winter cascading of cold water in lake geneva 1 during the winters of 1998 and 1999 observations were made of the cascading of cold water from the nearshore shallow shelf zones and down the sloping sides of lake geneva cascading starts on the average 10 hours after the onset of surface cooling the draining cold water descends like a gravity current and the downslope speed of the head of these slugs of cold water u has a mean value of 5 2 cm s 1 with slugs persisting on the average for 8 hours when the monin obukov length scale at the water surface l is negative implying convection occurs and d over bar lg 1 where d is the mean shelf depth the nondimensionalized speed of the front of slugs u b 1 3 is found to be 1 3 0 4 where b is the surface buoyancy flux integrated over the time period from one slug to the next each slug is unsteady the head being followed by several fronts in which the temperature of the current decreases and its thickness increases these fronts travel faster than the mean flow by a factor of r 1 38 0 3 dynamical similarities are found with roll waves observed in turbulent open channel flows the circulation induced by the cascade is found to give a positive skewness to the time derivatives of near surface temperature in shallow waters in contrast with negative values close to the slope the volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface the average volume carried by the slug across the 21 m depth contour is about 1 9 times the volume of water in shallower water i e that on the shelf between shore and a depth of 21 m implying that cascading is an efficient means of flushing shelf water integrated around the lake the mean total volume flux amounts to 11 5 the average winter river inflow

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

mixing and entrainment in the red sea outflow plume part i plume structure when the salty and heavy water of the red sea exits from the strait of bab el mandeb it continues downslope into the gulf of aden mainly along two channels the 130 km long northern channel nc is topographically confined and is typically only 5 km wide in it the red sea plume shows unanticipated patterns of vertical structure turbulent mixing and entrainment above the seafloor a 25 120 m thick weakly stratified layer shows little dilution along the channel hence this bottom layer undergoes only weak entrainment in contrast a 35 285 m thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing it is thus the interface that exhibits the bulk of entrainment of the red sea plume in the nc the interfacial layer also carries most of the overall plume transport increasingly so with downstream distance the southern channel sc is wider than the nc and is accessed from the latter by a sill about 33 m above the floor of the nc entrainment into the bottom layer of the sc is diagnosed to be strong near the entry into the sc such that the near bottom density and salinity are smaller in the sc than in the nc at the same distance from bab el mandeb in comparison with winter conditions the authors encountered weaker outflow with shallower equilibration depths during the summer cruise bulk froude numbers computed for the whole plume varied within the range 0 2 1 local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment

warming and weakening of the abyssal flow through samoan passage abstractthe abyssal flow of water through the samoan passage accounts for the majority of the bottom water renewal in the north pacific thereby making it an important element of the meridional overturning circulation here the authors report recent measurements of the flow of dense waters of antarctic and north atlantic origin through the samoan passage a 15 month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013 this allows for an update of the only prior volume transport time series from the samoan passage from woce moored measurements between 1992 and 1994 while highly variable on multiple time scales the overall pattern of the abyssal flow through the samoan passage was remarkably steady the time mean northward volume transport of about 5 4 sv 1 sv 106 m3 s 1 in 2012 13 was reduced compared to 6 0 sv measured between 1992 and 1994 this volume transport reduction is significant within 68 confidence limits 0 4 sv but not at 95 conf

late ordovician glaciation under high atmospheric co2 a coupled model analysis the analysis of the geologic record has revealed a question concerning how the late ordovician glaciation could have occurred simultaneously with high co2 levels 10 18x sensitivity studies using a coupled atmosphere ocean sea ice model show that it is possible to maintain a permanent snow cover which corresponds to 60 of all the glacial deposits found on gondwana under 10x co2 levels warm fall cool spring orbital parameters a 4 5 reduction in solar luminosity a length of day of 21 5 hours and an enhanced snow sea ice albedo of 0 3 a cold summer orbit experiment with 10x co2 and a reduced snow sea ice albedo of 0 1 also sustains a permanent albeit less extensive snow cover the geographic configuration of the late ordovician results in an up to 42 increase in the global ocean poleward heat transport in the southern hemisphere relative to present day and a significant asymmetry relative to the equator

aeolian sands in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin abstract aeolian sands occur widespread in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin middle europe in the eifel area cross bedded sands were mainly deposited by grainfall on lee slopes of barchanoid type dunes in unimodal wind regime in both low energy and high energy environments occasionally sets are internally truncated by reactivation surfaces crest height of solitary set dunes varies in the range of some meters locally sequences of cosets both of tabular planar and wedge planar type capped by flat or gently windward dipping topset beds represent larger dune complexes of about 10 20 m in height and at least 80 100 m in lateral extent horizontal laminated deposits originated as sand sheets in interdune areas the dune sands were deposited by trade winds of the northern hemisphere in low palaeolatitude predominantly by southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north aeolian sands built up an extensive dune belt in the eifel intersected by braided to anastomosing rivers fluviatile incursions or heavy ephemeral rainfall led to aquatic redeposition of aeolian sediments and origin of shallow lakes in interdune depressions distribution of aeolian and fluviatile sediments in karlstal schichten is different in the southern western and northern eifel middle buntsandstein sequence in the southern eifel reflects an evolution of the fluviatile depositional environment also incorporating the occurrence and distribution of aeolian sands evolution is characterized by increasing spacing and sinuosity of channels weakening supply of coarse detritus from the source areas and decreasing braiding of the river systems aeolian sands occur in the final phase of this evolution which led from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes and braided to anastomosing rivers matching with the climax of regional diversification of depositional milieu the aeolian sand sea in the eifel covered an area of nearly 2500 km2 at the time of its largest extension the dune belt ended in the northern eifel the alternating sequence passed into an entirely alluvial succession which graded northwards by continuously diminishing grain size into the central playa lacustrine to marine sediment series in northwest germany palaeogeography sedimentology and genesis of the middle buntsandstein at the western margin of the mid european basin and of rotliegendes in northwest germany appear to be partially similar as is revealed by certain characteristics of the palaeogeographical reconstructions concurrence of middle buntsandstein palaeogeographical models for both eifel and the whole german basin most clearly delineates the key position of the eifel for interpretation of the buntsandstein sedimentary environment and the palaeogeography in middle europe

topographic control of stratified flows upstream jets blocking and isolating layers optimal solutions to the nonlinear hydrostatic boussinesq equations are developed for steady density stratified topographically controlled flows characterized by blocking and upstream influence these flows are jet like upstream of an isolated obstacle and are contained within an asymmetric thinning stream tube that is accelerated as it passes over the crest a stagnant nearly uniform density isolating layer surrounded by a bifurcated uppermost streamline separates the accelerated flow from an uncoupled flow above the flows are optimal in the sense that for a given stratification the solutions maximize the topographic rise above the blocking level required for hydraulic control while minimizing the total energy of the flow hydraulic control is defined mathematically by the asymmetry of the accelerated flow as it passes the crest a subsequent analysis of the taylor goldstein equation shows that these sheared non uniformly stratified flows are indeed subcritical upstream critical at the crest and supercritical downstream with respect to gravest mode long internal waves the flows obtained are relevant to arrested wedge flows selective withdrawal stratified towing experiments tidal flow over topography and atmospheric flows over mountains

the impact of paleogeography pco2 poleward ocean heat transport and sea level change on global cooling during the late ordovician abstract we performed sensitivity experiments with the global climate model genesis on two stages of the upper ordovician caradocian 454 ma ashgillian 446 ma under a range of atmospheric p co 2 values 8 18 pal pre industrial atmospheric level high and low sea level and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling we then coupled a three dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation all simulations with a high sea level and normal heat transport remain free of ice sheets even with p co 2 levels as low as 8 pal in the caradocian simulations ice sheets form in three scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport 2 with p co 2 of 8 pal and a high sea level and reduced 50 of normal poleward ocean heat transport and 3 with p co 2 of 15 pal and a low sea level and reduced poleward ocean heat transport in the ashgillian simulations ice sheets form in only two scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport or 2 with p co 2 of 8 pal and a high sea level and reduced poleward ocean heat transport the ice sheets in the ashgillian experiments are larger and thicker than the ice sheets in the caradocian simulations because the southward movement of gondwana increased land area in the higher southern latitudes where ice sheets could grow the threshold for glaciation under ashgillian paleogeography is 8 pal and either a low sea level exposed shelves or a reduced poleward ocean heat transport while the paleogeographic evolution and a drop in p co 2 during the late ordovician cooled the global climate changes in additional factors were required to initiate ice sheet formation such as a drop in sea level a reduction in poleward ocean heat transport or a combination of both

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

evolution of fluvial sedimentation in the buntsandstein lower triassic of the eifel germany abstract the continental redbeds of the buntsandstein lower triassic in the western eifel germany are composed mainly of fluvial and aeolian sediments the terrestrial sequence reflects an evolution of fluvial depositional environments in the basisbildungen at the beginning of triassic sedimentation alluvial fans and mudflows distally intertounged with sandy braided rivers in local depressions of the pre triassic continent surface in the trifels schichten slow subsidence and rapid shifting of narrowly spaced channels in low sinuosity highly braided multi channel river systems often resulted in reworking of upper parts of cyclothems thus piling up thick sequences of stacked bar deposits in the rehberg schichten large flood plains developed due to increasing spacing and sinuosity of the channels and decreasing braiding of the river systems faster subsidence favoured preservations of predominantly complete cyclothems ideally terminated by thin aeolian sands in the karlstal schichten the climax of regional diversification of sedimentary environment matched the development of a large aeolian sand sea which was intersected by generally moderately braided rivers aeolian sands were deposited as barchanoid type dunes and interdune sheet sands and intertongued with fluvial channel and topstratum deposits at the top of the middle buntsandstein the violette grenzzone marks a hiatus in evolution resulting in recurrent change in fluvial style in the usch schichten coarse detritus accumulated in multi channel low sinuosity pebbly highly to moderately braided rivers rapid channel shifting and slow subsidence often led to erosion of topstratum deposits and palaeosols in the malbergweich schichten increase of channel spacing and sinuosity was accompanied by increasing subsidence sandy moderately to weakly braided streams and occasionally also meandering rivers deposited cyclic alluvial sediments at the beginning of the kyllburg schichten another minor recurrence interrupted the evolution channel braiding increased once more and rapid channel shifting accompanied by slower subsidence led to widespread erosion of overbank fines at the top of the kyllburg schichten sinuosity increased more and more and sandy moderately to weakly braided rivers graded into meandering streams in the voltziensandstein fluvial environment passed into a deltaic and or tidal milieu in front or at the margin of the prograding muschelkalk sea transgression of the shallow marine muschelkalk terminated the terrestrial period of the buntsandstein in the eifel evolution of fluvial sedimentary environment thus led in the middle buntsandstein from local alluvial fans via pebbly highly braided rivers to sandy moderately braided streams and finally to an intertonguing of aeolian dunes and multi channel rivers with regionally different degree of braiding tectonic activity in the source areas and climatic change at the beginning of the upper buntsandstein resulted in reversion to conditions existing in the inferior part of the middle

tidally forced lee waves drive turbulent mixing along the arctic ocean margins in the arctic ocean limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong however mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to arctic ocean state are poorly understood we present measurements from the shelf north of svalbard that show abrupt isopycnal vertical displacements of 10 50 m and intense dissipation associated with cross isobath diurnal tidal currents of 0 15 m s 1 energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water during a 6 hr turbulent event high frequency internal waves were present the full 300 m depth water column became turbulent dissipation rates increased by a factor of 100 and turbulent heat flux averaged 15 w m 2 compared with the background rate of 1 w m 2 plain language summary turbulent mixing in the arctic ocean water column affects sea ice variability through transport of subsurface heat toward the surface this turbulent mixing is concentrated along the margins mainly driven by tidal flow over sloping topography however processes of energy transfer from tides to turbulence in the arctic are poorly understood and the magnitudes and locations of mixing are poorly constrained here we present detailed measurements from the shelf north of svalbard showing a turbulent event driven by moderate tidal currents the energy is trapped and accumulated at the time of maximum downslope flow and is released at the turn of the tide when the entire water column becomes highly turbulent our observations imply that this process is an important source of mixing in the arctic ocean

turbulent mixing and hydraulic control of abyssal water in the samoan passage 1 we report the first direct turbulence observations in the samoan passage sp a 40 km wide notch in the south pacific bathymetry through which flows most of the water supplying the north pacific abyssal circulation the observed turbulence is 1000 to 10 000 times typical abyssal levels strong enough to completely mix away the densest water entering the passage confirming inferences from previous coarser temperature and salinity sections accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence specifically the flow accelerates substantially at the primary sill within the passage reaching speeds as great as 0 55 m s 1 a strong hydraulic response is seen with layers first rising to clear the sill and then plunging hundreds of meters downward turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill in addition to the primary sill other locations along the multiple interconnected channels through the samoan passage also have an effect on the mixing of the dense water in fact quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers suggesting that abyssal mixing depends sensitively on bathymetric details on small scales

the environments of deposition of the triassic main buntsandstein formation in the p and q quadrants offshore the netherlands interpretations of core and log data within the p and q quadrants were used to quantify the relative change in the distribution of environments of deposition within each of the five members of the main buntsandstein formation this triassic formation was deposited in a semi arid continental basin sediments reached the p and q quadrants in braided stream complexes from a hinterland to the southeast the braided stream complexes were flanked by aeolian interdune flood plain and crevasse splay environments and terminated in playa lakes the distribution and systematic arrangement of environments of deposition migrated subtly across the study area this is attributed to slight fluctuations in climatic conditions rainfall and windstrength or direction

pacific abyssal transport and mixing through the samoan passage versus around the manihiki plateau abstractthe main source feeding the abyssal circulation of the north pacific is the deep northward flow of 5 6 sverdrups sv 1 sv 106 m3 s 1 through the samoan passage a recent field campaign

fluvial conglomerates in continental red beds of the buntsandstein lower triassic in the eifel f r g and their palaeoenvironmental palaeogeographical and palaeotectonic significance abstract fluvial conglomerates occur in a variety of facies within the continental red bed sequence of the buntsandstein lower triassic in the eifel at the western margin of the mid european triassic basin the alluvial rudites are classified into channel floor lag channel bar channel top lag crevasse splay and floodplain lag conglomerates the conglomerates were predominantly deposited in scott type and in subordinate amounts also in donjek type and south sasketchewan type braided rivers the vertical distribution of clast size and frequency within the megacycles reflects the palaeotectonic sequence of uplift events in the source area resulting in supply of large amounts of coarse detritus which cause recurrences in fluvial style gradually upwards diminishing size and abundance of gravel indicate continuous lowering of source area relief during weakening endogenic activity or even tectonic quiescence after final peneplanation the palaeoenvironmental interpretation of the individual members and the palaeotectonic analysis of the conglomerate successions are combined with a reconstruction of the depositional history of the eifel buntsandstein which comprises an evolution of fluvial deposition the terminal stage of the palaeoenvironmental evolution is highlighted by a climax of regional diversification of conglomerate sedimentary milieu the horizontal distribution of clast size and abundance delineates the palaeogeographical evolution of the elongated eifel basin giving evidence of a progressive sidewards extension of the depositional area with time lateral supply from parts of the western margin and mixing of proximally derived coarse clasts with distantly delivered fine detritus result in highly inhomogeneous gravel distributions within the sedimentary region often not being characterized by gradually decreasing size and abundance of gravel in downstream direction the stepwise southwestwards shift of the marginal centre of supply of coarse gravel with time during the course of the lateral extension of the basin results in ascension of the fluvial conglomerate wedges into successively younger formations comprising consecutively middle buntsandstein upper buntsandstein lower muschelkalk middle muschelkalk and finally middle keuper the extraformational conglomerates mainly delineate the palaeogeographical framework whereas the intraclastic rudites primarily give evidence of processes operating within the depositional area reflecting erosion and reworking of overbank sequences with floodplain fines palaeosols and aeolian dune sands during lateral channel shift the brockelbank intraformational carbonate breccias to conglomerates originating from reworking of calcrete palaeosols are a peculiar facies element with particularly high palaeoenvironmental significance they document the frequency spatial extension and temporal persistence of pedogenesis within the alluvial plain the size of the extraformational clasts gives evidence of the range of current velocities during aggradation of the channels the distribution of conglomerate type and clast characteristics within cyclothems indicates the presence of stage fluctuations and variations in discharge in parts of the sequence the preservation of complete cyclothems or the multistoreying of channel bar conglomerates reflects the differential importance of the two main mechanisms controlling accumulation of sediments in the alluvial plain primary depositional restriction to suppression of formation and secondary erosional removal to destruction of finer grained channel sediments floodplain deposits and aeolian dune sands or palaeosols condensation of a series of depositional events within monotonous stacked conglomerate sequences obliterates the full range of sedimentary processes and mimics periods of long term stability of conglomerate formation waning flow mud drapes are particularly significant instruments to split up the individual genetical units of formation of fluvial conglomerates often comprising discrete flood phases separated by stage fluctuations of different magnitude the homogeneous or microcyclic composition of the coarse member reflects uniphase or multiphase aggradation of watercourses with gradually declining or abruptly waning individual flood periods the fluvial rudites thus are of high significance in the reconstruction of the depositional milieu in the eifel buntsandstein and provide a key to detailed interpretation of palaeoenvironmental processes palaeogeographical constellations and palaeotectonic events which could not have been identified to such an extent in exclusively sandy sequences

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

mixing and entrainment in the red sea outflow plume part ii turbulence characteristics abstract south of the strait of bab el mandeb saline red sea water flows downslope into the gulf of aden mainly along the narrow 130 km long northern channel nc and the shorter and wider southern channel sc in the nc the red sea plume simultaneously exhibited weak entrainment into a 35 120 m thick weakly stratified bottom layer while a 35 285 m thick interfacial layer above showed signs of vigorous mixing overturns up to 30 m thick and extensive zones of gradient richardson numbers below 1 4 turbulent overturning scales or thorpe scales are extracted from regular ctd profiles and equated to ozmidov scales on this basis interfacial mixing is quantified in terms of estimated turbulent dissipation rates vertical turbulent salt flux and interfacial stress even though these estimates are subject to significant uncertainty they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities k on the order of 10 2 m2 s 1 the large k occur in strong strati

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

surface pressure change through loewe s phenomena and katabatic flow jumps study of two cases in adelie land antarctica abstract preliminary results from the 1985 iago interaction atmosphere glace ocean field programme in adelie land antarctica allow for a detailed description of several katabatic events especially of their vertical structure and time and space variations several loewe s phenomena where a sudden transition from shooting to tranquil flow takes place have been observed two of them have been well documented and are shown here for both cases as observed at a downstream station close to the coast the mean wind speed suddenly decreased from about 20 m s 1 to almost zero while a large pressure increase was recorded 5 7 hpa on 3 december 1985 and 2 1 hpa on 18 december 1985 this paper aims at explaining such large surface pressure changes as more usual approaches cannot for both cases it is found that the flow is stratified upwards with a surface well mixed cold air layer a very stable capping inversion layer an overlying unstable layer thickening from upstream to downstream and a stable transi

intense mixing of antarctic bottom water in the equatorial atlantic ocean the spreading of antarctic bottom water the densest global scale water mass is highly constrained by ocean floor topography in the atlantic ocean the mid atlantic ridge confines this water mass mainly to the western basins the bottom waters in the eastern basins being renewed by flows through gaps in the ridge1 one such gap is the romanche fracture zone a large offset of the ridge which straddles the equator it has been observed2 that sills within this fracture zone block the passage of waters colder than 0 9 c warmer less dense waters passing over the sills appear to cascade downslope where they are modified by mixing here we present direct measurements which quantify these processes the flow is vertically sheared and exhibits remarkably intense turbulence comparable to that seen at the ocean surface in the presence of winds of l0ms 1 this turbulence mixes the densest waters passing through the fracture zone with the warmer overlying waters so that the coldest waters exiting this region have been warmed by 0 6 c during transit topographic obstructions and turbulent mixing together thus determine the properties of the flows renewing the deepest waters of the atlantic ocean s eastern basins

general circulation model sensitivity experiments with pole centered supercontinents although high co2 levels appear to be required to explain most times of ice free climates in earth history previous calculations with a two dimensional energy balance model ebm have suggested that the low thermal inertia associated with pole centered supercontinents might allow seasonally snow free conditions to develop in the absence of high co2 this conclusion has been challenged by other ebm calculations with lower geographic resolution but more explicit model physics to address this problem more directly we have undertaken a series of general circulation model gcm simulations with a new model genesis developed at the national center for atmospheric research for idealized pole centered supercontinents we obtain mid summer south polar temperatures of 22 c for the present orbital configuration and 15 c with a modified orbital configuration favorable to cool summers these results support the two dimensional ebm calculations however their application to real world situations of the paleozoic is hampered by the fact that due to extremely high sea levels and decreased solar luminosity the best time periods for applying this concept 420 360 ma have supercontinent areas too small to yield an ice free climate for all orbital configurations snow free conditions with summer temperatures at the pole of 25 c are simulated for a hot orbital configuration for one realistic geography configuration carboniferous 300 ma these results indicate that while the basic idea of snow free pole centered supercontinents is still theoretically possible the real world application to specific past time periods requires some modifications our reframed hypothesis states that for realistic geography and high co2 levels pole edged supercontinents could have permanent snow cover but pole centered supercontinents would be snow free in summer

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change

genesis of the buntsandstein lower triassic in the western eifel germany abstract the buntsandstein lower triassic of the western eifel germany is composed of sediments of different origin these are alluvial fan fluviatile aeolian lacustrine and deltaic deposits the fluviatile deposits are subdivided into channel lag bar and topstratum deposits alluvial fan deposits occur in local depressions of the pre triassic morphology aeolian sediments build up the main part of the middle buntsandstein locally interfingering with fluviatile deposits paleowind and paleocurrent directions generally coincide the upper buntsandstein consists of fluviatile sediments only which can be subdivided into well developed small cyclothems average thickness of sedimentation units 3 8 m capped by paleosoils the uppermost buntsandstein is of deltaic origin the ingression of the lower muschelkalk sea terminates the terrestrial buntsandstein sequence