erosional and climatic effects on long term chemical weathering rates in granitic landscapes spanning diverse climate regimes abstract we used cosmogenic nuclide and geochemical mass balance methods to measure long term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes our 42 study sites encompass widely varying climatic and erosional regimes with mean annual temperatures ranging from 2 to 25 c average precipitation ranging from 22 to 420 cm year 1 and denudation rates ranging from 23 to 755 t km 2 year 1 long term chemical weathering rates range from 0 to 173 t km 2 year 1 in several cases exceeding the highest granitic weathering rates on record from previous work chemical weathering rates are highest at the sites with rapid denudation rates consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock a simple empirical relationship based on temperature precipitation and long term denudation rates explains 89 95 of the variation in long term weathering rates across our network of sites our analysis shows that for a given precipitation and temperature chemical weathering rates increase proportionally with fresh material supply rates we refer to this as supply limited weathering in which fresh material is chemically depleted to roughly the same degree regardless of its rate of supply from breakdown of rock the temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter comparisons of catchment mass balance data from the field our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought at least in actively eroding unglaciated terrain similar to our study sites to the extent that chemical weathering rates are supply limited in mountainous landscapes factors that regulate rates of mineral supply from erosion such as tectonic uplift may lead to significant fluctuations in global climate over the long term

mississippi river subaqueous delta is entering a stage of retrogradation abstract the subaqueous delta of the mississippi river the largest river system in the conterminous u s has entered a stage of retrogradation caused by multiple natural and anthropogenic activities since the 1950s the suspended sediment load of the mississippi river has decreased by 50 due primarily to the construction of 50 000 dams in the mississippi basin the impact of this decreased sediment load has been observed in subaerial environments but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined to identify historic trends in sedimentation patterns we compiled bathymetric datasets including historical charts industry and academic surveys and national oceanic and atmospheric administration hydrographic data collected between 1764 and 2009 the progradation rate measured at the 10 m depth contour of southwest pass which receives 69 of the suspended sediment load reaching head of passes has decreased from 67 m yr between 1874 and 1940 to 26 m yr between 1940 and 1979 with evidence of further deceleration from 1979 to 2009 at south pass and pass a loutre the delta front has entered the destructive phase with the 10 m contour retreating at rates 20 m yr at both passes since 1979 advancement of the delta front also decelerated in deeper water in some areas out to 180 m depth except locally where mudflow lobes are advancing deeper contours show a pattern of decreasing progradation rate between 1874 1940 and 1979 2005 time periods furthermore based on differences measured between available bathymetric datasets the sediment accumulation rate across the delta front decreased by 73 for the same period the retention rate of mississippi river sediment on the delta front ranged from 67 to 81 for the time periods assessed with total sediment load stored on the delta front equal to 317 54 mt yr from 1874 to 1940 145 25 mt yr from 1940 to 1979 and 87 15 mt yr from 1979 to 2005 we document for the first time that the mississippi river delta front has entered a phase of retrogradation which will likely be accelerated by future upstream activities that divert a portion of the sediment load to the upper delta for coastal protection and restoration projects the decline of the subaqueous mississippi river delta has critical implications for biogeochemical cycling subaqueous mass wasting and sediment dispersal to the coastal ocean

the tidal spectrum underneath antarctic ice shelves a year long tidal record has been obtained from beneath the george vi ice shelf antarctica an unusual feature of the record is a significant response in tidal species 3 to 7 these harmonics are practically absent from records further north on the west coast of the antarctic peninsula but are present in all tidal height records from george vi sound a strong ter diurnal signal also exists in the tidal currents under the ice shelf nonlinearity also occurs in the tidal motion of the ronne and ekstrom ice shelves but has not been reported from the ross ice shelf the tidal dynamics of several antarctic ice shelves have therefore been modified by a region of strong nonlinear response to tidal forcing if nonlinear tides in the semidiurnal band are present on the ross ice shelf they could account for difficulties in modeling the area s weak semidiurnal tides an anelastic component in the deformation of the ice at the grounding line is tentatively proposed as the mechanism responsible the positioning of recording pressure sensors in pairs on the seafloor and at the ice shelf base will allow this hypothesis to be tested and also provide a value for the power dissipated by tidally induced flexure at the grounding line

diurnal tides near the yermak plateau recent current measurements obtained from drifting platforms over the yermak plateau in the eastern arctic ocean confirm that the plateau is a region of greatly enhanced diurnal tidal currents modulation of the diurnal currents is clearly related to the plateau topography as has been previously proposed we show however that temporal variability due to spring neap modulation must also be considered in interpreting records from drifting platforms we review simple models of tidal current amplification in this region and find that the previous assumption of near resonant barotropic shelf waves propagating around the plateau s entire perimeter is inconsistent with the true topography instead we propose that the diurnal variability is due to topographic shelf waves at the k1 and o1 tidal frequencies that are generated at points on the plateau s perimeter where the waves group velocities are near zero observed cross slope variations in ellipticity orientation and magnitude of tidal oscillations are consistent with the presence of topographic waves generated in this manner the topographic enhancement of the diurnal tide near the yermak plateau has important consequences for the sea ice cover hydrography and general circulation of this region for example the stress divergence applied by the tidal currents at the ice base greatly exceeds the typical divergence of the surface wind stress and tides may therefore be important to local ice deformation the strong cross slope tidal currents also appear to be responsible for the production of high frequency internal wave packets which are associated with energetic diapycnal mixing in the pycnocline we also consider the possibility that tidal rectification is responsible for a mean current transporting atlantic water clockwise around the plateau

observed tides on the northeastern bering sea shelf bottom pressure observations november 1981 to august 1982 show that the diurnal tides dominate the outer reaches of the northeastern bering sea shelf the diurnal tides decrease inshore from the shelf break maximum k1 amplitude of 38 cm and appear to be evanescent sverdrup poincare waves on the outer shelf the k1 amplitude 2 3 cm in the bering strait is 6 of that at the shelf break the diurnal phase lags vary little 322 359 g for the k1 tide between the stations north and eastward from the shelf break the tides become smaller and more semidiurnal the semidiurnal amplitudes vary considerably between stations these observations show the effects of semidiurnal amphidromic systems predicted by numerical models the m2 amplitude 7 6 cm in the bering strait is about a third of that at the shelf break m2 amplitude of 23 cm the semidiurnal phase lags are earliest at the shelf break 87 g for m2 and appear to increase toward st lawrence island 336 g for m2 in shpanberg strait and the bering strait 202 g for m2 the tidal harmonic constants fluctuated seasonally possibly in response to the variations in ice cover the diurnal amplitudes generally increased during winter while the semidiurnal amplitudes decreased at the shelf break the k1 amplitude increased 2 5 cm or 7 but the o1 and semidiurnal m2 amplitudes 0 4 cm or 2 for m2 showed little variation in bering strait the variations in m2 amplitude up to 3 0 cm amounted to 37 of the mean amplitude the decrease may be due to attenuation of the m2 tide from the arctic ocean in addition to damping of the m2 tide on the northeastern bering sea shelf shorter term fluctuations were also observed in the tides the tidal fluctuations may contribute to those observed in the deep sea numerical models show a set of amphidromic systems including a semidiurnal system off cape navarin and rapid phase transitions in the vicinity of some of the stations these models are in relatively good agreement with the observations

influence of granite mineralogy rainfall vegetation and relief on stream water chemistry vosges mountains north eastern france abstract the influence of rock mineralogy climate vegetation and relief on stream water composition and export as a result of weathering is a major scientific concern both at the local scale because of the link between stream chemistry and biodiversity and at the global scale in relation to global climate changes environmental factors influencing stream chemistry at baseflow were studied for 95 forested catchments located in the granite part of the vosges mountains ne france mean annual rainfall slope altitude catchment area forest cover and granite composition were attributed to each catchment using a geographic information system gis and spatially distributed databases these variables were then used to model stream water concentrations in so4 ca mg k na si anc acid neutralizing capacity and ph via linear regression analyses sulfate concentration is mainly negatively correlated to the mean altitude of the catchment silica and na concentrations are strongly correlated and negatively related to the mean annual rainfall and not to altitude ph is positively related to the sum of ca mg and k but not to so4 for any type of granite ca mg and ph are generally negatively related to the mean annual rainfall and positively related to the catchment area calcium is best explained by the rock content in hornblende mg by the rock contents of biotite and hornblende k by the content in k feldspar nevertheless models differ for each granite type in relation to the availability of weatherable minerals and probably to water pathways this study shows the interest of spatial analysis methods for understanding stream water concentrations at the mountain range scale in combination with local process studies and modelling at the catchment scale

flexural waves in the ross ice shelf flexural waves related to the ocean swell are identified more than 600 km from the open sea in the ross ice shelf antarctica where the ice cover is in places more than 500 m thick an equation relating the power spectra of the flexural wave and the ocean swell is derived based on the continuity of pressure in the fluid layer across the ice front correcting for the effect of the ice the power spectrum of the wave in the ice compares to that of the ocean swell elsewhere in the pacific ocean

a model of ice shelf ocean interaction with application to the filcher ronne and ross ice shelves a simple analytical model has been developed to study the formation of ice shelf water isw isw is assumed to flow as a buoyancy driven layer underneath the ice shelf a relation between potential temperature and salinity in the isw layer is calculated from the mass and energy balance this temperature salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates the model results are obtained without making any assumptions about entrainment and melt rates the model is in good agreement with observations under the ronne ice shelf and it indicates that isw in the filchner depression is formed from western shelf water wsw with salinity higher than 34 75 practical salinity units such high salinity water is only observed in the ronne depression in the western part of the continental shelf this implies a circulation of wsw under the filchner ronne ice shelf from the ronne depression into the filchner depression similarly the model shows that the isw observed under j9 at the ross ice shelf has been formed from low salinity shelf water lssw from the eastern parts of the ross sea continental shelf lssw must therefore circulate under the eastern parts of the ross ice shelf

understanding fine grained river sediment dispersal on continental margins studies of fine grained sediment dispersal in the marine environment have documented diversity in the behavior of depositional systems on continental margins with different oceanographic conditions and morphologies based on the pattern and nature of sediment accumulation for twenty three case studies we propose there are five basic types of dispersal systems and these are related to river wave tide and margin characteristics data suggest that the type of dispersal system on any margin can be predicted with knowledge of sediment load mean significant wave height tidal range and continental shelf width at a river mouth and from these a hierarchical decision tree is developed analysis of the dispersal system type of more than 100 river mouths reveals that proximal accumulation dominated pad and marine dispersal dominated mdd systems are most abundant but estuarine accumulation dominated ead systems also are anticipated to be numerous globally research quantifying marine sediment dynamics in these system types is needed although less common the size and nature of subaqueous delta clinoform sdc and canyon captured cc systems also warrant future attention to their study strong correlations between physical characteristics i e significant wave height and tidal range and the depth and distance to the nearest maximum shelf depocenter provide evidence that in reality a continuum exists between the system types the process related partitioning of sediment in the five different types of marine dispersal systems has important implications for understanding the stratigraphic record and the cycling of carbon

three dimensional modeling of tidal currents and mixing quantities over the newfoundland shelf major semidiurnal m2 s2 and n2 and diurnal k1 and o1 barotropic tides over the newfoundland shelf are computed using a three dimensional nonlinear primitive equation model with the vertical eddy viscosity calculated from a level 2 5 turbulence closure scheme computed elevation cotidal charts for the five constituents are generally consistent with previous knowledge for this region comparisons based on a statistical analysis of the differences between the computed elevations and currents and in situ observations indicate good agreement while m2 tidal currents up to 20 30 cm s are dominant there are locally intensified diurnal currents up to 5 10 cm s in some outer shelf locations the diurnal current intensification is attributed to first mode continental shelf waves an examination is carried out for the vertical structure of the computed m2 current and for the temporal and spatial variability of model turbulent kinetic energy mixing length scale vertical eddy viscosity and bottom friction velocity the examination indicates that large vertical eddy viscosity magnitudes and bottom friction velocities are associated with strong currents in shallow regions where strong vertical shears produce large turbulent kinetic energy solutions with both specified and evolving vertical stratification indicate that the stratification has a significant influence on the vertical profile of tidal mixing parameters and currents in shallow areas tidally induced turbulence is substantially reduced in the bottom boundary layer and completely suppressed above it tidal currents decrease in the log layer increase significantly in the rest of the bottom boundary layer and decrease in the upper and middle water column

deltaic morphodynamics and stratigraphic evolution of middle barataria bay and middle breton sound regions louisiana usa implications for river sediment diversions abstract river sediment diversions have been recognized as a key strategy for offsetting land loss in coastal louisiana recently much attention has been focused on sediment capture from the main stem river and conveyance of that material through diversion outflows yet the performance and long term feasibility of diversions as land building agents also hinges upon the subsurface stratigraphic architecture of receiving basins a concept that has largely been unexplored a major core collection and analysis program was undertaken to study the geological properties of substrates in the middle barataria bay and middle breton sound diversion receiving basins louisiana usa over a region of 200 square kilometers in both basins 50 vibracores up to 5 5 m in length were collected in spring and summer of 2015 cores were logged for bulk density and imaged with a geotek multi sensor core logger split cores were then subsampled for granulometry organic content and 14c dating both receiving basins are characterized by 1 2 m of organic rich surficial strata underlain by 2 m of mineral rich sand and silt beds that display a greater bulk density additionally some cores contain deeper peaty strata intercalated within muds and sands 14c dates of these buried peats range between 1910 and 3203 calendar years before present demonstrating that relatively old deltaic strata exist at shallow depths age comparisons of stratigraphically similar peat beds from this study and previous delta lobe chronological models indicate that the st bernard lobe prograded eastward at a time averaged rate of 75 m yr from a geotechnical standpoint the surficial uncompacted

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

studies of the grounding line location on ice streams d and e antarctica landsat thematic mapper tm images were used prior to the 1991 92 field season to infer the position of the grounding line at the mouths of ice streams d and e west antarctica our field plan for mass balance studies was based on this determination and thus the imagery played a central role in both the scientific and logistics planning a radar profile along the flow direction was made across the inferred grounding line at one location and ice thickness measurements together with surface surveying enable us to compare the hydrostatic surface and the actual topography to determine the point at which the ice becomes grounded the profile transits from floating to grounded ice at the same location as the grounding line inferred from the imagery changes in the radar echo strength also occur at this location giving further support to this interpretation tilt studies of the ice flexure caused by tidal variations at locations on either side of this grounding point give additional evidence that grounding is occurring close by the combination of these three measurements thus confirms the grounding line location derived from the satellite imagery and supports the use of this technique to determine grounding

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

simulation of subice shelf melt rates in a general circulation model velocity dependent transfer and the role of friction two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks the parameterizations differ in the transfer coefficients for heat and freshwater fluxes in their simplest form they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base an augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law in simulations with both parameterizations for idealized as well as realistic cavity geometries under pine island ice shelf west antarctica significant differences in melt rate patterns between the velocity independent and velocity dependent formulations are found while patterns are strongly correlated to those of thermal forcing for velocity independent transfer coefficients melting in the case of velocity dependent coefficients is collocated with regions of high boundary layer currents in particular where rapid plume outflow occurs both positive and negative feedbacks between melt rates boundary layer temperature velocities and buoyancy fluxes are identified melt rates are found to increase with increasing drag coefficient inline image in agreement with plume model simulations but optimal values of cd inferred from plume models are not easily transferable uncertainties therefore remain both regarding simulated melt rate spatial distributions and magnitudes

northern latitude chemical weathering rates clues from the mackenzie river basin canada abstract the main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate more than 110 river water samples from the mackenzie river system northern canada have been sampled and analyzed for major and trace elements and sr isotopic ratios in the dissolved phase the three main morphological units are reflected in water chemistry rivers from the canadian shield are very dilute dominated by silicate weathering millot et al 2002 whereas the rivers of the rocky and mackenzie mountains as well as the rivers of the sedimentary interior platform are dominated by carbonate weathering and are so 4 rich compared to the rivers of the mackenzie and rocky mountains the rivers of the interior plains are organic silica and na rich and constitute the dominant input term to the mackenzie river mainstream rivers of the canadian shield area do not significantly contribute to the mackenzie river system using elemental ratios and sr isotopic ratios a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering carbonate weathering rates are mostly controlled by runoff which is higher in the mountainous part of the mackenzie basin these rates are comparable to the carbonate weathering rates of warmer areas of the world it is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering but its extent remains difficult to assess conversely to what was stated by edmond and huh 1997 overall silicate weathering rates in the mackenzie basin are low ranging from 0 13 to 4 3 tons km 2 yr na k ca mg and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates in contrast to what has been observed in other large river systems such as the amazon and ganges rivers silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters this contradicts the raymo hypothesis raymo and ruddiman 1992 isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains e g by glaciers and deposited in the plains rather the relatively high chemical denudation rates in the plains are attributed to lithology uncompacted shales high mechanical denudation and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation the three to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by moulton et al 2000 between unvegetated and vegetated watershed this study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors such as organic matter associated with northern watersheds are able to counteract the effect of temperature this acceleration by a factor of 4 in the plains is equivalent to a 6 c increase in temperature

sub decadal submarine landslides are important drivers of deltaic sediment flux insights from the mississippi river delta front submarine mass failures triggered by energetic forcing events such as hurricanes and earthquakes are relatively well studied due to the potential for infrastructure damage and tsunami generation such failures are common on heavily sedimented margins where underconsolidated deposits are preconditioned to fail however studies of seafloor sediment movement between large events remain scarce using repeat bathymetric surveys of the mississippi river delta front mrdf we document substantial seafloor movement in absence of major hurricanes about 1 m yr of deepening was observed within preexisting failures with downslope sediment transport on the order of 10 5 m 3 yr outside failure features seafloor depths remained stable or showed minor 20 cm yr accretion mrdf volumetric sediment flux during hurricane driven mass failures is an order of magnitude greater than the annual flux during a quiescent interval when normalized by time however sediment flux during the quiescent interval 5 5 10 5 m 3 yr was half that of hurricane driven mass failures 1 1 10 6 m 3 yr these observations corroborate our wave modeling results which infer that even waves of 1 yr recurrence interval can generate differential seafloor pressures sufficient to trigger submarine landslides this does not exclude the possibility of river floods also being agents of failure these findings indicate that sub decadal submarine landslides are important to mrdf dynamics comparable to the role of major hurricanes and observation during seemingly quiescent periods is necessary to holistically assess sediment flux the periodicity and prevalence of moderate scale mass transport documented here corroborates similar recent studies offshore other deltas globally indicating that highstand mass and time budgets of shelf to deep sea sediment flux in addition to organic carbon and bioreactive particles may need to be revised

dynamics diffusion and geomorphological significance of tidal residual eddies many shallow tidal areas show the occurrence of ebb and flood surplusses organized in cell like structures or tidal residual eddies they seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients the dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed

large multidecadal salinity trends near the pacific antarctic continental margin abstract ocean temperature and salinity measurements on and near the antarctic continental shelf in the southwest pacific sector are evaluated for evidence of temporal change shelf water in the southwest ross sea has declined in salinity by 0 03 decade 1 from 1958 to 2008 while its temperatures have increased in proportion to the influence of salinity on the sea surface freezing point modified deep water intrusions that reach the central ross ice shelf have freshened at a similar rate and cooled by 0 5 c since the late 1970s salinity has decreased by 0 08 decade 1 in the westward coastal and slope front currents consistent with increased melting of continental ice upstream in the amundsen sea overturning of those near surface waters during winter sea ice formation and mixing across the slope front is sufficient to account for the 5 decade shelf water salinity change a strong correlation between the freshening and change in the southern annular mode index suggests a link with the large scale atmosp

modeling tidal currents beneath filchner ronne ice shelf and on the adjacent continental shelf their effect on mixing and transport a depth averaged tidal model has been applied to the southern weddell sea the model domain covers the southern continental shelf including the ocean cavity beneath filchner ronne ice shelf reasonable agreement with the available current meter data has been achieved our results confirm that in areas with shallow water and large topographic gradients tidal oscillations with peak velocities up to 1 m s 1 play a significant role in the vertical mixing and transport of water masses the estimated energy dissipation beneath filchner ronne ice shelf due to surface friction is 25 gw approximately 1 of the world s total tidal dissipation tidally induced lagrangian residual currents converging at the ice front an area of strong mixing draw together water masses from the continental shelf and sub ice shelf cavity the model indicates that lagrangian residual currents have fluxes of up to 250 000 m3 s 1 and speeds of over 5 cm s 1 along the ice front with over 350 000 m3 s 1 being exchanged between the sub ice shelf cavity and adjacent continental shelf these currents are particularly efficient in ventilating the sub ice shelf cavity within 150 km of ronne ice front such strong tidal mixing will significantly modify the properties of water masses that flow through this region particularly to the west of berkner island the model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of ronne ice front shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography

stratigraphic evolution of the nakdong river valley in response to late quaternary sea level changes abstract sequence analysis using borehole samples and high resolution seismic data in the nakdong river valley yielded a good example of a river valley sequence influenced by the complex interplay between the bottom morphology of the steep paleo valley varying sediment supply and holocene sea level change associated with coastal currents the late quaternary stratigraphic sequence in the nakdong river estuary approximately 60 70 m thick forms a high frequency sequence consisting of a set of lowstand transgressive and highstand systems tracts that corresponds to a fifth order 20 ka sea level cycle four main depositional systems including ten sedimentary facies constitute these systems tracts fluvial estuarine coastal shoreface and deltaic the lowstand systems tract lst consisting of gravelly sand forms a fluvial depositional system unit i which fills the thalweg of river valley mainly developed before 12 ka the transgressive systems tract tst showing a succession of retrograding or backstepping depositional arrangements can be divided into two depositional systems unit ii and iii the riverine sediments were trapped within the paleo estuary forming an estuarine depositional system unit ii developed between 12 and 7 ka as the transgression continued the coarse sediments were deposited and redistributed by coastal processes resulting in coastal shoreface depositional system unit iii it is characterized by an isolated sand body and thin sand veneer the hst is composed of deltaic depositional system including delta plain delta front and prodelta unit iv during the delta progradation most coarse grained sands derived from the river were deposited in the lower delta plain and delta front forming sand bars and shoals less than 15 m deep the remaining fine grained sediments were transported further offshore in a suspension mode and deposited in the inner shelf off the present river mouth forming a subaqueous prodelta radiocarbon and optically stimulated luminescence osl dating suggest that the holocene deltaic system was initiated by aggradational and progradational stacking patterns at approximately 8 ka during the last stage of decelerated sea level rise and was then followed by a prograding clinoform after the highest sea level at approximately 7 ka

diurnal lake level cycles on ice shelves driven by meltwater input and ocean tidal tilt diurnal depth cycles of decimeter scale are observed in a supraglacial lake on the mcmurdo ice shelf antarctica we evaluate two possible causes 1 tidal tilt of the ice shelf in response to the underlying ocean tide and 2 meltwater input variation we find the latter to be the most likely explanation of our observations however we do not rule out tidal tilt as a source of centimeter scale variations and point to the possibility that other larger supraglacial lake systems particularly those on ice shelves that experience higher amplitude tidal tilts such as in the weddell sea may have depth cycles driven by ocean tide the broader significance of diurnal cycles in meltwater depth is that under circumstances where the ice shelf is thin tidal tilt amplitudes are high and meltwater runoff rates are large there may be associated flexure stresses that can contribute to ice shelf fracture and destabilization for the mcmurdo ice shelf 20 50 m thickness 1 m tidal amplitude and 10 cm water depth variations these stresses amount to several 10 s of kpa

a continuing discussion about the correlation of tidal gravity anomalies and heat flow densities on the basis of several objections rydelek et al phys earth planet inter 68 215 219 1991 hereafter noted rzh denied the existence of a correlation between residuals of tidal gravity r and heat flow h suggested by other workers to re examine the whole matter on safe grounds we previously revised and reanalysed all the original tidal data from 300 stations taking care of the rzh objections by eliminating what they considered as the important sources of errors in the data essentially calibration errors of the instruments in amplitude and phase using this new data base we meet here one by one the other criticisms not related to the original tidal measurements and show that most of them are not valid in particular those concerning oceanic load and attraction corrections the findings are as follows 1 no correlation between diurnal tide and semi diurnal tide residuals is normal 2 distance from the sea is not a valid criterion for elimination of several excellent stations 3 unexpected residues at the south pole are not due to imperfections of the cotidal schwiderski maps but to ice shelf tides not considered by rzh 4 the correlation of the tidal gravity residues with the age of tectonic provinces is proposed as an alternative to the correlation with heat flow the data used in 1991 having been corrected are recalculated for comparison with 1986 1991 statistics the correlation is confirmed with the same degree of probability also new tidal gravity data obtained since 1990 with nullified instruments as recommended by rzh in africa 31 stations and latin america 34 stations are included in new statistics which also confirm the correlation the global result obtained from 174 revised and new stations on all continents gives a correlation coefficient k 0 691 with a linear regression r 0 0201 h 57 0 063 0 0013 where r is in microgals and h is in mw m 2 as there is at present no practical or theoretical means to take into account the role of fluids lubrication and stress corrosion owing to deep seated saline waters or fluids in crustal processes such as tidal deformations such small tidal gravity anomalies cannot be correctly calculated the use of a correlation technique therefore remains the only way to treat unexplained residues even if this does not constitute an unequivocal proof other arguments have to be found to deny the existence of this correlation

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

modeling antarctic tides in response to ice shelf thinning and retreat tides play an important role in ice sheet dynamics by modulating ice stream velocity fracturing and moving ice shelves and mixing water beneath them any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability here we show that removal or reduction in extent and or thickness of the ross and ronne filchner ice shelves would have a significant impact on the tides around antarctica the ronne filchner appears particularly vulnerable with an increase in m2 amplitude of over 0 5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf sheet dynamics these results highlight the importance of understanding tidal feedbacks on ice shelves streams due to their influence on ice sheet dynamics

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

coastal barriers northern gulf last eustatic cycle genetic categories and development contrasts a review abstract a large body of chronologically well constrained and detailed quaternary data accumulated in the last decades from the coastal shelf region of the northern gulf of mexico consequently a preliminary synthesis of major aspects of the late quaternary barrier and mainland coastal development on the northern gulf of mexico is timely several major factors account for the striking differences between barrier size and development styles in the three northern gulf coast regions barriers are nearly continuous in the nw where they dominate by the greatest dimensions and continuity sand supply from shelf and mainland sources continued at variable rates throughout six stages of the last eustatic cycle lec differences in antecedent topography in fluvial runoff volumes and relative sea level rise including rapid overstepping flooding events were critical in the regional distinctions leaving fewer and smaller relict fluvial delta behind drowned shore and nearshore landforms on the shelf only two large fluvial systems impacted the ne the nw shelf and nearshore received abundant sediment supply from the mississippi and lesser streams indirectly by ravinement erosion of relict landforms and landward directed cross shelf sediment transfer sands originated mainly in relict deltas fluvial tidal channel and inlet fill as well as from submerged shore ridge remnants on the continental shelf shelf margin deltas represented major secondary sources barrier formation categories and their respective importance in specific areas represent critical aspects of coastal development the authorship of various formation concepts e g penck gilbert mcgee haage ganong and keilhack often is still miscredited or remains unrecognized in the literature recognition of the stratigraphic and sedimentological diagnostic characteristics of the basic genetic barrier categories plays a key role in testing the validity of barrier evolution models closely knit process form relationships prevail between hydrodynamic sea level change storm tidal current overwash sedimentary shoal and island aggradation rollover and eolian accumulation processes and the resulting landform morphology the most detailed accounts that deal with the development of the alabama louisiana and apalachicola barrier chains are particularly instructive almost all modern gulf barriers with identifiable genetic background formed by shoal aggradation between 5 5 and 2 0 ka during a marked deceleration in sea level rise concurrently a sizable mainland strand plain complex extended the ne coastal plain tide and storm driven sediment transport to the large mobile bay ebb delta the focal role of composite east dauphin island in establishing the ms la island chain and the island blocking role of mississippi delta advance were prime factors in a complex barrier history migration paradoxically may also occur by transgressive rollover concurrently with regressive landward progradation of island strand plains stream discharge and sand reworked from relict lithosomes on the shelf were insufficient in compensating for wave and tide related sediment loss gulf levels stayed below the present throughout the mid and late holocene

major ion chemistry of the yarlung tsangpo brahmaputra river chemical weathering erosion and co2 consumption in the southern tibetan plateau and eastern syntaxis of the himalaya the yarlung tsangpo brahmaputra river drains a large portion of the himalaya and southern tibetan plateau including the eastern himalayan syntaxis one of the most tectonically active regions on the globe we measured the solute chemistry of 161 streams and major tributaries of the tsangpo brahmaputra to examine the effect of tectonic climatic and geologic factors on chemical weathering rates specifically we quantify chemical weathering fluxes and co2 consumption by silicate weathering in southern tibet and the eastern syntaxis of the himalaya examine the major chemical weathering reactions in the tributaries of the tsangpo brahmaputra and determine the total weathering flux from carbonate and silicate weathering processes in this region we show that high precipitation rapid tectonic uplift steep channel slopes and high stream power generate high rates of chemical weathering in the eastern syntaxis the total dissolved solids tds flux from the this area is greater than 520 tons km 2 yr 1 and the silicate cation flux more than 34 tons km 2 yr 1 in total chemical weathering in this area consumes 15 2 10 5 mol co2 km 2 yr 1 which is twice the brahmaputra average these data show that 15 20 of the total co2 consumption by silicate weathering in the brahmaputra catchment is derived from only 4 of the total land

