does the mid atlantic ridge affect the distribution of abyssal benthic crustaceans across the atlantic ocean abstract a trans atlantic transect along the vema fracture zone was sampled during the vema transit expedition in 2014 15 the aim of the cruise was to investigate whether the mid atlantic ridge mar isolates the abyssal fauna of the western and eastern abyssal basins based on two genetic datasets of macrostylidae and desmosomatidae nannoniscidae studied by riehl et al and brix et al in this issue we found that most of the therein delimitated species were found at only one side of the mar we analysed those species of macrostylidae and desmosomatidae that were sampled across the mar and complemented these with one species of a third family munnopsidae with these datasets we were further able to consider the effect of different niche adaptations macrostylidae are infaunal burrowing munnopsidae are considered epifaunal with pronounced swimming capabilities and desmosomatidae and nannoniscidae are partly able to swim but are not as well adapted to swimming as munnopsidae we concluded that the mar seems to be a dispersal barrier for the non swimming macrostylidae as well as weakly swimming desmosomatidae and nannoniscidae however four species of macrostylidae and desmosomatidae did cross the mar but evidence for regular unrestricted gene flow is still lacking for the swimming munnopsidae we were able to detect persistent gene flow across the mar

how many vent fields new estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations abstract decades of exploration for venting sites along spreading ridge crests have produced global datasets that yield estimated mean site spacings of 12 220 km this conclusion demands that sites where hydrothermal fluid leaks from the seafloor are improbably rare along the 66 000 km global ridge system despite the high bulk permeability of ridge crest axes however to date exploration methods have neither reliably detected plumes from isolated low temperature particle poor diffuse sources nor differentiated individual closely spaced clustered within a few kilometers sites of any kind here we describe a much lower mean discharge spacing of 3 20 km revealed by towing real time oxidation reduction potential and optical sensors continuously along four fast and intermediate rate 55 mm yr spreading ridge sections totaling 1470 km length this closer spacing reflects both discovery of isolated sites discharging particle poor plumes 25 of all sites and improved discrimination at a spatial resolution of 1 km among clustered discrete and diffuse sources consequently the number of active vent sites on fast and intermediate rate spreading ridges may be at least a factor of 3 6 higher than now presumed this increase provides new quantitative constraints for models of seafloor processes such as dispersal of fauna among seafloor and crustal chemosynthetic habitats biogeochemical impacts of diffuse venting and spatial patterns of hydrothermal discharge

early cambrian braid delta deposits mackenzie mountains north western canada latest neoproterozoic to earliest cambrian strata in north western canada provide an example of a pre vegetation braid delta depositional system depositional environments represented in the succession include braided fluvial and braid delta distributary channels aeolian dune fields and interdistributary lagoons bays as well as mouth bar beach to shoreface and prodelta to distal shelf settings three formations have been investigated the ingta formation formed in wave dominated nearshore to offshore shelf environments with little or no apparent deltaic influence whereas the overlying backbone ranges and vampire formations contain an extensive record of braid delta deposits ranging from braidplain to distal prodelta facies on the braid plain river channels reached widths of up to several kilometres such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial dominated mouth bar deposition with lesser wave influence wave dominated deltaic successions are rare in the succession interdeltaic areas were characterized by wave dominated prograding shorelines interdistributary lagoons probably formed primarily in abandoned distributary channels delta front prodelta deposits are silt rich and contain abundant soft sediment deformation including slumps the deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants this is illustrated in the dominance of braided fluvial deposition and of silt rich offshore facies that may have resulted from enhanced aeolian transport of loess the non actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies however the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures while the hypothesized latest proterozoic earliest cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession

shelf wide erosion deposition and suspended sediment tranport during cyclone winifred central great barrier reef australia abstract observation of shelf sediments collected immediately before and after cyclone winifred crossed the central great barrier reef shelf 1 february 1986 confirmed that the storm produced a normally graded mixed terrigenous carbonate storm layer extending 30 km offshore in water up to 43 m deep distinct post winifred changes in the cross shelf distribution of organic carbon and carbonate in the mud fraction of the sediment suggest that suspended sediment transport was extensive and that the storm layer had multiple sediment sources on a shelf wide scale the storm layer is composed almost entirely of reworked shelf sediment erosion depths were greater on the mid shelf 20 40 m water depth than on the inner shelf 6 9 cm and 5 1 cm respectively particles finer than medium sand were eroded and transported out of the mid shelf the inner shelf portion of the storm layer formed by the combination of three sediment sources including 1 seaward transport of terrigenous sediment in buoyant freshwater flood plumes 2 resuspension and settling of inner shelf sediment and 3 resuspension and shoreward transport of mid shelf sediment mass balance calculations predict that at least 10 30 of the inner shelf storm layer is composed of mid shelf mud combined wave and wind forced currents probably resuspended mid shelf material and drove the suspended fraction alongshelf and shoreward a minimum distance of 15 km the results suggest that tropical cyclones are capable of sporadic but efficient cross shelf transport of suspended sediment on shallow cyclone prone shelves suspended sediment may easily be exchanged between adjacent sedimentary facies in ancient shelf sequences the transport history of the mud will be complex and stratigraphically equivalent facies may have similar mud types but completely different sands

deep sea fluid and sediment dynamics influence of hill to seamount scale seafloor topography abstract deep sea sediments play a central role in a wide range of subject areas a number of important controls on the formation of sedimentary deposits have been studied however to date the impact of submarine landscape geometry as a possible control has received comparatively little attention this seems to be particularly true for intermediate scale topographic features such as abyssal hills knolls and seamounts that can be found in many regions of the global seafloor recent estimates suggest that in the deep open oceans away from continental margins there might be as many as 25 106 abyssal hills knolls and seamounts despite this large number very little is known about how they influence environmental complexity and patchiness biogeochemical fluxes and the formation of sedimentary records this paper reviews the currently known types of fluid flow interactions with abyssal hills knolls and seamounts that could potentially influence the way sediments are formed the main types of relevant flow components are quasi steady to eddying background flow internal lee and near inertial waves barotropic and baroclinic tides and seamount trapped waves previous studies looking into systematic links between fluid dynamics and sediments at hills knolls and seamounts are reviewed finally a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid flow components leaves a detectable imprint in the recent sediments on and around a short seamount the main conclusions and implications are as follows 1 topographically generated flow field geometries that are composed of a number of different prevailing fluid flow components can be reflected and detected in properties of the underlying sediments 2 tidal and other higher frequency lee wave near inertial components of deep ocean currents can be essential for locally driving total current velocities across threshold values for non deposition erosion resuspension of freshly deposited deep sea sediments moreover there is evidence suggesting that not only maximum current speeds but also intensities of higher frequency tidal and or near inertial current direction variability might control sediment dynamics and sediment formation this relativises the view that current speed is the main or even only controlling factor for sediment dynamics and sediment formation 3 when it comes to the reconstruction of paleo flows these findings imply that certain sedimentary records may well reveal more about variability in the higher frequency flow components than about variability in the basin scale net flow component that often is the focus of paleoceanographic studies 4 single core paleo records from hill seamount or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features to arrive at unbiased paleo records for non fluid dynamic parameters the influence of the flow field geometry would have to be removed from the record first 5 it seems the mechanistic understanding of hill and seamount related flow topography interactions and their links to sediment dynamics is approaching a level that may a facilitate improved interpretation of topographically controlled sedimentary paleo records b help fill in the knowledge gap that exists for functional deep sea biodiversity at intermediate space scales and c improve predictive capabilities for exploration of economically relevant iron manganese fe mn crusts on seamounts

on the strength of oceanic fracture zones and their influence on the intraplate stress field we use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere if fracture zones are weaker than the surrounding lithosphere they should be sites of concentrated intraplate seismicity and the lithospheric stress field should be perturbed in their vicinity the 77 earthquakes selected for the study have well determined focal mechanisms and epicenters in regions where fracture zones are well mapped we have searched for dependence of faulting style fault orientation or principal stress direction on the distance from the nearest fracture zone if fracture zones were generally weaker than the surrounding lithosphere one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner there is a slight tendency for earthquakes to occur near fracture zones and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress these patterns are not demonstrable at high significance however if oceanic transform fault zones are weak relative to the surrounding lithosphere these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere

decadal warming of the coldest antarctic bottom water flow through the vema channel a decadal warming trend of antarctic bottom water flowing through the vema channel is reanalyzed our data base consists of 94 high precision full depth stations from 19 visits to the vema sill plus twelve stopovers at two additional key locations originally a long term temperature increase in the near bottom jet was noted from 1992 onward after a period of rather constant abyssal temperatures since 1972 from today s perspective the apparent stagnant temperature level until 1991 can be interpreted as a period of feeble rising in comparison with a perspicuous warming trend of 2 8 mk yr 1 in the following 15 years however the clearly manifested temperature rise is superimposed with fluctuations for the first time the available time series appears long enough to indicate an associated slight freshening of the bottom water an attempt is made to trace the observed variability back to its source region in the weddell sea

eastward flow through the mid atlantic ridge at 11 n and its influence on the abyss of the eastern basin abstract the dilute antarctic bottom water of the north atlantic eastern trough is supplied from the western trough through fractures in the mid atlantic ridge in particular the influence on eastern trough property distributions of flow through the romanche and vema fracture zones near the equator and 11 n respectively has been noted previously here new observations are reported that document the abyssal circulation of the northeastern atlantic basins gambia abyssal plain south canary basin and north canary basin in particular the dominance of vema influence the absence of romanche influence and the existence of a system of deep western boundary currents and estimated transport deep isopycnals slope steeply across the vema s eastern end near 39 w corresponding to a geostrophic transport through the vema of 2 1 to 2 3 106 m3s 1 colder than 2 0 c this is half or more of the estimated bottom water that flows north across the equator into the subtropical western north atlantic this transp

late quaternary sedimentation in the paraiba basin northeastern brazil landform sea level and tectonics in eastern south america passive margin abstract late quaternary deposits in the northeastern brazil have been scarcely investigated despite their relevance to the discussion of the post rift evolution of the south american passive margin within the context of landform sea level and tectonic deformation sedimentological stratigraphic and morphological characterization of these deposits referred as post barreiras sediments led to their distinction from underlying early middle miocene strata based on optically stimulated luminescence osl dating two sedimentary units pb1 and pb2 were recognized and related to the time intervals between 74 8 9 3 and 30 8 6 9 ka and 8 8 0 9 and 1 8 0 2 ka respectively unit pb1 consists of indurated sandstones and breccias either with massive bedding or complex types of soft sediment deformation structures generated by contemporaneous seismic activity unit pb2 is composed of massive sands or sands related to structures developed by dissipation of dunes the present work focusing on the post barreiras sediments discusses landform sea level and tectonics of the eastern south american passive margin during the latest quaternary non deposition and sub aerial exposure related to the tortonian worldwide low sea level combined with tectonic quiescence followed the miocene transgression tectonic deformation in the latest pleistocene created space to accommodate unit pb1 in downthrown faulted blocks and perhaps also synclines produced by strike slip deformation although deposition of this unit was simultaneous with the progressive fall in sea level that followed the last interglacial maximum punctuated rises combined with land subsidence led to marine deposition close to the modern coastline renewed subsidence in the holocene gave rise to accommodation of the post barreiras sediments most of unit pb2 was deposited during the holocene transgression but it is not composed of marine sediments which suggests either an insignificant rise in relative sea level or aeolian reworking of thin transgressive sands the data presented here lead to a review of the evolution of the south american passive margin based on assumptions of uniform sedimentation and undeformed planation surfaces over a wide coastal area of the northeastern brazil

trace fossils as paleoenvironmental indicators in the taylor group devonian of antarctica abstract the taylor group of southern victoria land antarctica contains varied and abundant trace fossils which have been interpreted by palaeontologists as indicating marine deposition however sedimentologists have largely favoured non marine deposition on the basis of desiccation polygons fining upward cycles unidirectional paleocurrents small channels red beds and paleosols the debate is reconciled by a reinterpretation of the trace fossil assemblage it is shown that widespread and well preserved cruziana diplichnites rusophycus and skolithos which have previously been used as marine indicators comprise an ichnofacies scoyenia which is entirely typical of fresh water

the fine structure of the subducted investigator fracture zone in western sumatra as seen by local seismicity the sumatran margin suffered three great earthquakes in recent years aceh andaman 26 december 2004 mw 9 1 nias 28 march 2005 mw 8 7 bengkulu 12 september 2007 mw 8 5 here we present local earthquake data from a dense amphibious local seismic network covering a segment of the sumatran margin that last ruptured in 1797 the occurrence of forearc islands along this part of the sumatran margin allows the deployment of seismic land stations above the shallow part of the thrust fault in combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available in this region the investigator fracture zone ifz which consists of 4 sub ridges is subducted below the sunda plate this topography appears to influence seismicity at all depth intervals a well defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced ifz sub ridges more intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor the plate interface beneath siberut island which ruptured last in 1797 is characterised by almost complete absence of seismicity

carbon isotope stratigraphy across the silurian devonian transition in podolia ukraine evidence for a global biogeochemical perturbation the carbon and oxygen isotope composition of marine carbonates 13 c and 18 o respectively are studied in the fossiliferous stratigraphically well constrained and remarkably expanded successions of podolia sw ukraine spanning the silurian devonian transition significant isotopic shifts are directly comparable to previously published global secular trends in well preserved brachiopod calcite isotopic ratios from this region and therefore may be taken as a reliable primary record of seawater 13 c changes the sections reveal a major positive 13 c excursion with an amplitude above 6 beginning in the upper pridoli and reaching peak values as heavy as 4 2 in the lowermost lochkovian this turnover in carbon cycling is followed by a general trend toward more negative 13 c values in the upper lochkovian the podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the silurian devonian transition reflecting a complex combination of palaeogeographical biogeochemical and evolutionary processes in the late caledonian geodynamic setting with a likely undervalued role of the expanding vegetation in vast near coastal shallows and deltas

flexural uplift of a lithospheric slab near the vema transform central atlantic timing and mechanisms abstract the vema transverse ridge vtr is a prominent long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the vema transform at 11 n in the atlantic it rises abruptly about 140 km from the axis of the mid atlantic ridge mar in 10 myr old crust and runs continuously up to 25 myr old crust it reaches over 3 km above the predicted lithospheric thermal contraction level it is absent in crust younger than 10 myr thus the uplift of the vtr must have ended roughly 10 ma the vtr is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long mar segment based on satellite gravimetry imagery this mar segment was born roughly 50 ma and increased its length at an average rate of 1 6 mm yr multibeam data show that the mar parallel sea floor fabric south of the vtr shifts its orientation by 5 to 10 clockwise in 11 12 myr old crust indicating a change at that time of the orientation of the mar axis and of the position of the euler rotation pole this change caused extension normal to the transform followed between 12 and 10 ma by flexure of the edge of the lithospheric slab uplift of the vtr at a rate of 2 to 4 mm yr and exposure of a lithospheric section vema lithospheric section or vls at the northern edge of the slab parallel to the vema transform ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the vls at different distances from the mar axis suggest that the entire vtr rose vertically as a single block within the active transform offset a 50 km long portion of the crest of the vtr rose above sea level subsided was truncated at sea level and covered by a carbonate platform subaerial and submarine erosion has gradually removed material from the top of the vtr and has modified its slopes spreading half rate of the crust south of the transform decreased from 17 2 mm yr between 26 and 19 ma to 16 9 mm yr between 19 and 10 ma to 13 6 mm yr from 10 ma to present the slowing down of spreading occurred close in time to the change in ridge transform geometry suggesting that the two events are related a numerical model relates lithospheric flexure to extension normal to the transform suggesting that the extent of the uplift depends on the thickness of the brittle layer consistent with the observed greater uplift of the older lithosphere along the vtr

