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

effects of strait mixing on ocean stratification the density distribution in the abyssal atlantic ocean suggests that mixing associated with overflows across deep sills may account for substantial amounts of deep mixing estimates of the strait mixing are made from published estimates of the overflows and the difference between antarctic bottom water densities across the vema channel and the romanche fracture zone to demonstrate that the strait mixing is an order of magnitude larger than the abyssal mixing estimated for a standard diffusivity of 1 times 10 ndash 4 m2 s ndash 1

230th and 231pa on geotraces ga03 the u s geotraces north atlantic transect and implications for modern and paleoceanographic chemical fluxes abstract the long lived uranium decay products 230th and 231pa are widely used as quantitative tracers of adsorption to sinking particles scavenging in the ocean by exploiting the principles of radioactive disequilibria because of their preservation in the pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean this includes the vertical flux of particulate matter to the seafloor the lateral flux of insoluble elements to continental margins boundary scavenging and the southward flux of water out of the deep north atlantic in a section of unprecedented vertical and zonal resolution the distributions of 230th and 231pa across the north atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the mid atlantic ridge boundary scavenging is clearly expressed in the water column along a transect between mauritania and cape verde which is used to quantify a bias in sediment fluxes calculated using 230th normalization and to demonstrate enhanced 231pa removal from the deep north atlantic by this mechanism the influence of deep ocean ventilation that leads to the southward export of 231pa is apparent the 231pa 230th ratio however predominantly reflects spatial variability in scavenging intensity complicating its applicability as a proxy for the atlantic meridional overturning circulation

acraman impact ejecta and host shales evidence for low temperature mobilization of iridium and other platinoids geochemical investigations of a widely dispersed late proterozoic impact ejecta horizon and its host shales bunyeroo formation australia have provided strong evidence for low temperature mobilization of the platinum group elements pge including ir pge anomalies are associated with both the ejecta horizon ir up to 2 0 ppb pt up to 270 ppb and the green shales that envelop it several pge anomalies 0 073 0 45 ppb ir 3 11 314 ppb pt also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence average red shale background of 0 019 ppb ir and 0 89 ppb pt all green shale horizons analyzed regardless of their stratigraphic position have relatively high levels of ir and other pge the nonextraterrestrial postdepositional origin of the pge at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin permeable green beds in a predominantly red shale sequence their association with enrichments in cu v zn ni and their nonchondritic pge interelement ratios the impact ejecta horizon has a more chondritic pge geochemistry consistent with a meteoritic origin a redox precipitation model similar to that invoked for redbed cu u v deposits is proposed to explain the pge anomalies in the green shales

long period changes in the bottom water flowing through vema channel further details of the warming of bottom water flowing through the vema channel first reported by zenk and hogg 1996 are given because cross channel gradients of temperature are large and the channel is narrow careful analysis is required to determine whether or not the bottom water temperature minimum was adequately sampled by a particular station or cruise during a period from 1972 to 1996 at least 14 visits with quality hydrographic measurements have been made to the region of these 11 were judged successful and their data indicate an abrupt rise in potential temperature by 0 03 c from 0 18 c to 0 15 c in the early 1990s that has remained until the latest observation in the spring of 1996 although this observation is consistent with the report of warming of antarctic bottom water in the argentine basin by coles et al 1996 their associated conclusion that this water mass has also freshened by 0 008 psu on potential density surfaces is in contradiction with our finding of no measurable change

physical and biogeochemical transports structure in the north atlantic subpolar gyre 1 physical mass heat and salt and biogeochemical nutrients oxygen alkalinity total inorganic carbon and anthropogenic carbon transports across the transoceanic world ocean circulation experiment woce a25 section in the subpolar north atlantic 4x line obtained previously alvarez et al 2002 2003 are reanalyzed to describe their regional and vertical structure the water mass distribution in the section is combined with the transport fields to provide the relative contribution from each water mass to the final transport values the water mass circulation pattern across the section is discussed within the context of the basin scale thermohaline circulation in the north atlantic eastern north atlantic central influenced antarctic intermediate mediterranean and subarctic intermediate waters flow northward with 10 3 5 6 1 7 and 2 9 sv respectively the total flux of lower deep water ldw across the section is 1 sv northward a portion of ldw flowing northward in the iberian basin recirculates southward within the eastern basin and about 0 7 sv flow into the western north atlantic about 7 sv of labrador seawater lsw from the labrador sea cross the 4x section within the north atlantic current and 12 sv of lsw from the irminger sea flow southward within the east greenland current denmark strait overflow water flows over the western flank of the irminger basin 1 5 sv and iceland scotland overflow water isow is mainly transported in the charlie gibbs fracture zone 6 sv isow remnants flow southward to the eastern basin contributing to the eastern basin deep ventilation lsw and the overflows transport cant southward across the section this southward cant transport is overcome by the northward transport in upper layers finally cant is injected into the subpolar gyre where it accumulates

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

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

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

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

the marine lockne impact structure jamtland sweden a review the lockne impact structure in jamtland 63 00 20 n 14 49 30 e formed in the middle ordovician at approximately 455 ma the structure is a concentric crater with a total diameter of 13 5 km the impact took place in a marine environment seawater played an important role in the cratering process and in crater morphology and the amount of melt remaining in the structure seawater rushed back into the crater in a resurge eroding and redepositing the ejecta among the resurge deposit seawater furthermore facilitated the hydrothermal system which was driven by the residual heat in the structure the lockne structure hosts shocked quartz and an iridium anomaly the rim wall round the crater collapsed in the modification stage of the crater and was annihilated by the resurge the fractured basement and the impact breccia were initially rich in open cavities these became partly filled with dominantly calcite the filling contributed to a low density contrast generating a negative gravity anomaly of 22 gu the gravity model indicates a central uplift and a nw directed tilt of the structure this tilt is also seen in the magnetic models the apparent absence of any impact melt is probably real and related to the environment of impact

identification of the projectile at the brent crater and further considerations of projectile types at terrestrial craters abstract impact melt samples from drill hole b1 59 at the 3 8 km diameter brent crater ontario have been analysed for siderophile trace elements indicative of meteoritic contamination samples from the basal melt zone at 823 857 m depth are enriched in ir os pd ni co cr and se over basement with the abundance pattern suggesting a chondritic projectile for brent from a ni cr correlation of 10 melt samples an l or ll chondrite is inferred the contribution of an ultramafic country rock alnoite in the melt is too small to significantly influence its ni cr ratio glass rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component interelement ratios e g ni cr are however fractionated relative to the melt zone samples this as well as the low au content of all brent samples is probably a product of alteration additional data on impact melts from the 65 km diameter crater manicouagan still did not reveal a meteoritic component as also for the mistastin crater 28 km diameter where cr analyses set an upper limit of 1 of an achondritic projectile component in the melt irghizites tektite like glasses from the zhamanshin impact structure have been found to contain high ni and co concentrations and our data show that ir is also enriched it is however not possible to define the projectile type enrichment of an ivory coast tektite in ir is confirmed there are large differences in siderophile element concentrations among tektites with otherwise similar chemical composition there are now four known craters formed by chondrites clearwater east lapparjarvi wanapitei and brent with brent being the smallest of these for smaller craters the projectiles appear to be limited to iron or stony iron meteorites because of atmospheric destruction of relatively small stony meteorites it appears however that all major classes of meteorites are represented among the projectiles at terrestrial impact craters

highly siderophile elements re os ir ru rh pd au in impact melts from three european impact craters saaksjarvi mien and dellen clues to the nature of the impacting bodies twenty two large 10 g impact melt samples from three scandinavian craters i e saaksjarvi finland and mien and dellen sweden were analyzed for highly siderophile elements hse platinum group elements rh and au by the nickel sulfide technique in combination with neutron activation the ten impact melt samples from saaksjarvi are enriched in ir and other highly siderophile elements ir 2 48 0 73 ng g relative to average upper crust concentrations 0 03 0 02 ng g ir the twelve dellen and mien samples are marginally enriched in ir 0 48 0 23 ng g for dellen and 0 37 0 23 ng g for mien the amount of meteoritic component corresponds to 0 5 of a nominal ci component for saaksjarvi and about 0 1 for mien and dellen the saaksjarvi pattern is fractionated relative to ci chondrites normalized abundances increase from the refractory to the more volatile siderophile elements ir ni the trend is qualitatively similar to magmatic iron meteorites and corresponds to 0 4 of a nominal magmatic iron meteorite component tamarugal iiiab for mien and dellen no projectile assignment can be made all samples from the three impact craters have low osir ratios compared to chondritic ratios either the projectile had low osir ratios or os was lost during the impact as volatile oso4 based on the results of this study and on a compilation of literature data average upper crustal abundances of the highly siderophile elements in the target area baltic shield are estimated as 0 03 ng g for ir and os 1 1 ng g for ru 0 38 ng g for rh 2 0 ng g for pd 8 g g for co and 37 g g for cr these data are representative of the upper crust they allow for the first time reliable estimates of crustal abundances for rh and ru upper crustal abundances also show that ir and os are the most favourable elements for the identification of meteoritic signatures fractionated osir ratios in magmatic iron meteorites or potential losses of os during the impact leaves ir as the best indicator element for meteoritic contamination with ir extraterrestrial components as low as 2 104 ci may be identified

anthropogenic carbon accumulation in the subtropical north atlantic 1 recent data suggest the accumulation of anthropogenic carbon dioxide canth in the subtropical north atlantic is not occurring at a steady rate throughout the water column carbon measurements from three transatlantic cruises along 24 5 n in 1992 1998 and 2004 were investigated for changes in canth using both a back calculation shortcut technique and extended multiple linear regression for three time periods 1992 1998 1998 2004 and 1992 2004 we observed spatial and vertical changes in canth storage along with a general increase in total concentration in the surface layers total dissolved inorganic carbon tco2 and canth concentrations increased in line with atmospheric co2 levels tco2 8 8 0 5 mol kg 1 for 1992 1998 and 8 6 0 5 mol kg 1 for 1998 2004 and canth 8 0 0 2 mol kg 1 for 1992 1998 and 6 8 0 3 mol kg 1 for 1998 2004 in deeper waters canth was significantly different than zero for all depths above 5000 dbar between 1992 and 2004 while on a subdecadal timescale significant variability was observed for canth at a depth range of 800 1000 dbar evidence is presented for the arrival at 24 5 n at depth of freshly ventilated labrador sea water from the subpolar north atlantic between 1992 and 1998 as well as consistent smaller canth signals alongside the mid atlantic ridge this is in addition to low level stable increases identified in the deep eastern basin between 1992 and 2004 the first time that canth has been detected and confirmed by new measurements of carbon tetrachloride and cfc 11 from 2004 these results highlight the importance of the subtropics as a site for long term canth storage away from the surface

extensive soft sediment deformation structures in the early darriwilian middle ordovician shallow marine siliciclastic sediments formed on the baltoscandian carbonate ramp northwestern estonia abstract the nearshore middle ordovician early darriwilian deposits of the baltoscandian basin formed on the flank of tectonically stable fennoscandian shield comprise numerous soft sediment deformation structures the areal extent of the deformed interval extends up to 9000 km 2 from nw estonia to se sweden the deformation structures occur within a meter scale sandstone horizon that includes load casts flame structures ball and pillow morphologies up to 2 m wide sedimentary dykes autoclastic breccias sand volcanoes and other related soft sediment deformation structures evidently these deformation structures formed during a single seismic event that caused liquefaction and fluidization of the water saturated unconsolidated shallow marine sediments within almost the entire sandstone unit pakri formation probably a unique catastrophic earthquake of magnitude 7 or higher occurred in this region penecontemporaneously with the deposition of these sediments the ultimate cause of seismic event of such magnitude in the tectonically stable baltoscandian basin is unclear however deposition and deformation of the pakri formation fall within the middle ordovician meteoritic bombardment period which resulted from the disruption of a large parent body in the asteroid belt approximately 470 ma ago this in turn suggests a potential impact origin for the single catastrophic earthquake in the tectonically stable study region although impact crater of suitable age is currently not known from the region then previous studies have documented some impact metamorphic shock lamellae i e pdfs and abundant extraterrestrial chromite grains from the deformed sediments

recent variability of the north atlantic thermohaline circulation inferred from surface heat and freshwater fluxes abstract an annual mean surface forced component of the north atlantic thermohaline circulation thc in density space surf is diagnosed from observed surface heat and freshwater fluxes the climatological mean of surf over 1980 97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing 8 sv of greenland sea deep water gsdw 4 sv of labrador sea water lsw and 4 sv of 18 water stmw obtained for each year over 1980 97 surf reveals interannual to decadal variability in the renewal and overturning of these water masses the total surface forced overturning rate varies in the range 10 20 sv reaching peak values in 1989 90 and generally decreasing over the period 1990 97 more dramatic changes are apparent in the overturning of different water masses the overturning of stmw exhibits large amplitude interannual variability a background positive trend over 1980 97 is dominated by variance of 13 4 sv by contrast the overtur

seasonal dynamics in the azores gibraltar strait region a climatologically based study abstract annual and seasonal mean circulations in the azores gibraltar strait region north eastern atlantic are described based on climatological data an inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume salt and heat and the equations of the thermal wind the large scale gyre circulation azores current azores counter current canary current and portugal current is well represented in climatological data the azores current annual mean transport was estimated to be 6 5 0 8 sv 1 sv 106 m3 s eastward exhibiting a seasonal signal with minimum transport in the spring 5 3 0 8 sv and maximum transport in autumn 7 3 0 8 sv the azores current transport is twice that of the azores counter current in spring and autumn and is four times higher in summer and winter the southward portugal and canary currents show similar seasonal cycles with maximum transports in spring 3 5 0 6 and 6 6 0 4 sv respectively the overturning circulation within the area has an annual mean magnitude of 2 2 0 1 sv and two seasonal extremes the highest in summer 2 6 0 1 sv and the lowest in winter 1 7 0 1 sv of the annual mean about two thirds 1 4 sv of the overturning circulation results from water mass transformation west of the strait of gibraltar the downwelling and recirculation of upper central water 0 6 sv in the intermediate layer the entrainment of central water 0 6 sv into the mediterranean outflow and the contribution of antarctic intermediate water 0 2 sv to the mediterranean outflow the remaining 0 8 sv relates to the overturning in the mediterranean sea through the two layer exchange at the gibraltar strait accordingly the density level dividing the upper inflowing and lower outflowing limbs of the overturning circulation was found to be 1 31 65 kg m 3 1 potential density referred to 1000 db which is above the isopycnal that typically separates central and mediterranean water 1 31 8 kg m 3 in terms of water masses we describe quantitatively the water mass composition of the main currents focusing on the spread of mediterranean water we found that when the northward mediterranean water branch weakens in spring and autumn the westward mediterranean water vein strengthens and vice versa the maximum net transports of mediterranean water across the northern and western sections of the box were estimated at 1 9 0 6 sv summer and 0 8 0 2 sv spring respectively within the error bar 0 2 sv we found no significant net volume transport of mediterranean water across the southern section

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

hydrography of the romanche and chain fracture zones the romanche and chain fracture zones rfz and cfz are known as pathways for the lower north atlantic deep water lnadw and the antarctic bottom water aabw from the western trough to the eastern trough of the equatorial atlantic ocean across the mid atlantic ridge we report hydrographic observations describing the evolution of the lnadw and aabw properties during their transit through the equatorial fracture zones to the west of the rfz and cfz sills the lnadw and aabw cores are separated by a deep thermocline pycnocline which marks the transition between the two water masses the deep thermocline is eroded eastward and vanishes in the eastern basin this zonal asymmetry is the signature of a zonal baroclinic pressure gradient associated in each fracture zone with eastward flows of lnadw and aabw below 3800 m depth during the eastward transit through the rfz the near bottom aabw becomes less dense saltier 0 07 and warmer 0 7 c richer in oxygen 0 3 ml l 1 but less rich in silicate 30 mol l 1 simultaneously the lnadw core becomes fresher and colder at a given density and the oxygen maximum characterizing the lnadw core weakens by 0 2 ml l 1 we argue that the property changes within the aabw and lnadw are due to intense vertical mixing in addition to vertical mixing with the aabw it seems likely that isopycnal mixing of the lnadw with the eastern basin deep water contributes to the property evolution of the lnadw exiting the rfz in the cfz evidence is also given of mixing occuring within the aabw and the lnadw a sudden change in the aabw properties observed across the cfz main sill suggests the blocking of the eastward progression of the densest aabw by the cfz main sill however this dense water can reach the eastern basin by flowing northeastward through valleys the sills of which are deeper than thoses of the cfz finally a simple hydraulic control model applied to the rfz and cfz flows predicts a transport of 1 4 0 1 106 m3 s 1 of water colder than 1 9 c across the rfz cfz main sill

evolution of deepwater mixing and weathering inputs in the central atlantic ocean over the past 33 myr the isotopic composition of nd in present day deep waters of the central and northeastern atlantic ocean is thought to fingerprint mixing of north atlantic deep water with antarctic bottom water the central atlantic romanche and vema fracture zones are considered the most important pathways for deep water exchange between the western and eastern atlantic basins today we present new nd isotope records of the deepwater evolution in the fracture zones obtained from ferromanganese crusts which are inconsistent with simple water mass mixing alone prior to 3 ma and require additional inputs from other sources the new pb isotope time series from the fracture zones are inexplicable by simple mixing of north atlantic deep water and antarctic bottom water for the entire past 33 myr the distinct and relatively invariable nd and pb isotope records of deep waters in the fracture zones appear instead to have been controlled to a large extent by contributions from saharan dust and the orinoco amazon rivers thus the previously observed similarity of nd and pb isotope time series from the western and eastern north atlantic basins is better explainable by direct supply of labrador seawater to the eastern basin via a northern pathway rather than by advection of north atlantic deep water via the romanche and vema fracture zones

canary current and north equatorial current from an inverse box model 1 the large scale canary basin circulation is estimated from a box inverse model applied to hydrographic data from a quasi synoptic survey carried out in september 2003 the cruise consisted of 76 full depth ctd and oxygen stations circulation is required to nearly conserve mass and anomalies of salinity and heat within layers bounded by neutral surfaces it permits advective and diffusive exchange between layers and an adjustment of the ekman transport and the freshwater flux divergences the canary current at the thermocline layer transports a net mass of 4 7 0 8 sv southward north of the canary islands from the african coast to 19 w it is divided into a northward circulation at a rate of 1 1 0 5 sv between the african coast and lanzarote island and a southward transport of 5 8 0 6 sv it transports north atlantic central water and organic matters advected offshore by the filaments protruding from the upwelling system off northwest africa at 24 n the canary current feeds the north equatorial current that transports a mixture of north and south atlantic central waters westward in the intermediate layer a southwestward flow of 1 2 1 1 sv transports mediterranean water to the subtropical gyre though the highest salt flux is transported by a meddy oxygen distribution and mass transport suggest a northeastward deep flow of a water mass colder than 2 2 c consisting of diluted antarctic bottom water the heat and freshwater divergences and the average dianeutral velocity and diffusion between the sections and the african coast are negligible

primary productivity response to heinrich events in the north atlantic ocean and norwegian sea 1 the last glacial was punctuated by several massive ice sheet surges into the north atlantic that impacted surface water hydrology especially where icebergs melted however the links between variations in surface water hydrology and surface water productivity during these heinrich events hes remain uncertain to address this issue diatoms and organic carbon were examined across heinrich event 1 he 1 and heinrich event 4 he 4 in seven sediment cores spanning 40 n to 63 n latitude our results show low diatom abundances during hes consistent with decreased surface water productivity diatom dilution by increased sediment flux was tested by normalizing diatom abundance to a constant 230th flux although the particle rain rate was enhanced during hes this does not explain the sharp drop in diatoms during he 4 surface productivity decreased at all latitudes examined probably because of strong year round stratification the same inferred changes occurred during he 1 within the area of maximum iceberg melting however at northern latitudes above 50 n the summer insolation increase of the glacial termination drove increased surface productivity during the whole period including he 1 marine organic carbon taken as independent proxy for export production supports the diatom data trends shown by the productivity proxies evolve generally in parallel with the hydrographic proxies with an increase in productivity when sea surface temperature increases

dynamical pathways of antarctic bottom water in the atlantic abstract a reduced gravity model is developed to represent the flow of antarctic bottom water aabw over realistic bathymetry in an atlantic domain the dynamics are based on the steady planetary geostrophic shallow water equations including a linear bottom friction and a uniform diapycnal upwelling through the top of the model layer the model solutions are broadly consistent with observations of the distribution and transport of aabw the flows occur predominantly along potential vorticity contours which are in turn broadly oriented along bathymetric contours the characteristic weak flow across potential vorticity contours of the stommel arons model is present as a small addition to this stronger forced mode along potential vorticity contours as a consequence mass balance is maintained not by hypothesized western boundary currents as in the stommel arons model but by the interplay between topographic slope currents and interior recirculations in particular a transposition is found in the flow

extraterrestrial chromite in the resurge deposits of the early late ordovician lockne crater central sweden the distribution of extraterrestrial chromite grains 63 mu m has been studied in the resurge deposit of the early late ordovician 458 ma lockne impact structure in central sweden a 1 kg sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite in total the sample contains 125 chromium spinel grains kg 1 most grains being chromite due to post depositional alterations the origin of several of the grains is dubious although it is clear that the major part of the chromites is of extraterrestrial origin 75 grains kg 1 most likely derived from the impactor the alterations are primarily due to the hydrothermal system induced by the impactor with zinc enrichment in the chromites as the most common characteristic the element chemistry of the least altered chromite grains indicates that the impactor was an ordinary l chondrite this concurs with a suggested increase in the flux of l chondritic asteroids to earth following the disruption of the l chondrite parent body at ca 470 ma more than 170 impact craters are known from around the earth but only few of the impactors have been identified mainly due to the low survival rate of projectile material further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the lockne structure c 2006 elsevier b v all rights reserved less

on deep western boundary currents the stommel and arons 1960a b theory of the deep circulation of the world ocean does not directly address the physics of deep boundary currents in the present paper a maximally simple steady state physics for deep western boundary currents is developed and integrated into the stommel arons model this is accomplished by modifying the assumptions of the latter to include effective horizontal eddy viscous terms everywhere in the deep water including in the interior the resulting solution for the meridional velocity in a rectangular ocean on a beta plane is mathematically analogous to that of munk 1950 for the wind driven circulation in the interior the meridional velocity reduces identically to that of stommel and arons among other benefits the solution eliminates the theoretical difficulties associated with the crossing of the equator by the deep western boundary current as an initial test of the theory s consistency with observation theoretical plots of pressure and meridional velocity as a function of longitude are qualitatively compared with hydrographic sections and inferred meridional velocity fields in the eastern and western basins of the abyssal north atlantic also the theoretical profile for the meridional velocity as a function of longitude is qualitatively compared with observations of longshore velocity as a function of offshore distance in the deep western boundary current of the north atlantic deep water

transport of bottom water in the romanche fracture zone and the chain fracture zone abstract two moored arrays deployed in the romanche fracture zone and chain fracture zone in the equatorial atlantic ocean provide two year long time series of current and temperature in the lower north atlantic deep water and the antarctic bottom water total time averaged transport of antarctic bottom water potential temperature 1 9 c across the mid atlantic ridge amounts to 1 22 106 m3 s 1 eastward with a standard deviation of 0 25 106 m3 s 1 a time averaged transport of 0 36 0 23 106 m3 s 1 eastward is found for the lower north atlantic deep water in the 1 9 2 1 c temperature range but this may represent only a fraction of the total flow of this water mass across the ridge contributions of the romanche fracture zone and chain fracture zone to the antarctic bottom water transport are similar while the chain fracture zone has the greater share of lower north atlantic deep water transport semiannual and annual periods are detected in the transport time series and together expl

the ordovician chondrite from brunflo central sweden i general description and primary minerals abstract the brunflo chondrite was found in the rodbrottet quarry near brunflo central sweden lat 63 5 n long 14 53 e the meteorite ca 11 cm 9 5 cm occurs in a polished slab of middle ordovician limestone which based on its content of conodonts belongs to the aserian segerstad limestone the meteorite fell in the iapetus ocean close to its eastern coast line the original minerals except chromite are replaced by secondary minerals mainly calcite remarkably even minute details of textures and structures are preserved in some areas the various types of chondrules and their frequencies could therefore be studied they are similar to present falls of ordinary chondrites of petrologic types 3 and 4 chromite occurs as up to ca 0 1 mm large grains microprobe analyses of 59 grains show that except for zn the composition is fairly constant and similar to h4 h5 chromites the zn content varies from 0 2 to 6 8 wt zno mean ca 0 6 wt two analyses show that also chrome spinel is present tentatively brunflo is classified as h4 h5

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

reconstruction of east west deep water exchange in the low latitude atlantic ocean over the past 25 000 years radiogenic neodymium isotopes have been used as a water mass mixing proxy to investigate past changes in ocean circulation here we present a new depth transect of deglacial neodymium isotope records measured on uncleaned planktic foraminifera from five cores spanning from 3300 to 4900 m on the mauritanian margin in the tropical eastern atlantic as well as an additional record from 4000 m on the ceara rise in the equatorial western atlantic despite being located under the saharan dust plume the eastern atlantic records differ from the composition of detrital inputs through time and exhibit similar values to the western atlantic foraminiferal nd across the deglaciation therefore we interpret the foraminiferal values as recording deep water nd isotope changes all six cores shift to less radiogenic values across the deglaciation indicating that they were bathed by a lower proportion of north atlantic deep water during the last glacial maximum lgm relative to the holocene the eastern atlantic records also show that a neodymium isotope gradient was present during the lgm and during the deglaciation with more radiogenic values observed at the deepest sites a homogeneous water mass observed below 3750 m in the deepest eastern atlantic during the lgm is attributed to the mixing of deep water by rough topography as it passes from the western atlantic through the fracture zones in the mid atlantic ridge this implies that during the lgm the low latitude deep eastern atlantic was ventilated from the western atlantic via advection through fracture zones in the same manner as occurs in the modern ocean comparison with carbon isotopes indicates there was more respired carbon in the deep eastern than deep western atlantic during the lgm as is also seen in the modern atlantic ocean

the origin of the brunflo fossil meteorite and extraterrestrial chromite in mid ordovician limestone from the garde quarry jamtland central sweden the brunflo fossil meteorite was found in the 1950s in mid ordovician marine limestone in the garde quarry in jamtland it originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the thorsberg quarry at kinnekulle 600 km to the south based primarily on the low tio2 content about 1 8 wt of its relict chromite the brunflo meteorite had been tentatively classified as an h chondrite the meteorite hence appears to be an anomaly in relation to the kinnekulle meteorites in which chromite composition chondrule mean diameter and oxygen isotopic composition all indicate an l chondritic origin reflecting an enhanced flux of meteorites to earth following the disruption of the l chondrite parent body 470 ma new chondrule size measurements for the brunflo meteorite indicate that it too is an l chondrite related to the same parent body breakup chromite maximum diameters and well defined chondrule structures further show that brunflo belongs to the l4 or l5 type chromites in recently fallen l4 chondrites commonly have low tio2 contents similar to the brunflo chromites adding support for brunflo being an l4 chondrite the limestone in the circle quarry is relatively rich about 0 45 grain kg 1 in sediment dispersed extraterrestrial chromite grains 63 mu m with chemical composition similar to those in l chondrites and the limestone 1 3 grains kg 1 at kinnekulle suggesting that the enhanced flux of l chondrites prevailed although somewhat diminished at the time when the brunflo meteorite fell less

mineralogy of meteorite groups approximately 275 mineral species have been identified in meteorites reflecting diverse redox environments and in some cases unusual nebular formation conditions anhydrous ordinary carbonaceous and r chondrites contain major olivine pyroxene and plagioclase major opaque phases include metallic fe ni troilite and chromite primitive achondrites are mineralogically similar the highly reduced enstatite chondrites and achondrites contain major enstatite plagioclase free silica and kamacite as well as nitrides a silicide and ca mg mn na cr k and ti rich sulfides aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates complex organic compounds magnetite various sulfates and sulfides and carbonates in addition to kamacite and taenite iron meteorites contain carbides elemental c nitrides phosphates phosphides chromite and sulfides silicate inclusions in iab iiicd and iie iron meteorites consist of mafic silicates plagioclase and various sulfides oxides and phosphates eucrites howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides ureilites are made up mainly of calcic chromian olivine and low ca clinopyroxene embedded in a carbonaceous matrix accessory phases include the c polymorphs graphite diamond lonsdaleite and chaoite as well as metallic fe ni troilite and halides angrites are achondrites rich in fassaitic pyroxene i e al ti diopside minor olivine with included magnesian kirschsteinite is also present martian meteorites comprise basalts lherzolites a dunite and an orthopyroxenite major phases include various pyroxenes and olivine minor to accessory phases include various sulfides magnetite chromite and ca phosphates lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase several meteoritic phases were formed by shock metamorphism martensite 2 fe ni has a distorted body centered cubic structure and formed by a shear transformation from taenite during shock reheating and rapid cooling the c polymorphs diamond lonsdaleite and chaoite formed by shock from graphite suessite formed in the north haig ureilite by reduction of fe and si possibly from olivine via reaction with carbonaceous matrix material ringwoodite the spinel form of mg fe 2sio4 and majorite a polymorph of mg fe sio3 with the garnet structure formed inside shock veins in highly shocked ordinary chondrites secondary minerals in meteorite finds that formed during terrestrial weathering include oxides and hydroxides formed directly from metallic fe ni by oxidation phosphates formed by the alteration of schreibersite and sulfates formed by alteration of troilite

structure of the deep spillway in the western part of the romanche fracture zone structure of the deep spillway was studied on the basis of conductivity temperature depth lowered acoustic doppler current profiler and mooring measurements in 2011 2014 in the romanche fracture zone 22 27 22 32 w the spillway exists quasi permanently in the regime of hydraulic control overflow and consists of two flows in the layer 1 20 c deeper than 4 150 m the first flow enters the fracture from the south through a gap in the southern wall the second flow is directed to the east along the narrow valley of the fracture the sill depths of these passages are 4 570 and 4 430 m respectively the depths over the surrounding ridges are shallower than 4 100 m both flows descend to the local depression of the fracture with 5 000 m depth bottom potential temperatures over both sills were 0 51 c and the measured velocities were 20 40 cm s when the flows descend down the slope they accelerate in supercritical regime maximal measured velocities were up to 65 cm s analysis of the 189 day time series of velocities on the mooring deployed in the flow descending from the gap showed a very high level of fluctuation energy 10 times greater than the energy of the barotropic tide at this point the only energy source of these fluctuations is the available potential energy of the deep thermocline in the west atlantic transforming to the kinetic energy of the deep spillway the total transport of the bottom water below 1 2 c based on the lowered acoustic doppler current profiler measurements is estimated at 0 29 0 35 sv which agrees with the predicted values

from the mid ordovician into the late silurian changes in the micrometeorite flux after the l chondrite parent breakup we present the first reconstruction of the micrometeorite flux to earth in the silurian period we searched 321 kg of condensed marine limestone from the late silurian cellon section southern austria for refractory chrome spinel grains from micrometeorites that fell on the ancient sea floor a total of 155 extraterrestrial spinel grains 10 grains 63 m and 145 in the 32 63 m fraction were recovered for comparison we searched 102 kg of similar limestone from the mid ordovician komstad formation in southern sweden this limestone formed within 1 ma after the breakup of the l chondrite parent body lcpb in the asteroid belt in the sample we found 444 extraterrestrial spinel grains in the 63 m fraction and estimate a content of at least 7000 such grains in the 32 63 m fraction our results show that in the late silurian 40 ma after the lcpb the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude almost down to background levels among the ordinary chondrites the dominance of l chondritic micrometeorites has waned off significantly from 99 in the post lcpb mid ordovician to 60 in the late silurian with 30 h and 10 ll chondritic grains in the late silurian primitive achondrite abundances are similar to today s value contrasting to the much higher abundances observed in pre lcpb mid ordovician sediments

on climate and abyssal circulation in the atlantic ocean during late pliocene marine isotope stage m2 3 3 million years ago abstract marine isotope stage mis m2 3 3 ma is an isolated cold stage punctuating the benthic oxygen isotope 18o stratigraphy of the warm piacenzian interval of the late pliocene epoch the prominent 0 65 18o increase that defines mis m2 has prompted debate over the extent to which it signals an early prelude to the rhythmic extensive glaciations of the northern hemisphere that characterise the quaternary and raised questions about the forcing mechanisms responsible recent work suggests that co2 storage in the deep atlantic ocean played an important role in these events but detailed reconstructions of deep ocean chemical stratification are needed to test this idea and competing hypotheses here we present new records of the nd isotope composition of fish debris and 13c and b ca ratios of benthic foraminifera from the northwest and southeast atlantic ocean our novel geochemical data show that in contrast to major quaternary glaciations such as mis 2 21 ka and mis 100 2 52 ma the deep north atlantic ocean was weakly chemically stratified during mis m2 we show that southern component water incursion into the atlantic ocean was limited to the deep south atlantic basin during mis m2 and peaked well before 10 15 kyr the atmospheric co2 minimum our findings imply that the deep atlantic ocean was not the principle sink of co2 sequestered from the atmosphere during mis m2 implicating a different co2 storage deep water reservoir mechanism presumably southern component water incursion into the pacific ocean weak chemical stratification in the deep atlantic ocean during mis m2 relative to mis 100 and 2 suggests comparatively active atlantic meridional overturning circulation that suggestion is consistent with the warmth of the high latitude north atlantic during mis m2 surface water temperatures cooled during m2 but only close to holocene values our findings may help to explain the paucity of evidence for extensive early glaciation of the northern hemisphere during m2 but leave open the possibility of ice sheet advance on antarctica

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

constraints from paleotracer data on the north atlantic circulation during the last glacial maximum we investigate the extent to which the paleotracers 13c and 18o constrain the north atlantic ocean general circulation during the last glacial maximum lgm the distinction is made in this work between the depiction of water mass distributions and the inference of the oceanic circulation the 13c data indicate that water masses formed in the north atlantic and in the southern ocean were distributed differently during the lgm but the extent of the differences cannot be quantified because of uncertainties in the end member composition reasonable choices of values for the end members produce volumes of lower north atlantic deep water nadw significantly smaller than in the modern atlantic in agreement with previous studies to explore the range of circulations consistent with this water mass distribution a coarse resolution inverse model of the north atlantic is constrained with 13c and 18o paleodata the model shows that the glacial period tracer distribution is fully consistent with the circulation rates estimated for the modern ocean however the paleodata are also consistent with a circulation rate of one half the modern flux of nadw and indeed there are an infinite number of rates of deep water movement that are possible to remove this ambiguity the central need is for a data type capable of setting the overall rates of water movement i e a clock such as that provided by geostrophy or a well sampled radioactive tracer

accessory chromium spinels their coexistence and alteration in serpentinites abstract accessory chromium spinel occurring in serpentinites is usually altered the extent of this alteration and its relationship to alteration processes such as serpentinization and metamorphism is not defined complicating the use of altered spinel as a petrogenetic indicator here the relationship between different alteration processes and spinel alteration is evaluated in terms of accessory chromium spinel occurring in serpentinites of progressive metamorphism in the eastern central alps two coexisting cr spinels were identified i e an altered fe rich cr spinel i with cr al and an fe poor cr spinel ii with al cr which exsolved a cr fe 3 phase and which resisted alteration alteration of cr spinel i is evident from decreasing al mg and cr values with a trend leading to ferrite chromite trend a an enrichment in cr trend b related to co 2 in the serpentinizing fluid indicated by the presence of carbonate talc or tremolite is superimposed to a alteration is a decomposition of the spinel structure caused by the dissolution of mg al and cr during serpentinization dissolution of cr is favored in a reduced environment whereas reprecipitation occurs under locally oxidizing conditions indicated by the presence of co 2 sio 2 detected in spinel is a relative measure of the degree of alteration metamorphism up to amphibolite facies recrystallized and homogenized preexisting spinel phases to various degrees whether or not and to what extent a spinel alters depends on its fe content fe 2 stabilized in the octahedral site high cfse is responsible for an inverse and less stable spinel structure fe rich spinels cr spinel i are therefore readily subjected to alteration whereas fe poor spinels cr spinel ii are resistant to alteration during the serpentinization processes

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

an ordinary chondrite impactor for the popigai crater siberia with a diameter of 100 km popigai in northern siberia is the largest crater known in the cenozoic the concentrations in platinum group elements pge were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile the method selected was preconcentration by nis fire assay followed by inductively coupled plasma mass spectrometry icp ms this technique measures all the pge except os and by using aliquots 10g the results are highly reproducible the major and trace element composition of the impact melt resembles that of gneissic lithologies of the anabar shield which are representative of the target rock the pge are enriched in the melt by factors of 3 to 14 compared to the main target lithology but the meteoritic contamination is only around 0 2 wt using plots of elemental ratios such as ru rh vs pt pd or ru rh vs pd ir the popigai impactor is clearly identified as an ordinary chondrite and most likely l chondrite this study indicates that pge elemental ratios allow discrimination of the type of impactor even in the case of low meteoritic contamination this study confirms that a significant fraction of the crater forming projectiles presently documented could have an ordinary chondrite composition their probable source the s type asteroids appears to form the majority of the bodies in the main asteroid belt and among near earth objects neos the ordinary chondrite origin of the popigai projectile supports an asteroidal origin for the late eocene impacts as a plausible alternative to the comet shower scenario proposed by farley et al 1998 copyright 2005 elsevier ltd

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

abyssal circulation change in the equatorial atlantic evidence from cenozoic sedimentary drifts off west africa abstract seismic profiling in the equatorial atlantic reveals deep water 4500 m sediment bodies formed by current controlled deposition near the intersection of large offset fracture zones with the african margin a 600 km long drift accumulation the ivory coast rise lies north of the st paul transform near 3 n a smaller drift deposit has been identified along the northern side of the guinea transform at 10 n antarctic bottom water aabw which presently enters the eastern atlantic basins through the romanche and vema fracture zones probably played an important role in the development of these features proto aabw may have reached the equatorial region as early as mid eocene time before the establishment of permanent ice sheets in antarctica the ivory coast rise existed as a distinct sedimentary drift by the mid eocene as a result of deposition from bottom water moving southwards along the african margin and westwards parallel to the st paul fracture zone this early flow pattern is in the opposite sense to the present movement of deep water in the sierra leone basin a reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the cenozoic

chromite in komatiites ii modification during greenschist to mid amphibolite facies metamorphism chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes particularly above 500 c metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors as a result of morphism of host rocks this modification is the subject mg fe exchange with silicates and carbonates chromite metaof this paper morphosed to amphibolite facies is enriched in zn and fe and metamorphic modification of chromite has been disdepleted in ni relative to lower metamorphic grades relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions cr al and fe are not ophiolitic or alpine ultramafic complexes onyeagocha greatly modified by metamorphism up to lower amphibolite facies 1974 ulmer 1974 evans frost 1975 hoffman although minor fe depletion occurs during talc carbonate alwalker 1978 loferski lipin 1983 kimball 1990 teration at low temperature significant al is lost from chromite burkhard 1993 and in a few studies of komatiitic rocks cores above 550 c as a result of equilibration with fluids in bliss maclean 1975 donaldson 1983 gole hill equilibrium with chlorite elevated zn content in chromite is 1990 and in a detailed study of the pechenga intrusions restricted to rocks with low metamorphic mg fe ratios and is abzalov 1998 these studies have highlighted two the result of introduction of zn during low temperature alteration important effects first chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or fermetamorphism cobalt and mn also behave similarly except where ritchromit second chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage progressively modified during prograde metamorphism chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite this is the result of incomplete metamorphic reaction silicate minerals evans frost 1975 abzalov 1998 between chromite and chlorite bearing silicate assemblages magthis paper examines the nature and magnitude of these netite compositions at the inner chromite magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade eastern goldfields province of the archaean yilgarn block in western australia fig 1 brief locality descriptions are given in appendix b

on the mediterranean water composition abstractthe mediterranean outflow water mow spills from the mediterranean sea east north atlantic basin west off the strait of gibraltar as mow outflows it entrains eastern north atlantic central waters enacw and intermediate waters to form the neutrally buoyant mediterranean water mw that can be traced over the entire north atlantic basin its high salinity content influences the thermohaline properties of the intermediate deep water column in the north atlantic and its dynamics here the composition of mw in its source region the gulf of cadiz west off strait of gibraltar is investigated on the basis of an optimum multiparameter analysis the results obtained indicate that mixing of mow 34 1 0 3 occurs mainly with overlying enacw 57 1 0 8 in a process broadly known as central water entrainment a diluted form 80 of dilution of the antarctic intermediate water aaiw reaches the region and also takes part in mw formation 8 3 0 5 finally the underlying labrador sea wat

antarctic bottom water flux in the equatorial western atlantic a moored array at the equator in the western basin of the atlantic provides a 604 day time series of abyssal currents and temperatures spanning the full breadth of the antarctic bottom water aabw flowing from the brazil basin to the guiana basin mean aabw transport is estimated to be 2 0 sv sv 106 m3 s21 comprising organized westward flow of 2 24 sv and return flow of 0 24 sv the low frequency variability is dominated by a quasi annual transport cycle of amplitude 0 9 sv and a 120 day period of amplitude 0 6 sv maximum transports occur in september october while minimum transports occur in february march allowing for this quasiannual cycle and extrapolating the 604 day record to a full two years adds about 7 to the estimated mean aabw transport the array also provides limited sampling in the overlying lower north atlantic deep water lnadw where a southern boundary intensified flow of lnadw gives the strongest recorded mean speed through the array 9 9 cm s21 into the brazil basin the lnadw records also have a quasi annual cycle with strong lnadw flow episodes occurring in april may time series of temperature indicate that the lnadw aabw transition layer rises and falls in synchrony with the quasi annual aabw transport cycle uplifted transition layer during strong aabw transport periods an observed overall warming trend appears to be accompanied by a decline in aabw transport

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

spin up and breakdown of source driven deep north atlantic flow over realistic bottom topography in this study we examine the time dependent behaviour of the deep north atlantic dynamics we use a 1 layer nonlinear shallow water model with realistic topography and high spatial resolution it is driven by an inflow of deep water from a northern source and by parameterized density exchange processes at the interface the evolving circulation pattern is complex the mid atlantic ridge with the charlie gibbs fracture zone has a fundamental influence on the large scale flow field a reduction of the inflow causes a temporary reversal of the interior currents the perturbation travels along the geostrophic contours the response times of the currents for the spin up and breakdown experiments are of the order of years they differ for different areas of the north atlantic ocean the area which is affected last by the perturbation is the northwesternmost interior corner which is close to the position of the source

dissolved al in the zonal n atlantic section of the us geotraces 2010 2011 cruises and the importance of hydrothermal inputs abstract the distribution of dissolved aluminium determined during ga03 the us geotraces north atlantic transects us gt nazt shows large inputs to the basin from three main sources atmospheric deposition outflow from the mediterranean and inputs from hydrothermal sources along the mid atlantic ridge mar the partial dissolution of atmospheric aerosols emanating from the sahara yield high concentrations of dissolved al in the surface waters of the basin and are used to estimate the geographical pattern of dust deposition the mediterranean outflow delivers a large source of dissolved al to the intermediate waters of the eastern basin and its subsequent distribution within the basin can be explained by simple isopycnal mixing with surrounding water masses hydrothermal venting at the trans atlantic geotraverse tag hydrothermal field in the mar produces a neutrally buoyant plume that introduces copious quantities of dissolved al with concentrations of up to 40 nm to the deeper waters of the north atlantic that can be seen advecting to the west of the mar the concentration of dissolved al in the deep waters of the eastern basin of the atlantic can be accounted for by admixing the mar al enriched plume water and antarctic bottom water aabw as they pass through the vema fracture zone the data sets show no evidence for biological remineralisation of dissolved al from si carrier phases in deep waters

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

chemical analyses of bosumtwi crater target rocks compared with the ivory coast tektites abstract the bosumtwi crater ghana was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 ma old lower birimian system with a granodiorite intrusion at pepiakese on its northeastern side new major and trace element analyses are presented for 7 phyllites 5 greywackes 2 microgranites 3 pepiakese intrusion rocks and 1 suevite using xrf and inna means and standard deviations were calculated using all available modern analyses for each element in the bosumtwi target rocks bosumtwi suevite glasses and ivory coast tektites good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the pepiakese intrusion and some extra silica derived from quartz veins as well as the metasediments the new data provide evidence for vapour phase fractionation of p2o5 and na2o in the tektites in addition to the previously reported pb and rb evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements ni and ir

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

the distribution of neodymium isotopes and concentrations in the eastern tropical north atlantic abstract dissolved neodymium nd and its radiogenic isotope composition 143nd 144nd expressed as end belong to the key parameters of the international geotraces program which aims to investigate the processes controlling the distribution of trace elements and their isotopes in the global ocean we present nd isotope and concentration nd data from eleven full depth water column profiles from the eastern sub tropical atlantic ocean between 2 n and 29 n and from the romanche fracture zone sampled during meteor cruise m81 1 geotraces cruise ga11 in february march 2010 the end signatures range from 12 9 in upper north atlantic deep water unadw at the equator to 8 1 in the mixed layer near the canary islands nd concentrations range from 11 9 pmol kg observed within the equatorial undercurrent to 40 2 pmol kg in antarctic bottom water aabw in the romanche fracture zone large variations in surface water nd isotope compositions 12 7 end 8 1 and concentrations 15 7 pmol kg nd 27 7 pmol kg are caused by partial dissolution of saharan dust particles between 2 n and 13 n and volcanic island weathering canaries between 25 n and 29 n in contrast to dust inputs which predominantly affect the nd concentrations and isotope compositions of surface waters and underlying south atlantic central water sacw contributions originating from the canary islands affect the nd isotope composition of the entire surrounding water column without significantly altering nd concentrations thus confirming the concept of boundary exchange in its strictest sense the results confirm that the composition of lower north atlantic deep water lnadw end 12 1 in the abyssal plains of the eastern north atlantic is exclusively set by the mixing ratio of lnadw and antarctic bottom water aabw upper north atlantic deep water unadw in contrast is characterized by end signatures between 12 7 and 12 0 between 2 n and 10 n whereas further north it is clearly affected by admixture of mediterranean water mw and radiogenic inputs from the canary islands and likely also the cape verde islands this article is part of a special issue entitled cycles of trace elements and isotopes in the ocean geotraces and beyond edited by tim m conway tristan horner yves plancherel and aridane g gonzalez

hafnium and neodymium isotope variations in ne atlantic seawater the 176hf 177hf ratio of seawater is measured directly with 143nd 144nd in a composite vertical profile in the ne atlantic the value of hf of intermediate and deep water exhibits little variation and averages 1 0 0 8 surface water shows a wider range 5 7 to 3 3 while intermediate and deep water samples plot on or close to the seawater hf nd isotope array defined by ferromanganese deposits near surface seawater samples plot between the seawater and terrestrial rock hf nd arrays it is likely that the shallow water variability in hf arises from a short residence time of hf in surface water and isotopically heterogeneous inputs the difference between surface and intermediate deep water hf may be maintained by the release of surface scavenged hf at depth from a subset of sinking particles the composition of end at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter uniform hf values below 1000 m suggest the residence time of hf may be longer than that of nd the radiogenic isotopic composition of seawater hf relative to nd and terrestrial rocks suggests that minerals with high lu hf ratios may be preferentially weathered providing a source of radiogenic hf to the oceans

dynamically consistent hydrography and absolute velocity in the eastern north atlantic ocean the problem of mapping a dynamically consistent hydrographic field and associated absolute geostrophic flow in the eastern north atlantic between 24 deg and 36 deg n is related directly to the solution of the so called thermocline equations a nonlinear optimization problem involving needler s p equation is solved to find the hydrography and resulting flow that minimizes the vertical mixing above about 1500 m in the ocean and is simultaneously consistent with the observations a sharp minimum at least in some dimensions is found apparently corresponding to a solution nearly conserving potential vorticity and with vertical eddy coefficient less than about 10 exp 5 sq m s estimates of residual quantities such as eddy coefficients are extremely sensitive to slight modifications to the observed fields boundary conditions vertical velocities etc are a product of the optimization and produce estimates differing quantitatively from prior ones relying directly upon observed hydrography the results are generally insensitive to particular elements of the solution methodology but many questions remain concerning the extent to which different synoptic sections can be asserted to represent the same ocean the method can be regarded as a practical generalization of the beta spiral and geostrophic balance inverses for the estimate of absolute geostrophic flows numerous improvements to the methodology used in this preliminary attempt are possible

extraterrestrial chromite in middle ordovician marine limestone at kinnekulle southern sweden traces of a major asteroid breakup event the distribution of sediment dispersed extraterrestrial chromite grains and other cr rich spinels 63 mu m has been studied in middle ordovician orthoceratite limestone from two quarries at kinnekulle southern sweden in the thorsberg quarry all similar to 3 2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment dispersed extraterrestrial chromite grains typically 1 3 grains are found per kilogram of limestone in the nearby hallekis quarry the same beds show similarly high concentrations of extraterrestrial chromite grains but in samples representing the 9 m downward continuation of the section exposed at this site only 5 such grains were found in a total of 379 kg of limestone the extraterrestrial equilibrated ordinary chondritic chromite grains call be readily distinguished by a homogeneous and characteristic major element chemistry including 2 0 3 5 wt tio2 and stable v2o3 concentrations close to 0 7 wt terrestrial cr rich spinels have a wide compositional range and co exist with extraterrestrial chromite in some beds these grains may be derived for example from mafic dykes exposed and weathered at the sea floor considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied instead the difference may be primarily related to all increase in the ancient flux or extraterrestrial matter to earth in connection with the disruption of the l chondrite parent body in the asteroid belt at about this time the coexistence in some beds of high concentrations of chondritic chromite and terrestrial cr rich spinels however indicates that redistribution of heavy minerals on the sea floor related to changes in sea level and sea floor erosion and currents must also be considered less

computer modelling of the water resurge at a marine impact the lockne crater sweden abstract when a cosmic impact occurs at sea the resulting crater can form completely in the water mass or extend into the seafloor at intermediate to great water depths the collapse of the water cavity generates a forceful resurge of water along the seafloor towards the central part of the crater the erosive effects as well as certain conspicuous deposits from resurge flows are known from several craters this study uses numerical modelling to estimate the magnitude of the resurge in the case of the 455 myr old marine target lockne impact crater sweden the water depth at the lockne target site was at least 200 m possibly even as great as 1 km field evidence show that the resurge was strong enough to erode gullies hundreds of metres wide tens of metres deep and kilometres long with computer modelling we studied the behaviour of the flow to get a notion of its magnitude the calculations required simplification of the reality we used a fixed seafloor topography and pure water a rim in the water mass is expected but was neglected in the calculations the simplification likely led to underestimation of the magnitude of the resurge the effect of different target water depths was studied at 200 m target water depth about 1 2 10 11 m 3 of water were needed to fill the lockne crater including the water cavity this took about 2200 s it has been suggested that the main gully formation occurs when the resurge passes the inner rim of the crater that phase lasts for about 700 s with a 500 m target water depth these processes are 3 4 times faster the erosive force of the flow i e unit stream power is 1 9 10 5 wm 2 in the 200 m water depth model this is of the same magnitude as the lake missoula flood the greatest catastrophic flooding known on earth at 500 m target water the unit stream power is a magnitude greater

role of oceanic circulation on contaminant lead distribution in the south atlantic abstract both the relatively high lead concentrations and their characteristic anthropogenic isotopic compositions attest to the widespread contamination of industrial lead in the western equatorial and south atlantic ocean spatial gradients in those isotopic signatures evidence the conservative lateral transport of lead in oceanic water masses while the discrete isotopic signatures in deep oceanic waters substantiate the complementary hypothesis that the release of lead from settling particles is relatively small on a decadal time scale specifically the relatively low radiogenic lead e g 206pb 207pb 1 148 0 009 in the lower north atlantic deep water l nadw south of 10 north is primarily attributed to us industrial lead emitted in the northern hemisphere prior to 1965 and the more radiogenic lead e g 206pb 207pb 1 180 0 006 in the upper north atlantic deep water u nadw is primarily attributed to subsequent industrial lead emissions in that hemisphere in contrast the relatively radiogenic lead e g 206pb 207pb 1 186 0 007 in the antarctic bottom water aabw seemingly reflects a mixture of natural and anthropogenic lead sources within the southern hemisphere and its isotopic dissimilarity with that e g 206pb 207pb 1 159 0 002 of antarctic intermediate water aaiw and the aabw may be due to differences in either their aeolian or water mass inputs

quasi synoptic transport budgets and water mass transformation in the azores gibraltar strait region during summer 2009 abstract we describe the circulation patterns in the azores gibraltar strait region north eastern atlantic during the 2009 caibox cruise on the basis of hydrographic and direct current velocity measurements this study offers new data for a region where importation of central waters subpolar and subtropical modes of eastern north atlantic central water and exports of mediterranean water are strongly related to large scale dynamics in the north atlantic ocean azores current mediterranean water system the description is backed up quantitatively by the results of a box inverse model which was used to obtain absolute water mass transport values consistent with thermal wind equations and with conservation of volume salt and heat the contributions of water masses were determined in an extended optimum multiparameter analysis from a quasi synoptic point of view providing detail in addition to volume salt and heat transport the surface subsurface large scale current system in the region consists of the azores current 13 1 2 5 sverdrup sv 1 sv 10 6 m 3 s 1 the azores counter current 5 2 2 1 sv the portugal current 4 5 1 4 sv and the canary current 7 1 1 1 sv broadly speaking central waters are imported into the caibox region at a rate of 1 6 0 9 sv and mediterranean water is exported at a rate of 1 5 0 4 sv the downwelling of central waters west of gibraltar strait was quantified at 1 1 sv not all this volume participates in mw formation but 0 8 sv of entrained central waters of which 0 5 sv are from central waters of subpolar origin and 0 3 sv from subtropical central waters of the 4 9 sv of subtropical central waters advected by the azores current about 0 7 sv would reach the gulf of cadiz region either to take part in central water entrainment or to flow across the gibraltar strait as part of the atlantic inflow to the mediterranean sea

unaccounted role of mediterranean water in the drawdown of anthropogenic carbon a three sided box medbox comprising the strait of gibraltar 24 41 n east of 22 w was constructed using hydrographic and chemical data from three world ocean circulation experiment cruises in 1997 1998 an extended optimum multiparameter omp analysis resolves the water mass structure in the medbox the mass transport is estimated from data and models warranting mass conservation and no deep water formation within the region combining the mass transport the omp analysis and the spatial distribution of anthropogenic carbon cant some relevant questions about the mediterranean water mw formation transport and role in the global co2 cycle are resolved mw is produced at a rate of 2 7 0 2 sv in the eastern north atlantic it entrains 2 3 sv of central waters during its formation including also remnants of antarctic intermediate water the main advection route of mw is northward close to the iberian margin 2 2 sv while about 0 5 sv are exported into the subtropical eastern north atlantic the eastern north atlantic within the medbox is a strong region of air sea cant uptake 0 7 0 7 mmol m 2 d 1 where cant is accumulated at a rate of 66 21 kmol s 1 0 025 gtc yr 1 17 are taken up through the air sea interface and the rest is advected into the area by the circulation the entrainment of central waters to form mw drives a drawdown of 151 14 kmol s 1 0 06 gtc yr 1 from the surface to intermediate levels and 88 8 kmol s 1 0 03 gtc yr 1 are exported with mw into the north atlantic mainly by the horizontal eddy circulation

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

resurge morphology of the marine lockne impact crater jamtland central sweden in the brunflo area of jamtland central sweden a cambrian to ordovician sedimentary sequence rests on a proterozoic crystalline basement the area lies just outside the site of the middle ordovician lockne impact and it later experienced caledonian overthrusting the degree of caledonian tectonization of the palaeozoic varies but an autochthonous cambrian and ordovician sequence apparently occurs in the greater part of the area particularly in the north the pre impact sedimentary succession is 81 m thick in the autochthon with the middle ordovician furudal limestone as uppermost member brunflo village is located just outside the crater 8 9 km north of its centre but the area was affected by the impact the impact generated ejecta and resurge deposits rest on a surface which cuts the sedimentary strata at a low angle this surface cuts at progressively higher stratigraphic levels at increasing distance from the crater according to observations in the autochthon the impact generated an erosion surface dipping 1 2 towards the crater centre the sequence of events that shaped this surface began with bombardment with high speed ejecta closely followed by resurging water the resurge mixed ejecta clasts with the products of resurge erosion to form the resurge deposits no rim wall can be traced at the lockne impact structure probably because a rim wall if it formed collapsed in the modification stage owing to local lithological conditions and because the remains of it were completely eroded in the resurge phase it is suggested the rim wall formed in sedimentary strata with unlithified clays at their base and that this clay was unable to support it

geology of the early palaeozoic lockne impact structure central sweden abstract the diameter of the crater at lockne as seen on topographic maps is 7 8 km the tandsbyn breccia the principal evidence of impact consists of strongly crushed local basement its distribution follows the crater margin on the north margin basement granite rests on an inverted sub cambrian erosion surface on the lowermost middle cambrian tangential faults occur at the periphery of the structure after the impact occurred under the sea during the middle ordovician there was a resurgence of ejecta loaded sea water which deposited a chaotic breccia lockne breccia which has the appearance of a debris flow the lockne breccia together with a less coarsely grained turbidite loftarsten which lies immediately above it predominantly consist of clasts of lower to middle ordovician limestone and mainly subordinate inclusions of tandsbyn breccia and isolated clasts of crystalline basement fragments of impact melt are important components of the turbidite a protective cover of sediments formed after the impact is in turn overlain by an outlier of a caledonian overthrust nappe which occupies the central and topographically deepest part of the crater shocked quartz has been identified in the loftarsten

density and magnetic susceptibility of rocks from the lockne and tvaren marine impact structures the lockne and tv ren impact craters in sweden formed in a marine environment during the ordovician the contrast in density between the impact breccias and the surrounding target rock of these two craters is significantly lower than what has been found in craters formed in crystalline targets on land another marine target structure the estonian k rdla structure demonstrates intermediate contrast in impact breccia and target rock which we attribute to the interpreted shallowness of the sea at the k rdla impact site we conclude that the main cause for these low density contrasts is pore and fracture filling of calcite with subordinate quartz and fluorite calcite is the most abundant cement and its density differs most from that of fractured and brecciated bedrock with a low degree of cementation furthermore from the studied cases it is concluded that the target rock to impact rock contrast is generally the highest in craters formed on land in crystalline targets and the lowest in craters formed at sea while craters formed on land in sedimentary targets are intermediate the low density contrasts should decrease the negative gravity anomalies of marine craters

unmelted meteoritic debris in the late pliocene iridium anomaly evidence for the ocean impact of a nonchondritic asteroid ir bearing particles have been recovered from 2 piston cores in the antarctic basin in the southeastern pacific in core e13 3 the particles closely correspond to the late pliocene ir anomaly and have a fluence of 100 mg cm 2 in core e13 4 120 km to the southwest the particle fluence is 4 mg cm 2 particles with diameters from 0 5 to 4 mm contain at least 35 of the ir in this horizon three types of particles have been identified 1 vesicular 2 basaltic and 3 metal the vesicular particles appear to be shock melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component these particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of na k cl and radiogenic sr from the ocean water target the basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud estimates of the diameter of the projectile range from 100 to 500 m by many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall

meridional overturning transports at 7 5n and 24 5n in the atlantic ocean during 1992 93 and 2010 11 abstract transatlantic hydrographic sections along latitudes 7 5n and 24 5n have been repeated with about 20 years difference at the beginning of the 1990s and 2010s for each period an inverse model is applied to the closed box bound by both sections the model imposes mass conservation for individual layers defined by isoneutral surfaces and the whole water column using surface ekman transport and several transport constraints for specific ranges of longitudes and depths as a result the velocities at the reference layer for each station pair and the dianeutral velocities between layers are estimated and the horizontal velocity fields and the water heat and freshwater transports are calculated in particular we find that mass transport per stratum at 24 5n in 2011 is in good agreement with the transport estimates from the rapid watch array during both realizations the dianeutral velocities downwell from the upper north atlantic deep water unadw to the lower north atlantic deep water lnadw strata resulting in the merging of the two southward flowing strata at 24 5n into one deep southward moving stratum at 7 5n at 24 5n there is an increase in southward unadw transport between 1992 and 2011 compensated by a decrease of southward lnadw transport a descent in the upper limit of the antarctic bottom water aabw from 1992 to 2011 is also inferred the atlantic meridional overturning circulation amoc is larger in 1992 93 than in 2010 11 decreasing from 24 7 1 7 to 20 1 1 4 sv at 24 5n and from 29 2 1 7 to 16 9 1 5 sv at 7 5n much of this decrease arises because of the northward flow of antarctic intermediate water aaiw which was much more intense in 1992 93 than in 2010 11 as a consequence heat transport at 24 5n is not significantly different in 1992 1 4 0 1 pw and 2011 1 2 0 1 pw the estimation of heat transport at 7 5n strongly depends on the magnitude of the north brazil current over the american continental platform the freshwater flux into the box bounded by the two transoceanic sections decreases significantly from 1992 93 0 45 0 05 sv to 2010 11 0 36 0 3 sv meaning a decrease in net evaporation over this same area

vema transit an interdisciplinary study on the bathymetry of the vema fracture zone and puerto rico trench as well as abyssal atlantic biodiversity abstract the seafloor below 3500 m remains largely unexplored the paucity of knowledge of abyssal and hadal environments encompasses a wide spectrum of geological and biological patterns and processes as well as their interactions historically most marine research has been conducted in the north atlantic however the high proportion of undescribed taxa frequently discovered at greater depth there underline the need to fill in these knowledge gaps the vema transit campaign in northern winter 2014 2015 surveyed and sampled along almost the entire extent of one of the major offsets of the mid atlantic ridge mar the vema fracture zone vfz as well as the deepest trench in the atlantic the puerto rico trench prt the discoveries that were made include new data on deep sea habitats showing geologically complex features across all crust ages from 110 ma until present moreover some new species and genera of the abyssal and hadal benthos were described herein not only the taxa themselves but also their distributions and genetic structure were elucidated in this context significant differences in abundances community composition and species distribution were detected that were affected by the mar as well as by the depth transition between hadal prt and the adjacent abyss despite significant differences between eastern and western communities the mar does not represent an absolute barrier instead the vfz and especially the vtf may serve as a connecting feature between east and west and this may be exemplary for fracture zones across the whole atlantic nevertheless the mar as well as the 3000 m depth gradient between abyss and hadal appear to restrict gene flow for poor dispersers and thus contribute to speciation processes in the deep sea

use of platinum group elements for impactor identification terrestrial impact craters and cretaceous tertiary boundary abstract this study presents the abundances of ru ir pt pd rh and au platinum group elements pge in the ejecta layer at the cretaceous tertiary boundary and melt rocks from east clearwater lake brent new quebec and lake wanapitei impact craters all believed to have been formed by the impact of chondritic projectiles the objective was to evaluate the pure application of pge interelement ratios e g ru ir for impactor identification to compare the utility of both melt rocks and ejecta deposits for this purpose and to expand the pge database for these craters the similarity of pge interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition introduction of indigenous pge from country rock may contaminate the original meteoritic signature in addition the extent of pge fractionation during impact and melt sheet cooling and of post sedimentary pge remobilization must be evaluated in each case for east clearwater the pge chondrite normalized plot is flat indicating a chondritic impactor based on ru ir and cr ir ratios a cl or l chondrite is most likely however variation of ratios with depth in the core makes any fine classification of the projectile tenuous for the brent and new quebec craters the flat chondrite normalized pge plot indicates a chondritic impactor but finer classification is precluded by possible post syn depositional pge fractionation at lake wanapitei ni cr and cr ir ratios indicate a ll chondrite impactor wolf et al 1980 but sparse pge data for this chondrite type does not allow further classification of the projectile a flat chondrite normalized pge plot indicates a chondritic rather than an iron impactor at lake wanapitei the global ejecta layer at the cretaceous tertiary boundary has a chondritic pge signature but only when the integrated flux of pge on a global scale is considered individual sites may not show chondritic ratios due to differential pge remobilization resulting from local differences in depositional environment continental vs marine and to vaporization and or condensation pge fractionation occurring during impact the results for melt rocks and the k t ejecta deposit indicate that pge can be used for impactor identification but primarily in combination with other siderophile chalcophile meteoritic elements and their ratios

connecting flow over complex terrain to hydrodynamic roughness on a coral reef abstractflow over complex terrain causes stress on the bottom leading to drag turbulence and formation of a boundary layer but despite the importance of the hydrodynamic roughness scale z0 in pr