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

statistical geochemical and morphological analyses of stone line formation in uganda abstract the resolution of conflicting fluvial allochthonic erosion and deposition and bioturbation autochthonic faunal turbation theories of stone line formation has important implications for tropical geomorphology with the former indicating dynamic punctuated landscape development and the latter implying relative landscape stability with ongoing moderate erosion on a gentle hill in central uganda we quantitatively characterized and statistically modeled gravel distributions for 93 profiles elemental analysis of k selected rare earth elements ree and appropriate index elements were employed to test for compositional differences between soils above and saprolite below hilltop stone lines we also excavated and carefully described four indicative profiles the results of this study were difficult to reconcile with the bioturbation theory of stone line formation important findings included i the lack of a consistent gravel free biomantle with a weak connection between biomantle thickness and stone line depth ii highly variable stone line depth with a strong inverse relationship between stone line depth and maximum gravel content iii unweathered and rounded cobbles in a stone line over saprolite with angular highly weathered quartz and iv a statistically significant geochemical difference between soils and saprolites with evidence of a less weathered surface mantle over more weathered saprolite these findings indicated a sedimentary origin for soils at this site even for what is presently a flat geomorphically isolated hilltop evidence from this study and previous work on the geochemistry of this site geochim cosmochim acta 67 2003 2711 suggests that the contemporary hilltop once occupied a lower landscape position

an evaluation of mobile mud dynamics in the mississippi river deltaic region abstract large rivers are the primary interface between terrestrial and ocean environments a relatively small number of rivers account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are considered to be the main repositories of organic carbon in marine sediments particulate material in these environments are typically deposited and resuspended several times before permanent accumulation or transport off the shelf this sediment cycling is an important component influencing biogeochemical processes that occur in coastal environments during two cruises in april and october 2000 on the shelf adjacent to the mississippi river water and sediment samples were collected for analysis of suspended solids and particle reactive radionuclides 210 pb 137 cs 7 be and 234 th to evaluate the transport and fate of terrestrial and marine material a comparison of the distribution of these tracers provides insight about the pathways and residence times of particulate materials on the shelf inventories of these short lived radiotracers showed variations of more than two orders of magnitude indicating dramatic variations in sediment deposition between sampling events short lived radiotracers indicate that river borne materials are transported less than 30 km from the river mouth before initial deposition however seasonal variations in 7 be and 137 cs indicate significant remobilization of sediment and potential export of sediment out of the study area during the high energy e g wind wave winter months in addition depth profiles of 7 be and excess 234 th indicate sediment deposition rates between 0 8 and 3 9 cm month 1 0 4 2 1 g cm 2 month 1 at two shelf locations near river and open shelf environments these rates are much greater than those observed on decadal time scales 1 3 2 0 cm year 1 or 0 6 1 5 g cm 2 year 1 via 210 pb at the same sites this further substantiates active sediment reworking and potential export off the shelf as has been observed in other river dominated ocean margins

the temperature and salinity fine structure of the ocean under the ross ice shelf a series of temperature and salinity profiles was made in the ocean under the ross ice shelf at 82 22 5 s 168 37 5 w where the ice was 420 and the underlying seawater 240 m thick the water structure consisted of a fairly well mixed low salinity layer at the in situ freezing point of the ice water interface about 30 m thick a transition layer characterized by intrusions about 85 m thick a strongly stratified layer with increasing temperature and salinity about 50 m thick another transition layer characterized by intrusions about 45 m thick and an isohaline bottom layer with a nearly abiabatic temperature gradient about 30 m thick the temperature fluctuations in the two transition layers can be attributed mainly to intrusive activity even though internal wave activity was highest in these regions in the central stable layer internal waves probably contribute to the temperature fluctuations equally with intrusions the internal wave dropped displacement spectrum seems to be very similar to those found in the open ocean some profiles showed intrusions in the top transition layer colder than the top boundary layer and some showed intrusions in the bottom transition layer warmer than the bottom boundary layer indicating that horizontal shear must be present internal waves of near inertial frequency excited by the semidiurnal tides combined with shear induced instabilities seem to be the likely causes of the observed fine structure

assimilation of grace tide solutions into a numerical hydrodynamic inverse model 1 we assimilate localized mass anomalies inferred from grace ranging measurements into a hydrodynamic model to improve tidal solutions around antarctica for the m2 s2 and o1 constituents the variational approach used accounts for the spatial averaging of tidal elevations implicit in the mass anomaly parametrization used for the grace tidal analysis as well as spatial correlation of errors in the resulting estimates the inverse solution shows better agreement with independent station tide measurements around antarctica and it reduces cross over residuals for icesat laser altimeter data over the filchner ronne and ross ice shelves demonstrating that grace data can provide useful constraints for improving hydrodynamic tidal models at high latitudes

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

mass wasting on the mississippi river subaqueous delta abstract the mississippi river delta front mrdf is an excellent natural laboratory for studying seafloor instability and evaluating the preconditioning factors and triggering mechanisms that lead to seafloor failures the occurrence of seafloor landslides on the mrdf has been recognized since the 1950s but the bulk of research on the topic was conducted in the 1970s 80s after seafloor landslides triggered by hurricane camille damaged offshore infrastructure through these efforts it was recognized that seafloor instability features are present across the entire delta front that rapid sediment delivery from the mississippi river is a primary preconditioning factor for failures and that cyclic loading from hurricane waves is the main triggering mechanism for deep seated catastrophic events however mounting evidence shows that mrdf seafloor motion occurs even during periods of no hurricane activity which could be attributed to creep like motion of underconsolidated sediment or smaller episodic flows triggered by winter storms or mississippi river floods given the extensive spatial distribution of instabilities on the mrdf the multiple mechanisms of failure and variety of resulting morphology and the range of potential triggering mechanisms with short recurrence the mrdf is an exceptional location to assess the complex problem of seafloor landslides an improved understanding of seafloor landslides is crucial for understanding global sediment budgets and associated biogeochemical cycling as they can evacuate large volumes of sediment into deep water over short time periods additionally seafloor landslides pose a hazard to offshore infrastructure and to coastal communities through possible tsunami inundation and advancing research on preconditioning factors and triggering mechanisms would improve hazard assessments this paper presents a review and synthesis of geological and geotechnical research on the mrdf particularly with respect to seafloor landslides their occurrence geological and geotechnical controls triggering mechanisms and knowledge gaps related thereto this synthesis is conducted at times of rapidly developing technologies for studying seafloor instability and as the mrdf enters a stage of degradation characterized by reduced sediment delivery and accumulation

a morpho statistical classification of mountain stream reach types in southeastern australia abstract the concepts of sediment transport capacity q c and sediment supply q s have shown promise in broadly differentiating mountain streams the important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks this study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0 01 and 0 20 m m to describe mountain stream morphology in southeastern australia reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology spatial autocorrelation analysis based on moran s i is used to detect the frequency and occurrence of in stream features such as bars and pools these data are combined with the grain size data to describe eight dominant reach morphologies most channel morphologies display broad characteristics similar to previous accounts of mountain streams although a number of intermediate morphologies affecting subtle variations in form and process where evident there was no significant longitudinal arrangement of the observed morphologies particularly with respect to catchment area and slope this is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape regime diagrams applied to quantify the physical controls on the different reach morphologies identified different q c q s domains for most morphologies however some lithology dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape or sphericity in assigning dimensionless critical shear stress values in determining q s overall the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material

geochemistry and health aspects of f rich mountainous streams and groundwaters from sierras pampeanas de cordoba argentina symptoms of dental fluorosis have been observed in rural communities located in the sierras pampeanas de cordoba a mountainous area in central argentina the clinical assessment was performed in the charbonier department where the fluoride f intake was determined to be 3 90 0 20 mg day 1 n 16 in this community mild and severe fluorosis reach an incidence of 86 7 total teeth surface 636 teeth among the children population to determine the origin and distribution of fluorine in natural waters from the charbonier department and nearby regions sampling was performed in the area covering the san marcos river basin the obtained results show that f concentrations vary between 1 to 2 5 mg l 1 with an outlier value of 8 mg l 1 the spatial distribution of f shows that the lowest concentrations are found at the basin s catchments maximum values are located in two sectors of the basin the charbonier depression in the eastern part and at the san marcos village downstream the main collector in the western part of the basin in these two regions the f contents in ground and surface waters are 2 0 mg l 1 and nearly constant dissolved f in natural waters from the study area has its origin in the weathering of f bearing minerals present in the region s dominant lithology the extent of mineral weathering is mostly determined by the residence time of water within the aquatic reservoir longer residence times and a major solid water interaction lead to enhanced release of f this explains the higher f concentrations found in basin areas with lower run off the removal of f from water appears to occur by neither fluorite precipitation nor by adsorption hence variations in f concentrations seem to be more related to regional hydrological conditions