early devonian trace fossils in marine to non marine redbeds in podolia ukraine palaeoenvironmental implications abstract lochkovian redbeds in the dnester formation in podolia ukraine record an upward transition from marginal marine to fluvial conditions a 60 m thick section through transitional strata reveals a regressive stack of three facies associations laid down in offshore lagoon inshore lagoon or bay and a fluvial estuarine plain the offshore lagoonal association shows green to variegated alternation of fossiliferous bioturbated limestones quartz arenites and shales numerous coquinas contain schizohaline ostracods vermiform gastropods lingula charophyte gyrogonites all mixed with fragments of marine forms such as articulate brachiopods tabulates goniatites and leiospheres trace fossil assemblages contain abundant skolithos spirophyton teichichnus and simple tubular forms the inshore lagoon or bay association is a largely non fossiliferous locally bioturbated red succession of interbedded mudstones and sandstones which are arranged in coarsening up units capped by thin channelized sandstones the presence of lockeia monomorphichnus rusophycus and skolithos suggests that the coarsening up units could have originated as small bayhead deltas and washover fan channel complexes a new ichnospecies monomorphichnus podolicus described here occurs also in these deposits in the overlying fluvial estuarine association the only trace fossil recorded is skolithos which occurs in dense clusters along upper levels of cross bedded ribbon shaped channel fills until now piperock has been considered as an ichnofabric of the skolithos ichnofacies that is typical of sandy beaches bars and spits whereas the findings presented here expand landwards the environmental range of skolithos piperocks into more intracoastal marine influenced settings

early cambrian teichichnus dominated ichnofabrics and palaeoenvironmental analysis of the caerfai group southwest wales uk abstract the early cambrian caerfai group of pembrokeshire in southwest wales yields an intensely bioturbated teichichnus dominated ichnofabric and provides evidence for the early evolution of infaunal organisms three units of the caerfai group comprise the focus of this study in sections exposed at caerfai bay in the st non s sandstone few primary sedimentary structures are observed bedding being predominantly massive with subordinate parallel and current ripple cross lamination the topmost st non s sandstone contains common soft sediment deformation phenomena and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep fronted delta system teichichnus is the dominant trace with subordinate planolites palaeophycus and possible rhizocorallium the overlying caerfai bay shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front sandstone debrites are common as are tuffaceous beds with evidence of reworking by bottom currents sandstone and tuff event beds contain common skolithos and arenicolites with less frequent planolites interbedded mudstones contain infrequent teichichnus the caerfai bay shales coarsen and thicken upwards into the caerbwdy sandstone debrite sandstone beds dominate and bed thickening upsection indicates progradation of the delta system the dominant trace fossil within the unit is teichichnus which is constrained to the top few centimetres of individual sandstone beds less common are planolites and palaeophycus the size and abundance of teichichnus in the caerfai group are similar to or exceed those in contemporaneous sections in avalonia and baltica local environmental factors were probably the primary control on trace fossil distribution substrate and salinity are proposed as the main influences on ichnodiversity and size in the caerfai group teichichnus is most abundant in the st non s sandstone where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels burrow diameter progressively decreases upsection and bioturbation becomes less pervasive in subsequent units of the caerfai group this indicates that by the cambrian stage 2 significant advances into reduced salinity environments had occurred along with the dispersal of the teichichnus producer throughout the avalonia baltica seaway correlation within avalonia indicates that a transgression in the cambrian stage 2 initiated the deposition of the caerfai group marking the onset of the welsh basin

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

lower ordovician graptolite biozonation and lithofacies of southern bolivia relevance for palaeogeographic interpretations the interpretation of the lithofacies and basin evolution of the early ordovician of southern bolivia is based on a number of sections on an e w transect lithostratigraphic units are extremely diachronous and only the available data on the graptolite biostratigraphy enabled an interpretation of the basin evolution the newly proposed graptolite biozonation includes the biozones of rhabdinopora flabelliformis adelograptus sp araneograptus murrayi hunnegraptus copiosus tetragraptus phyllograptoides expansograptus protobalticus expansograptus holmi baltograptus minutus azygograptus lapworthi and isograptus victoriae isograptus victoriae is the first isograptid identified from gondwanan south america the early ordovician succession of southern bolivia is the most complete one documented from south america and can be used as a standard for this continent the faunas are most easily correlated with the faunal succession of scandinavia and without doubt belong to the atlantic graptolite faunal province they show distinct differences from coeval faunas of the argentine precordillera referable to the pacific faunal province

an analysis of dispersion in a stratified aquifer the dispersion of a conservative solute produced as a result of vertical variations of hydraulic conductivity in a horizontal stratified aquifer of finite thickness is analyzed by applying the moment method of aris to solve the governing advection dispersion equation describing mass transport in the analysis it is assumed that the aquifer is of constant thickness and of infinite lateral extent the hydraulic conductivity is a known function of the vertical coordinate only and the flow is unidirectional parallel to the stratification the applicable aris moment equations are developed in a suitable nondimensional form analytical solutions are obtained for the zeroth and first moments and for the time derivative of the second moment of the longitudinal concentration distribution for the case of an instantaneous plane source for several idealized hydraulic conductivity profiles parabolic linear step function and cosine profiles and their even periodic extensions the analysis gives the time dependent variation of the longitudinal macrodispersivity for these idealized cases throughout the transient development of the dispersion process the results of the analysis are applied to a field measured hydraulic conductivity profile and predicted values of the longitudinal macrodispersivity are compared with field results an important conclusion from the analyses is that nonuniformities in the hydraulic conductivity profile which persist over long distances may produce rather large values of longitudinal macrodispersivity which are comparable to those observed in some aquifers and which are much larger than those predicted by some previous stochastic analyses implications of the analytical results for field dispersion problems are discussed

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

composition of abyssal macrofauna along the vema fracture zone and the hadal puerto rico trench northern tropical atlantic abstract we analyzed composition and variations in benthic macrofaunal communities along a transect of the entire length of the vema fracture zone on board of rv sonne so 237 between december 2014 and january 2015 in order to test whether the mid atlantic ridge serves as a barrier limiting benthic taxon distribution in the abyssal basins on both sides of the ridge or whether the fracture zone permits the migration of species between the western and eastern abyssal atlantic basins the puerto rico trench much deeper than the surrounding abyssal west atlantic was sampled to determine whether the biodiversity of its hadal macrofauna differs from that of the abyssal atlantic the composition of the macrofauna from the epibenthic sledge catches yielded a total of 21 332 invertebrates crustacea occurred most frequently 59 with 12 538 individuals followed by annelida mostly polychaeta 26 with 5491 individuals mollusca 7 with 1458 individuals echinodermata 4 with 778 individuals nematoda 2 with 502 individuals and chaetognatha 1 with 152 and porifera 1 with 131 individuals all other taxa occurred with overall less than ten individuals hemichordata phoronida priapulida brachiopoda invertebrate chordata echiurida foraminifera here refereed to macrofaunal komokiacea only chelicerata platyhelminthes within the crustacea peracarida 62 6 with 7848 individuals and copepoda 36 1 with 44 526 individuals were the most abundant taxa along the abyssal vema fracture zone macrofaunal abundances ind 1000 m 2 were generally higher on the eastern side while the highest normalized abundance value was reported in the puerto rico trench at abyssal station 14 1 2313 individuals 1000 m 2 the lowest abundance was reported at station 11 4 with 120 ind 1000 m 2 located at the western side of the vema fracture zone the number of major macrofaunal taxa phylum class ranged between five stations 12 5 13 4 and 13 5 at hadal depths in the puerto rico trench and 14 station 9 8 in the western abyssal basin of the vema fracture zone differences are seen in the distribution of porifera at macrofaunal level between eastern and western sides of the vema fracture zone macrofaunal composition of the study area is compared with data from other expeditions in the atlantic and the northwest pacific ocean

fan delta sedimentation in the silurian coralliferous formation of sw wales implications for the structure of the southern margin of the welsh basin the uppermost llandovery to lower wenlock coralliferous group in sw pembrokeshire is here redefined as a single formation with two members the coralliferous formation is approximately 150 m thick and comprises a basal unit of granule to pebble grade rudite beds the renney slip member overlain by interbedded mudstones and fine sandstones of the deadman s bay member the renney slip member lies unconformably above the skomer volcanic group and includes 12 4 m of coarse grained granule and pebble rich rudites with beds up to 0 94 m thick three lithofacies are recognized within this unit coarse granule rich rudite beds are interpreted as a variety of mass flow deposits some of which have been reworked in a marine environment thick sandstones with planar and ripple lamination are shoreface to offshore transition zone deposits silty mudstones interbedded with very fine grained sandstones represent marine offshore deposits formed largely below mean storm wave base these facies associations and abundant bioturbation indicate an environment with a strong marine influence and a proximal source of coarse grained sediment the renney slip member is reinterpreted in the context of a fan delta depositional model at least seven cycles of deposition are recognized each showing an upwards fining pattern representing deposition from fan delta shoreface transition zone to open marine environments these patterns of deposition are attributed to localized tectonic movements causing variations in relative sea level at the time the renney slip member was deposited the southern welsh basin margin is interpreted as a fault block extensional margin with the landmass of pretannia to the south though fan delta deposition took place southwards against the uplifted footwall of the wenall fault the basin margin lay to the south of the ritec fault

palaeoenvironments of early devonian fish and other aquatic fauna of the campbellton formation new brunswick canada abstract the palaeoecology and palaeoenvironments of the classic fish bearing strata of the campbellton formation are here described a well documented fossil assemblage includes acanthodian placoderm chondrichthyan and cephalaspid fishes tracheophyte remains eurypterids ostracods and molluscs as part of the old red sandstone continent the emsian campbellton formation occupied a small basin within the mountainous terrain of the acadian orogen and may have connected through a narrow seaway to a closing foreland basin the highest diversity of vertebrate fossils was found above a basal unconformity with val d amour formation volcanics marked by fissure fills and unstratified breccia representing a craggy irregular palaeosurface that provided shelter for some fauna and sediment for rapid burial of skeletal remains overlying prodeltaic calcareous siltstones are also highly fossiliferous and represent a low energy habitat occurrences of pterygotid eurypterids cephalaspids and chondrichthyans in deltaic sandstones place these taxa in a habitat that may have been variably fresh or brackish the surrounding landscape was vegetated with primitive tracheophytes and abundant plant debris is preserved throughout the basin the diverse assemblage of fish arthropods gastropods plants and microbial growths suggests that a trophically complex ecosystem was in existence

conditions de fossilisation du genre lingula brachiopoda et implications paleoecologiques abstract in the light of recent ecological investigations on recent lingulid brachiopods some general concepts on lingulid paleoecology are reviewed under drastic environmental changes fossilization occurs 1 in life position e g rapid temperature decrease salinity increase desiccation or emersion of the subtratum very fine sedimentation 2 in flatlaying shells e g prolonged reduction in salinity coarse size sedimentation storms tolerances of lingulid populations distribution and species characteristics are briefly discussed in regard to the lingulid occurrences in recent and fossils ecosystems it is suggested that morphological conservatism of lingula since the paleozoic is related to the niche continuum in deltaic or estuarine ecosystems and to a large degree of independence of environmental constraints

turbulent mixing in a deep fracture zone on the mid atlantic ridge abstractmidocean ridge fracture zones channel bottom waters in the eastern brazil basin in regions of intensified deep mixing the mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones whether affected by the local or the nonlocal canyon topography are still subject to debate to discriminate those mechanisms and to discern the canyon mean flow two moorings sampled a deep canyon over and away from a sill contraction a 2 layer exchange flow accelerated at the sill transports 0 04 0 10 sv 1 sv 106 m3 s 1 up canyon in the deep layer at the sill the dissipation rate of turbulent kinetic energy e increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy cross sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides during spring to neap tides e varies from o 10 9 to o 10 10 w kg 1 below 3500 m around the 2 layer inte

faulting in madeira abyssal plain sediments abstract interpretation of high resolution seismic data revealed the occurrence of faults in the sediments of two north atlantic abyssal plain environments the madeira abyssal plain map and the nares abyssal plain nap most of the data are from the map where faulting appears to be confined to particular locations from our data individual faults cannot be traced from one seismic section to another a positive correlation exists between the concentration of faults and the maximum sediment thickness although the mechanism that caused the faulting is still not understood the faults apparently are related to processes which have been active for a long period of time

tidal and storm influenced sedimentation from a cambrian transgressive passive margin sequence the basal cambrian clastic sediments in northwest scotland record a transgressive sequence of barrier island to tidal shelf quartz arenites eriboll sandstone lower member and pipe rock overlain by a storm dominated interval of mixed clastic carbonate sediments fucoid beds two shallowing events interrupt this sequence the lower near the middle of the pipe rock and the upper at the top of the fucoid beds which led to the deposition of the quartz arenites of the salterella grit variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz arenites and then decreased until deposition of the mixed clastic carbonate deposits these features suggest amplification of tides as the shelf approached a resonant width during transgression further transgression produced a wide non resonant shelf where tides were frictionally damped and storm processes were dominant similar sequences of the same age in western newfoundland eastern greenland and spitsbergen may also have been produced by the shelf bordering the north american craton responding to tidal amplification followed by frictional damping

sedimentology and oceanography of early ordovician ironstone bell island newfoundland ferruginous seawater and upwelling in the rheic ocean abstract the bell island and wabana groups together comprise a ca 150 m thick succession of interbedded clastic and chemical sedimentary rocks composed of eight distinct lithofacies that accumulated along the northern margin of the rheic ocean lithofacies stacking patterns indicate that deposition occurred during a marine transgression with superimposed small scale sea level fluctuations producing at least six parasequences parasequences containing ironstone are 10 to 20 m thick and composed of hummocky cross stratified sandstone interbedded with organic rich mudstone and phosphatic fe silicate bearing siltstone which is overlain by hematitic granular ironstone capped by an erosive flooding surface this lithofacies association is interpreted to record the deposition of upwelling related ironstone on a storm dominated shelf the close relationship between fe silicates and phosphorite typical of upwelling systems suggests that fe was delivered from deep anoxic nutrient rich seawater that also stimulated high surface productivities the result was the precipitation of authigenic sedimentary apatite in anoxic organic rich sediments that accumulated near the upwelling front the gradual advection of fe rich waters away from the upwelling front initiated precipitation of fe silicate coated grains and cements in suboxic pore waters iron pumped into shallower environments through advection and fe redox cycling is interpreted to have precipitated fe oxyhydr oxide grains in sediment of the oxygenated middle shelf these coated grains were subsequently concentrated by fairweather and storm currents on the shoreface to create granular economic fe deposits we challenge conventional models of paleozoic ironstone deposition that rely on a continental source of fe by proposing a hydrothermal source that supplied fe2 to the shelf through upwelling it also highlights the potential connection between the delivery of anoxic ferruginous seawater to the margins of the rheic ocean and the early ordovician extinctions that punctuated the beginning of the great ordovician biodiversification event

trends in depth and extent of bioturbation in cambrian carbonate marine environments western united states cambrian carbonate strata of the great basin deposited below fairweather wave base but above mean storm wave base inner shelf and below mean storm wave base but above maximum storm wave base middle shelf were examined for extent and depth of bioturbation data indicate that initial extensive colonization of marine infaunal habitats occurred between tommotian and atdabanian time throughout the remaining 65 m y of cambrian time bioturbation increased although depth of bioturbation remained relatively constant at less than 6 cm during this time the amount of bioturbation in the inner shelf was greater than that in the middle shelf

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

vertical motions of the puerto rico trench and puerto rico and their cause 1 the puerto rico trench exhibits great water depth an extremely low gravity anomaly and a tilted carbonate platform between reconstructed elevations of 1300 m and 4000 m i argue that these features are manifestations of large vertical movements of a segment of the puerto rico trench its forearc and the island of puerto rico that took place 3 3 m y ago over a time period as short as 14 40 kyr i explain these vertical movements by a sudden increase in the slab s descent angle that caused the trench to subside and the island to rise the increased dip could have been caused by shearing or even by a complete tear of the descending north american slab although the exact nature of this deformation is unknown the rapid 14 40 kyr and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending the proposed shear zone or tear is inferred from seismic morphological and gravity observations to start at the trench at 64 5 w and trend southwestwardly toward eastern puerto rico the tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount

first evidence for high temperature off axis venting of deep crustal mantle heat the nibelungen hydrothermal field southern mid atlantic ridge during segment scale studies of the southern mid atlantic ridge mar 7 12 s we found evidence in the water column for high temperature hydrothermal activity off axis east of ascension island extensive water column and seafloor work using both standard ctd and deep submergence auv and rov deployments led to the discovery and sampling of the drachenschlund dragon throat black smoker vent at 8 17 87 s 13 30 45 w in 2915 m water depth the vent is flanked by several inactive chimney structures in a field we have named nibelungen the site is located 6 km south of a non transform offset between two adjacent 2nd order ridge segments and 9 km east of the presently active northward propagating a2 ridge segment on a prominent outward facing fault scarp both vent fluid compositions and host rock analyses show this site to be an ultramafic hosted system the first of its kind to be found on the southern mar the thermal output of this single vent based on plume rise height information is estimated to be 60 15 mw this value is high for a single black smoker vent but small for an entire field the tectonic setting and low he content of the vent fluids imply that high temperature off axis venting at drachenschlund is driven not by magmatic processes as at the majority of on axis hydrothermal systems but by residual heat mined from the deeper lithosphere whether this heat is being extracted from high temperature mantle peridotites or deep crustal cumulates formed at the duelling non transfrom offset is unclear in either case the drachenschlund vent provides the first direct observations of how cooling of deeper parts of the lithosphere at least at slow spreading ridges may be occurring

marine event beds and recolonization surfaces as revealed by trace fossil analysis turbidites and tempestites exhibit characteristic distributions of trace fossils to gain maximum advantage of the sedimentological and palaeoecological significance of these traces an attempt should be made to relate each trace to the associated colonization surface this approach allows traces initiated at the normal top of an event bed to be distinguished from traces that subsequently passed into the bed from a higher colonization level or were initiated on the eroded surface of the bed

mixing associated with sills in a canyon on the midocean ridge flank to close the global overturning circulation the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion hydrographic and microstructure observations from the brazil basin in the south atlantic ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge in previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents here the results from a detailed analysis of velocity hydrographic and microstructure data from a ridge flank canyon are presented two year long current meter records indicate that within the canyon there is a significant along axial mean flow down the density gradient toward the ridge crest five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2 the mean dissipation of kinetic energy inside the canyon exceeds that above the ridge flank topography by approximately a factor of 5 the largest dissipation values were observed downstream of a narrow 1000 m high sill that extends across the full width of the canyon along the entire canyon there is a strong association between the presence of sills and along axial density gradients while there is no similar association between the presence of depressions and density gradients together these observations suggest that sill related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal wave breaking which is not expected to be associated preferentially with sills while only 15 of the interfacial area between antarctic bottom water and north atlantic deep water in the brazil basin lie inside canyons the available data suggest that approximately one half of the diapycnal buoyancy fluxes take place there in comparison the region above the ridge flank topography accounts for about one third of the buoyancy fluxes the apparent importance of sill related processes for mixing in ridge flank canyons is therefore of global significance especially considering that such canyons occur on average every 50 km along 2 3 of the global midocean ridge system and that sills partially block the canyon axes every few tens of kilometers

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

the tectonic fabric of the ocean basins 1 we present a global community data set of fracture zones fzs discordant zones propagating ridges v shaped structures and extinct ridges digitized from vertical gravity gradient vgg maps we use a new semi automatic fz tracking program to test the precision of our hand digitized traces and find a mean absolute deviation of less than 3 4 km from the raw vgg minima that most clearly delineate each feature and less than 5 4 km from the fz location predicted by fitting model profiles to the vgg data that represent the morphology of the individual fzs these offsets are small considering gravity data only provide an approximation for the underlying basement morphology we further investigate the origin of non fz seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half spreading rates a residual abyssal hill height grid with spreading rate effects removed combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills where discordant zones do not overprint abyssal hill signals residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt depletion our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo faults and extinct ridges associated with ridge propagation and or microplate formation in the southern pacific ocean

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

paleoecology of the marine endobenthos endobenthic animals which reside within the sea bottom include stationary suspension feeders mobile deposit feeders and both stationary and mobile carnivores their activities especially with regard to dwelling feeding walking crawling and resting nesting are recorded as trace fossils abundance diversity and density of some kinds of trace fossils allow interpretation of the population strategies of the trace makers in terms of opportunistic r selected and equilibrium k selected strategies opportunistic ichnotaxa tend to be faciesbreaking traces which are highly localized in low diversity high density trace fossil associations in rocks representing environmental extremes e g variable salinities harsh temperatures low oxygen levels or shifting substrates equilibrium ichnotaxa usually are restricted to particular sedimentary facies and are characteristic of high diversity low dominance trace fossil associations in sediments reflecting stable predictable environmental conditions the most important environmental factors influencing the composition of trace fossil assemblages in marine settings are bathymetry substrate oxygen and hydrodynamic energy the four factors are closely interrelated because as water depth increases there is a general decrease in sediment grain size and hydrodynamic energy of the depositional environment as depth below the water sediment interface increases the firmness of the sediment due to compaction and dewatering increases and the oxygen content of interstitial waters drops drastically marine ichnofacies are largely substrate controlled soupgrounds are water saturated incompetent substrates typified by highly compressed and usually unidentifiable burrows softgrounds commonly contain numerous distinctive burrows and are zoned bathymetrically by the skolithos cruziana zoophycos and nereites ichnofacies firmgrounds are characterized by stiff compacted sediments in which traces of the glossifungites ichnofacies are excavated hardgrounds are cemented substrates in which bioerosion traces of the trypanites ichnofacies are bored woodgrounds are woody materials that have been exposed to the sea and bored by bivalves which produce characteristic traces of the teredolites ichnofacies tiering of endobenthic communities is common and is related to substrate preference of the burrowers and oxygen stratification of interstitial waters

phylogeny of the reedocalymeninae trilobita implications for early ordovician biogeography of gondwana

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

the peridotite ridge province in the southern iberia abyssal plain seismic constraints revisited the west iberia margin has become the type example of a magma starved margin in which extension during breakup led to the exhumation of mantle rocks apparently over a broad area much of our understanding of this process is based on geophysical and drilling data from the southern iberia abyssal plain where the seaward part of the region of exhumed mantle is comprised of a series of margin parallel basement ridges here we analyse wide angle and normal incidence seismic data from a series of intersecting profiles across this peridotite ridge province and develop a three dimensional picture of its structure using these seismic data we infer a location and age magnetic anomaly m1 125 127 ma of first formed oceanic crust in this region that are consistent with ocean drilling program data and data from the conjugate margin we infer that the peridotite ridge province is about 70 km wide with strong serpentinization 75 at the top of basement reducing to much lower degrees 25 around 2 km below we map the geometry of some prominent reflectors on the landward margin of the peridotite ridge province that likely represent detachment faults and show that one of these has a domal structure similar to that of detachment faults at slow to ultra slow spreading mid ocean ridges

ordovician increase in extent and depth of bioturbation implications for understanding early paleozoic ecospace utilization the infaunal habitat was colonized in the early phanerozoic primarily by soft bodied organisms to understand the early biotic history of this environment it is necessary to examine the overall record of bioturbation or ichnofabric we present here results documenting trends in ichnofabric from ordovician strata of the great basin in california nevada and utah collected by a method devised to semiquantitatively measure the extent of early paleozoic utilization of infaunal ecospace when combined with similar cambrian data a two phase stepwise pattern of increasing bioturbation contemporaneous with the cambrian and ordovician radiations of skeletonized metazoans is apparent these data represent evidence independent of the body fossil record of increasing ecospace utilization in the early paleozoic

how valid is cruziana stratigraphy the stratigraphic value of trilobite burrows has recently been questioned because of a seemingly ordovician form in the lower cambrian of canada closer examination shows however that the cambrian homeomorph cruziana pectinata n sp has a significantly different scratch pattern this raises the question whether the multiple scratches of the ordovician cruziana rugosa were also made by exopodites or by similarly shaped endopodites

discovery of widely available abyssal rock patches reveals overlooked habitat type and prompts rethinking deep sea biodiversity habitat heterogeneity and species diversity are often linked on the deep seafloor sediment variability and hard substrate availability influence geographic patterns of species richness and turnover the assumption of a generally homogeneous sedimented abyssal seafloor is at odds with the fact that the faunal diversity in some abyssal regions exceeds that of shallow water environments here we show using a ground truthed analysis of multibeam sonar data that the deep seafloor may be much rockier than previously assumed a combination of bathymetry data ruggedness and backscatter from a trans atlantic corridor along the vema fracture zone covering crustal ages from 0 to 100 ma show rock exposures occurring at all crustal ages extrapolating to the whole atlantic over 260 000 km2 of rock habitats potentially occur along atlantic fracture zones alone significantly increasing our knowledge about abyssal habitat heterogeneity this implies that sampling campaigns need to be considerably more sophisticated than at present to capture the full deep sea habitat heterogeneity and biodiversity

isotopic evidence 87sr 86sr δ7li for alteration of the oceanic crust at deep rooted mud volcanoes in the gulf of cadiz ne atlantic ocean the chemical and isotopic composition of pore fluids is presented for five deep rooted mud volcanoes aligned on a transect across the gulf of cadiz continental margin at water depths between 350 and 3860 m generally decreasing interstitial li concentrations and sr 87 sr 86 ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin although highest li concentrations at the near shore mud volcanoes coincide with high salinities derived from dissolution of halite and late stage evaporites clayey terrigenous sediments are identified as the ultimate li source to all pore fluids investigated light delta li 7 values partly close to those of hydrothermal vent fluids delta li 7 11 9 parts per thousand indicate that li has been mobilized during high temperature fluid sediment or fluid rock interactions in the deep sub surface intense leaching of terrigenous clay has led to radiogenic sr 87 sr 86 ratios similar to 0 7106 in pore fluids of the near shore mud volcanoes in contrast non radiogenic sr 87 sr 86 ratios similar to 0 7075 at the distal locations are attributed to admixing of a basement derived fluid component carrying an isotopic signature from interaction with the basaltic crust this inference is substantiated by temperature constraints from li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures 200 degrees c for the least radiogenic pore fluids of the most distal location advective pore fluids in the off shore reaches of the gulf of cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous fine grained sediments resulting in a chemical and isotopic signature similar to that of fluids in near shore ridge flank hydrothermal systems this suggests that deep rooted mud volcanoes in the gulf of cadiz represent a fluid pathway intermediate between mid ocean ridge hydrothermal vent and shallow marginal cold seep due to the thicker sediment coverage and slower fluid advection rates the overall geochemical signature is shifted towards the sediment diagenetic signal compared to ridge flank hydrothermal environments c 2009 elsevier ltd all rights reserved

cambrian trace fossils of the cruziana ichnofacies from the bikaner nagaur basin north western indian craton abstract the marwar supergroup of the bikaner nagaur basin is composed of sediments deposited from the late neoproterozoic ediacaran to upper cambrian the nagaur sandstone formation of the nagaur group uppermost division of the marwar supergroup preserves trace fossils significant for establishing early cambrian biostratigraphic zones and depositional facies fifteen ichnospecies and eight ichnogenera identified in the nagaur sandstone formation include treptichnus pedum cruziana cf tenella cruziana isp diplichnites ispp a b and c gyrophyllites isp lockeia isp merostomichnites isp monomorphichnus gregarius isp nov monomorphichnus isp planolites isp psammichnites isp rusophycus bikanerus isp nov rusophycus cf carbonarius rusophycus isp and radial trace fossils these trace fossils belong to ethological categories pascichnia repichnia cubichnia and fodinichnia and represent arthropod and worm like burrowing biota the assemblage and a regional comparison with contemporaneous trace fossils in the eastern gondwanan realm suggest that the sequence in the study area belongs to the cruziana tenella ichnozone and to stage 2 upper part of terreneuvian however the middle cambrian is not excluded the trace fossil assemblage belongs to the archetypal cruziana ichnofacies cross bedded sandstone mud cracks and rainprints in the ichniferous strata of the nagaur sandstone formation indicate deposition in an intertidal sand flat with channels that was exposed episodically

strike slip faults mediate the rise of crustal derived fluids and mud volcanism in the deep sea we report on newly discovered mud volcanoes located at 4500 m water depth 90 km west of the deformation front of the accretionary wedge of the gulf of cadiz and thus outside of their typical geotectonic environment seismic data suggest that fluid flow is mediated by a 400 km long strike slip fault marking the transcurrent plate boundary between africa and eurasia geochemical data cl b sr 87 sr 86 sr d 18 o dd reveal that fluids originate in oceanic crust older than 140 ma on their rise to the surface these fluids receive strong geochemical signals from recrystallization of upper jurassic carbonates and clay mineral dehydration in younger terrigeneous units at present reports of mud volcanoes in similar deep sea settings are rare but given that the large area of transform type plate boundaries has been barely investigated such pathways of fluid discharge may provide an important yet unappreciated link between the deeply buried oceanic crust and the deep ocean

the middle rhuddanian lower silurian hot shale of north africa and arabia an atypical hydrocarbon source rock an important hydrocarbon source rock a hot shale has been identified from the middle rhuddanian llandovery lower silurian of a number of localities in north africa and arabia it is unusual in having been deposited during a eustatic fall in sea level evidence for this sea level fall in libya and jordan is manifested in a wide variety of palynological proxy data including reduced acritarch and chitinozoan total abundance reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups and from the presence of a minor positive 13corg excursion at the same stratigraphical level as the hot shale the environment of deposition was anoxic before during and after deposition of the hot shale except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three dimensional pyrite internal moulds these incursions occurred just before and during deposition of the hot shale and are interpreted as resulting from the influx of sediment laden oxygenated currents from shallower water environments enhanced organic matter preservation within the hot shale is attributed to greater productivity resulting from increased nutrient input particularly from wind blown dust more rapid descent through the water column of organic matter in post bloom marine snow macroaggregates development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the rhuddanian

taphofacies models for epeiric sea environments middle paleozoic examples abstract skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and therefore are useful in discriminating taphonomic facies for purposes of taphonomic comparisons skeletonized marine taxa are reduced to five generally defined types massive arborescent univalved bivalved and multielement different skeletal types display different attributes which reflect specific responses to physical chemical and or biological processes the taphonomic properties of disarticulation reorientation and sorting fragmentation corrosion and abrasion skeletal dissolution and early diagenesis display patterns which are predictable with respect to a variety of inter related physico chemical parameters these can be quantified through indices which evaluate relative frequencies of occurrence sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event related properties thereby generating composite taphofacies such events produce taphonomic signature which in themselves correspond to specific environmental conditions i e water depth and event intensity indeed background and event related processes interact to generate complex but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments the comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties deductively derived taphofacies models corroborated by empirical data from paleozoic strata illustrate the distribution of taphonomic properties with respect to environmental energy turbulence background sedimentation rate and sediment oxygenation these parameters also indirectly influence other less obvious factors including bioturbation and sediment chemistry trends in the distribution of taphonomic properties taphonomic gradients permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore offshore transect within a generalized epeiric sea setting taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena moreover taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies

biostratigraphic precision of the cruziana rugosa group a study from the ordovician succession of southern and central bolivia cruziana ichnospecies have been repeatedly reported to have biostratigraphic significance this study presents a re evaluation of the arthropod ichnotaxa of the cruziana rugosa group from bio and or lithostratigraphically well defined lower to upper ordovician siliciclastic sections of southern and central bolivia with the exception of cruziana rouaulti the ichnofaunas contain all the members of the cruziana rugosa group throughout the ordovician arenig to caradoc successions in bolivia the bolivian material therefore indicates that these arthropod ichnofossil assemblages are suitable for recognizing ordovician strata in bolivia these findings cast doubt on their use as reliable indicators for a global intra ordovician arenig to caradoc biozonation of peri gondwanan sedimentary successions

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

ichnology of the laisong formation late eocene early oligocene of the naga hills indo burma range ibr paleoenvironmental significance abstract the late eocene early oligocene laisong formation basal barail group from the naga hills indo burma range has yielded a low diversity of trace fossils i e chondrites gyrolithes ophiomorpha palaeophycus planolites and thalassinoides three ichnofabrics namely ophiomorpha palaeophycus thalassinoides and chondrites planolites have been identified the ophiomorpha palaeophycus ichnofabric grades upsection into chondrites planolites ichnofabric in a fine to medium grained sandstone unit through a succession of alternating thinly bedded sandstone and mudstone containing thalassinoides ichnofabric the ichnofabric distribution is primarily controlled by the change in depositional energy levels from low to high a low diversity trace fossils and lack of graphoglyptids in the laisong formation of naga hills supports the presence of a proximal part of the hyperpycnal lobes of delta fed marine coarse grained turbidite system

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

habitat characterization of the vema fracture zone and puerto rico trench abstract although many of the regions on and close to the mid ocean ridges have been extensively mapped and sampled the abyssal intraplate regions remain essentially unsampled and unmapped leaving huge gaps in our understanding of their geologic history and present activity prominent bathymetric features in these intraplate regions are fracture zones here we present bathymetric and sampling information from a transatlantic transect along the vema fracture zone ca 11 n covering crustal ages from 109 0 ma on the african plate and 0 62 ma on the south american plate the vema fracture zone is the intraplate trace of the active vema transform plate boundary which offsets the present day mid atlantic ridge by ca 300 km left laterally juxtaposing zero age crust with crust of 20 million years age our results show clear evidence of tectonic activity along most of the fracture zone in most places likely associated with active fluid flow within the active vema transform at crustal ages of ca 10 ma we found clear indications of fluid flow both in the sediments and the overlying water column this region is 120 km from the nearest spreading axis and increases by almost an order of magnitude the maximum off axis distance that active hydrothermal discharge has been found on the oceanic crust sampling of the igneous seafloor was possible at all crustal ages and the accretionary fabric imprinted on the plate during its production was prominent everywhere seafloor sediments show signs of extensive bioturbation in one area high concentrations of spherical mn nodules were also found and sampled at the end of the transect we also mapped and sampled the puerto rico trough a 8000 m deep basin north of the caribbean arc here the seafloor morphology is more complicated and strongly influenced by transpressive tectonics

skolithos in a permian braided river deposit southern victoria land antarctica abstract skolithos burrows have been found in a permian braided river deposit the lower feather conglomerate of southern victoria land antarctica they are straight vertical 1 cm in diameter and up to 30 cm deep the host sediment varies from fine to very coarse sandstone and even in places fine conglomerate they are most abundant in coarse sandstone where they form densely packed clusters and have destroyed virtually all stratification they are less common in fine and medium sandstone and fine conglomerate they appear to be identical to skolithos linearis the sedimentary facies paleocurrent measurements and especially the well developed paleosols of the lower feather conglomerate provide a clear and consistent picture of rapid but intermittent deposition in relatively small shallow river channels in a high plains setting and a temperate sub humid to humid climate it is thought that the burrows are dwelling structures of worm like organisms that responded to a fluctuating water table in the same way that polychaetes in a modern coastal setting respond to tidal changes in water level we conclude that skolithos can no longer be used as it has in the past as an unequivocal indicator of coastal deposition

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

the sedimentary record of carboniferous rivers continuing influence of land plant evolution on alluvial processes and palaeozoic ecosystems abstract evidence from modern rivers and the deep time geological record attests to the fundamental importance of plant life for the construction of physical habitats within fluvial environments data from an extensive literature review and original fieldwork demonstrates that many landforms and geomorphic features of modern river systems appear in the palaeozoic stratigraphic record once terrestrial vegetation had adopted certain evolutionary advances for example stable point bars are associated with the onset of rooted plants in the siluro devonian and avulsive and anabranching fluvial systems become common at the same time as extensive arborescent vegetation in the carboniferous in this paper we demonstrate a correlation between the diversification of physical fluvial environments and the expansion of terrestrial fauna and flora with an emphasis on the culmination of these trends within carboniferous alluvial systems many extrinsic factors have been considered as possible controls on the evolutionary timelines of terrestrialization for organisms however a fundamental prerequisite for achieving terrestrial biodiversity was the variety of physical habitats especially riparian systems available for newly evolved organisms in association with abundant lowland meandering systems the widespread appearance across carboniferous alluvial plains of fixed channel and anabranching reaches created further physical landforms for colonization and would have promoted increasingly complex hyporheic flow regimes furthermore the associated increase in arborescent vegetation and supply of large woody debris to inland and coastal rivers would have created a wealth of microhabitats for continental organisms we argue that the expanding extent and diversity of physical alluvial niches during the palaeozoic is an underappreciated driver of the terrestrialization of early continental life the study of the deep time fossil and stratigraphic record also illustrates that vegetation is a fundamental prerequisite for the creation of biogeomorphic alluvial landforms and physical habitats and microhabitats

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

biodiversity and distribution of polynoid and spionid polychaetes annelida in the vema fracture zone tropical north atlantic abstract during the vema transit bathymetry of the vema fracture zone and puerto rico trench and abyssal atlantic biodiversity study expedition from december 2014 to january 2015 a transect along the vema fracture zone in the equatorial atlantic was surveyed and sampled at about 10 n the vema fracture zone is one of the largest fracture zones of the mid atlantic ridge and it is characterized by a large left lateral offset benthic communities of the transect and the abyssal basins on both sides were investigated to examine whether the mid atlantic ridge serves as a physical barrier for these organisms or if there is a potential connection from east to west via the vema fracture zone samples comprised 4149 polychaetes belonging to 42 families exemplary polynoidae and spionidae both typical deep sea families with high abundances in all investigated regions were identified up to species level the present results show significant differences in polychaete faunistic composition between both sides of the mid atlantic ridge moreover the eastern and western vema fracture zone characterizes divergent habitats since the two basins differ in sedimentology and environmental variables e g temperature salinity hence characterizing divergent habitats most species found were restricted to either eastern or western vfz but there was a trans mid atlantic ridge distribution of certain abundant species observed indicating that the mid atlantic ridge might rather act limiting to dispersal between ocean basins than as an absolute barrier given the abyssal valley formed by the vema fracture zone and its role in oceanic currents this seafloor feature may well represent exchange routes between eastern and western faunas

seismicity near the slip maximum of the 1960 mw 9 5 valdivia earthquake chile plate interface lock and reactivation of the subducted valdivia fracture zone understanding the processes behind subduction related hazards is an important responsibility and major challenge for the earth sciences few areas demonstrate this as clearly as south central chile where some of the largest earthquakes in human history have occurred we present the first observation of local seismicity in the villarrica region 39 40 s based on a temporary local network of 55 stations installed from the chilean coast into the argentinian back arc for one year while consistent with the chilean national catalog ssn our results allow us to observe smaller magnitudes with a completeness of about 2 0 and image the geometry of the wadati benioff zone from the chile trench down to 200 km offshore a gap in interplate seismicity is observed in the region of the 1960 valdivia earthquake slip above the interface two offshore seismicity clusters possibly indicate ongoing stress relaxation in the subducting nazca plate we find a prominent seismicity cluster along the extrapolated trace of the oceanic valdivia fracture zone vfz the seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike slip events it indicates weakening and reactivation of the major vfz by dehydration of oceanic crust and mantle interpreting the subducted vfz section as a localized reservoir of potential fluid release offers an explanation for the villarrica volcanic complex that is located above the reactivated vfz and shows the highest volcanic activity in south america crustal seismicity is observed near puyehue volcano which recently started to erupt june 2011 key points we observe local seismicity near the 1960 valdivia maximum slip region the subducted valdivia fracture zone is reactivated by fluid release the seismogenic zone is free of seismicity events occur at faults and volcanoes

ichnology palaeoecology and taphonomy of a gondwanan early vertebrate habitat insights from the ordovician anzaldo formation bolivia sacabambaspis janvieri is the most completely known pteraspidomorph fish from the ordovician in terms of its morphology histology and palaeogeographical range restricted to gondwana sacabambaspis has assumed a role as the text book ordovician fish as it is known from a large number of articulated specimens recovered from within the mid late ordovician anzaldo formation of central bolivia the research presented here provides insights into the nature of the habitats of this early vertebrate utilising new palaeoecological ichnological and taphonomic field data the anzaldo formation is interpreted as having been deposited in a number of shoreface and offshore transition environments that can be divided into four ichnofacies containing thirteen ichnotaxa arenicolites isp cruziana furcifera c goldfussi c rugosa curvolithus isp monocraterion isp palaeophycus striatus phycodes curvipalmatum planolites montanus rusophycus isp r latus skolithos linearis and teichichnus isp two of the ichnofacies are bathymetrically controlled the cruziana and skolithos ichnofacies and two are controlled by localised depositional and salinity conditions a mixed skolithos cruziana and restricted skolithos ichnofacies articulated specimens of sacabambaspis are confined to the restricted skolithos ichnofacies where they have been found in association with concentrations of lingulid brachiopods other brachiopod concentrations are found within the archetypal skolithos ichnofacies data is presented that suggests that both the fish and brachiopods were killed by a combination of freshwater inundations and influxes of terrigenous sediment that buried and preserved the organisms within nearshore obrution deposits subsequent colonization of these obrution deposits was then initially limited to opportunistic skolithos tracemakers 2007 elsevier b v all rights reserved

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

stratigraphic significance of cruziana new data concerning the cambrian ordovician ichnostratigraphic paradigm the classic cambrian ordovician ichnostratigraphic paradigm originally developed in europe is based on the assumption that cruziana ichnofossils presumably produced by trilobites can be used in much the same way as trilobite body fossils have been used in chronostratigraphy that these ichnofossils can be found in many otherwise unfossiliferous shallow marine siliciclastic deposits has made them extremely valuable as correlation tools the paradigm has been used to date lower paleozoic units in eastern canada and europe it has also been used as supporting evidence to show close affinities between 1 eastern north america wales and spain and 2 spain northern africa and southern asia ichnospecies indicative of the lower ordovician tremadocian and arenigian according to the paradigm have been recovered from the lower cambrian atdabanian gog group near lake louise alberta canada this discovery indicates that these ichnospecies cannot be used as global lower ordovician index fossils

animal substrate interactions in an early miocene wave dominated tropical delta delineating environmental stresses and depositional dynamics tacata field eastern venezuela abstract the miocene tacata reservoirs of the eastern venezuela basin contain deposits that accumulated as a result of the progradation of a wave dominated delta complex in a tropical setting integration of ichnologic and sedimentologic data helps to detect different stress factors allowing distinction between deltaic and associated along strike strandplain shoreface successions fluvial discharge was repeatedly interrupted by periods of storm wave reworking and suspension fallout leaving diagnostic ichnologic signatures in the deposits repeated storms rank among the most important controlling factors on the front of the tacata deltas lateral especially on the updrift side to the river mouths wave activity caused nearly continuous accretion of the associated strandplains evidence of tidal influence is essentially restricted to distributary channel and particularly interdistributary bay deposits although elements of the skolithos ichnofacies e g skolithos are present they tend to occur in subordinate numbers as a result of water turbidity distributary channel deposits are sparsely bioturbated but the presence of certain ichnotaxa e g ophiomorpha allows distinction from freshwater fluvial channels ichnofabrics from interdistributary bay deposits display all the characteristics expected from brackish water settings e g low ichnodiversity presence of monospecific suites and are also typically sparsely bioturbated teichichnus and planolites commonly associated with synaeresis cracks are dominant ichnotaxa in these fine grained low energy protected brackish water settings in contrast delta front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water settings such as chondrites and phycosiphon preservation of very thin storm layers is probably more common in wave dominated deltas than in strandplain systems because deltaic stresses preclude the establishment of an abundant infauna that otherwise would have completely reworked such thin sandstone layers amalgamated thick bedded storm deposits are typically unburrowed or contain deep vertical ophiomorpha scolicia ichnofabrics seem to be restricted to the fully marine parts of strandplain deposits and are therefore useful to detect possible delta asymmetry along depositional strike or vertical changes in degree of deltaic influence

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

a global biogeochemical perturbation across the silurian devonian boundary ocean continent biosphere feedbacks abstract the large scale global biogeochemical perturbation across the silurian devonian s d boundary recorded in the major positive excursion of the 13c time curve klonk event of an amplitude of 2 5 to 3 0 max 4 0 in europe and up to 5 0 in north america reflects a unique combination of palaeogeographic biogeochemical and evolutionary processes in the late caledonian geodynamic setting the steady sulfur isotopic ratios show an overall stability of the s d oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean continent biosphere feedback coastal zones of the latest silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation which explosively expanded to inhabit vast near coastal shallows and deltas large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for co2 drawdown which have resulted in reversed greenhouse effect and a global climatic cooling tendency this feedback was blocked when the sea level gradually dropped and led to shrinking of the silurian epicontinental seas and the growing climatic deterioration during the s d transition limited primary production furthermore continued processes of regressive abrasion and erosion limited the storage of organic carbon as well as the efficiency of the carbonate factory during the following early devonian greenhouse interval marine regression and active latest caledonian tectonism promoted progressive weathering of the sedimentary organic matter the oxidation of c rich deposits caused the subsequent growth of co2 levels in the early devonian culminating in the warming of global climate

vema fracture zone central atlantic tectonic and magmatic evolution of the median ridge and the eastern ridge transform intersection domain the eastern vema fracture zone intersection with the mid atlantic ridge mar axis was surveyed with the french submersible nautile during the vemanaute cruise in 1989 at this ridge transform intersection rti an elongated e w ridge more than 50 km long is present in the transform valley this median ridge rises up to 1000 m above the surrounding seafloor the crest of the median ridge is lower and presents an arcuate shape at the tip of the mar axis dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation this detrital cover consists of polymictic sedimentary breccias sandstones and siltstones composed of basaltic doleritic and gabbroic clasts with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation disaggregation mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls this event could be related to the uplift of the southern wall of the fracture zone the transverse ridge which took place probably between 10 and 3 ma ago since this uplift episode the transverse ridge which is now undergoing subsidence and the detritus covered transform valley floor separated by the transform fault zone have migrated westward and eastward respectively vertical tectonics of the median ridge at the approach of the rti can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors a possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western rti is more than 1 km out of isostatic equilibrium recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the mar axis have been recorded on the southern flank of the median ridge several stages in the very recent tectonic volcanic history of the eastern rti that is roughly during the last 300 000 years can thus be defined the lower elevation and narrow arcuate shape of the median ridge east of 40 57 w are inferred to have resulted from tectonic extension during the creation of the nodal basin

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems