glacial and syntectonic sedimentation the upper proterozoic kingston peak formation southern panamint range eastern california a sedimentological stratigraphic study of the upper proterozoic kingston peak formation in the southern panamint range shows that it was deposited under glacial conditions with contemporaneous volcanism and tectonic activity evidence for glaciation rests primarily upon 1 the homogeneity thickness and lateral extent of two diamictite units 2 the facies association of the diamictite and 3 presence of striated stones and dropstones within the formation elsewhere in the death valley area in the panamint range pillowed basalt interbedded with diamictite demonstrates synchronous subaqueous volcanism lower kingston peak units rest on a variable substrate and locally overlap faults indicating tectonism prior to deposition tectonism during kingston peak deposition is inferred from abrupt thickness changes and buried faults sedimentation was chiefly on a submerged continental platform and locally terrestrial during a period of incipient rifting two ice advances are recorded with associated sea level fluctuations the formation thickens northward from 40 to about 1 200 m over 40 km initial sediments were fine grained and in basins a few kilometres across nearby islands provided coarse debris overlying sandstone and conglomerate indicate regression predominantly massive diamictite as much as 450 m thick and interpreted as lodgment till or glacioma rine sediment records the first ice advance a northward increase in diamictite thickness decrease in clast size and facies change from diamictite to argillite and graywacke suggest a southern source overlying laminated limestone marks a transgression succeeding interbedded sandy limestone thick bedded graywacke and parallel laminated siltstone and sandstone double in thickness over a few kilometres demonstrating local subsidence and renewed terrigenous input trough cross laminated arkosic sandstone and conglomerate overlain by predominantly massive diamictite as much as 190 m thick with a locally erosive base represent glaciofluvial deposits and lodgment till and record the second ice advance

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

microstructural interpretations of modern and pleistocene subglacially deformed sediments the relative role of parent material and subglacial processes analysis of till micromorphology represents a relatively new technique that has been used most frequently to infer the importance of subglacial shear in till genesis this study aims to calibrate the technique by comparing pleistocene tills from united kingdom with a modern till the upb till from beneath ice stream b west antarctica despite the fact that all of the tills examined have been interpreted as deforming bed deposits the modern till has significantly less abundant and diverse microstructures than those found in the pleistocene tills seventeen examined thin sections of the upb till contain recognisable microstructures over only 0 30 of individual thin section area the most common microstructures are i birefringent clay patterns that are interpreted as shear zones and ii adherent matrix structures which we interpret as uncomminuted remnants of the parent glacial glaciomarine diamictons fourteen thin sections of the pleistocene tills were covered by microstructures in 10 95 of their area the pleistocene microstructures include birefringent clays and adherent matrix structures as in the upb till but also laminations and deformed pods made of chalk and sorted sediments we conclude that the same till forming process i e subglacial deformation may result in distinctly different till micromorphology this is a consequence of the fact that microstructural characteristics are strongly influenced by factors other than shear deformation we identify three controls that may be important for forming contrasting microstructural assemblages i strain magnitude ii the degree of heterogeneity of parent material and iii relative importance of sediment sorting by flowing water in the subglacial environment thus till micromorphology is sensitive to multiple factors which with proper calibration may enhance the existing capability to interpret past subglacial conditions from microstructures copyright 2001 john wiley sons ltd

the zonal currents and transports at 35 w in the tropical atlantic the total of 13 existing cross equatorial shipboard current profiling sections taken during the woce period between 1990 and 2002 along 35 w are used to determine the mean meridional structure of the zonal top to bottom circulation between the brazilian coast near 5 s and 5 n and to estimate mean transports of the individual identified shallow intermediate and deep current branches one of the results is that on the equator a mean westward equatorial intermediate current below the equatorial undercurrent exists

the cryogenian record in the southern kingston range california the thickest death valley succession in the hunt for a gssp abstract the kingston peak formation of the death valley area california allows valuable insight into both regional cordilleran stratigraphy and the number of glacial cycles preserved in the cryogenian record in the kingston range the eponymous strata have been previously interpreted to record both sturtian and marinoan pan glacial events in the context of a search for a global boundary stratotype section and point gssp for the cryogenian we provide the first detailed description of the thickest diamictite bearing interval in the western usa two clast poor muddy diamictite intervals within the succession one at the base and one near the top have been used to support sturtian and marinoan events previously however new data from the southern part of the kingston range suggest that the upper diamictite interval is genetically related to underlying strata the deposits are interpreted as glaciogenic debris flow deposits which probably represent the proximal tract of a subaqueous fan medial to distal portions of this fan are dominated by turbidites which were transported down a consistent se oriented palaeoslope lowermost beds of the upper diamictite interval are intercalated with graded sandstones and sandy matrix supported conglomerates the graded beds turbidites and matrix supported conglomerates debrites testify to a subaqueous setting with the compositionally and texturally distinct diamictites indicating a glacial origin

cap carbonate platform facies model noonday formation se california the neoproterozoic outcrop belt of the death valley region california preserves an oblique cross section of the noonday formation a mixed carbonate siliciclastic platform that hosts distinctive basal ediacaran cap carbonate affiliated sedimentary structures stromatolite textures and 13 c carb values the noonday platform encompasses two depositional sequences that reveal two cycles of relative sea level change within strata conventionally considered to record a single rapid postglacial sea level rise in updip localities facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite bearing tubestone textured reef rimmed platform downdip localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge base level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef overlying strata record the backfilling of a submarine incised valley and reestablishment of a back stepping carbonate dominated ramp prior to a second subaerial exposure event that defines the contact between the noonday and johnnie formations we address the relative contributions of syndepositional tectonism and recovery from low latitude deglaciation in dictating noonday platform architecture and the intra noonday formation sequence boundary noonday formation deposition coincided with extension of the laurentian margin during disaggregation of the rodinian supercontinent within this framework previous work has suggested that the intra noonday formation sequence boundary records growth faulting that reinforced differential topography uplifting reef rimmed horsts exposing the reef crest to karstic dissolution and downdropping grabens however we trace the intra noonday formation sequence boundary seaward of the reef crest and demonstrate that for a time wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks thus if the noonday margin were undergoing extension then the creation of the intra noonday formation sequence boundary required a concomitant decrease in accommodation due perhaps to postglacial isostatic uplift attendant with low latitude deglaciation we speculate that noonday formation sequence architecture records 1 immediate deglacial flooding 2 shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice sheet collapse against a background of global deglaciation and 3 resumed flooding following complete deglaciation as rift related tectonism could amplify or counter glacial isostasy inferences of the amplitude of local postglacial sea level change will require robust estimates of syndepositional extension across the noonday margin

equatorial subthermocline currents across the pacific recent sections crossing the equator at nine longitudes from 146 e to 86 w show zonal currents below 400 m similar to those found in a 16 month mean section on 159 w from the pacific equatorial ocean dynamics pequod program in 1982 1983 most of the new measurements were made with a lowered acoustic doppler current profiler eastward current extrema north and south intermediate countercurrents are found in all sections about 2 from the equator in the depth range 500 1500 m with the possible exceptions of 146 e where topography complicates the picture south of the equator and 150 e where the sections extend only from 1 s to 2 n poleward of the intermediate countercurrents westward flow is found near 3 from the equator in most of the sections on the equator from 250 500 m depth a westward equatorial intermediate current is found in many but not all of the sections similarly it is present in the pequod mean but not in all synoptic sections below the intermediate currents and countercurrents an eastward current near 3000 m was found at and south of the equator in the pequod mean and in the new sections from 150 e to 110 w near 4000 m there is westward flow south of the equator in the pequod mean and near the equator in the new sections from 179 e to 135 w geostrophic currents calculated from an 8 year mean hydrographic section at 165 e also resemble the pequod mean although not conclusive the evidence presented here indicates that these currents are basin scale components of the general circulation perhaps involving vigorous horizontal recirculation in a set of basin wide elongated gyres within a few degrees of the equator

contrasting sources for glacial and interglacial shelf sediments used to interpret changing ice flow directions in the larsen basin northern antarctic peninsula abstract marine sediment cores from the former larsen a ice shelf area reveal three lithological units deposited during ice sheet advance and retreat from the continental shelf the uppermost unit 1 consists of a diatom bearing silty clay with sparse ice rafted debris ird clasts include rock types which can all be matched to onshore outcrops along the eastern side of the antarctic peninsula most clasts are very angular angular or subangular the clasts in this unit are interpreted as supraglacial or englacial debris deposited below an ice shelf distal from the grounding line and or by icebergs in an open marine setting during the holocene in the underlying units 2 and 3 clast lithologies are quite different and mainly comprise cretaceous and jurassic sedimentary rocks these lithologies were derived locally from the continental shelf the larsen basin rather than the limited onshore outcrops of the 2750 clasts studied in units 2 and 3 most are subrounded or subangular and some are striated unit 2 is interpreted as being deposited beneath an ice shelf proximal to the grounding line unit 3 is interpreted as subglacial diamict deposited by grounded ice flowing across the shelf during the last glacial period the availability of sedimentary rocks at the seabed may have facilitated the development of a deforming till layer with low shear strength i e soft or deformation till within unit 3 there are no consistent differences in clast composition or roundness between the deformation till which was probably deposited at the base of an ice stream and an underlying till with high shear strength i e stiff till on the inner shelf where the bedrock lies very close to the seabed clast composition in the subglacial diamicts affords a way of identifying the presence of mesozoic stratigraphic units the contrasting provenance of the coarse fraction between unit 1 and units 2 and 3 is used to interpret ice sheet basal thermal regime and produce a conceptual model of changing palaeo flow directions between glacial and interglacial periods grain roundness and petrology are shown to be a potent method for interpreting different glacial interglacial processes and environments

zonal jets in the equatorial atlantic ocean abstract we use position data from argo floats smoothed out over 400 km 200 km zonal ellipses and interpolated onto a 0 5 grid to investigate the zonal jet structure of the flow field at the sea surface and on three subsurface layers central waters cw 200 m antarctic intermediate waters aaiw 1000 m upper north atlantic deep waters unadw 1500 m in the equatorial atlantic ocean 15 s to 15 n the annual mean fields exhibit narrow zonal jets typically 4 5 wide at the sea surface and only 2 at the subsurface levels with directions alternating in latitude and maximum speeds about 0 5 m s 1 at the surface 0 1 m s 1 at cw and unadw and 0 03 m s 1 at aaiw the available data also allows us to explore the seasonal variability of these jets at the surface and aaiw levels the surface currents are dominated by an annual cycle between 4 n and 10 n and to a lesser degree by a semi annual contribution close to the equator this variability is an outcome of evolving zonal recirculations with the north equatorial countercurrent necc arising from the diversion of the northern branch of the south equatorial current nsec the diversion begins in the eastern atlantic and propagates west between april and august following the inter tropical convergence zone itcz the aaiw current field is largely affected by westward propagating anomalies most visible near 3 s 0 3 n and 7 n which give rise to current reversals annual averaging produces the illusion of more 5 instead of 3 and slower peak values about 0 03 m s 1 instead of 0 1 m s 1 jets than found on any month

the bouma division a and the possible duration of turbidity currents abstract experimental data arnott and hand 1989 combined with theoretical arguments suggest that the transition in a turbidite from division a to division b occurs at a deposition rate of unsupported grains of about 6 7 10 4 m s 1 in the case of fine medium quartz sand a second rate may be calculated from properties measured at the transition from division b to division c allen 1971b these rates together permit an estimate to be made of the order of magnitude of the duration of deposition from a turbidity current the calculated order of a few tens of minutes for a mio pliocene turbidite described and interpreted hydraulically by komar 1985 is in broad agreement with his independent estimate based on the sequenc of sedimentary structures

cyclic cold climate during the nantuo glaciation evidence from the cryogenian nantuo formation in the yangtze block south china abstract geological and paleomagnetic data indicate that the ice sheets might have extended to low latitude regions in the cryogenian 720 635 ma the snowball earth hypothesis proposed that the earth was completely ice covered during global glaciation however the glacial sedimentary record seems to contradict with the snowball earth hypothesis detailed sedimentological investigations of the glacial deposits would provide the first order constraint on the nature of global glaciation the nantuo formation 654 635 ma in the yangtze block of south china has been correlated with the marinoan snowball earth in this study we conducted systematic sedimentological analyses of six sections cores of the nantuo formation three facies associations were recognized the proximal glaciomarine distal glaciomarine and non glacial marine facies associations the vertical stacking pattern of facies associations can be correlated among the five slope and basin sections while their correlation with the shelf section remains obscure our data indicate two episodes of glaciation that are separated by an interglacial interval during the nantuo glaciation the first glacial episode is recorded by successions of coarse grained facies e g massive diamictite in the lower part of the nantuo formation the re appearance of massive diamictite in the middle to upper part of the nantuo formation indicates onset of the second glacial episode these two glacial episodes were separated by a siltstone shale sequence of several 10 s m thick suggesting an interglacial period with limited influence from glaciation the top of nantuo formation consists of alternative distal glaciomarine and non glacial marine facies associations representing the deglacial sequence of the nantuo glaciation the sedimentary facies analysis indicates that the cold climate during the nantuo glaciation may be cyclic finally our sedimentary analysis confirms a lag between the deglaciation and precipitation of cap carbonate

sedimentation associated with antarctic peninsula ice shelves implications for palaeoenvironmental reconstructions of glacimarine sediments abstract recent disintegration of a number of antarctic peninsula ice shelves has given us a unique opportunity to investigate sub ice shelf sediments we characterize three sediment facies associations of two antarctic peninsula ice shelves larsen a and larsen inlet subglacial facies consist mainly of basal till diamicton with high shear strengths deposited under a grounded ice sheet progressive upward decrease in shear strength reflects a gradual decrease in the confining vertical effective pressure of grounded ice during till deposition proximal ice shelf glacimarine facies diamicton gravel rich and sand rich facies gravelly mud dropstone mud and sandy muds were deposited by sub ice shelf rain out bottom current activity and sediment gravity flows following decoupling of grounded ice from the sea floor distal ice shelf glacimarine and or open marine facies comprise terrigenous and diatom bearing bioturbated muds and gravelly muds that contain limited ice rafted debris these accumulated after recession of the grounding line to the coast with coarse grained surface sediments possibly documenting most recent ice shelf break up antarctic peninsula ice shelf sediments are more heterogeneous than ice shelf facies deposited elsewhere in antarctica cold polar antarctic ice shelves can be differentiated from temperate and sub polar marine terminated glacial sedimentary systems by the dominance of coarse grained proximal ice shelf glacimarine facies and an absence of subaqueous outwash meltwater sediment facies

glaciers flows and fans origins of a neoproterozoic diamictite in the saratoga hills death valley california abstract the kingston peak formation of death valley california provides an exceptional archive of cryogenian glaciation and concomitant rifting of rodinia in the saratoga hills an 800 m thick succession of diamictite dominated strata is exposed allowing lithofacies and clast compositions to be studied in detail and for the relative influence of glacial versus slope processes on sedimentation to be critically assessed for the first time two detailed sections 400 m apart allow four facies associations to be established 1 thinly laminated argillaceous sandstone interpreted as low density turbidites 2 laminated deformed and brecciated limestone interpreted as carbonate sourced turbidites 3 bedded massive diamictite interpreted as glacially fed debrite and 4 interbedded mudstone sandstone and diamictite interpreted as stratified diamictite deposited via ice rafted debris and passing gradually into turbidites separated by intensely deformed intervals bedded massive diamictite is dominant in the succession and shows a gradual up section transition into interbedded mudstone sandstone and diamictite this succession is interpreted to record a waning glacial influence from glacially sourced debris flows to stratified diamictite strongly influenced by ice rafted debris clasts in the diamictite include carbonate siliciclastic intraclasts granite diabase gneiss and quartzite clast presence profiles suggest that in spite of the comparable along strike facies profiles between the two logs clast content is significantly different within the same stratigraphic unit this finding suggests that clast content is of little use in aiding correlation and lithostratigraphic subdivision of the kingston peak formation the differences in clast compositions is envisaged to be due to complex drainage network examinations of the sedimentary structures and stacking pattern of the lithofacies complex drainage network and localized deformation features of fa4 all support deposition as part of a trough mouth fan system

oxygen tongues and zonal currents in the equatorial atlantic 1 equatorial zonal currents and associated oxygen distributions are studied using shipboard hydrographic data trajectories from isopycnic floats drifting at about 300 m depth and velocity time series from the upper 1100 m obtained at two equatorial moorings located at 35 w and 23 w mean profiles of zonal velocity measured by moored acoustic doppler current profilers yielded a westward flowing equatorial intermediate current eic below the equatorial undercurrent euc at both locations the eic consists of two westward current cores at about 250 and 450 m the upper core of the eic deepens by about 30 m from 23 w where it has a mean velocity of 6 2 cm s 1 to 35 w where the mean is 5 3 cm s 1 the lower core of the eic is about twice as strong with 12 1 cm s 1 at 23 w and 9 2 cm s 1 at 35 w the flow below the euc is characterized by substantial interannual variability from may to december 2005 a strong zonally coherent eastward jet occurred at 300 to 350 m depth found to be an expression of shallow stacked jets superimposed on the mean eic shipboard hydrographic observations in june july 2006 revealed the existence of a high oxygen tongue that can be traced from 35 w to 10 w in the depth range of the eastward jet prevailing during the preceding year on the basis of an advection diffusion balance it is suggested that the oxygen decrease from 35 w to 10 w within the oxygen tongue is mainly balanced by lateral eddy diffusivity and oxygen consumption with diapycnal turbulent diffusivity playing only a minor role

drifters reveal deep circulation circulation in the north atlantic at depths of 500 2 000 metres is largely fed by water from the labrador sea data from an armada of subsurface floats provide the first direct view of the flow of this water

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

1 08 ga diabase sills in the pahrump group california implications for development of the cordilleran miogeocline u pb baddeleyite ages of 1087 3 and 1069 3 ma for twodiabase sills that intrude the crystal spring formation of the pahrump group southeastern california provide a minimum estimate for the depositional age of the crystal spring formation the new ages high light the potential range in deposition ages for the pahrump group from 1087 ma to possibly as young as 600 ma for kingston peak glaciogenic sediments consequently there probably are significant disconformities within the pahrump group that separate unrelated tectonically distinct episodes of basin subsidence the crystal spring and beck spring formations are considered to be platform cover rocks preserved as a result of block faulting during deposition of the upper kingston peak formation the age of the pahrump sills is in excellent agreement with the age of sills that intrude the apache group central arizona and the unkar group grand canyon and supports earlier correlations of those units

subglacial meltwater supported aerobic marine habitats during snowball earth the earth s most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the cryogenian period 720 to 635 ma aerobic eukaryotes must have survived the snowball earth glaciations requiring the persistence of oxygenated marine habitats yet evidence for these environments is lacking we examine iron formations within globally distributed cryogenian glacial successions to reconstruct the redox state of the synglacial oceans iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice covered oceans but increasing oxidation with proximity to the ice shelf grounding line we propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans creating glaciomarine oxygen oases the confluence of oxygen rich meltwater and iron rich seawater may have provided sufficient energy to sustain chemosynthetic communities these processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations

role of eddy forcing in the dynamics of multiple zonal jets in a model of the north atlantic multiple zonal jets are observed in satellite data based estimates of oceanic velocities float measurements and high resolution numerical simulations of the ocean circulation this study makes a step toward understanding the dynamics of these jets in the real ocean by analyzing the vertical structure and dynamical balances within multiple zonal jets simulated in an eddy resolving primitive equation model of the north atlantic in particular the authors focus on the role of eddy flux convergences eddy forcing in supporting the buoyancy and relative potential vorticity pv anomalies associated with the jets the results suggest a central role of baroclinic eddies in the barotropic and baroclinic dynamics of the jets and significant differences in the effects of eddy forcing between the subtropical and subpolar gyres additionally diabatic potential vorticity sources and sinks associated with vertical diffusion are shown to play an important role in supporting the potential vorticity anomalies the resulting potential vorticity profile does not resemble a pv staircase a distinct meridional structure observed in some idealized studies of geostrophic turbulence

paleotectonic and stratigraphic implications of the kingston peak noonday contact in the panamint range eastern california geologic mapping in parts of the southern panamint range eastern california has documented critical stratigraphic relationships at the upper proterozoic contact between the kingston peak formation and noonday dolomite these relations bear significantly upon the timing and style of development of the latest proterozoic and paleozoic continental margin of western north america they include 1 an unconformity below wildrose diamictite interpreted as glaciogenic the uppermost kingston peak unit 2 diamictite similar to wildrose diamictite interbedded within carbonate strata of the overlying basal noonday dolomite and 3 interfingering between rocks of the kingston peak and noonday formations the kingston peak noonday contact is thus complex and variable in character and does not represent a regional unconformity instead it marks a transition from rift to post rift sedimentation without any hiatus and shows there was no regional doming during rifting the stratigraphic relations also demonstrate

ice shelf history from petrographic and foraminiferal evidence northeast antarctic peninsula a detailed record of late pleistocene deglaciation followed by mid holocene ice shelf breakup and late holocene re growth is contained in continental shelf sediments in the northern larsen area northeast antarctic peninsula the zero age of core tops is confirmed by new and published 210pb profiles and 70 accelerator mass spectrometer ams 14c dates on bulk organic carbon define sedimentation rates of 7 6 92 cm ka the varied geology in the local ice drainage basins facilitates the use of ice rafted debris ird provenance in determining the presence or absence of ice shelves all inshore cores contain an interval of non local ird in the post glacial section demonstrating widespread ice shelf breakup in the mid holocene both breakup and re growth may have taken centuries and there are no widespread debris layers associated with breakup cores beyond and up to 30 km inside the historical ice shelf limit exhibit a varied ird provenance throughout the holocene suggesting the maximum ice shelf limit may date only from the little ice age benthic foraminiferal assemblages are related to water masses and position on the continental shelf and have been modified by taphonomic processes nevertheless we discern a deglaciation signal in prince gustav channel of a calcareous spike in predominantly agglutinated assemblages and this is repeated at the time of mid holocene ice shelf breakup the inferred mid holocene warm period occurred later in the northern larsen area than on the west coast of the antarctic peninsula

architectural analysis of a volcaniclastic jokulhlaup deposit southern iceland sedimentary evidence for supercritical flow the 1918 eruption of the glacially capped katla volcano southern iceland generated a violent jokulhlaup or glacial outburst flood inundating a large area of m rdalssandur the proglacial outwash plain where it deposited ca 1 km3 of volcaniclastic sediment the character of the 1918 jokulhlaup is contentious having been variously categorized as a turbulent water flow a hyperconcentrated flow or as a debris flow based on localized outcrop analysis in this study outcrop based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi stationary antidunes and down current migrating antidunes and from regular based bedsets associated with transient chute and pool bedforms all of which are associated with turbulent transcritical to supercritical water flow conditions antidune wavelengths range from 24 to 96 m corresponding to flow velocities of 6 to 12 m sec 1 and average flow depths of 5 to 19 m this range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts architectural analysis of the 1918 jokulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jokulhlaup that could not have been achieved through localized outcrop analysis the observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions the physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations regarding modes of deposition and formative flow hydraulics are to be made

generation of the north equatorial undercurrent jets by triad baroclinic rossby wave interactions abstractformation processes responsible for the north equatorial undercurrent neuc jets that appear across the tropical north pacific ocean near 9 13 and 18 n are explored both theoretically and using numerical models with different complexities analyses of an eddy resolving global ocean general circulation model output reveal that the neuc jets have a mode 1 baroclinic vertical structure and are spatially persistent on the interannual and longer time scales this ogcm simulated vertical structure prompts the authors to adopt the simpler nonlinear layer reduced gravity model as well as the baroclinic rossby wave triad interaction theory to unravel the essential processes underlying the neuc jets the seed for the neuc jets originates in annual baroclinic rossby waves driven by the large scale surface wind stress forcing emanating from the ocean basin s eastern boundary these wind forced primary waves are subject to nonlinear triad interactions and break down offshore where the e folding tim

the paleoclimatic significance of deformation structures in neoproterozoic successions abstract this paper reviews the different types of soft sediment deformation structures that can form in glacial and non glacial settings and explores the potential use of these structures in resolving long standing debates in paleoenvironmental reconstructions of neoproterozoic glacigenic successions soft sediment deformation structures are created when compressional gravitational or shear stress is applied to unlithified sediments during or shortly after deposition in subglacial or ice marginal glacial settings shear and compressional stress imparted by ice moving on top of a deformable substrate or advancing ice buldozing unlithified ice marginal sediments can result in a wide range of folding faulting and shear structures in glaciofluvial or stagnant ice marginal setting gravitational collapse and remobilization of sediments associated with the melting of buried ice can result in normal faulting and broad folding in glaciolacustrine or glaciomarine settings compressional shear and gravitational types of deformation structures can occur as a result of grounding ice or icebergs rapid sedimentation and reworking downslope associated with high sedimentation rates in non glacial settings similar deformation structures can form as a result of slope instability and reworking of sediments downslope rapid sedimentation seismic shaking wave induced shearing or loading in this context two case studies are presented to demonstrate the type of paleoenvironmental information that an analysis of deformation structures can provide in the first case study analysis of deformation in the port askaig formation scotland reveals a distinctive stratigraphic distribution of deformation structures the types of deformation observed together with their recurrence over several 100s of meters and their basinal context are used to infer a seismic origin for the deformation which in turn suggests a significant tectonic control on sedimentation atop a record of ice margin fluctuations in a glaciomarine setting in the second example analysis of deformation in the smalfjord formation northern norway provides strong evidence for deformation by active ice overriding glaciofluvial deposits the types of deformation in this example together with its complexity scale and associated facies provide the strongest case for ice marginal deformation in sum analysis of deformation structures together with analysis of structural geology stratigraphy facies and facies associations can provide additional constraints on paleoenvironmental conditions at the time of deposition which can help us refine or test paleoenvironmental models proposed for this critical time period in earth history

sea ice free conditions during the sturtian glaciation early cryogenian south australia in the neoproterozoic snowball earth hypothesis shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations in the central flinders ranges south australia the yudnamutana subgroup of sturtian age includes diamictite sandstone and siltstone units of glaciomarine origin as much as 5000 m thick and is overlain by postglacial transgressive siltstone and shale of the tindelpina shale member tapley hill formation in the central flinders ranges the yudnamutana subgroup consists of 1 the pualco tillite gravity resedimented glacial deposits 2 the holowilena ironstone glacioturbidites 3 poorly stratified pebbly diamictite of the warcowie dolomite member lowermost wilyerpa formation gravity resedimented glacial deposits succeeded by 4 siltstones and sandstones with abundant hummocky cross stratification hcs storm deposits and 5 a lonestone bearing succession with cobble sized clasts in the upper wilyerpa formation ice rafted debris interpreted to record a glacial re advance in which hcs is absent because the action of oscillating waves is required to produce hcs on the seafloor its presence indicates an interval of significant meltback prior to glacial re advance given that the hcs occurs 2 km beneath the tindelpina shale member it signifies a major ice free interval during the sturtian glaciation

changes in the ventilation of the oxygen minimum zone of the tropical north atlantic changes in the ventilation of the oxygen minimum zone omz of the tropical north atlantic are studied using oceanographic data from 18 research cruises carried out between 28 5 and 23 w during 1999 2008 as well as historical data referring to the period 1972 85 in the core of the omz at about 400 m depth a highly significant oxygen decrease of about 15 mol kg 1 is found between the two periods during the same time interval the salinity at the oxygen minimum increased by about 0 1 above the core of the omz within the central water layer oxygen decreased too but salinity changed only slightly or even decreased the scatter in the local oxygen salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and or zonal jets acting on the background gradients here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the omz contribute to the ventilation of the omz a conceptual model of the ventilation of the omz is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the omz according to the model a weakening of zonal jets which is in general agreement with observed hydrographic evidences is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the north atlantic omz

micromorphology of a debris flow deposit evidence of basal shearing hydrofracturing liquefaction and rotational deformation during emplacement abstract micromorphological analysis has been applied to a very coarse debris flow deposit and associated sand dominated glaciofluvial sediments exposed in westend wood quarry carstairs central scotland the microstructures present folds faults plasmic fabrics within this diamicton and underlying predominantly sandy sediments are interpreted in terms of sedimentary processes active during emplacement of the debris flow these processes include the formation of a basal shear zone the deformation and mixing of fine grained sediments incorporated into the base of overriding debris flow features associated with liquefaction sediment remobilisation and water escape are also present and developed in response to increasing overburden pressure during debris flow emplacement these microstructures have also been recorded elsewhere within subglacial diamictons and an evaluation is made between microstructures in palaeo debris flow deposits and those found in modern debris flows

subthermocline and intermediate zonal currents in the tropical pacific ocean paths and vertical structure abstractthe mean subthermocline and intermediate zonal circulation in the tropical pacific is investigated using a compilation of shipboard adcp measurements and absolute geostrophic velocities constructed from a high resolution 0 2000 m argo climatology referenced to a 1000 m velocity field derived from argo float drifts this reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3 in regions where the sampling of sadcp data is sufficient the consistency between the two independent datasets is striking using the argo drift reference is crucial to capture the current structures two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10 a series of low latitude subthermocline currents llscs below the thermocline extending from about 200 to 800 m including the eastward tsuchiya jets and a series of low latitude intermediate currents llics extending from about 700 to at least 2000

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

oxygen minimum zone in the north atlantic south and east of the cape verde islands the open ocean oxygen minimum zone omz south and east of the cape verde islands is studied from ctd hydrography adcp velocities argo float trajectories and historical data with a focus on the zonal supply and drainage paths the strongest oxygen minimum is located north of the north equatorial countercurrent necc at about 400 to 500 m depth just above the boundary between central water and antarctic intermediate water aaiw it is shown that the necc the north equatorial undercurrent at 4 to 6 n and a northern branch of the necc at 8 to 10 n are the sources for oxygen rich water supplied to the omz in summer and fall a weak eastward necc at 200 m depth also exists in winter and spring as derived from argo floats drifting at shallow levels historical oxygen data from 200 m depth confirm this seasonality showing high low oxygen content in summer and fall spring within the supply paths compared to the strong oxygen supply at 150 to 300 m depth the ventilation of the omz at 300 to 600 m depth is weaker westward drainage of oxygen poor water takes place north of the guinea dome i e north of 10 n most pronounced at 400 to 600 m depth in july 2006 the total eastward transport of both necc bands above 27 1 kg m 3 at 23 w was about 13 sv 1 sv 106 m3 s 1 about half of this water volume circulates within the guinea dome or recirculates westward north of the guinea dome

quasi stationary north equatorial undercurrent jets across the tropical north pacific ocean 1 subthermocline circulation in the tropical north pacific ocean 2 n 30 n is investigated using profiling float temperature salinity data from the international argo and the origins of the kuroshio and mindanao current okmc projects three well defined eastward jets are detected beneath the wind driven westward flowing north equatorial current dubbed the north equatorial undercurrent neuc jets these subthermocline jets have a typical core velocity of 2 5 cms 1 and are spatially coherent from the western boundary to about 120 w across the north pacific basin centered around 9 n 13 n and 18 n in the western basin the neuc jet cores tend to migrate northward by 4 in the eastern basin vertically the cores of the southern central and northern neuc jets reside on the 26 9 27 2 and 27 3 surfaces respectively and they tend to shoal to lighter density surfaces by about 0 2 as the jets progress eastward

intermediate zonal jets in the tropical pacific ocean observed by argo floats abstractargo float data in the tropical pacific ocean during january 2003 august 2011 are analyzed to obtain lagrangian subsurface velocities at their parking depths maps of mean zonal velocities at 1000 and 1500 m are presented at both depths a series of alternating westward and eastward zonal jets with a meridional scale of 1 5 is seen at the basin scale from 10 s to 10 n these alternating jets with mean speeds about 5 cm s 1 are clearly present in the western and central parts of the basin but weaken and disappear approaching the eastern coast they are stronger in the southern hemisphere along the equator at both 1000 and 1500 m a westward jet is seen the jets closer to the equator are remarkably zonally coherent across the basin but the jets farther poleward appear broken in several segments in the western half of the basin the 1000 m zonal jets appear to slant slightly poleward from east to west at the western boundary in the south east of solomon islands and papua new guinea the al

tectonic controls on distribution and stratigraphy of the cryogenian rapitan iron formation northwestern canada paleogeographic data indicate that the neoproterozoic rapitan group accumulated in two spatially related silled basins during the first mid neoproterozoic glacial episode ca 711 ma yet the controls on glaciation reason for basin development spatio temporal relationships among stratigraphic units and constraints on iron formation deposition and distribution in this region and globally at this time have not been adequately explained the lack of a coherent understanding of the group s regional stratigraphy has been a significant barrier to deciphering the geologic history of this critically important succession based on new stratigraphic data and incorporating the results of a related geochemical study the rapitan group accumulated in tectonically complex sub basins of a young rift zone the spatial and temporal dynamics of these sub basins controlled the grain size volume and distribution of detrital sediment predominantly turbidites and diamictites deposited during two glacially influenced episodes with ice cover during an interglacial episode the rotated exposed upper surfaces of newly displaced rift blocks directed drainage away from basin margins precluding significant clastic sediment delivery and subaqueous margins of outboard tilted fault blocks acted as sills that isolated local bottom water allowing the accumulation of an entirely hydrogenous succession jaspilitic and hematitic iron formation two strike parallel normal faults that strongly controlled basin margin development are prominent thrust faults in the present day orogen syndepositional activity of strike normal transfer faults segmented the basins along their length as reflected in pronounced along strike variations in the presence absence thickness and composition of the three stratigraphic units mt berg sayunei and shezal formations and in the presence or absence of iron formation locally 100 m thick top of sayunei formation differential subsidence in actively rifting basins was essential for deposition of rapitan type iron formation but glaciation although coeval with rapitan group accumulation was not a strong control the timing of the onset of glacially influenced sediment deposition in the closely spatially related rapitan similar to 711 ma and mount harper groups similar to 717 ma in northwestern laurentia differs by 6 m y arguing for diachronous sturtian glaciation both locally and globally c 2016 elsevier b v all rights reserved

impact of the equatorial deep jets on estimates of zonal transports in the atlantic the structure and variability of the zonal equatorial flow in the atlantic is studied on the basis of velocity profiles obtained with lowered acoustic doppler current profilers during multiple surveys the vertical extent of the zonal currents is found to vary considerably it can be as small as 100 m or as large as 1000 m in the atlantic vertical scales of 400 600 m have been associated with the equatorial deep jets they are also frequently called deep jets or stacked jets typical amplitudes of the zonal velocity are about 20 cm s 1 an analysis of quasi synoptic surveys indicates that the zonal extent of most jets is likely to be at least 27d they can rise or deepen from west to east although the deepening was observed more often and is often more pronounced the west to east deepening can be as large as 320 m 10d basin wide mean depth changes of the jets are mostly on the order of 50 m 10d and the largest depth changes are typically observed between 35d and 23d w the existence of these changes indicates that vertically propagating equatorially trapped waves might be one cause for the jet structure however the dependence of the slope on the longitude indicates that other processes must be involved as well the typical vertical extent of the jets is small enough to result in several direction changes of the zonal flow in the antarctic intermediate water aaiw and the north atlantic deep water nadw layer from transport estimates for 14 meridional sections it is found that the transport for the westward component of the flow within the aaiw layer 500 1000 m can be as large as 24 sv 1 sv 10 6 m s 1 within 1d of the equator for the eastward component of the flow in the aaiw layer the transport can be as large as 8 sv adding the transport components for each section results in a range of total aaiw transports from 24 to 7 sv this suggest that the annual mean transport of aaiw is westward the only months with eastward total transports are june and july this is consistent with earlier lagrangian and some other observations that indicated that the aaiw flow along the equator is governed by an annual cycle in the nadw layer 1200 3900 m the transport for the westward eastward flow can be as large as 25 sv 23 sv within 1d of the equator this results in a range of total nadw transports from 10 to 18 sv the variations of the total transports of aaiw and nadw are anti correlated with a correlation coefficient of 0 86 since only eight sections reached deep enough to allow transport estimates in the nadw layer it is more difficult to come to a conclusion about the mean transport in this layer than for the transport in the aaiw layer for the latter layer 14 sections were available nevertheless the obtained estimates suggest that the total nadw transport may be eastward

anatomy of turbidites and linked debrites based on long distance 120 x 30 km bed correlation marnoso arenacea formation northern apennines italy much of our understanding of submarine sediment laden density flows that transport very large volumes ca 1 to 100 km3 of sediment into the deep ocean comes from careful analysis of their deposits direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic in order to understand how submarine sediment laden density flows evolve in space and time lateral changes within individual flow deposits need to be documented the geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events this study of the miocene marnoso arenacea formation italy provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence one hundred and nine sections were logged through a ca 30 m thick interval of time equivalent strata between the contessa mega bed and an overlying columbine marker bed correlations extend for 120 km along the axis of the foreland basin in a direction parallel to flow and for 30 km across the foredeep outcrop as a result of post depositional thrust faulting and shortening this represents an across flow distance of over 60 km at the time of deposition the correlation of beds containing thick 40 cm sandstone intervals are documented almost all thick beds extend across the entire outcrop area most becoming thinly bedded 40 cm in distal sections palaeocurrent directions for flow deposits are sub parallel and indicate confinement by the lateral margins of the elongate foredeep flows were able to traverse the basin in opposing directions suggesting a basin plain with a very low gradient small fractional changes in stratal thickness define several depocentres on either side of the verghereto high area the extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle substantially less than the thickness of flows thick beds contain two distinct types of sandstone ungraded mud rich sandstone intervals record evidence of en masse debrite deposition graded mud poor sandstone intervals are inferred to result from progressive grain by grain turbidite deposition clast rich muddy sandstone intervals pinch out abruptly in downflow and crossflow directions in a fashion consistent with en masse debrite deposition the tapered shape of mud poor sandstone intervals is consistent with an origin through progressive grain by grain turbidite deposition most correlated beds comprise both turbidite and debrite sandstone intervals intrabed transitions from exclusive turbidite sandstone to turbidite sandstone overlain by debrite sandstone are common in the downflow and crossflow directions this spatial arrangement suggests either i bypass of an initial debris flow past proximal sections ii localized input of debris flows away from available sections or iii generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and or abrupt flow deceleration a single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud rich and mud poor sandstone

classification of poorly sorted sedimentary rocks abstract the classification and nomenclature of sandstones has evolved over the years into a system which is now generally accepted as standard however no equivalent system exists for the poorly sorted sedimentary rocks which include grain sizes from clay to gravel a descriptive classification is proposed based on an earlier system but using updated nomenclature and lithological fields such a scheme is needed to clearly define lithological terms currently used in the description of sedimentary sequences

acquisition of the neodymium isotopic composition of the north atlantic deep water the north atlantic deep water nadw neodymium isotopic composition nd ic is increasingly used in oceanography and paleoceanography to trace large scale circulation and weathering processes notably to investigate past variations of the global thermohaline circulation although the present day nadw nd ic is well characterized at nd 13 5 the acquisition of this isotopic signature in other words the causes of this value has so far been very sparsely documented such an understanding is however fundamental to the interpretation of paleo records nd ic and rare earth element concentrations were measured at 9 stations within the north atlantic subpolar gyre signature cruise summer 1999 the comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the nadw the labrador sea water lsw nd 13 9 0 4 north east atlantic deep water neadw nd 13 2 0 4 and north west atlantic bottom water nwabw nd 14 5 0 4 acquire their nd ic through distinct water mass mixings and lithogenic inputs these different mechanisms acting upon water masses from very diverse sources seem to bring the nd ic of the three nadw layers to values close together and similar to that of the nadw it is suggested that sediment seawater interactions significantly lower the neadw and nwabw nd ic along the south east greenland margin since these interactions do not significantly modify the nd content of these water masses sediment remobilizations leading to the nd ic variations are probably associated with nd removal fluxes from the water mass toward the sediment a process called boundary exchange on the other hand lsw seems to acquire its nd ic from the subpolar mode waters from which it is formed by deep convection and no other mechanism needs to be invoked its unradiogenic signature could ultimately be linked to fresh water runoff from the canadian shield these conclusions should allow more precise interpretations of paleoceanographic nd ic records taking into account the distinct histories of the three nadw layers including distinct water mass mixings and distinct lithogenic inputs

middepth equatorial tracer tongues in a model of the atlantic ocean 1 observational estimates of middepth tracer tongues in the equatorial atlantic are reviewed and are compared with results from several eddy resolving model simulations local maxima of chlorofluorocarbon cfc concentrations along the equator at around 1500 m depth are related to mean eastward jet structures in the models at similar depth ranges and can also be identified in several simulated tracer distributions similar to the observations strong eastward jets are located in the simulations 1 2 north and south of the equator the model simulations show in addition consistent with the cfc observations weaker jets at around 4 6 n s and 8 10 n s suggestive of a large scale alternating eastward westward current system in the western tropical atlantic in this depth range lagrangian transport estimates in the model using float diagnostics show a transport of 1 3 sv in each of the eastward jets 1 2 n s off the equator compared to 3 12 sv throughflow into the south atlantic with no seasonal cycle apparent in the transport fractioning comparing different model solutions reveals the choice of the subgrid scale mixing parameterization as important for the amplitudes of the jets enhanced reduced diapycnal mixing is related to stronger weaker jets

sequencing the sturtian icehouse dynamic ice behaviour in south australia the cryogenian record of south australia includes the type region of the sturtian glaciation the oldest of three pan global icehouse intervals during the neoproterozoic data are presented from previously little described sections at holowilena creek oladdie creek and hillpara creek in the central and southern flinders ranges where five facies associations are recognized these are 1 diamictite and conglomerate 2 interbedded heterolithic deposits 3 hummocky cross stratified sandstone 4 lonestone bearing siltstone and 5 ferruginous siltstone and sandstone the succession reveals significant lateral and vertical facies variation which is linked to a complex inherited palaeotopography and distance from the sediment source repeated stratigraphic occurrences of striated clasts and abundant ice rafted debris strongly support recurrent glacial influence on sedimentation the intercalation of gravitationally reworked diamictites dropstone bearing siltstone and dropstone free siltstone testifies to dynamic sedimentation within a periodically glacially influenced subaqueous environment sequence stratigraphic analysis identifies four glacial advance systems tracts separated by three glacial retreat systems tracts wherein hummocky cross stratified sandstones attest to open water conditions these findings support dynamic ice sheet behaviour in south australia and provide clear evidence for repeated intra sturtian ice sheet recession

the neoproterozoic noonday formation death valley region california the neoproterozoic cambrian succession in the death valley region of the southwestern united states is among the best exposed and easily accessible in the world the largest single exposure of these strata occurs in the panamint range on the west side of death valley and it although variably metamorphosed contains the most complete sections of the noonday formation cap carbonate sequence new geological mapping measured sections and high resolution carbon isotope data for the noonday formation in this range enable establishment of a unified stratigraphy across the death valley region that consists of three units from the base upward they include the sentinel peak member of light gray massive to laminated fine dolostone locally containing vugs and tubes variably filled with micrite spar or quartz and rarely galena and varying from 2 to 200 m in thickness the radcliff member composed of feldspathic shale and sandstone and thin bedded limestone 0 200 m thick and the newly defined mahogany flats member of gray commonly stromatolitic thin to medium bedded fine dolostone that is 200 m thick at its type locality carbon isotopic trends in the panamint range match to within 1 2 reproducibility those known for the equivalent nonmetamorphosed strata in the eastern death valley sections a composite section of the noonday formation displays a chemostratigraphic profile with values near 3 through the cap dolostone of the sentinel peak member a decline to 6 in the lower part of the radcliff member followed by a recovery to near 0 and subsequent decline toward 2 in the remainder of the radcliff units and lower mahogany flats member and then a return to positive values 4 through the remainder of the mahogany flats member this pattern matches the ediacaran cap carbonate in namibia remarkably well and assuming our carbon isotopic correlations are correct it indicates that noonday deposition occurred at the beginning of the ediacaran period and that the immediately underlying wildrose diamictite of the kingston peak formation probably represents the younger cryogenian marinoan glacial episode

antarctic intermediate water circulation in the tropical and subtropical south atlantic recent hydrographic data from the south atlantic ventilation experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the antarctic intermediate water aaiw in the atlantic ocean the low salinity of the aaiw extends eastward across the south atlantic just south of the equator 3 4 s evidence of a weak eastward flow just north of the equator 1 2 n is also shown lateral and vertical homogenization of properties in the aaiw is found at the equator between 2 s and 2 n there is no clear zonal gradient in salinity just along the equator these observations suggest enhanced mixing within the equatorial baroclinic deformation radius the south atlantic tropical gyre is shown to consist of the following three cells one cyclonic cell centered at about 7 s another centered at about 19 s in the west and 23 s in the east and one anticyclonic cell centered at about 13 s these cells are associated with a westward extension at 10 s of high salinity and low oxygen which originates in the eastern tropical south atlantic and a front in these properties at about 15 s in the west and about 20 s in the east

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

duration of deposition from decelerating high density turbidity currents using recent advances in the stability analysis of current ripples a new model for the calculation of duration of sediment deposition from decelerating high density turbidity currents is proposed the model named tdure refines a duration model proposed by allen 1991 j sediment petrol 61 291 295 by calculating the accumulation time of the rippled tc division separately from the massive and plane parallel laminated tab divisions in bouma type turbidites tdure consists of three modules in the first module the accumulation time for theta and tb divisions is approximated by assuming a linear decrease in sedimentation rate with height in the turbidite as in the original model a straight line is fitted through inferred sedimentation rates at the tab and tbc boundaries in the second module angle of climb of ripples thickness of the tc division and grain size distribution are used in empirical relationships between ripple migration rate and a grain related mobility parameter to estimate the accumulation time of the tc division in the third module the expected development of ripple size across the tc division is calculated using empirical relationships between rate of development of ripple height and grain related mobility parameter and subsequently compared with the observed development of ripple size in the turbidite in this way the accuracy of the accumulation time calculated in module 2 can be verified independently tdure was tested using bouma type turbidites from the doheny channel capistrano fm ca usa and the flysch di motta calabria italy accumulation times of 12 and 10 75 min for the tc division and 19 and 16 min for the tabc sequence were calculated for the doheny channel and the flysch di motta turbidites respectively although module 3 underestimates the rate of development of current ripples near the tbcboundary in both beds ripple size at the tcd boundary is calculated accurately the underestimation of development rate may be caused by differences between flow conditions in experimental flumes on which the model calculations are based and turbiditycurrent dynamics q 2000 elsevier science b v all rights reserved

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

triassic olistostrome and shelf basin transition in the western great basin paleogeographic implications the pershing district of the southern humboldt range northwestern nevada contains a large upper triassic submarine olistostrome containing carbonate clasts from pebble size to 700 m in length that are supported mostly by a lime mud matrix the olistostrome comprises debris flows separated by intervals of hemipelagic sedimentation turbidity flows and density modified grain flows the olistostrome body is triangular prism shaped with a flat top and keel like base it lies in a thick sequence of fine grained siliciclastic rock the lower submarine gravity flows were deposited in a pre existing trough at a lower slope or base of slope the highest gravity flow filled the trough and spilled over its sides the source of olistostrome clasts and matrix was a carbonate platform dun glen formation to the northeast that formed on the surface of a pre existing shallow marine delta the olistostrome and older shelf basin transitional rocks of the humboldt range indicate the existence of a middle and upper triassic shelf basin transition with an arcuate trace that is convex to the southwest at the southern humboldt range this bend in the middle to late triassic margin of western north america probably existed since at least middle triassic and did not result from later tectonics the bend was apparently a controlling factor in the location and orientation of jurassic contractile deformation

the deep currents in the eastern equatorial atlantic ocean 1 in the atlantic ocean equatorial deep jets have been observed in the central and western equatorial basin here we present three meridional velocity sections carried out in boreal summer 2000 in the eastern equatorial atlantic at that time the deep equatorial jets were observed to weaken or to vanish in the gulf of guinea toward the african coast at 10 w the comparison with currents measured one year before in boreal summer 1999 along with tracer distributions confirm the existence of an eastward route along the equator of part of the north atlantic deep water

submarine slope stability on high latitude glaciated svalbard barents sea margin abstract slope stability is evaluated at two locations on high latitude deep sea fans along the svalbard barents sea margin based on available samples and using an infinite slope analysis the stability evaluation uses the mohr coulomb failure criterion and a semi analytical approach based on gibson s formulation for determining the excess pore pressure build up due to sedimentation the main results are presented in the form of contour plots of slope safety factors in a diagram with axes of time and thickness of deposit the results show that during rapid sedimentation which mostly takes place during periods of maximum glaciation with the ice front located along the shelf edge slope failure will occur with a frequency varying between 95 and 170 years only part of the upper sedimented layer will be mobilised 10 30 m while the remaining thickness 40 70 m will remain at the initial sedimentation site these results may explain why the continental slope is characterised by relatively uniform sediment thickness from upper to lower slope

lagrangian observations in the intermediate western boundary current of the south atlantic abstract subsurface float measurements at 800 m depth carried out from 1994 to 2003 in the brazil basin are used to characterise the equatorward intermediate western boundary current iwbc and its connections to the ocean interior transversally the boundary flow is less than 100 km wide and most intense at 10 20 km from the 800 m isobath its average velocities range from 0 1 ms 1 to 0 3 ms 1 depending on latitude with individual daily values as high as 0 7 ms 1 the flow meridional extent exhibits 3 contrasted domains i from 27 s to the vitoria trindade ridge at 20 30 s the boundary flow intensifies northward along a relatively smooth topography a counter current adjacent to it on its seaward side feeds it with intermediate water from the northern limb of the subtropical gyre ii at latitudes 20 15 s characterised by a very irregular topography the iwbc becomes weaker with even no real proof of its presence at 18 15 s an intense mesoscale variability prevails there which apparently takes over from the boundary flow to ensure the northward transport of water to 15 s where the iwbc re forms iii north of this latitude the boundary flow increases again to 10 s along smooth isobaths then decreases when encountering a rougher topography and the zonal jets of the equatorial current system a counter current present from 5 s to 14 s partly fed from the boundary flow contributes to its drainage the iwbc shows two main input locations at 27 23 s and 15 12 s in the southern parts of the two latitudinal domains of smooth topography where the northward current increases output locations coincide with major capes in the continental slope geometry at 20 s and 18 s the southeastern and northeastern corners of the abrolhos bank at 8 s near the recife plateau and at 5 s near cape sao roque

from the labrador sea to global change an understanding is emerging of the powerful changes that have occurred in convection in the north atlantic that understanding provides pointers not only to the regional influence of such changes but how they may bear on global shifts in climate

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

sedimentology of a sandur formed by multiple jokulhlaups kverkfjoll iceland abstract the kverkfjoll sandur in north iceland is the furthest upstream of a suite of fluvial landforms extending for 200 km along the jokulsa a fjollum river incision of the sandur exposes over 3 km of sedimentary sections up to 15 m in height a sandur wide well bedded succession of matrix rich cobble gravel and pebble granule gravel with individual beds 0 2 to 0 5 m thick indicates that the sandur is primarily the product of sandur wide sheet floods with sediment rich hyperconcentrated flows and also some debris flows and channelised turbulent flows this interpretation is evidenced by bedded hyperconcentrated flow deposits occurring as laterally extensive tabular depositional units that dominate the entire sandur reflecting the unconfined nature of the flow clast supported boulder gravel units interpreted as the product of macroturbulent flow occur in relatively narrow but deep channels the sedimentary succession is interpreted as the product of at least six volcanically generated catastrophic outburst floods jokulhlaups during the little ice age the sedimentology of these little ice age flood deposits on a small high gradient sandur contrasts strongly with the deposits of volcanically generated jokulhlaups on large low gradient coastal sandar and sandar associated with retreating glaciers which have been the basis for most previous models of jokulhlaup sedimentation

an interglacial on snowball earth dynamic ice behaviour revealed in the chuos formation namibia the sturtian is the oldest ca 716 ma of three pan global glaciations in the cryogenian at omutirapo in northern namibia a 2 km wide 400 m deep palaeovalley is filled by glaciogenic strata of the chuos formation which represents the sturtian glacial record sedimentary logging of an exceptionally high quality exposure permits detailed stratigraphic descriptions and interpretations allowing two glacial cycles to be identified at the base of the exposed succession strong evidence supporting glaciation includes diamictites ice rafted dropstones and intensely sheared zones of interpreted subglacial origin these facies collectively represent ice proximal to ice rafted deposits upsection dropstone free mudstones in the middle of the succession and the absence of diamictites imply sedimentation free from glacial influence however the reappearance of glacial deposits above indicates a phase of sturtian glacial re advance comparison with age equivalent strata in south australia where evidence for sea ice free sedimentation has been established previously suggests that a sturtian interglacial may have been extensive implying global scale waxing and waning of ice sheets during a cryogenian glacial event

a neoproterozoic ice advance sequence sperry wash california the kingston peak formation is an archetypal cryogenian succession that crops out across the death valley region of eastern california above pre glacial strata kp1 two distinct glacial phases have been recognized and are interpreted to be allied to the panglacial sturtian kp2 and kp3 and marinoan kp4 icehouse events the thickest and most extensive unit kp3 forms the entire exposed section at sperry wash at this locality ice distal turbidites are succeeded in turn by ice medial and ice proximal facies comprising a spectrum of ice rafted debris bearing turbidites debrites and shales these are overlain by ice marginal grounding line fan deposits interbedded with glacitectonically deformed heterolithics supporting local advance to an ice contact position the succession records accumulation within a glacier fed subaqueous shelf wherein the clear progradational signature is driven by ice advance towards the south east evolution of the subaqueous complex is five fold comprising i ice distal outwash ii build out of ice medial depositional lobes iii ice proximal deposition and increased calving iv resumed ice margin advance and v growth of ice contact grounding line fan this sequence is unique in the death valley region for recording the first evidence of advance to ice marginal and ice contact settings thereby enabling the location of the glacier terminus to be documented for the first time

an ice grounding line wedge from the ghaub glaciation 635 ma on the distal foreslope of the otavi carbonate platform namibia and its bearing on the snowball earth hypothesis our detailed examination of the ghaub formation possibly 635 ma on the distal foreslope of the otavi carbonate platform is part of a regional study of the congo paleocontinental margin in northwestern namibia detrital carbonates of the ghaub formation disconformably overlie the franni aus member of the ombaatjie formation a coarsening upward stack of carbonate turbidites and oolite clast debris flow breccias interpreted to be a glacioeustatic falling stand wedge within the main ghaub formation carbonate diamictites are interleaved with mesoscale laminated to cross laminated climbing rippled grainstones and mudstones and conglomeratic carbonates amalgamation of diamictite units is observed where interleaved facies grainstones mudstones are laterally discontinuous due to reactivation of erosion followed by renewed deposition the diamictite package is progradational overall and 80 m thick on average it is overlain by the 5 15 m thick bethanis member which is unique in its lateral continuity composite fining upward trend and distinctive interbedding of turbidite grainstones argillaceous siltstones climbing rippled mudstones and meter scale stromatolite dropstones dropstones are ubiquitous within the finer grained ghaub lithofacies and their presence along with the facies context for subglacial and near grounding line deposition indicates a glacigenic origin for the ghaub formation despite its subtropical paleolatitude and distal foreslope setting we infer a glacial maximum represented by the sub ghaub disconformity followed by the main ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude terminated by the bethanis episode of unusually widespread iceberg calving and slope instability the bethanis member is overlain conformably by the keilberg member of the maieberg formation reconstruction of the foreslope places the ghaub grounding line wedge 1 3 km vertically below the rim of the platform implying an enormous base level change upon deglaciation when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment the magnitude of base level change supports the pan glacial hypothesis that dynamic thick ice sheets existed simultaneously on virtually all continents the snowball hypothesis that the oceans were also covered by glacial ice sea glacier provides a simple explanation for the main ghaub to bethanis transition terminal deglaciation was triggered by collapse of the sea glacier

the shallow and deep western boundary circulation of the south atlantic at 5 11 s repeated shipboard observation sections across the boundary flow off northeastern brazil as well as acoustic doppler current profiler adcp and current meter records from a moored boundary array deployed during 2000 04 near 11 s are analyzed here for both the northward warm water flow by the north brazil undercurrent nbuc above approximately 1100 m and the southward flow of north atlantic deep water nadw underneath at 5 s the mean from nine sections yields an nbuc transport of 26 5 3 7 sv sv 106 m3 s 1 along the boundary at 11 s the mean nbuc transport from five sections is 25 4 7 4 sv confirming that the nbuc is already well developed at 11 s at both latitudes a persistent offshore southward recirculation between 200 and 1100 m depth reduces the net northward warm water flow through the 5 s section west of 31 5 w to 22 1 5 3 sv and through the 11 s section to 21 7 4 1 sv west of 32 0 w the 4 yr long nbuc transport time series from 11 s yields a seasonal cycle of 2 5 sv amplitude with its northward maximum in july interannual nbuc transport variations are small varying only by 1 2 sv during the four years with no detectable trend the southward flow of nadw within the deep western boundary current at 5 s is 25 5 8 3 sv with an offshore northward recirculation yielding a nine section mean of 20 3 10 1 sv west of 31 5 w for antarctic bottom water a net northward flow of 4 4 3 0 sv is determined at 5 s for the 11 s section the moored array data show a pronounced energy maximum at 60 70 day period in the nadw depth range which was identified in related work as deep eddies translating southward along the boundary based on a kinematic eddy model fit to the first half of the moored time series the mean nadw transfer by the deep eddies at 11 s was estimated to be about 17 sv given the large interannual variability of the deep near boundary transport time series which ranged from 14 to 24 sv the 11 s mean was considered to be not distinguishable from the mean at 5 s

two diamictites two cap carbonates two δ13c excursions two rifts the neoproterozoic kingston peak formation death valley california stratigraphic mapping of the neoproterozoic glaciogenic kingston peak formation death valley california provides evidence for two temporally discrete extensional deformation episodes these episodes are bracketed by the sourdough limestone and noonday dolomite the facies characteristics and 13 c data ranging between 2 15 and 2 56 and 1 88 and 4 86 respectively of which make them equivalent to sturtian and varangian age cap carbonates respectively this constrains the two extensional episodes along the southwestern margin of laurentia to ca 700 ma and ca 600 ma these observations and data show that the field evidence for mid neoproterozoic breakup and the predictions from tectonic subsidence curves for a latest neoproterozoic breakup are both correct thus neoproterozoic plate reconstructions must account for two discrete rift episodes separated by 100 m y or more confining rifting to within the kingston peak formation thereby places the younger proterozoic rocks of the southwestern great basin in the rift to drift tectonic phase

the ocean general circulation near 1000 m depth abstractthe mean ocean circulation near 1000 m depth is estimated with 100 km resolution from the argo float displacements collected before 1 january 2010 after a thorough validation the 400 000 or so displacements found in the 950 1150 dbar layer and with parking times between 4 and 17 days allow the currents to be mapped at intermediate depths with unprecedented details the antarctic circumpolar current acc is the most prominent feature but western boundary currents and their recirculations and alternating zonal jets in the tropical atlantic and pacific are also well defined eddy kinetic energy eke gives the mesoscale variability on the order of 10 cm2 s 2 in the interior which is compared to the surface geostrophic altimetric eke showing e folding depths greater than 700 m in the acc and northern subpolar regions assuming planetary geostrophy the geopotential height of the 1000 dbar isobar is estimated to obtain an absolute and deep reference level worldwide this is done by solving num

a diamictite dichotomy glacial conveyor belts and olistostromes in the neoproterozoic of death valley california usa multiple intercalations of glacially derived and slope derived diamictites testify to the drawbacks of correlating neoproterozoic diamictites more widely but shed new light on the close interrelationship of these processes in the cryogenian world in the neoproterozoic of death valley california usa rifting of rodinia occurred concomitantly with a major glacial event that deposited the kingston peak formation a new sedimentologic investigation of this formation in the silurian hills demonstrates for the first time that some diamictites are ultimately of glacial origin abundant dropstone textures occur in interstratified heterolithic deposits with clasts of identical composition gneiss schist granite metabasite quartzite to those of boulder bearing diamictites suggesting a common source the glacial conveyor belt in stark contrast megaclast bearing diamictites yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across are interpreted as olistostromes the occurrence of syn sedimentary faults within the succession allows glacial versus slope derived material to be distinguished for the first time

geochronological constraints on neoproterozoic rifting and onset of the marinoan glaciation from the kingston peak formation in death valley california usa

stratigraphic investigations of carbon isotope anomalies and neoproterozoic ice ages in death valley california an unusual richness of biogeochemical events is recorded in neoproterozoic cambrian strata of the death valley region california united states eight negative carbon isotope d13c excursions are found in carbonate units between 1 08 ga and the precambrian cambrian boundary four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial cap carbonates including one or more of the following laminite with rollup structures apparent tube rocks seafloor precipitates and sheet crack cements however only two of these units the sourdough limestone member of the kingston peak formation and the noonday dolomite rest directly upon glacial strata the basal beck spring dolomite and the rainstorm member of the johnnie formation each contain negative excursions and cap carbonate like lithofacies but do not rest on known glacial deposits if the negative d13c excursions are assumed to record depositional processes two equally interesting hypotheses are possible 1 the death valley succession records four glacial pulses in neoproterozoic time but glacial units are not preserved at two stratigraphic levels 2 alternatively other global oceanographic processes can cause negative excursions and cap carbonate like facies in addition to or independent of glaciation

a new estimate of the global 3d geostrophic ocean circulation based on satellite data and in situ measurements abstract a new estimate of the global ocean 3d geostrophic circulation from the surface down to 1500 m depth surcouf3d has been computed for the 1993 2008 period using an observation based approach that combines altimetry with temperature and salinity through the thermal wind equation the validity of this simple approach was tested using a consistent dataset from a model reanalysis away from the boundary layers errors are less than 10 in most places which indicate that the thermal wind equation is a robust approximation to reconstruct the 3d oceanic circulation in the ocean interior the surcouf3d current field was validated in the atlantic ocean against in situ observations we considered the andro current velocities deduced at 1000 m depth from argo float displacements as well as velocity measurements at 26 5 n from the rapid mocha current meter array the surcouf3d currents show similar skill to the 3d velocities from the glorys mercator ocean reanalysis in reproducing the amplitude and variability of the andro currents in the upper 1000 m high correlations are also found with in situ velocities measured by the rapid mocha current meters the surcouf3d current field was then used to compute estimates of the atlantic meridional overturning circulation amoc through the 25 n section showing good comparisons with hydrographic sections from 1998 and 2004 monthly averaged amoc time series are also consistent with the rapid mocha array and with the glorys mercator ocean reanalysis over the april 2004 september 2007 period finally a 15 years long time series of monthly estimates of the amoc was computed the amoc strength has a mean value of 16 sv with an annual resp monthly standard deviation of 2 4 sv resp 7 1 sv over the 1993 2008 period the time series characterized by a strong variability shows no significant trend

sediment reworking on high latitude continental margins and its implications for palaeoceanographic studies insights from the norwegian greenland sea geological evidence indicates that sediment reworking is common around the continental margins and abyssal depths of the norwegian greenland sea a high latitude setting with glacier influenced margins detailed analysis of 22 cores up to 5 in long placed in context by accompanying geophysical data including high resolution sub bottom profiles swath bathymetry and backscatter maps indicates that reworking is variable and ranges from debris flows and turbidity currents to bottom current activity as well as iceberg scouring reworking by debris flows appears to be restricted mainly to the main trough mouth fans and sediment slides elsewhere turbidity current activity frequently dominates although iceberg ploughing down to 600 in depth and current winnowing assume increasing significance on continental shelves reworking in the norwegian greenland sea reflects variations in ice sheet dynamics that in turn influence the rate of sediment delivery and location of depocentres spatial variations in the style of reworking may also reflect the influence of continental slope gradient and bedrock geology on continental shelves the widespread nature of sediment reworking has important implications for palaeoceanographic investigations in the region as reworking can result in erosion and disturbance of the sediment column it is estimated that less than 7 of material delivered to the norwegian greenland sea since the late weichselian is derived from hemipelagic and pelagic sedimentation this problem is significant where continuous high resolution records of hemipelagic and pelagic sedimentation are required and attempts are made to correlate with other high resolution proxy records such as ice cores at sub millennial scales bioturbation results in the smoothing of high resolution records and imposes a maximum resolution for sediment core time slices of generally 400 years or more in the norwegian greenland sea areas of high sedimentation such as trough mouth fans or contourite drifts are commonly associated with extensive reworking identification of reworking is particularly important where attempts are made to link records of iceberg rafted debris to past ice sheet dynamics as bottom current winnowing and mass flow processes can increase the concentration of coarse grained iceberg rafted debris such localized accentuation of the iceberg rafted debris signal may lead to erroneous palaeo environmental interpretations it is therefore critical that palaeoceanographic interpretations are firmly underpinned by an explicit sedimentological assessment of reworking

precambrian olistoliths masquerading as sills from death valley california olistolith production and magmatism are processes commonly associated with extensional tectonic settings such as rift basins we present a cautionary exemplar from one such precambrian basin in which we reinterpret metabasite bodies previously documented as sills to be olistoliths we nevertheless demonstrate that on the basis of field observation alone the previous but erroneous sill interpretation is parsimonious indeed it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed we present new data from metabasites and metasedimentary strata of the kingston peak formation cryogenian and crystal spring formation mesoproterozoic of death valley usa these include field observations u pb apatite ages u pb zircon ages detrital and igneous and whole rock geochemistry these data also provide a new maximum age for the base of the pahrump group and suggest that the crystal spring diabase was more tholeiitic than previously thought similar sill olistolith misinterpretations may have occurred elsewhere potentially producing erroneous age and tectonic setting interpretations of surrounding strata this is particularly relevant in precambrian rocks where fossil age constraints are rare this is illustrated herein using a potential example from the neoproterozoic literature of the lufilian belt africa we caution others against precambrian olistoliths masquerading as sills supplementary material details of a meta igneous boulder from p12 of the silurian hills la icp ms and whole rock geochemistry methods and standards and u pb apatite and zircon isotopic data including standards and selected cathodoluminescence images are available at https doi org 10 6084 m9 figshare c 3990639

a new depositional model for ice shelves based upon sediment cores from the ross sea and the mac robertson shelf antarctica we document the similarity of depositional facies occurring in gravity cores recovered from two disjunct regions of the antarctic continental shelf the ross sea and the mac robertson shelf the facies sequence model is represented in two cores one collected during the 1995 1 cruise of the r v nathaniel r palmer core nbp95 tc 18 and the other collected by the rsv aurora australis during cruise 149 in 1995 core 149 39gc38 both cores show a succession of facies indicative of ice shelf retreat during the late pleistocene to holocene transition distinct lithofacies range in thickness from a few tens of cm to 1 m and consist of from bottom to top a coarse granulated sandy mud laminated silt and clay structureless silly clay poorly sorted sandy siliceous mud and siliceous mud and ooze these facies represent the passage of distinct depositional regimes across the core sites including sub ice shelf beneath a basal debris zone sub ice shell distal to a debris zone calving line transition and open marine this facies model represents an advance in our understanding of glacial marine stratigraphy for the antarctic continental shelf and will provide the basis for more realistic palaeoglacial reconstructions

the intermediate depth circulation of the western south atlantic the subsurface oceanic circulation is an important part of the earth climate system subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances occasionally supplemented by current meter measurements neutrally buoyant floats however now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western south atlantic here our combined data set provides unprecedented observations and quantification of key flow patterns such as the subtropical gyre return flow 12 sv 1 sverdrup 10 6 m 3 s 1 its bifurcation near the santos plateau and the resulting continuous narrow and swift northward intermediate western boundary current 4 sv this northward flowing water passes through complex equatorial flows and finally enters into the north atlantic

neoproterozoic ice sheets and olistoliths multiple glacial cycles in the kingston peak formation california the kingston peak formation is a diamictite bearing succession that crops out in the death valley region california usa an exceptionally thick 1 5 km outcrop belt in its type area the kingston range provides clear insights into the dynamics of mid cryogenian sturtian ice sheets in laurentia seven detailed logs allow the lateral and vertical distribution of facies associations to be assessed we recognize 1 diamictite facies association ice proximal glacigenic debris flows 2 lonestone bearing facies association ice marginal hemipelagic deposits and low density gravity flows with iceberg rafting 3 pebble to boulder conglomerate facies association ice proximal cogenetic glacigenic debris flows and high density turbidites 4 megaclast facies association olistostrome and hemipelagic sediments subject to ice rafting and 5 interbedded heterolithics facies association low density turbidites and hemipelagic deposits the stratigraphic motif allows three glacial cycles to be inferred across the range ice minimum conditions interrupting the kingston peak succession are associated with the development of an olistostrome complex succeeded by a thick accumulation of boulder conglomerates deposited during ice readvance the data testify to a strong glacial influence on sedimentation within this ancient subaqueous succession and to highly dynamic ice sheet behaviour with clear glacial cycles during the sturtian glaciation

discriminating glacier thermal and dynamic regimes in the sedimentary record abstract this paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes with additional consideration given to the tectonic context new and previously published data are evaluated together and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal dynamic and topographic regimes in the context of landsystems representatives from terrestrial environments include temperate glaciers in the european alps patagonia new zealand the cordillera blanca peru cold glaciers in the dry valleys of antarctica and the antarctic peninsula region and polythermal valley glaciers in svalbard northern sweden the yukon and the khumbu himal nepal the glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the east antarctic continental shelf and from a polythermal tidewater glacier in svalbard along with general observations from temperate glaciers in alaska these data show that temperate glacial systems particularly in high relief areas are dominated by rockfall and avalanche processes although sediments are largely reworked by glaciofluvial processes debris in polythermal glaciers is both thermally and topographically influenced in areas of moderate relief debris is mainly of basal glacial origin and the resulting facies association is dominated by diamicton in high relief areas such as the himalaya the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs resulting in extensive accumulations of sandy boulder gravel cold glaciers are dominated by basal debris entrainment but sediments are little modified from the source materials which are typically sandy boulder gravel from older till and sand from glaciofluvial glaciolacustrine and aeolian sources similar facies associations but with different facies geometry and thickness occur in equivalent glaciomarine settings application of these concepts can aid the interpretation of glacier thermal regime and hence palaeoclimate in quaternary and ancient glacial systems

the water masses along the western boundary of the south and equatorial atlantic a quasi meridional hydrographic section located offshore from south america from 50 s to 10 n and three shorter transverse lines to the continental slope are used for a descriptive study of the water masses along the western boundary of the south and equatorial atlantic at the upper and intermediate levels the tracer analysis provides geographical limits of the wind driven circulation regimes and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate at depths shallower than about 200 m the subdivision of the subtropical gyre into two cells separated by the subtropical countercurrent near 28 s that was pointed out in a previous study is corroborated south of this front a warm variety 18 c of subtropical mode water in the inner recirculation of the brazil current appears despite its limited extent as a southern counterpart of the north atlantic 18 c water at the deep levels the upper circumpolar water and upper north atlantic deep water enter the south atlantic in a significantly overlapping density range the ensuing lateral encounter of both water masses occurs at 26 s near the western boundary where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre other tracer anomalies signal significant eastward escapes of north atlantic deep water within two jets at about two degrees of latitude on either side of the equator in another narrow current at 10 s and at 34 s the latter latitude marks the confluence and eastward deflection of the opposite boundary currents of lower north atlantic deep water and lower circumpolar water near the bottom of the argentine basin the weddell sea deep water that flows westward north of the zapiola ridge is more recently ventilated than the water carried by the boundary current near the falkland escarpment while a part of it flows anticyclonically around the ridge another part turns equatorward and enhances the southern property signatures of the water farther north

zonal intermediate currents in the equatorial atlantic ocean acoustic float data collected near 800 m depth are used to map zonal mean currents within the antarctic intermediate water aaiw tongue in the equatorial atlantic alternating zonal jets of 2 latitudinal width are revealed between 6 s and 6 n displacements from profiling floats drifting near 1000 m depth also reveal similar zonal jets at the base of the aaiw layer the strongest jets 15 cm s 1 peak are found at 4 s 2 s 0 2 n and 4 n they are coherent longitudinally over order of 3000 km and poleward of 1 s and 1 n generally coherent vertically between 800 m and 1000 m large seasonal fluctuations exist at both levels within 1 of equator aaiw at 800 m flows westward 8 cm s 1 mean in boreal summer and fall but eastward 3 cm s 1 mean in winter whereas the flow at 1000 m is eastward in late fall and winter

deep circulation in the equatorial atlantic ocean in the atlantic ocean the northward export of warm surface water is compensated by a southward flow of cold north atlantic deep water nadw the nadvv is transported southward along the american continental margin within the deep western boundary current dwbc some tracer and float observations have shown that part of the dwbc water flows eastward along the equator here we present three meridional velocity sections which give an instantaneous image of the lop to bottom zonal circulation along the equatorial atlantic they reveal the presence of equatorial deep jets edjs between 1 30 n and 1 30 s alternating eastward westward currents with short vertical scale surrounded by columns of eastward currents the extra equatorial jets or eejs at 2 n and 2 s in addition to direct velocity measurements tracer distributions give indications of water mass feeding of the edjs and eejs by the dwbc

precambrian sequence stratigraphy abstract sequence stratigraphy studies the change in depositional trends in response to the interplay of accommodation and sediment supply from the scale of individual depositional systems to entire sedimentary basin fills as accommodation is controlled by allogenic mechanisms that operate at basinal to global scales the change in depositional trends is commonly synchronized among all environments established within a basin thus providing the basis for the definition of systems tracts and the development of models of facies predictability all classical sequence stratigraphic models assume the presence of an interior seaway within the basin under analysis and are centered around the direction and types of shoreline shifts which control the timing of all systems tracts and sequence stratigraphic surfaces in overfilled basins dominated by nonmarine sedimentation the definition of systems tracts is based on changes in fluvial accommodation as inferred from the shifting balance between the various fluvial architectural elements the method of sequence stratigraphy requires the application of the same set of core principles irrespective of the age of strata under analysis from precambrian to phanerozoic the study of precambrian basins is often hampered by poorer stratal preservation and by a general lack of time control however where sedimentary facies are well preserved the lack of time control may be partially compensated by a good knowledge of facies architecture and relationships as well as by paleocurrent data the latter are particularly important to understand the stratigraphic record of tectonically active basins where abrupt shifts in paleoflow directions allow one to infer tectonic events and map the corresponding sequence bounding unconformities arguably the most important contribution of precambrian research to sequence stratigraphy is the better understanding of the mechanisms controlling stratigraphic cyclicity in the rock record and hence of the criteria that should be employed in a system of sequence stratigraphic hierarchy there is increasing evidence that the tectonic regimes which controlled the formation and evolution of sedimentary basins in the more distant geological past were much more erratic in terms of origin and rates than formerly inferred solely from the study of the phanerozoic record in this context time is largely irrelevant as a parameter in the classification of stratigraphic sequences and it is rather the stratigraphic record of changes in the tectonic setting that provides the key criteria for the basic subdivision of the rock record into basin fill successions separated by first order sequence boundaries these first order basin fill successions are in turn subdivided into second and lower order sequences that result from shifts in the balance between accommodation and sedimentation at various scales of observation irrespective of the time span between two same order consecutive events sequences identified in any particular basin are not expected to correlate to other first and lower order sequences of other basins that may be similar in age but may have different timing and duration

giant cross beds in the neoproterozoic port askaig formation scotland implications for snowball earth abstract the neoproterozoic port askaig formation is a thick succession of interbedded diamictite conglomerate sandstone and mudstone that records glacially influenced marine sedimentation in the extensional dalradian basin giant cross beds individual units up to 11 m thick are observed in sandstone at the base of member iii of the port askaig formation the cross bedded sandstones are closely associated with other sandstone diamictite and conglomerate facies and are characterized by both planar and trough shaped foreset laminae with an average maximum dip angle of 14 the cross bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south the sandstone is medium grained well sorted and quartzitic the giant cross beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the port askaig formation the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine ice free depositional conditions the sedimentology of the cross bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal with periods of both ice free and ice influenced conditions the environmental conditions recorded in these port askaig deposits appear inconsistent with deep freeze or rapid meltback associated with a snowball earth

pulsed iceberg delivery driven by sturtian ice sheet dynamics an example from death valley california the kingston peak formation is a cryogenian sedimentary succession that crops out in the death valley area california usa it is widely accepted to record pre glacial conditions kp1 followed by two glaciations of pan global extent the older sturtian kp2 to kp3 and younger marinoan glaciation kp4 in the type area the kingston range detailed facies analysis of the sturtian succession reveals that a basal diamictite unit and an upper boulder conglomerate were deposited by proglacial subaqueous sediment gravity flows an olistostrome unit punctuating the succession is interpreted to result from tectonically induced downslope mobilization during isostatic rebound triggered by significant ice meltback focussing on strata onlapping the olistostrome this article provides insight into the processes of glacial re advance following an intra sturtian glacial minimum the first 50 m of strata above the olistostrome are thinly bedded turbidites that are devoid of lonestones a trend towards thicker graded beds upsection in concert with the gradual appearance and then abundance of lonestones testifies to the influence of ice rafting and to the resumption of a direct ice sheet influence upon sedimentation stratigraphic organization into thickening and coarsening upward bedsets over a multi metre scale reveals a subaqueous gravity flow dominated succession composed of a spectrum of high to low density turbidites with thick graded boulder conglomerates at intervals the finer grained facies assemblage is heterolithic current ripple cross laminated sandstones intercalated with shales that bear delicate granule to pebble sized dropstones in abundance intervals of dropstone bearing and dropstone free strata are attributable to dynamic oscillation of the ice margin in the hinterland integrating palaeocurrent data with observations from neighbouring outcrop belts allows a detailed palaeogeographic map of the eastern death valley area to be compiled for the first time

on the spreading of south atlantic water into the northern hemisphere 1 the upper branch of the meridional overturning circulation in the north atlantic is fed by cross equatorial transport of various water masses from the southern hemisphere here we study the large scale spreading of south atlantic water saw into the western tropical north atlantic from the equator to 25 n the fractions of saw in the upper ocean water masses are quantified using a water mass analysis applied on a data set of conductivity temperature depth data from the hydrobase project and the argo float program to fill gaps in the data coverage and to gain insight into the mechanisms involved the observations are complemented with results from the high resolution family of linked atlantic model experiments model 1 12 which has been shown to realistically simulate the inflow of saw into the caribbean the analysis reveals the mean saw propagation pathways in the north atlantic and identifies the regions of largest variability high saw fractions in the thermocline and central water layers are limited to the region south of 10 n where the water body consists of 80 90 saw thus the zonal currents in the equatorial gyre are mainly formed of saw the weaker currents in the intermediate layer combined with a northward excursion of the north equatorial current allow the saw in this layer to intrude farther north compared to the layers above the transition into north atlantic water occurs gradually from 12 n to 20 n in the intermediate layer