tidal modulation of ice shelf flow a viscous model of the ross ice shelf three stations near the calving front of the ross ice shelf antarctica recorded gps data through a full spring neap tidal cycle in november 2005 the data revealed a diurnal horizontal motion that varied both along and transverse to the long term average velocity direction similar to tidal signals observed in other ice shelves and ice streams based on its periodicity it was hypothesized that the signal represents a flow response of the ross ice shelf to the diurnal tides of the ross sea to assess the influence of the tide on the ice shelf motion two hypotheses were developed the first addressed the direct response of the ice shelf to tidal forcing such as forces due to sea surface slopes or forces due to sub ice shelf currents the second involved the indirect response of ice shelf flow to the tidal signals observed in the ice streams that source the ice shelf a finite element model based on viscous creep flow was developed to test these hypotheses but succeeded only in falsifying both hypotheses i e showing that direct tidal effects produce too small a response and indirect tidal effects produce a response that is not smooth in time this nullification suggests that a combination of viscous and elastic deformation is required to explain the observations

tabular iceberg collisions within the coastal regime during 2000 07 five giant icebergs b15a b15j b15k c16 and c25 adrift in the south western ross sea antarctica were instrumented with global positioning system gps receivers and other instruments to monitor their behavior in the near coastal environment the measurements show that collision processes can strongly influence iceberg behavior and delay their progress in drifting to the open ocean collisions appear to have been a dominant control on the movement of b15a the largest of the icebergs during the 4 year period it gyrated within the limited confines of ross island the fixed ross ice shelf and grounded c16 iceberg interactions in the near coastal regime are largely driven by ocean tidal effects which determine the magnitude of forces generated during collision and break up events estimates of forces derived from the observed drift trajectories during the iceberg collision induced calving of iceberg c19 from the ross ice shelf during the iceberg induced break off of the tip of the drygalski ice tongue and the break up of b15a provide a crude estimate of the stress scale involved in iceberg calving considering the total area the vertical face of new rifts created in the calving or break up process and not accounting for local stress amplification near rift tips this estimated stress scale is 10 4 pa

geochemistry of surface waters and weathering effects in the upper catchment of the ctalamochita river cordoba s sierras pampeanas central argentina the chemical compositions of waters from the upper basin of the ctalamochita river draining the cordoba pampean ranges were analyzed to determine the chemical signature related to different forcings geochemical processes and solutes sources the catchment has four sub basins santa rosa river grande river quillinzo river and de los sauces river most of the riverine chemical compositions are dominated by the hco3 ca2 type to ca2 na mg2 the specific conductivity is higher in the santa rosa and de los sauces rivers 347 59 and 409 00 s cm respectively than in the grande and quillinzo rivers 125 56 and 87 54 s cm respectively mineral hydrolysis of andesine and oligoclase and the dissolution of calco magnesian limestone are the main dissolved solids contributors to the river waters higher contribution of bicarbonate ions is associated to those sub basin having quarry mining activity saturation indexes show that the santa rosa and de los sauces rivers are saturated with calcite dolomite and talc

causes of river avulsion insights from the late holocene avulsion history of the mississippi river u s a abstract the emphasis on gradient advantages in studies of avulsion is misleading while gradient advantages are necessary for an avulsion to occur the late holocene avulsion history of the mississippi river in louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important cross valley to down valley slope ratios of the modern floodplain range from 16 to 110 and are typically 30 the slope ratio is 35 at the location of the mississippi atchafalaya diversion old river yet slope ratios are 83 to 110 immediately upvalley of old river all values of mississippi river floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models shallow floodplain cores 14c dating of organic remains and geologic mapping show that the mississippi river has avulsed only four times over the past 5 ky in the southern lower mississippi valley lmv gradient advantages are widespread yet avulsions are rare these observations indicate that factors other than gradient advantage control mississippi river avulsion several examples of mississippi and red river avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion incipient mississippi river avulsion and development of the atchafalaya river involved reoccupation of abandoned mississippi river channels and a red river crevasse splay complex the modern atchafalaya river also incises buried mississippi river channel belt sands abandoned channel belts and crevasse splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the mississippi river abandoned channels provide ready made conduits for mississippi river flow that can efficiently develop into avulsive channels multi storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion these features record episodic reoccupation of channel belts which at least in some cases may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate

weathering assessment in the achala batholith of the sierra de comechingones cordoba central argentina ii major hydrochemical characteristics and carbon dynamics la trucha is a granitic second order catchment 1 9 km2 average slope 5 placed at 1400 m elevation in the sierra de comechingones argentina 31 54 s 64 45 w 31 53 64 44 w the main geochemical characteristics were studied during a hydrological year march 2005 february 2006 attending mainly seasonal and spatial variability and weathering induced carbon consumption subjected to a weathering limited denudation regime the catchment is representative of hundreds of the second order streams that dissect the achala batholith water chemistry is typical of streams draining f rich granites high discharges austral summer determine the dilution of na ca2 mg2 hco3 so42 and f conversely high summer rainfall increases cl concentrations the rainfall induced mobilization of k is likely masked by its affinity for adsorption onto fine grained particles and that of sio2 by its biological consumption la trucha s upper catchment shows that steep slopes and a shallow regolith layer result in a more diluted water flow chemically closer to rainfall subsurface flow increases the downriver positive rate of la trucha s electrical conductivity twice as much in winter 6 7 s cm 1 per 1000 m than during wet summer 3 9 s cm 1 per 1000 m phreeqc allowed the calculation of free co2 and the likelihood of its evasion in different seasons and areas of the catchment a mean carbon efflux of 180 mg c m 2 year 1 has been estimated for la trucha catchment the first and second order streams which occupy 62 5 km2 of the achala batholith s surface area are significant sources in the sierra de comechingones carbon cycle

tides of the northwestern ross sea and their impact on dense outflows of antarctic bottom water abstract moored and depth profile measurements of hydrography and currents during the 2003 2005 antarctic slope anslope program on the shelf slope region near drygalski trough sill in the northwest ross sea have revealed an energetic benthic outflow of cold dense antarctic bottom water aabw produced near the shelf break this outflow couples dense water production processes on the continental shelf to ventilation of the global deep ocean we use time series of bottom pressure and currents on the anslope moorings and a high resolution three dimensional primitive equation numerical model to explore the hypothesis that the tidal currents near the shelf break have a significant impact on outflow volume transport and hydrographic properties tidal currents exceed 1 m s 1 at spring tide over the outer shelf and upper slope tidal advection at spring tide carries water parcels 20 km across slope during a half tidal cycle sufficient to advect water from northern drygalski trough across the sill to the shelf break or similarly from the shelf break 500 m water depth down to the 1600 m isobath a tidally rectified lagrangian mean flow of order 0 05 0 1 m s 1 over the outer shelf and upper slope accompanies the spring tide thickness and water mass properties of the benthic layer over the sill and upper slope exhibit pronounced tidal cycles due to advection of hydrographic gradients and to energetic mixing that can extend more than halfway up from the seabed to the ocean surface maximum parameterized diapycnal diffusivity in the benthic layer at spring tides approaches 1 m 2 s 1 the addition of tides has a profound effect on time averaged outflows of aabw primarily through an increase in benthic layer thickness during spring tide downslope volume fluxes of aabw over the slope averaged over a tidal cycle are 2 5 times larger at spring tide than at neap indicating a need to include the effect of tides in large scale ocean models attempting to represent shelf water contributions to the global ocean circulation

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

barotropic tides of the southern indian ocean and the amery ice shelf cavity the 8 main tidal constituents were computed using a finite element hydrodynamic ocean tide model over the south indian ocean region the discretization of the domain is of the order of 100 km over the deep ocean and a few hundred meters near the coast such refinement in the grid resolution enables wave propagation and damping on the continental shelves to be solved correctly the model used the gebco 1 minute global bathymetric grid which was improved with updated topographic data the model solutions show good agreement with in situ observations and topex poseidon altimeter measurements and are significantly better than previously published solutions we obtain a combined standard deviation of 1 4 cm for differences of our new regional model against independent observations compared to about 2 5 cm for the other tide models the greatest improvements are found around the kerguelen islands around antarctica and beneath the amery ice shelf and can be explained by the high grid resolution used and the particular attention given to the accuracy of the bathymetry in those regions

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

ambient seismic hydroacoustic and flexural gravity wave noise on a tabular iceberg cross correlation of ambient seismic noise between four seismographs on tabular iceberg c16 ross sea antarctica reveals both the source and the propagation characteristics of signals associated with icebergs we find that noise correlation functions computed from station data are asymmetric about zero time lag and this indicates that noise observed on the iceberg originates primarily from a compact localized source associated with iceberg collisions between c16 and a neighboring iceberg b15a we additionally find two and possibly more distinct phases of noise propagation we believe that flexural gravity wave propagation dominates the low frequency noise 10 s period and that hydroacoustic wave propagation in the water column between the ice and seabed appears to dominate high frequency noise 10 hz faster seismic propagation dominates the intermediate band 2 6 hz however we do not have sufficient data to characterize the wave mechanisms more precisely e g by identifying distinct longitudinal and shear body waves and or surface waves secular changes in the amplitude and timing of ambient noise correlations e g a diurnal cycle and an apparent shift in the noise correlation of fast seismic modes between two periods of the deployment allow us to speculate that ambient noise correlation analysis may be helpful in understanding the sources and environmental controls on iceberg generated ocean noise as well as geometric properties such as water column thickness of subglacial lakes

weathering assessment in the achala batholith of the sierra de comechingones cordoba central argentina iii appraising chemical weathering la trucha is a small 1 9 km2 mountainous 1300 m a s l granitic drainage basin demonstrative of hundreds of second order basins in the achala batholith of the sierras pampeanas of cordoba argentina dominated by physical weathering and a weathering limited denudation regime the coarse and fine grained regolith has shown scant geochemical differences with the country rock erosion and sediment transport mainly occurs during torrential rain events austral summer whereas surface waters classify as dilute 750 t 375 eq l 1 or very dilute 375 t 185 eq l 1 water types the dynamics of chemical weathering at la trucha was approached through the use of major dissolved chemical components comparing the performance of different methodologies in its assessment mineral stability diagrams i e chemical equilibria the re chemical weathering index ternary diagrams and phreeqc inverse modeling the latter approach allowed to probe into subtleties of the geochemical processes indicating that besides the morphological dependency on mineral hydrolysis dissolution or precipitation there is a marked seasonal control that promotes or limits the occurrence of geochemical reactions

stick slip motion of an antarctic ice stream the effects of viscoelasticity stick slip behavior is a distinguishing characteristic of the flow of whillans ice stream siple coast antarctica distinct from stick slip on northern hemisphere glaciers which is generally attributed to supraglacial melt the behavior is thought be controlled by basal processes and by tidally induced stress however the connection between stick slip behavior and flow of the ice stream on long time scales if any is not clear to address this question we develop a new ice flow model capable of reproducing stick slip cycles similar to ones observed on the whillans ice plain the model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice till interface the model results suggest the long term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick slip behavior such as velocity magnitude during the slip phase and factors that regulate it e g elastic modulus furthermore it is found that factors controlling purely viscous flow such as temperature influence stick slip contribution to long term flow in much the same way additionally we show that viscous ice deformation traditionally disregarded in analysis of stick slip behavior has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick slip observations

phosphorus distribution c n p ratios and δ13coc in arctic temperate and tropical coastal sediments tools for characterizing bulk sedimentary organic matter abstract onshore offshore trends in phosphorus p organic carbon oc and total nitrogen tn concentration p distribution elemental organic c n p ratios and stable carbon isotopic composition of oc 13 c oc of surficial sediments are presented from three river dominated coastal regimes the mackenzie river beaufort shelf in the canadian arctic the mississippi delta and louisiana shelf in the temperate gulf of mexico and the tropical amazon shelf these parameters measured in surficial sediments from the three sites are used to assess changes in the importance of terrestrial and marine organic matter sources to sediments as a function of distance from the locus of riverine discharge trends in elemental ratio data from the arctic transect and a portion of the gulf of mexico transects can be explained in terms of a two end member mixture of terrestrial and marine phytodetritus in the arctic transect covariation of organic c p ratios with 13 c oc is consistent with a two end member mixture of terrestrial and marine organic matter a similar relationship between c p ratios and 13 c oc is not evident in gulf of mexico or amazon shelf sediments at these sites the two end member model fails to explain the data adequately the most striking feature of the onshore offshore trends in elemental oc op and oc tn ratios is the occurrence of low ratios in sediments from deep water stations in the gulf of mexico and all stations on the amazon shelf resulting from p and n enrichments exceeding the redfield ratio for marine plankton three explanations are discussed to account for these low ratios 1 the dominance of refractory op and on compounds in the residuum of degraded organic matter 2 differential sorption of op and n compounds regardless of lability on the surfaces of the fine grained sediments characteristic of these sites and 3 a dominance of bacterial biomass or components derived from bacterial biomass elemental ratios and 13 c oc signatures indicate that differential sorption may be most important in the deep water gulf of mexico sites in contrast elemental ratios and 13 c oc on the amazon shelf are most consistent with a sedimentary organic matter pool dominated by bacterial biomass or derivative substances

impact of river discharge on hydrodynamics and sedimentary processes at yellow river delta abstract during the anthropocene regulating river discharge by high dams may have met the need for water demands in river basins but resulted in carrying less freshwater and sediment to the sea inducing land degradation and shoreline retreat in worldwide mega river deltas in land ocean interaction tide response to water discharge changes plays an important role and is crucial for the river laden sediment transfer and dispersal affecting both nearshore and estuarine deposits the yellow river delta yrd which is under an increasing pressure of the new discharge regime of the yellow river has undergone drastic changes in terms of sediment dynamics and morphologic evolution to gain a better understanding of the overall fluvial and marine hydrodynamics and morphodynamic processes in the yrd in this study a full scale numerical model is built to investigate the interaction and impacts of changing environmental forcing and dynamics on flow and sediment transport in the estuary of yrd and its adjacent coasts the results show that the river discharge strongly affects the tidal dynamics and morphology of the delta particularly in the close vicinity of the outlet and the intertidal zone tidal constituents m2 and k1 which are the most significant ones in the yrd are found to be noticeably affected with a decreasing trend when the river discharge increases the model results also indicate that river discharge affects the location and intensity of the shear front that occurs in the nearshore areas of the yrd increasing the river discharge can induce a seaward movement of the shear front reduce its width and concentrate its shear intensity it is found that the reverse of the flow direction at each side of the shear front and strong longshore tidal current can act as a barrier for the sediment dispersal process by keeping suspended sediment in the inner zone thus to form a particular sediment deposition zone and the depo center

ocean variability contributing to basal melt rate near the ice front of ross ice shelf antarctica basal melting of ice shelves is an important but poorly understood cause of antarctic ice sheet mass loss and freshwater production we use data from two moorings deployed through ross ice shelf 6 and 16 km south of the ice front east of ross island and numerical models to show how the basal melting rate near the ice front depends on sub ice shelf ocean variability the moorings measured water velocity conductivity and temperature for 2 months starting in late november 2010 about half of the current velocity variance was due to tides predominantly diurnal components with the remainder due to subtidal oscillations with periods of a few days subtidal variability was dominated by barotropic currents that were large until mid december and significantly reduced afterward subtidal currents were correlated between moorings but uncorrelated with local winds suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the ice front estimated melt rate was 1 2 0 5 m a 1 at each site during the deployment period consistent with measured trends in ice surface elevation from gps time series the models predicted similar annual averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the ice base these results show that accurately modeling the high spatial and temporal ocean variability close to the ice shelf front is critical to predicting time dependent and mean values of meltwater production and ice shelf thinning

influence of geomorphological variables on mountainous stream water chemistry sierras pampeanas cordoba argentina abstract the relationship between geomorphological features and water geochemistry was studied for a group of mountainous rivers from 900 to 2200 m a s l with similar geology and climate in the sierras pampeanas of cordoba argentina 31 30 32 00 s and 64 30 65 10 w a multivariate approach was used to identify three morphological domains that describe the set of sampled rivers namely size dominance slope dominance and drainage density dominance the links between physicochemical and geomorphological variables show that size dominance is mostly related to major ions conductivity and ph which tend to increase downstream slope dominance is associated with the total concentration of heavy metal i e high heavy metal concentrations are associated with relatively flat areas with slightly acid to circumneutral ph which promotes desorption the drainage density dominance results in an association between well drained catchments and low cl concentration i e preserving the chemical signature of atmospheric precipitation

ice shelf dynamics near the front of the filchner ronne ice shelf antarctica revealed by sar interferometry fifteen synthetic aperture radar sar images of the ronne ice shelf also referred to as the filchner ronne ice shelf antarctica obtained by the european remote sensing satellites ers 1 and 2 are used to study ice shelf dynamics near two ends of the iceberg calving front interferograms constructed from these sar images are used to resolve the ice shelf displacement along several directions in response to both ocean tide and long term creep flow tidal motion is separated from creep flow using differential interferometry i e two or more interferograms in which fringe patterns common to all are predominantly associated with creep flow creep flow velocities thus determined compare well with prior ice shelf velocity surveys using these data we studied the influence of large scale rifts ice rises and coastal separation on the ice shelf flow many of the large rifts that appear to form the boundaries where tabular icebergs may eventually detach from the ice shelf are filled with a melange of sea ice ice shelf debris and wind blown snow the interferograms show that this melange tends to deform coherently in response to the ice shelf flow and has sufficient strength to trap large tabular ice shelf fragments for several decades before the fragments eventually become icebergs in many instances the motion of the tabular fragments is a rigid body rotation about a vertical axis that is driven by velocity shear within the melange the mechanical role of the rift filling melange may be to bind tabular ice shelf fragments to the main ice shelf before they calve this suggests two possible mechanisms by which climate could influence tabular iceberg calving first spatial gradients in oceanic and atmospheric temperature may determine where the melange melts and thus the location of the iceberg calving margin second melting or weakening of ice melange as a consequence of climate change could trigger a sudden or widespread release of tabular icebergs and lead to rapid ice shelf disintegration

a review of sediment diversion in the mississippi river deltaic plain abstract one of the proposed methods for restoring the disappearing mississippi delta is sediment diversion which uses channels and structures to divert water and sediment from the mississippi and atchafalaya rivers into adjacent basins this study presents a comprehensive review of geological and physical aspects of sediment dynamics in the mississippi river deltaic plain mrdp with special reference to diversion studies over the past two decades we synthesize these studies present the current understanding of sediment diversions in the context of sediment dynamics identify multiple key knowledge gaps and make recommendations for future studies to maximize net land building in the mrdp management strategies should be focused on a enhancing river sediment delivery both mud and sand b increasing sediment retention in receiving basins and c minimizing erosion in bays and estuaries compared with extensive studies of land building there have been relatively fewer studies of erosional processes a heterogeneous coastal geological framework cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the mrdp sediment retention rates are highly sensitive to spatial and temporal scales types of sediments and delivery season sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes structures and devices that improve sediment retention trap sediments dissipate waves and build living shorelines should be explored and cost to benefit analysis is needed long term planning should consider more landward diversions strategic community relocation and nonlinear response of the complex sedimentary system of the mrdp

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front

improving antarctic tide models by assimilation of icesat laser altimetry over ice shelves 1 assimilation of laser altimeter data from the ice cloud and land elevation satellite icesat significantly improves the accuracy of ocean tide models for the ross ice shelf ris for the most energetic tidal harmonic k1 assimilation reduces the root mean square error rmse between the model and a set of 16 independent tide records on and near the ris from 6 0 to 2 8 cm and the combined rmse for the six most energetic tidal harmonics from 7 7 to 5 4 cm when only the six most recent and highest quality tide records are considered the combined rmse is 4 8 cm this value is close to the uncertainty expected from tidal analyses of the short 1 2 month validation records indicating that assessing further improvements in tide model accuracy will require development of a higher quality validation data set

numerical simulations of the ross sea tides tidal currents below the floating ross ice shelf are reconstructed by using a numerical tidal model they are predominantly diurnal achieve maximum strength in regions near where the ice shelf runs aground and are significantly enhanced by topographic rossby wave propagation along the ice front a comparison with observations of the vertical motion of the ice shelf surface indicates that the model reproduces the diurnal tidal characteristics within 20 similar agreement for the relatively weak semi diurnal tides was not obtained and this calls attention to possible errors of the open boundary forcing obtained from global ocean tidal simulations and to possible errors in mapping zones of ice shelf grounding air sea contact below the ice shelf is eliminated by the thick ice cover the dominant sub ice shelf circulation may thus be tidally induced a preliminary assessment of sub ice shelf conditions based on the numerical tidal simulations suggests that 1 strong barotropic circulation is driven along the ice front and 2 tidal fronts may form in the sub ice shelf cavity where the water column is thin and where the buoyancy input is weak

the generation of diurnal period shelf waves by tidal currents abstract continental shelf waves of diurnal period are shown to be generated by tidally induced reynolds stresses within a bottom stokes scale boundary layer the theory is applicable to a uniformly rotating homogeneous ocean of two dimensional depth variability in which alongshore variations occur over scales large compared to the shelf width explicit solutions are derived for the shelf wave velocity components and for the cross shelf sea surface slope in the case of a traveling kelvin wave forcing numerical values are presented for an exponential depth profile h h0 exp 2 x where x is the coordinate normal to the coast results indicate that the amplitude of the shelf wave current can exceed that of the astronomical tidal current and that the alongshore component of the shelf wave current will consistently lead the alongshore component of the tidal current by 180 to 0 over one wavelength in the direction of phase propagation

the cavity under the amery ice shelf east antarctica ocean circulation under ice shelves and associated rates of melting and freezing are strongly influenced by the shape of the sub ice shelf cavity we have refined an existing method and used additional in situ measurements to estimate the cavity shape under the amery ice shelf east antarctica a finite element hydrodynamic ocean tide model was used to simulate the major tidal constituents for a range of different sub amery ice shelf cavity water column thicknesses the data are adjusted in the largely unsurveyed southern region of the ice shelf cavity by comparing the complex error between simulated tides and in situ tides derived from gps observations we show a significant improvement in the simulated tides with a combined complex error of 1 8 cm in comparison with past studies which show a complex error of 5 3 cm our bathymetry incorporates ice draft data at the grounding line and seismic surveys which have provided a considerable amount of new data this technique has particular application when the water column beneath ice shelves is inaccessible and in situ gps data are available

seismic observations of glaciogenic ocean waves micro tsunamis on icebergs and ice shelves seismometers deployed over a 3 year period on icebergs in the ross sea and on the ross ice shelf antarctica reveal that impulsive sources of ocean surface waves are frequent e g 200 events per year in the ross sea in the ice shelf and iceberg covered environment of coastal antarctica the 368 events recorded by our field deployment suggest that these impulsive events are generated by glaciological mechanisms such as 1 small scale calving and edge wasting of icebergs and ice shelf fronts 2 edge on edge closing and opening associated with iceberg collisions and 3 possibly the impulsive opening of void space associated with ice shelf rifting and basal crevasse formation the observations described here provide a background of glaciogenic ocean wave phenomena relevant to the ross sea and suggest that these phenomena may be exploited in the future using more purposefully designed observation schemes to understand iceberg calving and ice shelf disintegration processes

anatomy and growth of a holocene clinothem in the gulf of papua 1 high resolution seismic profiles and sedimentological data from grab samples and long cores provide an unprecedented picture of the structure sedimentology and late quaternary development of two gulf of papua gop clinothems one probably stage 3 and 4 in age and one holocene in age the older was partially eroded during stage 2 and partially covered by the younger clinothem during stage 1 the younger clinothem consists of three stratigraphic units separated by two surfaces of erosion bypass or correlative surfaces of lap the surfaces were formed by changes in accommodation and sediment supply the underlying physiography of the older clinothem also appears to play an important role in governing the shape of the younger clinothem in the northern gulf oblique clinoforms of the younger clinothem suggest that the rate of sediment supply from the northern rivers outstripped the formation of new accommodation whereas in the south sigmoidal clinoforms indicate that accommodation increased faster than sediment supply the origin of the new accommodation remains uncertain because of limited age constraints on the basis of sediment thickness stratal geometry and acoustic character off shelf transport appears to be the dominant sediment transport direction with preferential accumulation on the promontories and bypass in the valleys presently observed and computed modern flows and complex gyres in shallow water coupled with wave and current supported gravity flows or river floods can explain the form internal clinoform shapes and mineralogy of the younger gulf of papua clinothem

the tides in the arctic ocean from a finite element model the two main diurnal i e k1 and o1 and semidiurnal i e m2 and s2 tidal waves have been computed from a hydrodynamic finite element spectral model the spatial resolution of the model varies from a few kilometers at the coastline to 100 km in the deepest regions an optimization of the open boundary conditions has been performed to improve the solution accuracy the bottom topography used in this model has been originally extracted from the earth topography 5 minutes grid etopo5 and corrected from various sources of data the impact of the input parameters like the friction coefficient the loading effects and the parameterisation of the additional friction due to the arctic ice cover is investigated the tidal elevations are compared to a set of in situ observations and validated they also are compared to the kowalik and proshutinsky 1993 model these comparisons mainly show good agreement between the two models nevertheless significant differences are observed where small wavelength structures are produced by the topographic trapping of the tidal energy the velocities associated with the elevations are also presented the potential rate of work the dissipation and the energy fluxes are computed from these elevations and velocities because of the uncertainties of the velocities at the open boundaries we are restricted to a qualitative analysis of the energy equation terms which appears to be consistent with the previous studies on arctic tides

barotropic tides beneath the amery ice shelf 1 a barotropic tidal model has been applied to the amery ice shelf cavity and prydz bay region of east antarctica the sensitivity of the tidal solution of the model to variation of the water column thickness of the amery ice shelf cavity has been determined a best estimate water column thickness grid is presented which both fits available water column thickness data bed elevation and ice thickness and results in reasonable agreement with available tidal elevation data this is an important result for the amery ice shelf given the severe lack of sub ice shelf bed elevation and limited direct ice thickness measurements using the resulting topography simulated tidal current speeds in the sub amery ice shelf cavity are significantly less than those beneath other major embayed antarctic ice shelves with maximum tidal current speeds of 26 cm s 1 indicated for this cavity similarly the estimated energy dissipation beneath the amery ice shelf due to surface friction of 6 mw is low in comparison with the other ice shelves tidally induced vertical mixing is found to be too weak to destroy the stratification associated with the relatively warm water in the lower part of the cavity and ice shelf meltwater in the upper part of the cavity however it is proposed that buoyancy driven upwelling rather than vertical mixing is sufficient to bring the lower water mass into contact with the ice shelf the depth averaged model suggests that barotropic tidal processes have little influence on the oceanographic properties of the amery ice shelf cavity

sources of dissolved ree in mountainous streams draining granitic rocks sierras pampeanas cordoba argentina abstract stream waters draining granitic terrains from the highest part 850 to 2200 m a s l of sierras pampeanas cordoba argentina 32 s 65 w were sampled in order to define sources and distribution of dissolved rare earth elements ree and to describe the geochemical processes that govern their mobility the contribution of the regional granite to the dissolved ree pool in stream water is limited due to the physical conditions predominating in the area i e steep slopes and semiarid climate therefore precipitation is considered a seasonally significant source controlling ree concentration in stream water dissolved ree concentrations are inversely correlated with monthly precipitation and rainfall frequency during the rainy season i e the austral summer ree concentrations in stream water are lower than during the dry season i e austral winter such low concentrations reflect the balance between the ree input from precipitation and their removal by adsorption in contrast during the dry season the longer residence time of water within fractures and colluvium determines an increased ree concentration in the base flow lower ph values also contribute to raise ree concentration through desorption from mineral surfaces

the mississippi delta region past present and future the mississippi delta region of south louisiana houses a wealth of resources within a dynamic subsiding landscape foundations for the delta region reflect neogene evolution of the depocenter whereas details of the modern landscape reflect late pleistocene to holocene evolution of the alluvial deltaic plain the holocene delta plain was constructed by cyclical growth of deltaic headlands followed by avulsion and relocation of the fluvial sediment source abandoned headlands were then submerged and reworked while a new headland was created at the site of active fluvial sediment input historic period levees have decoupled the delta plain from its fluvial sediment source at the same time global sea level rise was accelerating which has accelerated delta plain submergence diversions of mississippi river water and sediment are necessary to achieve delta plain sustainability but upstream dams trap 50 of the total sediment load and there is not enough supply to keep pace with subsidence and accelerated

chemical weathering studies in relation to geomorphological research in southeastern canada abstract although chemical weathering provides fundamental information relevant to geomorphology the subject has been overlooked during the 20th century in canada this paper provides an overview of the current state of canadian research on chemical weathering in southeastern canada and takes into account three spatial and temporal contexts 1 the formation of bedrock morphology by chemical weathering 2 occurrences characteristics and age of saprolites and 3 contemporary chemical denudation rates long term geomorphological evolution of southeastern canadian landscapes shows that chemical weathering has played an important role an example is taken from the laurentide region of the canadian shield north of montreal quebec the present topography resulted from the stripping of the former weathering mantle and from the probable subsequent modification of the weathering front first by the action of hillslope processes and rivers and then by glaciers before and during plio pleistocene times the present landscape reflects the timing of the formation of erosion surfaces and of the stripping of the paleozoic cover rocks and exposure of the shield since the late seventies several isolated occurrences of saprolite soil profiles have been discovered in eastern canada and prompted a renewal of the study of these materials about the cenozoic evolution of these regions one of the problems in this field of research is the dating of saprolites and their inclusion in a chronostratigraphic framework along with the other cenozoic surficial deposits and landforms because of the multiple factors involved in the development of secondary minerals in saprolites during the course of weathering it is preferable to distinguish the dating of saprolites from the study of their mineralogical and geochemical evolution fortunately several new techniques are becoming available for the absolute dating of surficial deposits and saprolites including the use of cosmogenic radionuclides saprolites provide a strong potential field of research for our understanding of the geological evolution of eastern canada during the cenozoic contemporary weathering and erosion rates are fields of research that have gained increasing interest recently since modeling landscape geochemical response can be applied to various environmental stress situations such as acidification by rain and forest harvesting rock type may be the main factor explaining the large differences between watersheds in fact variability of cation removal in the temperate zone is probably most closely related to flow paths of water investigations at different scales from entire watersheds to slopes to individual pedons highlight the problem in the catamaran brook watershed new brunswick water chemistry is explained by a mix of groundwater and soil solution from the horizons at the base of the floodplain soils geochemical mass balances based on net outputs give little information on the weathering reactions of primary minerals the weathering products or on the nature of the weathering processes that provide the dissolved load of streams mineralogic and petrographic analysis of selected soil pedons are necessary to determine weathering reactions and their role as sources or sinks for bases silica aluminum and iron in the various compartments through which water percolates before it reaches the stream

high resolution modelling of the shelf and open ocean adjacent to south georgia southern ocean abstract the marine ecosystem on the shelf and open ocean adjacent to south georgia is extraordinarily rich with a history of commercial exploitation although much progress has been made attempts at modelling and hence better understanding this system have consistently been hampered by the poor representation of key physical processes in global or regional ocean general circulation models here we present the development of a high resolution 3d hydrodynamic model of the south georgia shelf and the adjacent open ocean including a novel method for prescribing freshwater fluxes the ability of the model to reproduce the observed oceanography of the region is quantified by comparisons with data from tide gauges at south georgia with satellite derived sea surface temperatures and with an extensive ctd dataset collected during january april 1995 predicted cotidal charts for the diurnal tides o1 and k1 show a periodic amplification in both the current and elevation fields at the shelf edge suggesting the presence of a diurnally forced continental shelf wave this could have important implications for processes such as larval transport and retention the comparison with ctd data reveals mean and root mean square errors in temperature salinity of 0 27 c 0 07 and 0 64 c 0 23 respectively vertical profiles of potential temperature and salinity on the shelf agree acceptably well with observations but there is a tendency for the model to under predict the density contrast between surface and bottom waters the main limitation on model accuracy is found to be the large scale forcing releasing a passive tracer into the model transport and retention pathways are identified including a prevalence for tracer export from the shelf to the west of south georgia and a transport pathway linking south georgia and shag rocks significantly the model suggests this to be a unidirectional link from south georgia to shag rocks with possible significance for fisheries management the implications of these results in the context of the south georgia ecosystem are discussed briefly demonstrating the usefulness of this new tool for interdisciplinary studies of the region

decadal link between longitudinal morphological changes in branching channels of yangtze estuary and movement of the offshore depo center in estuaries the morphology of inland and offshore areas usually evolves synergistically this study examines the decadal link between longitudinal changes in morphology of branching channels and movement of the offshore depo center where sediment deposition rate is maximum of the yangtze river estuary under intense human interference integrated data analysis is provided on morphology runoff discharge and ebb partition ratio from 1950 to 2017 channel volume reductions and change rates between isobaths in branching channels reflect the impact of estuarine engineering projects ebb partition ratio and duration of discharge 60 000 m3 s 1 act as proxies for the water excavating force in branching channels and runoff intensity it is found that deposition occurs in the lower upper sub reaches or further downstream upstream channels of the inland north south branching channels and the offshore depo center moves southward or southeastward as runoff intensity grows the reverse occurs as runoff intensity declines this is because the horizontal circumfluence in the yangtze estuary rotates clockwise as ebb partition ratios of the north south branching channels increase decrease for increasing runoff and conversely rotates anticlockwise for decreasing runoff land reclamation activities the deepwater channel project and the qingcaosha reservoir have impacted greatly on longitudinal changes of morphology in the north branch and the south passage and on ebb partition ratio variations in the north south channel and the north south passage dam induced runoff flattening has enhanced deposition in the upper lower sub reaches of the north south branching channels and caused northward movement of the offshore depo center except in areas affected by estuarine engineering projects dam induced longitudinal evolution of branching channel morphology and offshore depo center movement will likely persist in the future given the ongoing construction of large cascade dams in the upper yangtze and the completion of major projects in the yangtze estuary

changes in major element hydrochemistry of the pecos river in the american southwest since 1935 abstract the pecos river situated in eastern new mexico and western texas receives water from a drainage area of 91 000 km 2 there are primarily two major water inputs namely snowmelt from winter storms in the headwater region of the southern rocky mountains and runoff from warm season monsoonal rainfall in the lower valley the pecos river suffers from high levels of total dissolved solids tds 5000 mg l 1 under normal flow conditions this not only poses serious problems for agricultural irrigation and safe drinking water supply but also results in a permanent loss of biodiversity this study examines changes in stream flow and water chemistry of the pecos river over the last 70 a to better understand the long term variability in stream salinity and the role of agricultural practices in salt transfer a tds record from the lower pecos river near langtry texas back to 1935 was extracted to show a distinct pattern of decadal variability similar to the pacific decadal oscillation pdo in which stream salinity is overall above average when the pdo is in positive warm phase and below average when the pdo is in negative cold phase this is due to 1 the dissolved salts contributed to the river are largely from dissolution of nacl and caso 4 bearing minerals e g halite and gypsum in the upper basin 2 the amount of the dissolved salts that reach the lower basin is mainly determined by the stream flow yield in the upper basin and 3 the stream flow yield from the upper basin is positively correlated with the pdo index this further attests that large scale climatic oscillation is the major source of long term changes in stream flow and salinity of the pecos river on the other hand there is also a strong indication that the rate of salt export has been affected by reservoir operations and water diversions for agricultural practices

climatic and tectonic controls on chemical weathering in the new zealand southern alps abstract climatic and tectonic controls on the relative abundance of solutes in streams draining the new zealand southern alps were investigated by analyzing the elemental and sr isotope geochemistry of stream waters bedload sediment and hydrothermal calcite veins the average relative molar abundance of major cations and si in all stream waters follows the order ca2 50 si 22 na 17 mg2 6 k 5 for major anions the relative molar abundance is hco3 89 so42 7 cl 4 weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters but nearly all streams have a carbonate dominated ca2 and hco3 mass balance stream water ca sr and 87sr 86sr ratios vary from 0 173 to 0 439 mol nmol and from 0 7078 to 0 7114 respectively consistent with the ionic budget these ratios lie solely within the range of values measured for bedload carbonate ca sr 0 178 to 0 886 mol nmol 87sr 86sr 0 7081 to 0 7118 and hydrothermal calcite veins ca sr 0 491 to 3 33 mol nmol 87sr 86sr 0 7076 to 0 7097 streams draining regions in the southern alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain 10 more ca2 and 30 more sr2 from carbonate weathering compared to streams draining regions in drier more stable landscapes similarly streams draining glaciated watersheds contain 25 more sr2 from carbonate weathering compared to streams draining non glaciated watersheds the highest abundance of carbonate derived solutes in the most physically active regions of the southern alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock which contains trace amounts of calcite estimated to weather 350 times faster than plagioclase in this environment in contrast regions in the southern alps experiencing lower rates of uplift and erosion have a greater abundance of silicate versus carbonate derived cations these findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters using the southern alps as a model for assessing the role of active tectonics in geochemical cycles this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater