuarine and tidal delta deposits that were offset by flooding surfaces in general the sediment trapping capacity of bays increased as evolving barrier islands and peninsulas slowly restricted tidal exchange with the gulf and valley filling led to more shallow wider bays a widespread period of increased riverine sediment flux and delta growth is attributed to climate change during mis 1 between 11 5 and 8 0 ka and occurred mainly under cool wet climate conditions relatively small sea level oscillations during the mis 1 transgression 17 ka to 4 0 ka profoundly influenced coastal evolution as manifested by landward stepping shorelines on the order of tens of kilometers within a few thousand years the current barriers strand plains and chenier plains of the study area formed at different times over the past 8 ka due mainly to differences in sand supply and the highly variable relief on the mis 2 surface on which these systems formed modern highstand deposition on the continental shelf formed the texas mud blanket which occurs on the central texas shelf and records a remarkable increase in fine grained sediment supply this increase is attributed to greater delivery of sediments from the colorado and brazos rivers which had filled their lower valleys and abandoned their transgressive deltas by late holocene time and to an increase in westward directed winds and surface currents that delivered suspended sediments from the mississippi river to the texas shelf collectively our results demonstrate that source to sink analyses in low gradient basin settings requires a long term perspective ideally a complete eustatic cycle because most of the sediment that was delivered to the basin by rivers underwent more than one cycle of erosion transport and sedimentation that was regulated by sea level rise and fall climate was a secondary control the export of sediments from the hinterland to the continental shelf was not directly in step with temperature change but rather varied between different fluvial deltaic systems

impact induced seismic activity on asteroid 433 eros a surface modification process high resolution images of the surface of asteroid 433 eros revealed evidence of downslope movement of a loose regolith layer as well as the degradation and erasure of small impact craters less than 100 meters in diameter one hypothesis to explain these observations is seismic reverberation after impact events we used a combination of seismic and geomorphic modeling to analyze the response of regolith covered topography particularly craters to impact induced seismic shaking applying these results to a stochastic cratering model for the surface of eros produced good agreement with the observed size frequency distribution of craters including the paucity of small craters

return to comet tempel 1 overview of stardust next results on february 14 2011 stardust next sn flew by comet tempel 1 the target of the deep impact di mission in 2005 obtaining dust measurements and high resolution images of areas surrounding the 2005 impact site and extending image coverage to almost two thirds of the nucleus surface the nucleus has an average radius of 2 83 0 1 km and a uniform geometric albedo of about 6 at visible wavelengths local elevation differences on the nucleus reach up to 830 m at the time of encounter the spin rate was 213 per day period 40 6 h and the comet was producing some 130 kg of dust per second some 30 of the nucleus is covered by smooth flow like deposits and related materials restricted to gravitational lows this distribution is consistent with the view that the smooth areas represent material erupted from the subsurface and date from a time after the nucleus achieved its current shape it is possible that some of these eruptions occurred after 1609 when the comet s perihelion distance decreased from 3 5 au to the current 1 5 au much of the surface displays evidence of layering some related to the smooth flows and some possibly dating back to the accretion of the nucleus pitted terrain covers approximately half the nucleus surface the pits range up to 850 m in diameter due to their large number they are unlikely to be impact scars rather they probably result from volatile outbursts and sublimational erosion the di impact site shows a subdued depression some 50 m in diameter implying surface properties similar to those of dry loose snow it is possible that the 50 m depression is all that remains of an initially larger crater in the region of overlapping di and sn coverage most of the surface remained unchanged between 2005 and 2011 in albedo photometric properties and morphology significant changes took place only along the edges of a prominent smooth flow estimated to be 10 15 m thick the margins of which receded in places by up to 50 m coma and jet activity were lower in 2011 than in 2005 most of the jets observed during the sn flyby can be traced back to an apparently eroding terraced scarp the dust instruments detected bursts of impacts consistent with a process by which larger aggregates of material emitted from the nucleus subsequently fragment into smaller particles within the coma

hydrologic and climatic implications of stable isotope and minor element analyses of authigenic calcite silts and gastropod shells from a mid pleistocene pluvial lake western desert egypt abstract authigenic calcite silts at wadi midauwara in kharga oasis egypt indicate the prolonged presence of surface water during the marine isotope stage 5e pluvial phase recognized across north africa exposed over an area of 4 25 km 2 these silts record the ponding of water derived from springs along the libyan plateau escarpment and from surface drainage the 18 o values of these lacustrine carbonates 11 3 to 8 0 pdb are too high to reflect equilibrium precipitation with nubian aquifer water or water of an exclusively atlantic origin mg ca and sr ca of the silts have a modest negative covariance with silt 18 o values suggesting that the water may have experienced the shortest residence time in local aquifers when the water 18 o values were highest furthermore intra shell 18 o sr ca and ba ca analyses of the freshwater gastropod melanoides tuberculata are consistent with a perennially fresh water source suggesting that strong evaporative effects expected in a monsoonal climate did not occur or that dry season spring flow was of sufficient magnitude to mute the effects of evaporation the input of a second isotopically heavier water source to aquifers possibly indian ocean monsoonal rain could explain the observed trends in 18 o and minor element ratios

ultra high resolution stable isotope sampling of slow growing and fragile coral skeletons abstract the calcareous caco3 skeleton of massive scleractinian corals represents a reliable archive for environmental proxy data stable carbon and oxygen isotope records built from series of powder samples drilled along the axis of maximum growth represent the backbone of most coral sclerochronologies because isotope analyses require a certain minimum amount of sample powder commonly used drill bit diameters are 400 m in order to achieve monthly or higher sampling resolution annual upward growth of the corals must therefore exceed 5 mm while this threshold represents no limitation for most coral sclerochronologies of massive corals from the tropics it hampers the preparation of significant datasets from slow growing and delicate corals from subtropical and temperate regions and the fossil record here we present a modified milling technique which allows sampling carbonate powder from slow growing very porous and delicate corals with a minimum spatial resolution of 50 m by using dental composite resin as a stabilising medium in combination with a newly developed mounting apparatus accurate high resolution sampling along straight growth axes becomes feasible with two case studies we demonstrate applications of this method to two different kinds of slow growing corals a recent phaceloid cladocora caespitosa from the columbretes islands spain mediterranean sea and a middle miocene massive porites sp from the vienna basin austria central paratethys sea compared to conventional milling based methods the modified sampling design allows minimizing averaging effects caused by the integration over skeletal elements formed at different times and enhancing the highest spatial resolution hitherto achieved by factor 1 5 for porites and even factor 4 for cladocora in this respect the herein presented method represents a significant step forward for exploring also sub annually resolved proxy time series from slow growing and delicate corals extant and fossil thus allowing the broadening of sclerochronology to so far underexplored environmental coral archives

corals paleotemperature records and the aragonite calcite transformation oxygen isotopic records from corals play an increasing role in paleoclimatic research and climatic modeling proxy records of sea surface temperature are produced by isotopically analyzing samples drilled from aragonitic caco 3 coral skeletons however drilling aragonite causes a polymorphic inversion to calcite accompanied by 18 o enrichment the isotopic enrichment may occur through atmospheric exchange co 2 degassing or fractionation during digestion regardless of the mechanism dry drilling produced isotopic shifts as large as 0 8 corresponding to an erroneous temperature decrease of 3 c this error is larger than the global temperature rise since the industrial revolution and a substantial part of the global temperature variation over the past 10 ka because the isotopic shift is inconsistent and can occur without the production of measurable calcite detecting and correcting the isotopic shift may be difficult

geochemical gradients within modern and fossil shells of concholepas concholepas from northern chile an insight into u th systematics and diagenetic authigenic isotopic imprints in mollusk shells abstract seriate geochemical measurements through shells of one modern one holocene and two sangamonian concholepas concholepas from marine terraces of northern chile were performed to document diagenetic vs authigenic geochemical signatures and to better interpret u series ages on such material subsamples were recovered by drilling from the outer calcitic layer to the inner aragonitic layer of each of the studied shells unfortunately this sampling procedure induces artifacts notably the convertion of up to 20 of calcite into aragonite and of up to 6 of aragonite into calcite as well as in the epimerization of a few percent of isoleucine into d alloisoleucine l isoleucine negligible sampling artifacts were noticed for stable isotope and total amino acid contents diagenetic effects on the geochemical properties of the shells are particularly pronounced in the inner aragonitic layer and more discrete in the outer calcitic layer the time dependent decay of the organic matrix of the shell is illustrated by a one order of magnitude lower total amino acid content in the sangamonian specimens by comparison with the modern shell conversely the sangamonian shells u contents increase by a similar factor and 13c 18o enrichments as high as 2 to 3 seem also to occur through the same time interval possibly due to partial replacement of aragonite by gypsum the decay of the organic matrix of the aragonitic layer of the shell is thought to play a major role with respect to u uptake processes and stable isotope shifts nevertheless asymptotic 230th ages 100 ka in the inner u rich layers of the sangamonian shells and 234u 238u ratios compatible with a marine origin for u suggest u uptake within a short diagenetic interval when marine waters were still bathing the embedding sediment thus u series ages on fossil mollusks from such a hyper arid environment should not differ much from the age of the corresponding marine unit deposition however the diagenetic enrichments in stable isotopes raise concerns about their use for paleoenvironmental reconstructions under such climate conditions

sources and transport of land derived particulate and dissolved organic matter in the gulf of mexico texas shelf slope the use of ligninphenols and loliolides as biomarkers the fate and transport of terrestrial organic matter across the continental margin in the gulf of mexico was studied in 1992 and 1993 using chemical biomarkers lignin phenols were utilized as biomarkers for terrestrial inputs and indicated that much of the terrestrial organic matter inputs were deposited on the shelf break and slope the lignin phenol concentrations normalized to carbon in poc hmw doc and sediments in slope waters were considerably higher than at other open ocean sites studied previously the dominant mechanism for transport of terrestrially derived poc and hmw doc across the shelf and slope was hypothesized to be advection of riverine and estuarine discharges through benthic nepheloid layers based on loliolide concentrations in the water column we believe that lateral transport of these materials at the shelf break through extensions of benthic nepheloid layers may have been an important mechanism for the injection of terrestrially derived organic matter into deep slope waters