dust deposition pulses to the eastern subtropical north atlantic gyre does ocean s biogeochemistry respond 1 enhancement of primary and export production following dust deposition pulses is well established for high nutrient low chlorophyll regions but the effect of atmospheric dust on the biogeochemistry of oligotrophic gyre regions remains unclear here we report atmospheric dust concentrations measured on gran canaria canary islands concomitantly with upper water biogeochemistry at the oligotrophic time series station estoc european station for time series in the ocean canary islands during a 2 year period from winter 1997 to 1999 estoc is located in the eastern subtropical atlantic gyre about 100 km north of gran canaria and 500 km west of the sahara and receives aeolian dust episodically mainly in winter and summer we found a close correlation of the magnitude of atmospheric dust concentration in winter with the magnitude of downward particle flux at estoc mainly due to a relative increase in lithogenic matter and carbonate sedimentation higher aerosol concentration was not accompanied by higher primary or export production however indicating that phytoplankton production remained unaffected by atmospheric nitrogen supply on a seasonal or yearly timescale however by estimating bioavailable iron input and the need of the phytoplankton population we found that the highly episodic dust pulses might exert a feast and famine effect on the phytoplankton despite the high lithogenic matter input carbonate mainly stemming from coccolithophorids exceeded in importance the role of lithogenic matter as ballasting agent of sinking organic matter

late quaternary palynology in marine sediments a synthesis of the understanding of pollen distribution patterns in the nw african setting abstract after a review of the first steps in marine palynology we show that the understanding of the northwest african setting is crucial to evaluate the potential of marine palynological studies elsewhere we studied distribution patterns of pollen grains in recent marine sediments off nw africa and were able to clearly relate patterns to modern pollen source areas vegetation belts and operating transport systems wind belts and ocean currents in particular patterns of quercus artemisia chenopodiaceae amaranthaceae ephedra gramineae and wet forest trees are very indicative of the position of the vegetation belts on the adjacent continent aeolian pollen transport is carried out by the northeast trade winds and the african easterly jet aej in the rain forest belt transport of pollen and fern spores also occurs by rivers a detailed comparison between recent pollen rain samples from terrestrial and marine sites between 21 and 12 n showed that the latitudinal range of vegetation belts is clearly reflected in the pollen samples of both environments a migration of the southern border of the sahara is reflected by the changing ratio between chenopodiaceae amaranthaceae pollen from the desert and gramineae pollen from the savannah belt distribution patterns of pollen for 9000 and 18 000 14 c yr bp last glacial maximum time slices based on pollen records from eleven marine cores between portugal and the gulf of guinea show significant latitudinal migrations of vegetation belts but a stable position of the main wind trajectories the aej had a stable position around 21 n the belt with trade winds had a stable position from morocco southwards changing vigour of the trade winds is clearly reflected by the patterns of isopollen contours and by changes in pollen influx records

234th 238u disequilibria within the california current1 profiles of dissolved and particulate sup 234 th were determined at several stations within the california current modeling of the disequilibria between the sup 234 th and sup 238 u within the surface waters provides for estimates of the residence time of dissolved thorium with respect to particle scavenging the particle residence time and the particulate sup 234 th flux exiting the surface layer the model derived first order scavenging rate constant for dissolved thorium is observed to be proportional to the rate of primary production particle residence times seem to be governed by the rate of zooplankton grazing and the types of zooplankton present model derived particulate sup 234 th fluxes are in good agreement with direct measurements by sediment traps

rare earth element fluxes and geochemical budget in the eastern equatorial pacific rare earth element ree patterns and flux measurements of sediment traps sediments and nodules from the eastern tropical pacific demonstrate the dominance of ree supply to the sea floor by particle settling quantitative partitioning of the particle flux reveals that biogenic debris is the major ree carrier in shallow depths however the labile particles are surpassed in importance by alumino silicate particles at greater depths regeneration of biogenic associated ree appears responsible for la ce variations in the particle flux with depth and provides the primary source of ree accreted by nodules

investigations of the stillwater complex part v apatites as indicators of evolving fluid composition variations in the f cl and oh contents of apatite are not constrained by crystal chemical factors in contrast to micas and amphiboles and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks microprobe analysis of intercumulus apatites from the stillwater complex reveal that cl rich apatites typically containing 6 0 wt cl occur throughout the lower 1 3 of the complex excluding the basal series a change from cl rich to more f rich apatite occurs within olivine bearing zone i ob i of the banded series the host zone of the platiniferous j m reef although apatite compositions are somewhat variable above the j m reef more f rich apatites predominante and typically contain 1 2 wt f and 3 0 wt cl the most f rich apatites occur in the uppermost exposed cumulates pristine apatites from coeval sills and dikes from below the complex and from the basal series are similarly f rich in all apatites the cl and f contents are lower in rocks affected by later metamorphic fluids rare earth element ree concentrations in chlorapatites show a marked peak in the olivine rich rocks of the j m reef and contain up to 2 wt ce2o3 la2o3 the trend of first increasing then decreasing cl f ratios with stratigraphic height is modeled by a vapor driven zone refining process occurring within the cumulate pile causing cl enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of cl from the magma as crystallization proceeded the abrupt change from cl rich to more f rich apatites within ob i is interpreted as the result of a breakdown of the cl rich zone refining front and mixing with cl poor supernatant melt any high temperature fluids that exsolved and circulated through the lower 1 3 of the complex must have been enriched in cl and could have transported ree and trace metals

sedimentation under deep sea storms fast currents high concentrations of suspended sediment and grooved mud beds are associated with erosion in frequent abyssal storms where a fast deep mean flow is augmented or reversed by intense intermittent currents this occurs about 5 km below the gulf stream or its rings the waning phase of a storm results in development of bedforms and rapid deposition of a mud blanket several other regions of the world ocean display evidence of abyssal storm activity

aspects of atmospheric circulation the late pleistocene 0 950 000 yr record of eolian deposition in the pacific ocean rea d k 1990 aspects of atmospheric circulation the late pleistocene 0 950 000 yr record of eolian deposition in the pacific ocean palaeogeogr palaeoclimatol palaeoecok 78 217 227 the eolian component of pelagic sediments provides a proxy record of atmospheric circulation intensity and of dust transport examination of those records indicates that the atmosphere responds to orbital forcing but that during the past 875 000 years the variability in atmospheric circulation is at much shorter periods than the 100 000 year variation in ice volume the mid brunhes climatic event is well characterized in the eolian grain size records and in records of sea surface phenomena it is not seen in proxy indicators of either deep water or ice volume variability the increase in the amplitude of paleoclimatic variability that denotes the early late pleistocene transition occurs suddenly about 875 000 years ago in the eolian grain size record which is about 20 000 to 25 000 years earlier than the transition in the deep sea caco3 dissolution record or in the 8180 proxy record of ice volume

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

vertex manganese transport through oxygen minima abstract manganese transport through a well developed oxygen minimum was studied off central mexico 18 n 108 w in october november 1981 as part of the vertex vertical transport and exchange research program refractory leachable and dissolved mn fractions associated with particulates caught in traps set at eight depths 120 1950 m were analyzed particles entering the oxygen minimum had relatively large mn loads however as the particulates sank further into the minimum total mn fluxes steadily decreased from 190 nmol m 2 day 1 at 120 m to 36 nmol m 2 day 1 at 400 m manganese fluxes then steadily increased in the remaining 800 1950 m reaching rates of up to 230 nmol m 2 day 1 at 1950 m manganese concentrations were also measured in the water column dissolved mn levels 1 were consistently observed within the 150 600 m depth interval in contrast suspended particulate leachable mn amounts were especially low at those depths and never exceeded 0 04 nmol kg 1 the combined water column and particle trap data clearly indicate that mn is released from particles as they sink through the oxygen minimum rate of change estimates based on trap flux data yield regeneration rates of up to 0 44 nmol kg 1 yr 1 in the upper oxygen minimum 120 200 m however only 30 of the dissolved mn in the oxygen minimum appears to be from sinking particulate regeneration the other 70 probably results from continental slope release horizontal transport processes dissolved mn scavenges back onto particles as oxygen levels begin to increase with depth scavenging rates ranging from 0 03 to 0 09 nmol kg 1 yr 1 were observed at depths from 700 to 1950 m these scavenging rates result in mn residence times of 16 19 years and scavenging rate constants on the order of 0 057 yr 1 manganese removal via scavenging on sinking particles below the oxygen minimum is balanced by mn released along continental boundaries and transported horizontally via advective diffusive processes manganese appears to be very weakly associated with particulates nevertheless the amounts of mn involved with sinking biogenic particles are large and the resulting fluxes are on the same order of magnitude as those necessary to explain the excess mn accumulating on the sea floor the overall behavior of mn observed in this and other studies strongly suggests some type of equilibrium occurring between dissolved and particulate phases this equilibrium appears to shift in direct or indirect response to dissolved oxygen levels

sedimentation resuspension and chemistry of particles in the northwest atlantic abstract sediment traps were deployed to sample vertical fluxes of particulate matter both immediately above and within the nepheloid layer on the continental slope and rise of the western north atlantic the data from the traps are used to estimate the settling flux of particles and to attempt to distinguish between the primary flux settling material from the upper water column and the resuspended flux settling material that has been resuspended from some region of the sea floor from these data and from measurements of the net nepheloid standing crop of particles estimates of the mean residence time for particles resuspended in the nepheloid layer are on the order of weeks to months from 15 to 500 m above bottom horizontal advection and diffusion are more important than vertical mixing in maintaining the upper portion of the nepheloid layer the estimated flux of al to the north atlantic as atmospheric dust is one to two orders of magnitude less than the primary flux of al measured just above the nepheloid layer yet the range of accumulation rates of al measured on the sea floor with radioisotopes is close to the values we define as the primary flux horizontal transport of al rich particles mostly aluminosilicates must therefore be significant even above the nepheloid layer the size of 80 90 of the particles collected in the traps both above and within the nepheloid layer was less than 63 m the mean size of particles trapped in the nepheloid layer was about 22 m and above the nepheloid layer 11 m the particles were probably in large aggregates when they encountered the traps but identifiable fecal pellets are only a small fraction of the material collected in the traps between 6 and 15 of the primary production of particulate organic carbon in the euphotic zone was collected in floating sediment traps at the 100 m depth indicating that most particulate organic carbon is recycled in the surface water only 2 3 of primary production of organic carbon was collected as primary flux 500 m above the sea floor from an intercomparison of the composition of particles falling rapidly collected in traps falling slowly or not at all collected in water bottles and resting on the sea floor from core tops it was found that elements associated with biogenic matter such as ca sr cu and i were carried preferentially by the rapidly falling particles once these particles reached the sea floor the concentration of those elements was decreased through decomposition respiration or dissolution dissolution appears to be rapid near or on the sea floor because radiolarians diatoms and juvenile foraminifera were rare in core samples even though abundant in all traps the dynamic nature of the nepheloid layer makes it possible for particles to be resuspended many times before they are finally buried enabling sediment to be carried long distances from its origin the recycling of particles near the sea floor may also increase dissolution of siliceous and carbonate matter

palladium platinum and rhodium contents of rocks near the lower margin of the stillwater complex montana palladium platinum and rhodium concentrations are reported for rocks belonging to the peridotite zone of the ultramafic series and the basal series of the late archcan stillwater complex a suite of stillwater associated sills and dikes and the metamorphic rocks lying within the contact aureole of the complex mean values do not exceed 20 ppb for any rock unit and the tenth percentile rarely exceeds 200 ppb the concentration of pd is greater than pt for most rock groups both of these elements have higher concentrations than rh a large part of the variability in the data appears to be related to rock types containing different proportions of sulfide it is also notable that sulfur does not correlate with the platinum group element concentrations for many lithologies and when it does it commonly is not a useful predictor of platinum group element content further calculated values of r the mass ratio of silicate liquid which has equilibrated with sulfide liquid are low 1 to 320 analysis of this information suggests that the immiscible sulfide liquids from which the sulfides formed did not completely equilibrate with the silicate liquids with respect to their platinum group element contents and that the silicate liquids were characterized by different concentrations and ratios of platinum group elements

pelagic sedimentation of aragonite its geochemical significance the relative importance of the pelagic flux of aragonite as compared to calcite to the deep sea floor has been evaluated by means of a quantitative x ray diffraction study of samples collected from sediment traps and from an unusually shallow portion of the open atlantic ocean the rio grande rise the results suggest that the aragonite flux constitutes at least 12 percent of the total flux of calcium carbonate on a worldwide basis the presence of high magnesium calcite in several samples suggests that some of the calcareous material falling to the deep sea floor may be derived from the long distance transport of debris from shallow water beenthic organisms as well as from the settling of planktonic remains this observation supports the contention that 12 percent represents a minimum value aragonite and high magnesium calcite transported laterally from shallow water regions upon dissolution during settling into deeper water may contribute to the neutralization of excess anthropogenic carbon dioxide added to the oceans

sedimentation of lithogenic particles in the deep ocean abstract investigation of lithogenic particles collected by sediment traps in open ocean stations revealed that the sediment flux increased linearly with depth in the water column this rate of increase decreased with distance of the station from the continent it was largest at the panama basin station and almost negligible at the e hawaii abyssal plain station at the panama basin station smectite flux increased with depth we suggest that smectite resuspended from bottom sediments of the continental slope west of the sediment trap station is advected by easterly deep currents and the suspended particles are then possibly entrapped by large settling particles on the other hand the flux of hemipelagic clay particles kaolinite and chlorite was nearly constant at all depths this can be explained by incorporation of these particles in fecal pellets which then settle from the surface water at the demerara abyssal basin station flux of illite and chlorite particles increased with depth and the flux of smectite was constant a sudden increase of the flux of illite and chlorite was observed near the bottom traps at the sohm abyssal plain station the flux of quartz and feldspar was 10 to 15 of the clay flux

rates and mechanisms of iceberg ablation in the d urville sea southern ocean l action des vagues provoque l ablation dans une zone etroite de la ligne de flottaison d un iceberg jusqu a 0 3 m par jour dans de l eau a 1 c avec des vagues allant jusqu a 0 4 m de hauteur des velages subaeriens de glace s en suivent au meme rythme a partir des flancs de l iceberg la fusion sous marine a une action d un ordre de grandeur inferieur a celui de l action des vagues mais agit sur la face la plus etendue a l iceberg les vitesses d ablation prevues theoriquement ou statistiquement ailleurs pour des conditions comparables de l environnement sont en bonne concordance avec les mesures faites ici les icebergs en derivant trainent dans leur sillage un panache d eau de mer melangee un peu refroidie et diluee

image analysis and composition of platinum group minerals in the j m reef stillwater complex detailed imaging bulk geochemical analyses and laser ablation icp ms analyses were conducted on mineralized samples from the platinum group element pge bearing j m reef stillwater complex montana the aims of the study were to determine which phases are the main hosts of the pge to examine the textural relationships of the pge bearing minerals and the associated base metal sulfides silicates and secondary magnetites the platinum group minerals pgm observed in the studied samples consist of pd or pt sulfides pt fe alloy isoferroplatinum pd or pt tellurides and pd cu alloy skaergaardite with minor native pd ru ir os s sub 2 laurite and au pd ag alloy palladian electrum these minerals account for all the pt half the pd ru and ir and a smaller proportion of the os in whole rocks the balance being found in the base metal sulfide most of these pgm are closely associated with base metal sulfides either included in the base metal sulfide or located at the contact between the base metal sulfide and the silicate or oxide minerals the textures of the pgm within and at the margins of the base metal sulfide suggest that they exsolved from the sulfides we suggest that the pge ni cu and te partitioned into a magmatic sulfide liquid that crystallized into base metal sulfide the pt fe alloys and some of the pd sulfides exsolved from the base metal sulfide during desulfurization at fairly high temperatures in contrast the skaergaardite and some pd sulfides are found in association with secondary magnetite and probably formed at relatively low temperatures approximately 250 degrees 465 degrees c during a second fluid migration event the pd pt ratio of the reef approximately 3 3 is higher than most mafic magmas and the pd pt ratio of the footwall is lower 0 3 0 7 it is possible that some of the pd in the reef was leached from the underlying rocks and deposited in the reef as pgm associated with secondary magnetite

a study of brash ice in the proximal marine zone of a sub polar tidewater glacier abstract intermittent calving of tidewater maar glacier into arthur harbor anvers island antarctic peninsula produces mainly brash ice fragments 2 samples of dense brash are closely approximated by rosin probability distributions for an average population of freshly calved brash subjected to mean open water melt rates observed in arthur harbor over 90 of the bergs will completely melt within 24 hours consequently little brash escapes the proximal marine zone and most debris rafted by such bergs is probably deposited within a few hundred meters of the glacier terminus

highly siderophile and strongly chalcophile elements in magmatic ore deposits an ore deposit by definition must be economically viable that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit bates and jackson 1987 this requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth at the oxygen fugacities found in the crust native fe is not normally stable and thus the highly siderophile elements defined as ru rh pd re os ir pt and au cannot behave as siderophile elements except in rare cases such as on disko island klock et al 1986 where the magma is sufficiently reduced for native fe to be present however if mafic magmas become saturated in a base metal sulfide liquid the highly siderophile elements behave as highly chalcophile elements table 1 thus these elements are generally found in association with base metal sulfide minerals which crystallized from a magmatic sulfide liquid namely pyrrhotite pentlandite chalcopyrite cubanite pyrite an exception to this is au although au is strongly chalcophile and is produced as a by product from many platinum group element pge deposits table 2 most primary au deposits consist of native au groves et al 1998 these will not be discussed in this chapter there are many pge deposits i e accumulations of pge minerals and base metal sulfides containing pge bates and jackson 1987 around the world but most of these do not constitute pge ore deposits because they are either too small or their grade is too low or other political or infrastructure factors prevent the economic exploitation of the deposit bates and jackson 1987 for the purpose of this work we have defined pge ore deposits as those which have significant production 2 of the annual world

orbital scale vegetation change in africa abstract palynological records of middle and late pleistocene marine sediments off african shores is reviewed in order to reveal long term patterns of vegetation change during climate cycles whether the transport of pollen and spores from the source areas on the continent to the ocean floor is mainly by wind or predominantly by rivers depends on the region despite the differences in transportation accumulation rates in the marine sediments decline exponentially with distance to the shore the marine sediments provide well dated records presenting the vegetation history of the main biomes of western and southern africa the extent of different biomes varied with the climate changes of the glacial interglacial cycle the mediterranean forest area expanded during interglacials the northern saharan desert during glacials and the semi desert area in between during the transitions in the sub saharan mountains ericaceous scrubland spread mainly during glacials and the mountainous forest area often increased during intermediate periods savannahs extended or shifted to lower latitudes during glacials while the representation of the tropical rain forest fluctuated with summer insolation and precession that of the subtropical biomes showed more obliquity variability or followed the pattern of glacial and interglacials

glacial cold water coral growth in the gulf of cadiz implications of increased palaeo productivity a set of 40 uranium series datings obtained on the reef forming scleractinian cold water corals lophelia pertusa and madrepora oculata revealed that during the past 400 kyr their occurrence in the gulf of cadiz goc was almost exclusively restricted to glacial periods this result strengthens the outcomes of former studies that coral growth in the temperate ne atlantic encompassing the french iberian and moroccan margins dominated during glacial periods whereas in the higher latitudes irish and norwegian margins extended coral growth prevailed during interglacial periods thus it appears that the biogeographical limits for sustained cold water coral growth along the ne atlantic margin are strongly related to climate change by focussing on the last glacial interglacial cycle this study shows that palaeo productivity was increased during the last glacial this was likely driven by the fertilisation effect of an increased input of aeolian dust and locally intensified upwelling after the younger dryas cold event the input of aeolian dust and productivity significantly decreased concurrent with an increase in water temperatures in the goc this primarily resulted in reduced food availability and caused a widespread demise of the formerly thriving coral ecosystems moreover these climate induced changes most likely caused a latitudinal shift of areas with optimum coral growth conditions towards the northern ne atlantic where more suitable environmental conditions established with the onset of the holocene

tidal influence on particulate organic carbon export fluxes around a tall seamount as tall seamounts may be stepping stones for dispersion and migration of deep open ocean fauna an improved understanding of the productivity at and food supply to such systems needs to be formed here the 234th 238u approach for tracing settling particulate matter was applied to senghor seamount a tall sub marine mountain near the tropical cape verde archipelago in order to elucidate the effects of topographically influenced physical flow regimes on the export flux of particulate organic carbon poc from the near surface topmost 100 m into deeper waters the comparison of a suitable reference site and the seamount sites revealed that poc export at the seamount sites was 2 4 times higher than at the reference site for three out of five seamount sites the calculated poc export fluxes are likely to be underestimates if this is taken into account it can be concluded that poc export fluxes increase while the passing waters are advected around and over the seamount with the highest export fluxes occurring on the downstream side of the seamount this supports the view that biogeochemical and biological effects of tall seamounts in surface ocean waters might be strongest at some downstream distance from rather than centred around the seamount summit based on measured vessel mounted adcp and modelled regional flow field aviso internal tides at senghor mitgcm flow dynamics it is proposed that tidally generated internal waves result in a screen of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased poc export above the seamount 1 sudden increased upward transport of nutrients into the euphotic zone driving brief pulses of primary production of new particulate matter followed by the particles export into deeper waters and 2 pulses of increased shear driven aggregation of smaller slower settling into larger faster settling particles this study shows that under certain conditions there can be an effect of a tall seamount on aspects of surface ocean biogeochemistry with tidal dynamics playing a prominent role it is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic pelagic communities at and near tall seamounts

control of organic carbon accumulation in the late quaternary equatorial atlantic ocean drilling program sites 664 and 663 productivity versus terrigenous supply organic geochemical records of the last 940 kyr are presented for equatorial atlantic ocean drilling program odp sites 663 and 664 and discussed with regard to the development of ocean productivity and african paleoclimate proportions of marine and terrigenous organic matter om are estimated from elemental pyrolytic isotopic and petrologic data spectral analyses reveal a strong power at the eccentricity and obliquity band indicating a close response of tropical organic sedimentation to the climatic evolution at high latitudes the orbital covariance of organic carbon with biogenous opal and terrigenous records favor that glacially enhanced dust supply and surface water mixing were primary controls for deposition of organic carbon wind borne supply of terrigenous om contributes 26 to 55 and 0 to 39 to the bulk om based on microscopic and isotopic records respectively admixture of c4 plant matter was approximated to contribute up to 16 to the bulk organic fraction during peak glacial conditions

tropical atlantic terrigenous fluxes since 25 000 yrs b p twelve tropical atlantic cores each 14c dated and with 18o records were examined for burial fluxes of terrigenous mostly eolian sediment and biogenic opal and caco3 sediment over the last 25 000 years the similar flux trends through time of these three components within each core indicates that post depositional redistribution of sediment is the strongest control on burial fluxes in the tropical atlantic increased terrigenous dust fluxes apparent during the last glacial maximum in several cores are consistent with several previous studies the lack of significant dust flux maxima following the early holocene monsoon maximum argues strongly against changes in source area humidity aridity being a significant factor in orbital scale eolian dust transport and deposition in this region this suggests that the enhanced glacial influxes of dust to the tropical atlantic are mainly due to changes in transport by winter winds rather than to glacial hyper aridity

a three year time series of mineral dust deposits on the west african margin sedimentological and geochemical signatures and implications for interpretation of marine paleo dust records keywords saharan dust deposition flux grain size clay mineralogy sr and nd isotopes amma program a b s t r a c t mineral dust deposits in the northeastern tropical atlantic ocean netao are an important contribution for reconstructing paleoenvironments and paleoclimates of west africa however the interpretation of the changes in the sedimentological and geochemical characteristics of the dust deposits recorded in the marine sediments from the netao remains incomplete in order to improve our understanding of dust proxies in particular its mineralogical and geochemical provenance tracers present day dust deposition has been monitored at mbour 80 km south of dakar on the senegalese margin here we report a multi proxy investigation of a unique three year continuous time series of mineral dust deposits collected at a weekly or better temporal resolution over the march 2006 march 2009 period mass deposition flux and mean modal grain size display marked but reverse seasonal features with higher flux during the winter spring seasons and coarser grain size mode in summer when flux is minimal reflecting contrasting transport patterns throughout the year similarly clay mineralogy the illite kaolinite ratio in particular shows seasonal fluctuations manifesting the latitudinal displacement of the contributing domains of provenance in response to the seasonal migration of the itcz position and the associated wind systems our three year record also reveals the occurrence of major deposition events superimposed on the seasonal pattern generally during the winter spring dry seasons and most frequently during the month of march our study shows that these major events which contribute a large fraction of the total annual deposition flux all originate from the western sahara sahel a major area of emission in the region beside the bodele depression stretching from the mauritanian and western saharan coasts to the hoggar mountains combined with air mass tri dimensional back trajectories and satellite images the mineralogical and sr nd isotopic compositions of these large dust events enable us to identify several mineralogically and geochemically distinct provenance sectors within this vast western area unlike the background dust deposits the 87 sr 86 sr and the e nd isotopic signatures of the major dry events closely match that of the netao late holocene sediments supporting the hypothesis that these events account for most of the aeolian terrigenous supply reaching the ocean floor although this database needs to be expanded our results already provide useful constraints for the interpretation of the dust proxies variations in marine sedimentary archives off mauritania senegal in terms of changes in wind regimes and aridity over west africa

geochemical cycles of elements and human perturbation abstract the pollution inputs of various elements are omnipresent in aerosols snows rivers and the ocean of the northern hemisphere the mid ocean ridges are not important sinks or sources for most of the elements the settling of fecal pellets is the predominant transport mechanism of most elements except mn co and ni from the surface to deep oceans dissolved mn co and ni are directly incorporated into pelagic clays at the sediment water interface

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

seasonal impact of mineral dust on deep ocean particle flux in the eastern subtropical atlantic ocean abstract seasonal lithogenic particle and al fluxes were obtained from a deep ocean sediment trap deployment during 1992 and 1993 off nw africa and were compared concurrently with atmospheric al concentrations and two dimensional backward trajectories of windfields from two barometric levels in the lower and mid troposphere marine al fluxes lithogenic particle fluxes and grain size distributions in the area were found to be directly linked to airmass pathways and surface mineral aerosol concentrations at 1000 m water depth highest al fluxes 10 77 mg m 2 day 1 lithogenic particle fluxes 99 25 mg m 2 day 1 and smallest mean grain sizes 11 9 m occurred during the winter and spring season concurrent with highest atmospheric dust load and al concentrations 15 300 ng m 3 in the lower troposphere a strong seasonal change of the main atmospheric dust transport from low altitude winds during winter spring to higher altitudes during summer is clearly reflected at depth by a significant coarsening of mean grain sizes 18 6 m and lowest al 0 81 mg m 2 day 1 and lithogenic particle fluxes 11 3 mg m 2 day 1 found in the sediment traps the comparison of marine atmospheric and model derived data used within this study highlights the close temporal coupling between atmospheric dust transport and the deep ocean particle stock

sulfide saturation history of the stillwater complex montana chemostratigraphic variation in platinum group elements a platinum group element pge investigation of a 5 3 km thick stratigraphic section of the stillwater complex montana was undertaken to refine and test a geochemical technique to explore for platiniferous horizons in layered mafic ultramafic complexes pge au major and trace elements were determined in 92 samples from outcrops along traverses in the chrome mountain and contact mountain areas in the western part of the stillwater complex where the j m reef occurs 1 460 m above the floor of the intrusion a further 29 samples from a drill hole cored in the immediate vicinity of the j m reef were analyzed to detail compositional variations directly above and below the j m reef below the j m reef background concentrations of pt 10 ppb and pd 7 ppb are features of peridotites with intermediate s concentrations mostly 100 200 ppm and rocks from the bronzitite norite i and gabbronorite i zones mostly pt au within the middle and upper banded series s abundances fluctuate considerably but exhibit an overall upward increase the behavior of these elements records periodic sulfide saturation during deposition of the peridotite zone followed by crystallization under sulfide undersaturated conditions until saturation is achieved at the base of the j m reef following formation of the reef sulfide saturated conditions persisted throughout the deposition of most of the remaining lower layered series this resulted in a pronounced impoverishment in pge abundance in the remaining magma a condition that continued throughout deposition of the remainder of a succession which is characterized by very low pt 1 5 ppb and pd 0 7 ppb abundances because only unmineralized rock was selected for study in the 5 3 km thick section the results provide an unbiased picture of the variation in background pge levels during crystallization of the stillwater complex in contrast the variations in the drill core samples through the reef provide a detailed record of ore formation plots of pt pd pd s and pt pd as a function of stratigraphic height in the intrusion show that the location of the j m reef is defined by an abrupt change in these concentrations and ratios although this is the most abrupt change three other anomalies in pge abundance and ratios are apparent in the profiles and coincide with known laterally extensive sub economic sulfide concentrations above the j m reef the uppermost of these is the pge bearing picket pin sulfide horizon the relative ease with which mineralized horizons can be pinpointed in these diagrams indicates that a similar approach could be used in exploration programs in other ultramafic mafic intrusions our observations exclude the possibilities of either magma mixing within the stillwater chamber or the fluxing of a volatile rich fluid as the mechanisms responsible for the genesis of the j m reef rather our data indicate that the j m reef formed from a parental magma that was strongly enriched in pge this magma likely formed at depth below the stillwater magma chamber by the interaction of the parental magma with s rich meta sedimentary rocks followed by the re dissolution of these sulfides in the stillwater magma

variability in terrigenous sedimentation processes off northwest africa and its relation to climate changes inferences from grain size distributions of a holocene marine sediment record variations in deposition of terrigenous fine sediments and their grain size distributions from a high resolution marine sediment record offshore northwest africa 30 51 0 n 10 16 1 w document climate changes on the african continent during the holocene end member grain size distributions of the terrigenous silt fraction which are related to fluvial and aeolian dust transport indicate millennial scale variability in the dominant transport processes at the investigation site off northwest africa as well as recurring periods of dry conditions in northwest africa during the holocene the terrigenous record from the subtropical north atlantic reflects generally humid conditions before the younger dryas during the early to mid holocene as well as after 1 3 kyr bp by contrast continental runoff was reduced and arid conditions were prevalent at the beginning of the younger dryas and during the mid and late holocene a comparison with high and low latitude holocene climate records reveals a strong link between northwest african climate and northern hemisphere atmospheric circulation throughout the holocene due to its proximal position close to an ephemeral river system draining the atlas mountains as well as the adjacent saharan desert this detailed marine sediment record which has a temporal resolution between 15 and 120 years is ideally suited to enhance our understanding of ocean continent atmosphere interactions in african climates and the hydrological cycle of northern africa after the last deglaciation

planktonic foraminiferal flux to the deep ocean sediment trap results from the tropical atlantic and the central pacific abstract the flux of planktonic foraminifera between 100 m and 1 mm to the seafloor has been estimated for the central pacific abyssal plain east of hawaii and the tropical atlantic demerara abyssal plain based on sediment trap samples collected from various water depths the faunas in each region are quite similar with 4 to 5 species generally accounting for 70 80 of the total assemblage at both study sites the total foraminiferal flux and the carbonate flux tend to decrease with depth in addition the flux of individual species of planktonic foraminifera varies significantly with depth with the number of small solution susceptible species decreasing with increasing water depth these results suggest that there is significant dissolution of small m foraminifera as they settle through the water column material collected from the sediment water interface directly below the pacific sediment trap array contains no planktonic foraminifera suggesting that the residence time of an individual on the seafloor before it dissolves is extremely short

the interaction of plankton and suspended sediment in fjords abstract the interaction of plankton with suspended sediment affects both the plankton and the sediment trace metals organics and nutrients that are adsorbed onto suspended particles affect the supply of these materials to phytoplankton and zooplankton ingestion of suspended sediment by zooplankton exposes the particulate minerals to physical and chemical breakdown occasionally strong enough to produce changes in particle mineralogy fecal pellets produced from undigested material package fine particulate material and thus increase their rate of sedimentation this is particularly important in fjords an aqueous environment that is dominated by the vertical flux of suspended sediment pelletization also allows the transport of undigested food to greater depths providing an important source of organic material to benthic sediments and food to deep water organisms

southern california inner basin sediment trap calibration the deployment of particle interceptor traps pits in the three inner basins of the southern california bight santa barbara santa monica and san pedro where preserved laminated bottom sediments occur provides a natural calibration between the parameters determined with the pits and those derived from the historical sedimentary deposits the accumulation rates and chemical composition of the pit materials compare favorably with these recently deposited bottom sediments the ratio of the measured particle collection rate to the sediment accumulation rate for these three basins averaged0 93 0 20 radionuclide 210pb 228th 232th and trace element fe cu zn cd pb compositions of the pit materials and surface sediments agree within generally 30 in each basin an application of pits as a sampling system in an outer basin san nicolas where the sedimentary record has been obscured by bioturbation is also presented to further demonstrate the utility of pits for spatial and temporal studies of various aspects of particle transport and sediment deposition based on the calibration of the pits in the three inner basins we conclude from the comparison of the radionuclide and trace element compositions between the pit materials and surface sediments from the san nicolas basin that recently deposited sedimentary material has been mixed by bioturbation with older material from the last several hundred years excess210pb and the228th 232th activity ratio in the pit material are 6 and 15 times higher than in the surface sediment total pb and1n hno3 leachable pb are 4 and 8 fold higher in the pit material

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

thermohaline steps induced by melting of the erebus glacier tongue a vertically stable step like thermohaline structure is observed throughout a continuous 400 m conductivity temperature depth ctd profile taken near the erebus glacier tongue mcmurdo sound antarctica the pattern is best developed between the sea surface and 250 m depth the interval corresponding to that of the irregular underwater profile of the glacier tongue the steps average 17 m in thickness and typically display discontinuities of 0 1 c in temperature 0 04 in salinity and 3 5 10 4 g cm 3 in density the observations are compared with theory and laboratory experiments of cell development and lateral flow near ice melting into vertically stratified salt water at this location subsurface seawater is inferred to remain above the in situ freezing point year round and contains sufficient heat to account for much of the glacier tongue thinning by basal melting an adequate volume of meltwater would result to produce the measured salinity steps we discuss related observations and some implications of this process for ocean circulation and biological productivity in the antarctic

a 300 kyr record of aridity and wind strength in southwestern africa inferences from grain size distributions of sediments on walvis ridge se atlantic abstract the terrigenous fraction of sediments recovered from walvis ridge se atlantic ocean reveals a history of southwestern african climate of the last 300 kyr end member modelling of a data set of grain size distributions n 428 results in three end members the two coarsest end members are interpreted as eolian dust the third end member as hemipelagic mud the ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the se trade winds trade winds were intensified during glacials compared to interglacials changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern african aridity late quaternary southwestern african climate was relatively arid during the interglacial stages and relatively humid during the glacial stages owing to meridional shifts in the atmospheric circulation system during glacials the polar front shifted equatorward resulting in a northward displacement of the zone of westerlies causing increased rainfall in southwestern africa the equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient leading to enhanced atmospheric circulation and increased trade wind intensity

multiple s isotope studies of the stillwater complex and country rocks an assessment of the role of crustal s in the origin of pge enrichment found in the j m reef and related rocks abstract the j m reef in the stillwater complex of montana is the highest grade pd deposit in the world averaging approximately 14 ppm pd one suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal s because 34s values alone are often insufficient to document in igneous rocks the assimilation of s from archean sedimentary rocks a combined study of both 33s and 34s values and computed 33s values was undertaken 34s and 33s values of cordierite bearing hornfels as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations one is characterized by anomalous positive 33s values from 0 to 0 32 and 34s values from 0 5 to 1 6 whereas the other is characterized by anomalous negative 33s values from 0 to 0 22 and 34s values from 1 5 to 5 8 34s and 33s values for rocks from the basal series of the complex range from 0 9 to 3 0 and 0 01 to 0 18 respectively the anomalous s isotope ratios indicate that for some samples at least 50 of the their s was of crustal origin in contrast most of the rocks from the j m reef are characterized by 34s and 33s values between 1 3 and 1 8 and 0 06 to 0 06 respectively these values deviate only slightly if at all from those thought to be representative of mantle s and suggest that i contamination was not an important process for pge enrichment in the complex ii the contaminant was also characterized by near 0 33s values or iii s isotopic exchange in the staging chambers was an important process upgrading of pge values via reaction between sulfide in the chamber and uncontaminated mantle derived magma may also have resulted in the conversion of s isotopic ratios that may have been indicative of crustal s addition to mantle like values rare 34s values up to 7 2 in the j m reef and 8 2 in chromitite from the ultramafic series provide strong evidence for localized assimilation of crustal s which did not experience isotopic homogenization the s isotopic ratios of the j m reef are non unique in terms of a specific genesis as they may be interpreted in either a downward sulfide collection model or an upward fluid transport model

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

geochemistry and mineralogy of platinum group element mineralization in the river valley intrusion ontario canada a model for early stage sulfur saturation and multistage emplacement and the implications for contact type ni cu pge sulfide mineralization the river valley intrusion within the 2 48 ga east bull lake intrusive suite in ontario canada is an example of a mafic igneous intrusion with contact type ni cu pge sulfide mineralization along its base whereas many contact type deposits are thought to form from in situ contamination of the magma by the addition of crustal s during emplacement there are some intrusions including the river valley intrusion which appear to have a much more complex history where the timing of s saturation and thus the critical ore genesis processes may have occurred much earlier prior to emplacement the river valley intrusion is made up of a basal 100 m of unlayered inclusion bearing units overlain by layered cumulates the basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites all within a gabbroic matrix platinum group element rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides the matrix and inclusions can be separated into two distinct textural types hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages the platinum group mineral pgm assemblages vary only between textural types and not between inclusions and matrix being dominated tellurides in all rock types the hydrothermally altered rocks however have fewer tellurides and an increased amount of sb and as bearing pgm indicative of minor fluid interaction although the pgm have not been mobilized significantly away from the base metal sulfides precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements pge indicating the presence of a single well homogenized pge rich sulfide liquid however au and cu appear to be decoupled from the pge at low concentrations although correlate well with each other which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins the overlying layered units above the mineralized units are not pge depleted trace element data including th yb pm and nb th pm ratios demonstrate that all river valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber although some samples have s se ratios indicative of crustal s most have s se ratios lower than the mantle range indicative of s loss we propose a multistage model for the formation of the mineralization in the river valley intrusion with a major contamination event at depth with the addition of s from local crustal rocks inducing sulfide saturation sulfide droplets were then enriched in pge within a conduit system with possible further upgrading of sulfide metal tenors and reduction of s se ratios via partial dissolution of sulfide the pge enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid eroded blocks of precrystallized and mineralized gabbro and footwall rocks and emplaced an inclusion bearing package as the lower 100 m or so of the river valley intrusion later emplacement of main river valley magma was from an s undersaturated pge fertile magma the river valley intrusion is thus an example whereby contact type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization rather than contamination triggering sulfide saturation in situ in such cases processes at depth determine the generation and subsequent tenor of the mineralization in particular dissolution of the sulfide can upgrade metal tenor but subsequently will reduce s se ratios masking the signature of crustal contamination in addition a multistage emplacement such as this will not necessarily preserve the characteristic increase in cu pd ratios in the overlying cumulates that is often used in exploration for pge deposits in mafic intrusions thus a full understanding of all the field geochemical and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of s se and cu pd ratios as an indicator of crustal contamination and the presence of pge mineralization