structure of the gulf stream and its recirculations at 55 w abstract two years of direct current and temperature observations from an army of 13 current meter moorings deployed near 55 w as part of the synop synoptic ocean prediction experiment have been used to explore the spatial and temporal variability of the gulf stream from three points of view in the geographic reference frame mean eastward velocities were observed from the surface to 4000 m there was no evidence of westward flow south of the eastward jet suggesting that the worthington recirculation gyre was located south of the array during this time period westward flow was observed north of the jet only at 4000 m where it had a magnitude comparable to the mean gulf stream 5 10 em s 1 these data also indicate that the mean eastward jet is much more vertically aligned than was depicted in an earlier picture constructed from noncontemporaneous observations in the lagrangian or streamwise reference frame it was found that at the thermocline level the width of the average synoptic gulf str

the long term hydrographic record at bermuda abstract a long by oceanographic standards time series of hydrographic observations at bermuda was begun in 1954 and continues to the present analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer 0 200 dbar where integral timescales for salinity are 50 longer than for temperature and surface salinity changes are correlated with decadal changes in the 18 c water production temperature salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of 50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of 300 kim salinity changes in the deepest layer observed at bermuda station s 1500 2500 dbar are uncorrelated with temperature masked by measurement errors in the early years of the time series inclusion of earlier

complex trace fossils from the terminal proterozoic of namibia we document for the first time the occurrence of the complex trace fossil treptichnus from the terminal proterozoic nama group namibia these traces differ in detail from the cambrian index fossil treptichnus pedum but document precursors with the same basic behavior the producer of the trace is not known in its complexity however this trace fossil provides strong evidence for advanced latest proterozoic bilaterians

evidence in hydrography and density fine structure for enhanced vertical mixing over the mid atlantic ridge in the western atlantic received 24 august 2001 revised 22 february 2002 accepted 25 march 2002 published 15 october 2002 1 anomalous conditions exist in the salinity oxygen and nutrient fields over the western flank of the northern hemisphere mid atlantic ridge we examine possible advective sources for this anomaly but determine that vertical mixing is the most likely cause we proceed to use knowledge gained from the brazil basin tracer release experiment in the south atlantic where microstructure and fine structure were obtained to explore the intensity spatial distribution and mechanisms of mixing in the deep ocean to interpret density fine structure from common conductivity temperature depth data in the north atlantic these data support the hypothesis that the anomalous hydrographic conditions are associated with enhanced levels of vertical mixing the inferred levels of vertical diffusivity over the northern hemisphere mid atlantic ridge are as high as in the south atlantic 1 10 10 4 m 2 s index terms 4532 oceanography physical general circulation 4536 oceanography physical hydrography 4568 oceanography physical turbulence diffusion and mixing processes keywords diapycnal mixing fine structure recirculation mid atlantic ridge mar hydrography silicate

hemispheric asymmetry of deep water transport modes in the western atlantic subtropical studies of the atlantic meridional cold water flow show a hemispheric contrast in the dominant southward transport mode below 2000 m in the north atlantic lower deep water ldw 1 8 2 4 c dominates with small transport of middle deep water mdw 2 4 3 2 c while in the south atlantic the opposite is observed we use numerous observations in the western basins of the tropics to show that the transition occurs rapidly near the equator in the western atlantic a meridional section in the central brazil basin suggests zonal flows are responsible for the transition ldw transport from the guiana basin north of the equator flows eastward in the northern brazil basin and is inferred to continue on through the romanche fracture zone into the eastern atlantic an opposing flow of mdw from the eastern tropical atlantic flows toward the western boundary where it bifurcates to supply mdw to the deep western boundary current dwbc of the brazil basin as well as to feed the northward flow of mdw in the guiana basin offshore of the dwbc the magnitude of each of these oppositely directed flows is roughly 7 sv we furthermore speculate that they are connected predominantly by upwelling from ldw to mdw within the low latitude eastern basin the overall deep water transport system below 2000 m in the western basins of the mid and low latitude atlantic is thus found to comprise the following three distinct components 1 a strong dwbc transport of ldw with associated recirculation dominates the guiana basin north of the equator 2 in the northern brazil basin just south of the equator a narrow eastward flow absorbs the ldw and carries it eastward while a somewhat broader westward flow imports mdw into the western basin 3 this mdw flow then bifurcates with the southward branch causing the mdw dominance in the brazil basin where the mdw dominated dwbc and associated recirculations are the third component of the deepwater transport system

changes across 66 w the caribbean sea and the western boundaries of the north atlantic subtropical gyre abstract a world ocean circulation experiment hydrographic program along with a global repeat hydrography program occupied sections along 66 w in the north west atlantic ocean in 2003 and 2012 hydrographic variables tracers and ladcp measurements were collected in situ this section together with the north american and south american coasts create a closed volume excluding any flow through the panama canal combining mass silica and ladcp information with constraints in an inverse box model we obtain a strong eastward flow of the gulf stream 100 1 4 6 sv in 2003 and 123 8 4 4 sv in 2012 and westward flows of the caribbean current 24 4 1 0 sv in 2003 and 24 2 1 1 sv in 2012 and deep western boundary current dwbc the dwbc carries the ventilated waters of the labrador sea and the nordic seas comparing the results of 2003 and 2012 an insignificant reduction is observed in the dwbc transport 17 3 2 9 sv and 15 0 2 5 sv for 2003 and 2012 respectively the net heat fluxes do not show major changes through the section during both cruises 0 23 0 08 pw in 2003 and 0 21 0 12 pw in 2012 in contrast freshwater fluxes present positive values in the dry season 0 24 sv in spring 2012 and negative values in the wet season 0 32 sv in fall 2003 the difference between both cruises may be due to seasonal variability in the area

where and how long ago was water in the western north atlantic ventilated maximum entropy inversions of bottle data from woce line a20 1 a maximum entropy me method is used to deconvolve tracer data for the joint distribution of locations and times since last ventilation the deconvolutions utilize world ocean circulation experiment line a20 repeat hydrography for cfc 11 potential temperature salinity oxygen and phosphate as well as global ocean data analysis project glodap radiocarbon data combined with surface boundary conditions derived from the atmospheric history of cfc 11 and the world ocean atlas 2005 and glodap databases because of the limited number of available tracers the deconvolutions are highly underdetermined leading to large entropic uncertainties which are quantified using the information entropy of relative to a prior distribution additional uncertainties resulting from data sparsity are estimated using a monte carlo approach and found to be of secondary importance the me deconvolutions objectively identify key water mass formation regions and quantify the local fraction of water of age or older last ventilated in each region ideal mean age and radiocarbon age are also estimated but found to have large entropic uncertainties that can be attributed to uncertainties in the partitioning of a given water parcel according to where it was last ventilated labrador irminger seawater l water is determined to be mostly less than 40 a old in the vicinity of the deep western boundary current dwbc at the northern end of a20 but several decades older where the dwbc recrosses the section further south pointing to the importance of mixing via a multitude of eddy diffusive paths overflow water lies primarily below l water with young waters 40 a at middepth in the northern part of a20 and waters as old as 600 a below 3500 m

origin and impact of the oldest metazoan bioclastic sediments the emergence of soft bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of ediacaran environments leading to the genesis of biogenic hard part deposits for the first time in earth s history the impact of bioclast origin on sedimentary processes is analyzed herein focusing on the sedimentology and taphonomy of shell concentrations dominated by the ediacaran index fossil cloudina from the itapucumi group paraguay skeletal concentrations include both dense accumulations of parautochthonous disarticulated specimens type 1 deposits and in situ specimens preserved as loosely packed assemblages type 2 deposits at that time cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts the simple fabric and geometry of these accumulations are typical of cambrian style shell beds despite their precambrian age these deposits indicate that the establishment of the phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes

zonal circulation in the nw atlantic and caribbean from a meridional world ocean circulation experiment hydrographic section at 66 w a world ocean circulation experiment hydrographic program section along 66 w in the north atlantic was made in 1997 in addition to the usual variables hydrographic and tracer measured in woce onetime sections we made lowered acoustic doppler current profiler ladcp measurements at nearly all of the stations the section closed off a portion of the western north atlantic to the west of the line making a closed volume for constraining the circulation in addition the deep portions of the caribbean do not communicate with the rest of the basin by combining mass silica and ladcp information as constraints an inverse calculation obtained reveals a strong eastward transport in the gulf stream bounded by westward flowing water on either side within these energetic flows we see evidence for recently ventilated classical labrador sea water which has not reached the deep western boundary current north of puerto rico in any significant amounts within the caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep atlantic water through the major deep sill the anegada jungfern passage the signature of the deep atlantic source water is most prominent in cfcs and both a bottom and mid depth maximum are present in the caribbean to the south of puerto rico off the coast of venezuela however only the deeper cfc maximum is found for the entire section the net overturning circulation heat flux and freshwater fluxes are all consistent with expectations based on water mass formation and air sea exchanges to the west of our section but the annual mean air sea fluxes of heat and freshwater from comprehensive ocean atmosphere data sets appear somewhat too small in comparison with our single section result

long term hydrographic changes at 52 and 66 w in the north atlantic subtropical gyre caribbean abstract in july september 1997 two hydrographic lines were done in the western n atlantic along longitudes of 52 and 66 w as part of the woce one time hydrographic survey of the oceans each of these two lines approximately repeated earlier ones done during the international geophysical year s igy and the mid 1980s because of this repeated sampling long term hydrographic changes in the water masses can be examined in this report we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35 n and compare our results to those presented by bryden et al 1996 who examined changes along a zonal line at 24 n most recently occupied in 1992 since this most recent 24 n section in 1992 substantial changes have occurred in the western part of the subtropical gyre at the depths of the labrador sea water lsw in particular we see clear evidence for colder fresher labrador sea water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24 n at shallower depths inhabited by waters that are an admixture of mediterranean mw and antarctic intermediate waters aaiw our recent survey shows an increase in salinity which can only be attributed to changes in water masses on potential temperature or neutral density surfaces furthermore waters above the mw aaiw layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40 year period spanned by our sections these latter changes have been dominantly due to a vertical sinking of density surfaces as t s changes in density surfaces are small but depths of individual t s horizons have increased with time the net change since the igy shows a mean temperature increase between 800 and 2500 m depth at a rate of 0 57 c century with a corresponding steric sea level rise of 1 mm yr and a net downward heave with small values near the top and bottom and a maximum rate of 2 7 m yr at 1800 m depth changes in the deep caribbean indicate a warming since the igy due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth but no significant change in the deep salinity

advection of north atlantic deep water from the labrador sea to the southern hemisphere recently formed labrador seawater lsw and overflow water from denmark strait dsow are main components of the atlantic meridional overturning circulation both exhibit a distinct chlorofluorocarbon cfc maximum here we use 25 years of cfc observations in the atlantic to study the main features of the circulation of lsw and dsow from the cfc data the age and fraction of young deep water are inferred due to the superior spatial data resolution compared to former attempts regional differences in the spreading velocity and pathways of young deep water become evident dependent on the regional circulation the observed distributions of young lsw and dsow showed that the dwbc is the fastest pathway to reach the southern hemisphere the downstream decrease of the fractions of young lsw in the dwbc is slower compared to model studies from 47 n to 42 n dwbc transports of young lsw and dsow decrease by 44 and 49 respectively at 26 n the dwbc transport of young water is still 39 of the lsw formation rate and 44 of the dsow overflow transport interior pathways also exist especially in the subpolar north atlantic and in the transition zone between the subpolar and subtropical gyre compared to dsow the distributions indicate a higher tendency for lsw to follow additional interior pathways north of 45 n the major part of lsw is younger than 20 years the general weakening of new lsw formation since the 1990s worked toward a homogenization between the lsw in the western and the eastern subpolar north atlantic

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

shell structure and distribution of cloudina a potential index fossil for the terminal proterozoic cloudina bearing biosparites and biomicrites in the lower part of the nama group namibia contain a wide morphological diversity of shell fragments that can all be attributed to the two named species c hartmannae and c riemkeae the curved to sinuous tubular shells of cloudina were multi layered each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed growth appears to have been periodic with successive shell layers forming within older layers each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer this type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements the thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid caco3 impregnated organic rich material often the shell layers are preferentially dolomitized suggesting an original mineralogy of high magnesian calcite both species in the nama group formed thickets or perhaps bioherms and this sedentary and gregarious habit suggests that cloudina was probably a filter feeding metazoan of at least a cnidarian grade of organization the unusual shell structure of cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify cloudina bearing deposits within the nama group and in other terminal proterozoic deposits around the world species of cloudina occur in limestones from brazil spain china and oman in sequences consistent with a latest proterozoic age assignment in addition supposed lower cambrian pre trilobitic shelly fossils from northwest mexico and the white inyo mountains in california and nevada including sinotubulites nevadatubulus and wyattia are all either closely related to or con generic with cloudina hence it is probable that these outcrops are latest proterozoic in age and that cloudina or cloudina like organisms were widely distributed at that time it is possible moreover to suggest that metazoan biomineralization occurred on a global scale by the latest proterozoic at the same time that evidence for complex multicellularity and locomotion in animals appears in siliciclastic ediacaran rocks in the form of body and trace fossils

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

observational evidence of alternating zonal jets in the world ocean 1 multiple zonal jets with the east west velocity direction alternating with latitude are discovered in satellite altimetry data the time varying jets are shown to populate every part of the world ocean and its marginal seas and are best seen in the anomaly of geostrophic vorticity at midlatitudes the jets have a meridional wavelength of about 300 kms with r m s sea level velocity and vorticity values of 2 4 cm 6 9 cm s and 1 5 10 6 s 1 correspondingly realistic data from the high resolution ogcm run on the earth simulator are used to justify high vertical coherence of the jets structure and relevance of the jets to an evolving mesoscale eddy field strong coupling between the jets and mesoscale eddies is hypothesized

zonal circulation across 52 w in the north atlantic 1 in july august 1997 a hydrographic acoustic doppler current profiler adcp tracer section was occupied along 52 w in the north atlantic as part of the world ocean circulation experiment hydrographic program underway and lowered adcp ladcp data have been used to reference geostrophic velocities calculated from the hydrographic data additional small velocity adjustments provided by an inverse model constraining mass and silicate transports in 17 neutral density layers yield the absolute zonal velocity field for 52 w we find a vigorous circulation throughout the entire section with an unusually strong gulf stream 169 sv and southern deep western boundary current dwbc 64 sv at the time of the cruise at the northern boundary on the west side of the grand banks of newfoundland we find the westward flowing labrador current 8 6 sv whose continuity from the labrador sea east of our section has been disputed directly to the south we identify the slopewater current 12 5 sv eastward and northern dwbc 12 5 sv westward strong departures from strictly zonal flow in the interior which are found in the ladcp data make it difficult to diagnose the circulation there isolated deep property extrema in the southern portion associated with alternating bands of eastward and westward flow are consistent with the idea that the rough topography of the mid atlantic ridge directly east of our section causes enhanced mixing of antarctic bottom water properties into overlying waters with distinctly different properties we calculate heat and freshwater fluxes crossing 52 w that exceed estimates based on air sea exchanges by a factor of 1 7

proterozoic modular biomineralized metazoan from the nama group namibia we describe a proterozoic fully biomineralized metazoan from the omkyk member 549 million years before the present of the northern nama group namibia namapoikia rietoogensis gen et sp nov is up to 1 meter in diameter and bears a complex and robust biomineralized skeleton it probably represents a cnidarian or poriferan namapoikia encrusts perpendicular to the walls of vertical synsedimentary fissures in microbial reefs this finding implies that large modular metazoans with biologically controlled mineralization appeared some 15 million years earlier than previously documented

redox sensitivity of p cycling during marine black shale formation dynamics of sulfidic and anoxic non sulfidic bottom waters abstract a high resolution geochemical record of a 120 cm black shale interval deposited during the coniacian santonian oceanic anoxic event 3 odp leg 207 site 1261 demerara rise has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo redox conditions high contents of organic matter sulfur and redox sensitive trace metals cd mo v zn as well as continuous lamination point to deposition under consistently oxygen free and largely sulfidic bottom water conditions however rapid and cyclic changes in deep ocean redox are documented by short term 15 20 ka intervals with decreased total organic carbon toc s and redox sensitive trace metal contents and in particular pronounced phosphorus peaks up to 2 5 wt p associated with elevated fe oxide contents sequential iron and phosphate extractions confirm that p is dominantly bound to iron oxides and incorporated into authigenic apatite preservation of this fe p coupling in an otherwise sulfidic depositional environment as indicated by fe speciation and high amounts of sulfurized organic matter may be unexpected and provides evidence for temporarily non sulfidic bottom waters however there is no evidence for deposition under oxic conditions instead sulfidic conditions were punctuated by periods of anoxic non sulfidic bottom waters during these periods phosphate was effectively scavenged during precipitation of iron oxyhydr oxides in the upper water column and was subsequently deposited and largely preserved at the sea floor after 15 25 ka sulfidic bottom water conditions were re established leading to the initial precipitation of cds zns and pyrite subsequently increasing concentrations of h2s in the water column led to extensive formation of sulfurized organic matter which effectively scavenged particle reactive mo complexes thiomolybdates at site 1261 sulfidic bottom waters lasted for 90 100 ka followed by another period of anoxic non sulfidic conditions lasting for 15 20 ka the observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface to mid waters from the south atlantic resulting in a lowering of the oxic anoxic chemocline in the water column alternatively sea water sulfate might have been stripped by long lasting high rates of sulfate reduction removing the ultimate source for hs production

absorption and fluorescence of dissolved organic matter in the waters of the canadian arctic archipelago baffin bay and the labrador sea the optical properties of dissolved organic matter dom were investigated for the first time in the canadian arctic archipelago caa baffin bay bb and labrador sea ls as part of the international polar year canada s three oceans project c3o the dynamics and composition of absorbing dom i e colored dom or cdom and fluorescent dom fdom were evaluated in several distinct water masses occupying the three regions surface waters arctic outflow waters west greenland intermediate waters wgiw upper labrador sea waters ulsw and bottom baffin deep water bbdw four fluorescent components were identified by applying parallel factor analysis parafac to 522 excitation emission matrix eem spectra three humic like and one protein like the fdom in surface waters of the caa and bb differed considerably in character from those of the ls with higher fluorescence intensity in the former the fluorescence intensities of the two terrestrial humic like components c1 and c3 were linearly correlated with apparent oxygen utilization aou in the caa arctic outflow and the wgiw whereas only humic like c3 was significantly correlated with aou in ulsw these findings suggest that the humic like components were produced in situ as organic matter was bio oxidized the slopes of the linear relationship between humic like intensity and aou were significantly greater in the wgiw relative to the arctic outflow waters which implies that fdom in the arctic derived deeper layer was less prone to mineralization

mean atlantic meridional overturning circulation across 26 5 n from eddy resolving simulations compared to observations 1 observations along 26 5 n are used to examine the time mean structure of the atlantic meridional overturning circulation amoc in eddy resolving simulations with the hybrid coordinate ocean model hycom the model results yield a 5 year mean amoc transport of 18 2 sv compared to 18 4 sv based on data the modeled northward limb of the amoc has a vertical structure similar to observations the southward limb is shallower than observed but deeper than other ocean general circulation models and includes a secondary transport maximum near 4000 m corresponding to nordic seas overflow water the modeled flow through the florida strait and the deep western boundary current dwbc east of abaco bahamas are also approximately consistent with observations the model results are used to clarify the sources of the northward amoc transport and to explore the circulation pattern of the southward transport in the western subtropical north atlantic in the range 18 33 n about 14 1 sv of the modeled northward amoc transport is through the florida strait and the remainder through the mid ocean primarily in the ekman layer but also below 600 m the modeled amoc transport is about 2 3 surface water and 1 3 antarctic intermediate water with no contribution from the thermocline water in between in the western subtropical north atlantic the model results depict a complicated deep circulation pattern associated with the complex bathymetry the dwbc flows southward then eastward in both the upper and lower north atlantic deep water nadw layers but with different offshore recirculation pathways and there exists a second more northern branch of eastward flow in the lower nadw layer

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

dynamic redox conditions control late ediacaran metazoan ecosystems in the nama group namibia the first appearance of skeletal metazoans in the late ediacaran 550 million years ago ma has been linked to the widespread development of oxygenated oceanic conditions but a precise spatial and temporal reconstruction of their evolution has not been resolved here we consider the evolution of ocean chemistry from 550 to 541 ma across shelf to basin transects in the zaris and witputs sub basins of the nama group namibia new carbon isotope data capture the final stages of the shuram wonoka deep negative c isotope excursion and these are complemented with a reconstruction of water column redox dynamics utilising fe s c systematics and the distribution of skeletal and soft bodied metazoans combined these inter basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation the strongly negative 13c values in the lower parts of the sections reflect both a secular global change in the c isotopic composition of ediacaran seawater as well as the influence of local basinal effects as shown by the most negative 13c values occurring in the transition from distal to proximal ramp settings critical though is that the transition to positive 13c values postdates the appearance of calcified metazoans indicating that the onset of biomineralization did not occur under post excursion conditions significantly we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive 13c that marks the end of the shuram wonoka excursion thus if the c isotope trend reflects the transition to global scale oxygenation in the aftermath of the oxidation of a large scale isotopically light organic carbon pool it was not sufficient to fully oxygenate the deep ocean both sub basins reveal highly dynamic redox structures where shallow inner ramp settings experienced transient oxygenation anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity these settings supported short lived and monospecific skeletal metazoan communities by contrast microbial thrombolite reefs found in deeper inner and mid ramp settings supported more biodiverse communities with complex ecologies and large skeletal metazoans these long lived reef communities as well as ediacaran soft bodied biotas are found particularly within transgressive systems where oxygenation was persistent we suggest that a mid ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea level rise our data support a prominent role for oxygen and for stable oxygenated conditions in particular in controlling both the distribution and ecology of ediacaran skeletal metazoan communities

the nama foreland basin sedimentation major unconformity bounded sequences and multisided active margin advance abstract the nama foreland basin developed in neoproterozoic times as a peripheral foreland basin between the kalahari craton in the east and the partly contemporaneous damara and gariep orogenic belts in the north and west the foreland basin is subdivided into three basins from north to south the zaris witputs and vanrhynsdorp basins deposits of the nama group proper fill the first two basins whereas the third contains the vanrhynsdorp group which differs in many aspects from the nama group only during its latest phase of development did the foreland basin form one continuous depository the stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis flexural bulging and sea level changes that affected the basin throughout its history palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin as mountain belts emerged along the interactive northern and western orogenic margins transport directions switched to southerly and southeasterly foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens three major unconformity bounded sequences composite sequences probably of second order cyclicity can be recognised the uppermost composite sequence cs3 comprises the fish river subgroup whereas the lower composite sequence cs1 is composed of the flaminkberg formation and the kwanous kuibis and lower and middle schwarzrand subgroups cs2 comprises the knersvlakte brandkop and upper schwarzrand subgroups cs1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a type 1 unconformity at the base of the nomtsas formation cs2 contains a conformable lowstand flysch delta partly derived from cs1 deposits eroded on flexural bulges as well as an orogenic source in the northwest cs3 is entirely composed of orogen derived molasse onlapping a basal type 2 unconformity and overstepping older sequences in a cratonward direction a number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading the age of the nama and vanrhynsdorp groups is estimated at between 600 ma and 530 ma as determined by the age of the basement the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast which affected nama and vanrhynsdorp rocks along the active basin margins marginal tectonogenesis of the nama foreland basin resulted from northwesterly subduction of the kalahari plate underneath the congo and south american plates through a process of sinistral transpression along the western kalahari margin leading to sequential closure of the proto atlantic adamastor ocean from north to south

anoxia in the terrestrial environment during the late mesoproterozoic a significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the precambrian in contrast the oxygenation history of terrestrial aquatic environments has received little attention despite the significance of such settings for early eukaryote evolution to address this we provide here a geochemical and isotopic assessment of sediments from the late mesoproterozoic nonesuch formation of central north america we utilize rhenium osmium re os geochronology to yield a depositional age of 1078 24 ma while os isotope compositions support existing evidence for a lacustrine setting fe s c systematics suggest that the nonesuch formation was deposited from an anoxic fe rich ferruginous water column thus similar to the marine realm anoxia persisted in terrestrial aquatic environments in the middle to late proterozoic but sulfidic water column conditions were not ubiquitous our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution