lead precipitation fluxes at tropical oceanic sites determined from 210 pb measurements concentrations of lead 210 pb and 210 po were measured in rain selected for least influence by local sources of contamination at several tropical and subtropical islands enewetak pigeon key florida and american samoa and shipboard stations near bermuda and tahiti ratios expressed as ng pb dpm 210 pb in rain were 250 900 for pigeon key assuming 12 adsorption for 210 pb and no adsorption for lead depending on whether the air masses containing the analyzed rain came from the caribbean or from the continent respectively about 390 for the northern sargasso sea downwind from emissions of industrial lead in north america 65 for enewetak remote from continental emissions of industrial lead in the northern hemisphere and 14 near tahiti a remote location in the southern hemisphere where industrial lead emissions to the atmosphere are much less than in the northern hemisphere the american samoa sample yielded a higher ratio than tahiti the reason for this is not clear but may be due to local pb sources the corresponding fluxes of lead to the oceans based on measured or modeled 210 pb precipitation fluxes are about 4 ng pb cm 2y for tahiti 10 for enewetak and 270 for the sargasso sea site and between 110 to 390 at pigeon key

propagating rift during the opening of a small oceanic basin the protector basin scotia arc antarctica abstract the opening of oceanic basins constitutes one of the key features of plate tectonics because it determines the rifting and displacement of the continental crustal blocks although the mechanisms of development of large oceans are well known the opening and evolution of small and middle size oceanic basins have not been studied in detail the protector basin located in the southern scotia sea is a good example of a small oceanic basin developed between two thinned continental blocks the pirie bank and the terror rise poorly studied up to now a new set of multibeam bathymetry multichannel seismic reflection and gravity and magnetic anomaly profiles obtained on the scan 2001 cruise led us to determine that the protector basin probably opened during the period comprised between c5dn 17 4 ma and c5acn c5abr chrons 13 8 ma forming a n s oriented spreading axis the end of spreading is slightly younger to the north the start of spreading is clearly diachronous with the most complete set of chrons up to c5dn in the southern profile c5cn in the middle section and only up to c5adn in the northern part of the basin the spreading axis propagated northwards during the basin development producing the wedge shape of the basin in addition at the ne part of the basin a reverse fault developed in the border of the pirie bank after basin opening accentuates the sharp northern end moreover the northwestern part of the pirie bank margin is an extremely stretched continental crust with n s elongated magnetic anomalies related to incipient oceanic southward propagating spreading axes the protector basin shows the oldest evidence of e w continental stretching and subsequent oceanic spreading during middle miocene related with the eastward development of the scotia arc that continues up to present the relative rotation of continental blocks during the development of small sized oceanic basins by continental block drifting favoured the opening of wedge shape basins like the protector basin and conjugate propagating rifts

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

the correction for sediment loading on the seafloor available measurements of sediment velocity and density indicate that a factor of 600 m s 1 of two way sediment reflection time will correct seafloor depth to yield basement depth in the absence of sediment loading this simple linear relation is derived from the results of six deep drill holes in abyssal sediments of the north and south atlantic oceans the relation predicts the sediment correction with an accuracy of about 50 m regardless of the lithology sampled down to a depth of 1 5 s of reflection time the cost wells on the u s continental shelf and empirical relations among velocity density porosity and depth provide estimates of the deeper behavior of abyssal sediments and suggest that the 600 m s 1 factor continues to provide a satisfactory sediment correction to a depth of 3 s

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

the age and origin of the central scotia sea summary opening of the drake passage gateway between the pacific and atlantic oceans has been linked in various ways to cenozoic climate changes from the oceanic floor of drake passage the largest of the remaining uncertainties in understanding this opening is in the timing and process of the opening of the central scotia sea all but one of the available constraints on the age of the central scotia sea is diagnostic of or consistent with a mesozoic age comparison of tectonic and magnetic features on the seafloor with plate kinematic models shows that it is likely to have accreted to a mid ocean ridge between the south american and antarctic plates following their separation in jurassic times subsequent regional shallowing may be related to subduction related processes that preceded backarc extension in the east scotia sea the presence of a fragment of jurassic cretaceous ocean floor in the gateway implies that deep water connections through the scotia sea basin complex may have been possible since eocene times when the continental tips of south america and the antarctic peninsula first passed each other

oceanographic and climatic consequences of the tectonic evolution of the southern scotia sea basins antarctica abstract the scotia sea is a complex geological area located in the southern ocean which evolution is closely linked to the opening of the drake passage structural highs of continental nature derived from the former continental bridge between south america and the antarctic peninsula surround the abyssal plains of the scotia sea restricting small isolated sedimentary basins along its southern margin morpho structural and seismo stratigraphic analyses of multichannel seismic reflection profiles and additional geophysical data available in the region have been conducted decoding regional and global implications of the basins evolution the main aim of this work is to describe the stratigraphic evolution of the southern scotia sea basins from their opening in the back arc tectonic context of the scotia sea to the last oceanographic changes which have carried on global climatic implications the evolution of the south scotia sea occurred through two major tectonic stages registered in the sedimentary record of the region 1 the end of the subduction in the northwest part of the weddell sea during the early miocene which shortened the back arc subduction trench generating a major change in the regional tectonic field that determined the evolution of the southern basins towards two different types of passive margins magma poor and magma rich and 2 the full development of the southern scotia sea basins during the middle miocene that led to the opening of deep oceanic gateways along the south scotia ridge interplay among tectonics oceanography and climate is proposed to control the regional sedimentary stacking pattern with coeval changes globally identified

the record of magnetic mineral and heavy metal deposition at regent street bog fredricton new brunswick canada measurements of isothermal remanence and of heavy metal concentrations are presented for two recent moss increment dated cores from hummocks of ombrotrophic peat from regent street bog fredricton new brunswick canada the magnetic record from both profiles appears to be closely related to the documented history of industrial and urban development in the region the heavy metal record displays greater variation between elements and profiles and mirrors less consistently the documentary record some vertical mobility within the peat column is inferred for all the metal elements measured

recent estuarine sedimentation rates from shallow inter tidal environments in western scotland implications for future sea level trends and coastal wetland development during the mid late holocene large sections of the scottish coastline have been characterized by falling relative sea levels resulting from differential glacio isostatic uplift of this region of northern britain the complex interplay between crustal and sea level movements continues to influence the morphological development of the scottish coast a number of geophysical models predict ongoing uplift of the scottish landmass however a number of recent studies based upon the analysis of satellite altimetry data indicate a late 20th century acceleration in the rate of eustatic sea level rise detailed geochemistry radiometric dating and diatom analysis on selected sediment cores from four mature coastal marsh environments in argyll western scotland provides an opportunity to investigate the linkages between twentieth century crustal movements eustatic sea level rise and recent rates of sedimentation recorded within marsh sediments across the proposed scottish glacio isostatic uplift dome solid phase major and trace element geochemistry has been used to examine the extent to which post depositional physical disturbance and or chemical reactions may have influenced the reliability of the radiometric dating methods geochemical data indicate that the evolution of these marsh environments has not been significantly influenced by physical disturbance and overall the supply of minerogenic material to the marshes has been quite uniform vertical distributions of 210pbexcess and 137cs activity have been measured and used to develop models of recent marsh vertical accretion dating of the cores reveals subtle variations in the rates of sediment accumulation over the last c 70 years between sites for much of the last hundred years or so sedimentation rates have been in good overall agreement with various estimations for sea level rise although at the more easterly sites these estimates are generally exceeded however quasi equilibrium between marsh sedimentation and sea level rise for much of the twentieth century is indicated from the diatom analysis over the most recent period of marsh development 10 years a significant increase in the rate of surface sedimentation is recorded at all sites across the study area diatom analysis of these surface layers reveals an increase in the relative abundance of marine polyhalobous taxa in the near surface sediments this signifies a very recent increase in the rate of regional relative sea level rise indicating that a regional threshold in coastal forcing has now been exceeded these findings provide clear evidence that recent relative sea level rise is now outpacing estimated rates of glacio isostatic adjustment gia across the proposed scottish uplift dome

anomalous temperatures in central scotia sea sediments bottom water variation or pore water circulation in old ocean crust we report low temperature gradients from 60 to 12 of geothermal and extrapolated temperatures offset from modern bottom water temperatures in sediments from the central scotia sea we examine possible causes bearing in mind similar anomalous measurements 18 years previously attributed at the time to instrumental error small temperature offsets 0 1 c may reflect short term bottom temperature variation within eddies of the antarctic circumpolar current low sediment temperature gradients may be caused by horizontal advection of cold water within the upper oceanic basaltic layer documented in younger ocean floor elsewhere or by northward encroachment of colder bottom waters from the antarctic peninsula shelf or weddell sea for several years prior to measurement

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

210pb in the sediments and water column of the northeast atlantic from 47 to 59 n along 20 w abstract over much of the deep north atlantic ocean the water column and atmospheric input deficit of the radionuclide 210 pb is not matched by the inventories found in the underlying sediments this implies that an unidentified extensive or intense sink or sinks for 210 pb must exist in the atlantic to counterbalance the large area of open ocean in deficit one likely sink area which has been suggested is the atlantic north of 50 n because of its moderately high surface productivity and the high inventories observed in the limited number of cores available this work tests the hypothesis by investigating the water column at 48 n 20 w and a set of cores from 47 to 59 n along 20 w the water column deficit is 21 dpm 210 pb cm 2 in a water column of 4500 m depth and a 226 ra content of 54 dpm cm 2 the important unmeasured parameter is the atmospheric input of 210 pb to the surface ocean but the 100 yr average fluxes for the majority of the cores eight out of eleven can be modelled to within 40 of their observed values with a value of 0 30 dpm cm 2 y 1 for the atmospheric input added to the flux implied by the water column deficit this leads to best estimate values of the characteristic water column parameters scavenging effectiveness and scav of 48 and 35 yrs respectively which are typical north atlantic values it is concluded that removal from the water column and steady state supply to the sediments are in approximate balance along this transect and that this part of the north atlantic is not a preferential sink for 210 pb two cores from a sediment drift area do show active enhanced 210 pb excess accumulation but this is ascribed to current driven fine sediment redistribution rather than to productivity driven enhanced removal from the water column

empirical reflection travel time versus depth and velocity versus depth functions for the deep sea sediment column numerous correlations of one way reflection travel times and depths to reflecting horizons have been proposed in the initial reports of the deep sea drilling project dsdp having carefully reviewed proposed correlations we selected 233 correlated depths ranging from 27 m to 1 4 km and one way travel times ranging from 0 02 to 0 7 s from 154 dsdp sites worldwide assuming that the velocity depth function for deep sea sediments can be approximated by an exponential form we have used these data to establish statistically well constrained global models relating depth to travel time and velocity to depth there is remarkably little scatter in the data based on nonlinear least squares fits the best depth time function is z km 3 03 1n 1 0 52t s the rms error in depth is 26 m the corresponding average velocity depth function is v km s 1 59exp 0 33z we estimate that the real site to site variability of velocities at depth is less than 0 20 km s any dependence of velocity on composition is below the resolution of the data and available evidence suggests that age has no significant effect these results imply that to a depth of 1 4 km the physical state of sediments depends on in situ overburden pressure and temperature which are related to depth of burial and that chemical and mechanical equilibrium is achieved in a period of time that is geologically short our results are consistent with laboratory measurements of p wave velocities in deep sea sediments but velocity depth functions for terrigenous and calcareous sections derived from sonobuoy wide angle reflection surveys are incompatible with the time and depth data and overestimate velocities by as much as 10 and 30 respectively these systematic errors may result from the fact that the marked anisotropy of deep sea sediments is neglected in the interpretation of reflection data

a geological map of the scotia sea area constrained by bathymetry geological data geophysical data and seismic tomography models from the deep mantle abstract the scotia sea is one of the tectonically most complex and least understood back arc basins on earth which partly results from its remote location making the acquisition of data challenging here we provide a review of current publicly available geophysical and geological data in the scotia sea realm including magnetic bouguer gravity anomaly high resolution bathymetric heat flow and reflection seismic data and rock type data from cored and dredged samples with this inter disciplinary data set we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged scotia sea domain cross sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area the data review revealed that basalt is the dominant lithology in the scotia sea area occupying most of the west and east scotia sea wss and ess andesitic and more felsic lithologies are identified in the central scotia sea css and the northern east scotia sea ess mesozoic palaeozoic metamorphic sedimentary lithologies are reported from the highs along the north and south scotia ridges nsr and ssr these highs originate from a land bridge that until the late mesozoic connected the south american and antarctic continents scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the scotia plate but our mapping exercise allowed us to confirm the presence of the early oligocene late miocene ancestral south sandwich arc assa in the east of the css setting the minimum age of the older segment of the css crust to eocene earliest oligocene three cross sections cross cutting the scotia sea reveal two high velocity zones indicating cold mantle material one is situated below the structural highs along the ssr which we interpreted as remnant slab material of the assa and another below south georgia and the css which we interpreted as the front of the slab that currently subducts at the south sandwich trench ssat the neutral velocity discontinuity below the wss implies mantle conditions of a recently extinct spreading centre the upper mantle low velocity anomaly below the css is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the ssat we have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote inaccessible offshore domains this is a less time consuming economically advantageous exercise which re uses existing geological and geophysical data for a new purpose it is the most data inclusive study there is today of the scotia sea region and serves as a guideline for future expeditions targeting the css and the structural features along the nsr to identify their age and origin a georeferenced version of the map is provided in the supplementary material

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

circum antarctic age modelling of quaternary marine cores under the antarctic circumpolar current ice core dust magnetic correlation sediments in the belt under the antarctic circumpolar current acc contain high quantities of siliceous microfossils mainly diatoms and very little to zero carbonate this prevents establishment of the age by the usual methods of oxygen isotope stratigraphy at most coring sites downcore variation in magnetic susceptibility ms from cores from the scotia sea closely resembles the temporal pattern of dust concentration and flux in east antarctic ice cores the validity of the temporal correlation between marine ms and epica dome c edc dust on the edc3 time scale is established through 14c dating of the acid insoluble organic matter back to 30 ka and radiolarian abundance stratigraphy of cycladophora davisiana back to 180 ka the correlations are good r 0 80 and give credence to the use of ms dust as a means of establishing chronostratigraphy in carbonate free pelagic sediments from the southern ocean this correlation has been established in 10 cores in the scotia sea the circum antarctic validity of the pattern is shown in correlations to ms in sediment cores from the indian and sw pacific sectors of the southern ocean during mis 3 57 29 ka when sedimentation rates at correlated core sites exceeded 10 cm ka 1 three ms maxima in the marine sediments and dust peaks at edc respectively correspond to the cool periods between antarctic warm events a1 a4 we have not established the precise cause of the correlation but its validity around the circum antarctic west wind belt suggests that the material was either all transported by wind or was supplemented by wind driven current transport the fact that it occurs also in sections with a low terrigenous content suggests a fine grained high susceptibility material such as magnetite as the likely source

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

210pb and 137cs in air and soils measure the rate and vertical profile of aerosol scavenging we have determined the deposition rates of sup 210 pb and sup 137 cs from the atmosphere to the landscape of the eastern and mid western united states by measuring their inventories in core samples of undisturbed soils the mean inventories in lowland sites are 27 7 dpm cm sup 2 for sup 210 pb and 21 8 dpm cm sup 2 for sup 137 cs as of january 1984 the mean annual concentration of sup 210 pb in surface air at the 9 sites of the sure network decreases smoothly from about 0 060 dpm m sup 3 near the mississippi river to about 0 030 near the atlantic coast the ratio of deposition rate to surface air concentration is 0 62 0 03 cm s sup 1 for sup 210 pb and 1 36 0 08 cm s sup 1 for sup 137 cs using the activity vs altitude profiles of moore et al 1973 for sup 210 pb and the fission product nuclide sup 90 sr we calculate tropospheric aerosol mean residence times for sup 210 pb and sup 137 cs of 4 8 and 5 5 days respectively this near equality precludes a rapid decrease of the aerosol scavenging rate constant in the mid troposphere the sup 137 cs sup 210 pb ratios in soils from elevations above 1000 m in the appalachian mountains aremore lower than those in adjacent lowland soils indicating the preferential removal of aerosols from the lower layer of the troposphere presumably by cloud droplet impaction or by seeder feeder clouds less

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

late quaternary glacial history of the south orkney plateau antarctica abstract piston cores from the south orkney plateau penetrated overcompacted diamictons in water depths of up to 250 m detailed textural and petrological analyses of these diamictons indicate that they are basal tills seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material the ice rafted debris in these glacial marine sediments is composed almost exclusively of material derived from the south orkney islands this implies deposition beneath an ice shelf as opposed to iceberg rafting in contrast diatomaceous muds contain relatively abundant exotic iceberg rafted material and reflect a glacial maritime setting similar to that of today the sharp contact separating diatomaceous surface sediments from basal tills and sub ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago the homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time

radiometric dating 210pb 137cs 241am of recent ombrotrophic peat accumulation and evidence for changes in mass balance the paper summarizes and evaluates the results of 210pb 137cs and 241am analysis on 37 recent ombrotrophic peat profiles from these and previously published data it is concluded that radiometric measurements alone cannot be relied on to give an accurate chronology of peat accumulation where constrained by profiles of 241am activity in the upper part and one or more pollen dated horizons below this 210pb can provide good chronologies of peat accumulation provided the constraint does not displace the calculated 210pb dates by more than c 10 profiles of 137cs activity in most cases fail to contribute significantly to dating recent peat at somewhat degraded sites the residual dry mass of peat representing the last c 150 years is reduced further analysis of well dated profiles suggests that this may reflect loss of catolelm peat newly exposed to aerobic decomposition processes after a fall in water table several mechanisms are proposed for the displacement and net loss of lead including 210pb

an early miocene ridge crest trench collision on the south scotia ridge near 36 w abstract preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest trench collision zone southeast of the south orkney islands the existence of a series of such collision zones along the south scotia ridge had earlier been hypothesised to explain changes in scotia sea evolution the collision zone takes the form of a double ridge with dissecting trough aligned ne sw and about 200 km long oceanic magnetic lineations in the northern weddell sea young northwestward towards the zone with the youngest identified as anomaly 6 20 ma reaching the central trough volcanics dredged from jane bank the northwestern ridge are chemically very similar to the more siliceous of the low k tholeiite series which characterises the presently active south sandwich arc jane bank is interpreted as an in situ remnant of the arc and upper fore arc of an intra oceanic arc produced by subduction of south american oceanic lithosphere like the present south sandwich islands part of the upper fore arc and all of any accretionary wedge appear to have been subducted unlike ocean floor at collision zones off chile and the antarctic peninsula the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age depth relationship the estimated collision time 20 ma is close to the time of onset of n s extension in the central scotia sea the limited data set described here however does not establish the synchroneity of the collision over the entire 200 km of jane bank so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey

diagenetic metal profiles in recent sediments of a scottish freshwater loch integrated total elemental phase specific and pore water analyses of sediment cores from loch ba scotland show that early diagenetic processes have promoted extensive metal enrichment immediately beneath the sediment water interface the accumulation of mn pb zn cu and co in sedimentary solids upward of 3 cm depth is accompanied by an increasing residence of these elements in adsorbed and hydrous oxide phases such phases are formed through oxidative precipitation from the interstitial pore fluids following the upward migration of metals from more deeply buried anaerobic sectors of the sediment pile there is good evidence that fe and ni are subject to similar influences although their total abundances near the sediment surface are less conspicuously modified in the loch ba sediments the oxic conditions promoting metal precipitation are entirely confined to strata of postindustrial age in the absence of fully diagnostic pore water and sequential chemical data similar diagenetic profiles could plausibly be misinterpreted as the product of anthropogenic contamination

heat flow in jane basin northwest weddell sea the jane basin is a marginal basin situated immediately to the east of the south orkney microcontinent in the northwest section of the weddell sea thirty five heat flow measurements made in the jane basin ranged in value from 67 5 4 3 to 92 1 3 0 mw m2 excluding values that were corrected for tilting or were on the very edge of the basin the remaining 28 values range between 75 0 8 0 and 84 6 9 1 mw m2 magnetostratigraphy on the recovered core from ocean drilling program hole 697 which was drilled in jane basin to a depth of 322 m allowed sedimentation rates to be calculated back to 4 5 ma single channel seismic reflection data from rss shackleton allowed estimations of total sediment thickness for the jane basin to be made we calculate that the measured heat flow is only 86 89 of the actual heat flow as a result of sedimentation heat generation in the sediments contributes 1 5 1 9 mw m2to the total heat flow the corrected heat flow gives an age for the jane basin of between 25 and 32 ma from age versus heat flow comparisons similar to the age determined from basement depth the scotia sea located to the north of jane basin and the south orkney microcontinent has been dated as anomaly 10 30 ma and younger our calculated age for the jane basin would indicate that it may have been created prior to the initiation of seafloor spreading in the scotia sea evidence from major plate motions indicate that antarctica began to rotate clockwise away from south america at about 65 ma such motion may have triggered subduction along the southeast side of jane bank and the opening of jane basin as a back arc basin subduction at jane bank ended at anomaly 6a time 22 ma as evidenced by the age of the identified magnetic anomalies on the antarctic plate found immediately to the east of jane bank we conclude that jane basin opened prior to the opening of the scotia sea and that the spreading center that opened jane basin may have jumped to the scotia sea and produced the seafloor spreading there

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

sedimentation and sediment accretion in michigan coastal wetlands u s a abstract sediment transport and accretion rates were studied for three different types of coastal wetlands of lake michigan u s a a forested intradunal wetland cranberry bog no 1 near stevensville michigan displayed sediments in the form of detritus and exported them very little sedimentary material entered by streamflow these features of bottom land wetlands were confirmed by radioisotope studies on cores which showed low accretion rates of 210 pb the mass fluxes were found to range from 0 01 to 0 2 g cm 2 yr 1 the equivalent linear growths are 0 5 9 0 mm yr 1 the suspended material from this wetland entered a filling shrub bog lake complex known as cranberry bog no 2 a considerable fraction of the incoming sediments were retained in this intradunal basin wetland this was confirmed via radioisotope studies with cores showing higher accretion rates than in the bottom land wetland the river delta wetland at pentwater michigan had little sediment trapping ability large amounts of water and suspended materials entered this riverine wetland but no annual average removal was found radioisotope studies of cores at selected locations within this system showed little or no accretion in stream channels and normal accumulation of organic material in non channel areas no net annual effect on water quality of the streams as they discharged to lake michigan was found storm events caused no notable import or export of suspended material presumably the sand substrate in the vicinity of all three wetlands caused runoff events to be mediated by subsurface flow through this porous soil these three sand based coastal wetlands appear to have little net effect on suspended solids the 210 pb method proved more valuable than 137 cs because of the small depth of occurrence of the latter isotope interpretation of isotope profiles required information on acid fraction soluble profiles results are expressed in terms of mass units since compaction of cores distorts the linear accretion rate delayed transfers of radionuclides appear to have occurred perhaps as a result of integration of atmospheric inputs by the watersheds the total activities of 210 pb and 137 cs are well correlated which indicates particle trapping efficiency as the primary factor for affecting accumulation of both isotopes

heat flow determinations of basement age in small oceanic basins of the southern central scotia sea results are reported from seven heat flow stations in small basins of the southern part of the central scotia sea css undertaken in order to determine basement ages the basins are small which makes magnetic anomaly based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors the fact that these small basins formed in a back arc setting adds additional uncertainty to depth based age estimates the results confirm that basin extension commenced in the eocene and indirectly support a relatively young back arc origin for the northern css but do not affect previously published suggestions of the age of onset of the antarctic circumpolar current

tectonic activity evolution of the scotia antarctic plate boundary from mass transport deposit analysis the spatial distribution and temporal occurrence of mass transport deposits mtds in the sedimentary infill of basins and submerged banks near the scotia antarctic plate boundary allowed us to decode the evolution of the tectonic activity of the relevant structures in the region from the oligocene to present day the 1020 mtds identified in the available data set of multichannel seismic reflection profiles in the region are subdivided according to the geographic and chronological distributions of these features their spatial distribution reveals a preferential location along the eastern margins of the eastern basins this reflects local deformation due to the evolution of the scotia antarctic transcurrent plate boundary and the impact of oceanic spreading along the east scotia ridge esr the vertical distribution of the mtds in the sedimentary record evidences intensified regional tectonic deformation from the middle miocene to quaternary intensified deformation started at about 15 ma when the esr progressively replaces the west scotia ridge wsr as the main oceanic spreading center in the scotia sea coevally with the wsr demise at about 6 5 ma increased spreading rates of the esr and numerous mtds were formed the high frequency of mtds during the pliocene mainly along the western basins is also related to greater tectonic activity due to uplift of the shackleton fracture zone by tectonic inversion and extinction of the antarctic phoenix ridge and involved changes at late pliocene the presence of mtds in the southern scotia sea basins is a relevant indicator of the interplay between sedimentary instability and regional tectonics

time averaged fluxes of lead and fallout radionuclides to sediments in florida bay recent unmixed sediments from mud banks of central florida bay were dated using 210 pb 226 ra and chronologies were verified by comparing sediment lead temporal records with pb ca ratios in annual layers of coral montastrea annularis located on the ocean side of the florida keys dates of sediment lead peaks 1978 2 accord with prior observations of a 6 year lag between the occurrence of maximum atmospheric lead in 1972 and peak coral lead in 1978 smaller lags of 1 2 years occur between the maximum atmospheric radionuclide fallout and peaks in sediment temporal records of 137 cs and pu such lags are consequences of system time averaging sta in which atmospherically delivered particle associated constituents accumulate and mix in a sedimentary reservoir before transferring to permanent sediments and coral sta model calculations using time dependent atmospheric inputs produced optimized profiles in excellent accord with measured sediment 137 cs pu lead and coral lead distributions derived residence times of these particle tracers 16 1 15 7 0 7 19 3 and 16 2 years respectively are comparable despite differences in sampling locations in accumulating media and in element loading histories and geochemical properties for a 16 year weighted mean residence time sta generates the observed 6 year lead peak lag evidently significant levels of nondegradable particle associated contaminants can persist in florida bay for many decades following elimination of external inputs present results in combination with sta model analysis of previously reported radionuclide profiles suggest that decade scale time averaging may occur widely in recent coastal marine sedimentary environments

modern and historic atmospheric mercury fluxes in both hemispheres global and regional mercury cycling implications 1 using two different natural archiving media from remote locations we have reconstructed the atmospheric deposition of mercury hg over the last 800 1000 years in both hemispheres this effort was designed 1 to quantify the historical variation and distributional patterns of atmospheric hg fluxes in the midlatitudes of north america at nova scotia n s and at a comparable midlatitude region in the southern hemisphere at new zealand n s 2 to identify and quantify the influence of anthropogenic and natural hg contributions to atmospheric hg fluxes 3 to further investigate the suitability and comparability of our two selected media lake sediments and ombrotrophic peat for hg depositional reconstructions and 4 to assess the relative importance of wet and dry deposition to the study areas significant findings from the study include the following 1 the lake sediments examined appear to faithfully record the contemporary flux of hg from the atmosphere e g 1997 n s lakes approximately 8 3 g m 2 yr 1 n s rain 8 g m 2 yr 1 the upper 10 cm approximately 10 yr of ombrotrophic peat cores from nova scotia were dated using a biological chronometer polytrichum and were also consistent with the flux data provided by current direct sampling of precipitation these observations place limits on the contribution of dry deposition 40 50 of wet flux unfortunately the peat samples could not be dated below 10 cm this was due to the apparent diagenetic mobility of the geochronological tracer 210pb 2 there is no evidence of a significant enhancement in the atmospheric hg flux as a result of preindustrial 1900 c e common era activities such as the extensive au and ag mining in the americas 3 a factor of 3 and 5x increase in the deposition of hg to the lake sediment archives was observed since the advent of the industrial revolution in new zealand and nova scotia respectively suggesting a worldwide increase in the atmospheric deposition of hg furthermore this increase is synchronous with increases in the release of co2 from combustion of fossil fuels on a global scale the magnitude of increase since industrialization appears larger in nova scotia than in new zealand this may be due to enhanced deposition of hg as a result of either regional emission of hg or enhanced regional oxidation of hg

mercury contamination chronologies from connecticut wetlands and long island sound sediments sediment cores were used to investigate the mercury deposition histories of connecticut and long island sound most cores show background pre 1800s concentrations 50 100 ppb hg below 30 50 cm depth strong enrichments up to 500 ppb hg in the core tops with lower hg concentrations in the surface sediments 200 300 ppb hg a sediment core from the housatonic river has peak levels of 1 500 ppb hg indicating the presence of a hg point source in this watershed the hg records were translated into hg contamination chronologies through 210pb dating the onset of hg contamination occurred in 1840 1850 in eastern connecticut whereas in the housatonic river the onset is dated at around 1820 the mercury accumulation profiles show periods of peak contamination at around 1900 and at 1950 1970 peak hg hg hg measured minus hg background accumulation rates in the salt marshes vary dependent on the sediment character between 8 and 44 ng hg cm2 per year whereas modern hg accumulation rates range from 4 17 ng hg cm2 per year time averaged hg accumulation rates are 15 ng hg cm2 per year these hg accumulation rates in sediments are higher than the observed hg atmospheric deposition rates about 1 2 ng hg cm2 per year indicating that contaminant hg from the watershed is focused into the coastal zone the long island sound cores show similar hg profiles as the marsh cores but time averaged hg accumulation rates are higher than in the marshes 26 ng hg cm2 a year because of the different sediment characteristics in situ atmospheric deposition of hg in the marshes and in long island sound is only a minor component of the total hg budget the 1900 peak of hg contamination is most likely related to climatic factors the wet period of the early 1900s and the 1950 1970 peak was caused by strong anthropogenic hg emissions at that time spatial trends in total hg burdens in cores are largely related to sedimentary parameters amount of clay except for the high inventories of the housatonic river which are related to hg releases from hat making in the town of danbury much of the contaminated sediment transport in the housatonic river basin occurs during floods creating distinct layers of hg contaminated sediment in western long island sound the drop of about 40 in hg accumulation rates between the 1960s and 1990s seems largely the result of reduced hg emissions and to a much lesser extent of climatic factors

reconstructing historical trends in metal input in heavily disturbed contaminated estuaries studies from bilbao southampton water and sicily estuaries may be important reservoirs for contaminants as they tend to act as sinks for fine contaminant reactive sediments and historically they have acted as centres for industrial and urban development analysis of dated sediment cores from these areas may allow historical trends in heavy metal input to be reconstructed and recent and historical inputs of metal contaminants to be compared undisturbed saltmarsh settings have been used widely in the reconstruction of historical trends in metal input as saltmarshes provide a stable vegetated substrate of dominantly fine sediments and are less prone to erosion and reworking than adjacent mudflat areas in comparison much less research on historical pollution trends has been undertaken at estuarine sites which are prone to severe local disturbance such as intertidal areas which are routinely dredged or where sedimentary processes have been modified by human activities such as shipping salt working port activities land claim etc this paper assesses the usefulness of 210pb and 137cs dating combined with geochemical studies in reconstructing historical trends in heavy metal input and sediment accretion in 3 heavily modified industrialised estuarine areas in europe the bilbao estuary spain southampton water uk and the mulinello estuary sicily of these sites only a salt marsh core from the mulinello estuary provides a high resolution record of recent heavy metal inputs in southampton water only a partial record of changing metal inputs over time is retained due to land claim and possible early diagenetic remobilisation while at bilbao the vertical distribution of heavy metals in intertidal flats is mainly controlled by input on reworked sediment particles and variations in sediment composition where 137cs and 210pb distributions with depth allow a chronology of sediment deposition to be established and early diagenetic remobilisation has been minimal mudflat and saltmarsh cores from even the most heavily disturbed estuarine sites can provide useful information on variations in historical contaminant input when the sediments have been vigorously mixed or reworked large scale compositional variations are present or significant early diagenetic remobilisation has occurred however only general information on the scale of contamination can be obtained

mobility and diagenesis of pb and 210pb in peat peatlands long have been considered to preserve the record of atmospheric deposition of anthropogenic contaminants such as pb in the past two decades sup 210 pb has been widely used to data recent strata of peat and to calculate accumulation rates the assumption that pb and sup 210 pb are immobile and not subject to diagenesis in peat has been questioned but not rigorously tested the authors attempted to determine if pb is mobile in peatlands and if pb profiles are altered by diagenic processes by constructing a mass balance for pb about a small peatland by comparing inventories concentrations and accumulation rates of pb and sup 210 pb in peatlands across northeastern north america and by examining the relationship between concentrations of pb in bog waters and peat in numerous sites the results clearly demonstrate that pb and sup 210 pb are mobilized by the organic rich waters of peatlands profiles of pb and sup 210 pb at depths below the water table do not preserve the record of atmospheric deposition and inventories of pb and sup 210 pb are depleted in peatland hollows concentrations of pb in bog waters are regulated by the concentration of pb in the peat and the concentration of dissolved organicmore carbon the mass balance for one bog indicated that over the specific three year period of study more than 30 of inputs of pb were not retained within the peat as a result of this mobility dates based on sup 210 pb can be biased and inaccurate by as much as 30 years dates based on sup 210 pb should be verified by other techniques especially when the inventory of sup 210 pb is less than that expected from local rates of deposition less

past mining activities in the vosges mountains eastern france impact on vegetation and metal contamination over the past millennium coring was carried out in a soligenous marsh in the vosges mountains in the past mining district of sainte marie aux mines eastern france high resolution palynological non pollen palynomorph and geochemical analyses were performed along the core correlations between the herbal composition of the landscape and trace metals in the core reveal a specific palynological pattern during mining activities two main periods of anthropogenic impacts on vegetation and trace metal contamination are shown during the 16th 17th centuries for mining and smelting activities and the beginning of the 20th century for smelting and the industrial revolution no drastic deforestations occurred near the study site contrary to historical descriptions and prints of the valley controlled forest practices were implemented from the beginning of the record that is since cal ad 1000 so the impact of mining activities seems to be less significant than expected near mining sites we demonstrate that the minerotrophic cha

reply to comment by william shotyk on does within bog spatial variability of mercury and lead constrain reconstructions of absolute deposition rates from single peat records the example of store mosse sweden reply to comment by william shotyk on does within bog spatial variability of mercury and lead constrain reconstructions of absolute deposition rates from single peat records the example of store mosse sweden

boron beryllium and sulfur in holocene sediments and peats of the nile delta egypt their use as indicators of salinity and climate abstract the concentrations of 14 major and trace elements b ba be co cr cu fe mn ni p s v zn and organic c in bulk sediment samples of holocene age from bore holes in the nile delta egypt are considered in terms of sediment grain size and depositional setting using this background information an attempt is made to evaluate paleosalinity and paleoclimatic change by geochemical analyses of peats in every sediment facies examined we find that grain size is a major controlling factor of geochemical composition but the b be ratio is helpful in paleoenvironmental interpretations organic matter content is the major factor affecting element concentrations in peats zn cr fe be cu mn concentrations on an organic matter free basis are lower in these organic rich layers than in other nile delta sediment facies the sulfur content of the peats indicates a decreased importance of freshwater input to holocene lagoons from about 8000 to 3400 years bp high boron beryllium ratios and the presence of gypsum in peats deposited between 6300 and 4900 years bp record the most evaporitic phase affecting this region this geochemical finding is in close agreement with results of other independent paleoclimatic studies which indicate the onset of arid conditions in northeast africa at about 5000 years bp

contourite deposits in the central scotia sea the importance of the antarctic circumpolar current and the weddell gyre flows abstract new swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift sediment wave morphologies and seismic facies in the central scotia sea the deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean the eastward flowing antarctic circumpolar current acc and the northward outflow of the weddell sea deep water wsdw the contourite drifts are wedge like deposits up to 1 km thick that exhibit aggradational reflectors along axis thinning towards the margins the contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles the elongate mounded drifts are best developed along the left hand margins of channelized bottom current flows due to the coriolis force a contourite fan has a main channel and two distributary channels that expand over a gentle seafloor the proximal fan exhibits sediment waves with the distal fan incised by furrows sediment wave fields are well developed in areas of intensified bottom flows without channels particularly at the confluence of the acc and the wsdw small sediment waves occur where unidirectional bottom current flows predominate sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea bottom relief the stratigraphic sequence above the oceanic crust of early to middle miocene age contains six seismic units separated by major reflectors all the units were shaped under the influence of strong bottom current flows although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time the age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep sea deposits in the region the oldest units vi iv are of early to middle miocene age and developed under the influence of the acc they are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5 8 cm ky unit iii with an estimated middle miocene age evidences the first incursion of wsdw into the central scotia sea when plate movement caused openings in the south scotia ridge and allowed the connection with the northern weddell sea through jane basin and gaps in the ridge unit ii estimated to be of late miocene to early pliocene age extends over the area and is characterized by internal unconformities a major unconformity at the base of unit ii records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the antarctic peninsula shelf unit i of late pliocene to quaternary age shows intensified bottom currents the unconformity at the base of unit i probably predates the onset of major northern hemisphere glaciations and the greater expansion of antarctic ice sheets during the late pliocene the extensive distribution of contourite deposits above the oceanic crust testifies to the long term production of antarctic bottom water cold deep water was swept northward from the weddell gyre interacting with the acc and possibly exerting profound influences on the global circulation system and the onset of major glaciations

estimation and uncertainty of recent carbon accumulation and vertical accretion in drained and undrained forested peatlands of the southeastern usa the purpose of this study was to determine how drainage impacts carbon densities and recent rates past 50 years of vertical accretion and carbon accumulation in southeastern forested peatlands we compared these parameters in drained maple gum mapl atlantic white cedar cdr and pocosin poc communities in the great dismal swamp national wildlife refuge gds of virginia north carolina and in an intact undrained cdr swamp in the alligator river national wildlife refuge ar of north carolina peat cores were analyzed for bulk density percent organic carbon and 137cs and 210pb an uncertainty analysis of both 137cs and 210pb approaches was used to constrain error at least partially related to mobility of both radioisotopes gds peats had lower porosities 89 6 sd 1 71 versus 95 3 0 18 and higher carbon densities 0 082 0 021 versus 0 037 0 009 g c cm 3 than ar vertical accretion rates 0 10 0 56 cm yr 1 were used to estimate a time period of 84 362 years for reestablishment of peat lost during the 2011 lateral west fire at the gds carbon accumulation rates ranged from 51 to 389 g c m 2 yr 1 for all sites in the drained gds versus intact ar cdr sites carbon accumulation rates were similar with 137cs 87gds versus 92ar g c m 2 yr 1 and somewhat less at the gds than ar as determined with 210pb 111gds versus 159ar g c m 2 yr 1 heightened productivity and high polyphenol content of peat may be responsible for similar rates of carbon accumulation in both drained and intact cdr peatlands

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

reconstructing tropical cyclone frequency using hydrogen isotope ratios of sedimentary n alkanes in northern queensland australia a peat record from quincan crater queensland australia spanning the past 200 years was used to test if hydrogen isotope ratios of leaf wax long chain n alkanes derived of higher plants can be used to reconstruct past tropical cyclone activity queensland is frequently impacted by tropical cyclones with on average 1 2 hits per year the most abundant n alkanes in the peat are c 29 and c 31 possible sources for long chain n alkanes in the peat core are ferns and grasses which grow directly on the peat layer and the tropical forest growing on the crater rim hydrogen isotope ratios of c 27 c 29 and c 31 n alkanes vary between 155 and 185 parts per thousand vsmow with the largest variability in the upper 30 cm of the record for the period 1950 2000 ad the variability in delta d of c 29 alkanes resembles a smoothed record of historical tropical cyclone frequency occurring within a 500 km radius from the site this suggests that the high number of tropical cyclones occurring in this period strongly impacted the delta d signal and on average resulted in more depleted values of precipitation in the period before 1900 ad the variability in the hydrogen isotope record is relatively small compared to the period 1950 2000 ad this might be the result of lower variability of tropical cyclones during this time period more likely however is that it results from the increasing age span per sampled interval resulting in a lower temporal resolution average delta d values between 1900 and 2000 ad are around 167 parts per thousand which is similar to average values found for the period between 1800 and 1900 ad this suggests that on average tropical cyclone frequency did not change during the past 200 years this study demonstrates the potential of stable hydrogen isotope ratios of long chain n alkanes for the reconstruction of past tropical cyclone frequency

changes of lead and barium with time in california off shore basin sediments during ancient times the natural deposition fluxes of lead which can be leached with dilute acid from sediments in santa barbara santa monica and san pedro basins offshore from the los angeles urban complex were about 0 7 0 1 and 0 2 g pb cm 2 yr respectively since there was little difference in biological productivity in surface waters of these basins it is proposed that clay is a major transport vehicle for sequestered soluble lead which then explains why the lead deposition flux within the santa barbara basin was so much larger compared to the other basins the fluxes of silicate mud in the basins in ancient times were about 92 19 and 30 mg cm 2 yr in santa barbara santa monica and san pedro basins respectively today deposition fluxes of acid soluble lead within these three basins are 3 to 9 fold greater being about 2 1 1 1 and 1 8 g pb cm 2 yr respectively partly in the form of directly deposited large sewage particles which account for none 2 3 and 3 4 of the total industrial lead deposition fluxes in the respective basins concentrations of leachable lead in varve dated sediment layers increase with time and isotopic compositions of these leads change in accordance with corresponding known changes of isotopic compositions of industrial lead in the los angeles atmosphere lead remaining in acid leached sediment residues originates from igneous and clay minerals exhibiting no change in concentration or isotopic composition since pre industrial times deposition fluxes of total barium in sediments among the three basins were proportional to mass deposition fluxes before 1950 in the same manner as for lead afterwards there are barium concentration maxima with time in both santa monica and san pedro basin sediments which are attributable to industrial sewage rather than to episodic erosion from barium rich sedimentary evaporite strata exposed locally along the shore an increase of barium concentrations in present day santa barbara basin sediments may reflect dispersal of barium rich drilling mud from local drilling operations

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

mangrove development and its response to environmental change in yingluo bay sw china during the last 150years stable carbon isotopes and mangrove pollen abstract located at the interface of terrestrial and marine ecosystems mangroves are particularly sensitive to environmental change they provide a sedimentary sink for organic carbon whereby cores can provide detailed records of mangrove species we aimed to trace the history of mangrove development over the past 150 years in yingluo bay sw china sedimentation rates avg 0 32 and 0 37 cm year were calculated on the basis of ln 210 pb ex vs mass depth and offset the rate of relative sea level rise 0 22 0 24 cm year leading to a seaward expansion of new mangrove habitats chemical tracers 13 c org and c n and an isotope mixing model were utilized to trace the contribution of mangrove derived organic matter mom changes in the relative abundance of pollen from mangrove plants was used to compensate for diagenetic alteration of the stable isotope values and potential overlaps in isotope values for different sources of organic matter the result of pearson correlation analysis showed that the mom was moderately positive correlated with total mangrove pollen indicating that stable carbon isotopes and mangrove pollen provide similar information for tracing mangrove ecosystems based on results from this study compositional changes in mangrove communities could be divided into two main stages a degradation period 1870 1930 ad and a flourishing period 1930 2011 ad corresponding to colder temperature and warmer temperature respectively owing to the location being far away from any industrial area and human activity temperature may be a key factor for mangrove development

records of radionuclide deposition in two salt marshes in the united kingdom with contrasting redox and accumulation conditions single cores from two salt marshes in the united kingdom located near different nuclear facilities were investigated to compare chronostratigraphic estimates derived from the natural radionuclide 210pbexcess with estimates from the known times of introduction of artificial radionuclides to the environment both cores selected had clear visual indications of redox zonation and evidence for diagenetic redox perturbation of the radionuclide records was also sought in the core from beaulieu marsh on the south coast of england the redox zonation was revealed by the profiles of the redox sensitive elements mn i fe p as and s and the records of nuclear discharges were entirely contained within oxidized conditions in the upper 40 cm the constant flux constant sedimentation 210pbexcess accumulation estimate was 76 of that derived from the 1963 fallout 137cs level 0 35 vs 0 46 g cm 2 yr 1 dry mass but the constant flux 210pbexcess method indicated that accumulation rates were lower at beaulieu before 1950 on any timescale 137cs appears earlier in the sediment record than its introduction to the environment but although downward diffusion of 137cs relative to 241am has clearly occurred the 137cs peak still appeared in place and there was negligible penetration of 137cs into underlying reduced conditions this core also contained a peak of the 60co discharges from either or both the winfrith and la hague nuclear plants that peaked in 1980 and 1984 respectively the sediments in the core from wyre marsh on the eastern coast of the irish sea had accumulated more rapidly than those at beaulieu and in this case the redox zonation could be established only from mn and s profiles here the constant initial activity 210pbexcess accumulation rate estimate was 125 of that derived from the 137cs peak correlated with the 1975 sellafield discharge maximum 0 79 vs 0 64 g cm 2 yr 1 sellafield discharge 137cs swamps fallout or chernobyl 137cs signals in this core but the 137cs and 241am sediment records are well separated and remain consistent with the slightly different discharge patterns over time this is so despite the fact that the maximum activity levels of both isotopes are now located well into reducing conditions out of which mn must have migrated the 210pb profile appeared similarly unaffected by the oxidized reduced boundary in this case this core was too short to define the limits of any downward 137cs migration 210pbexcess accumulation rate estimates for salt marshes should be viewed with some caution because of the steady state assumptions inherent in all 210pbexcess methods and the potential for fluctuating accumulation conditions and open system behavior in salt marshes

depositional processes and growth patterns of isolated oceanic basins the protector and pirie basins of the southern scotia sea antarctica abstract sedimentary processes in small isolated oceanic basins that form adjacent to continental margins but detached from continents remain poorly understood this work describes two such basins located in the southern scotia sea the protector and pirie basins we analysed multichannel seismic profiles to interpret morphostructural features and stratigraphy of these basins sedimentary stacking patterns and depocentre distribution illustrate basin development patterns basal units infill basement depressions formed by the submerged banks of thinned continental crust that abut the basin plains these lower and middle deposits of the sedimentary record are interpreted as pre and syn rift deposits the laterally extensive upper deposits are interpreted as post rift deposits these include five discrete units evident in seismic profiles a prominent regional reflection referred to as reflector c separates in these upper deposits two sets of seismic units that have recorded major shifts in the dominant sedimentary processes stacking patterns and paleo environmental conditions the most important processes controlling deposition of the older units those beneath reflector c include down slope gravity processes that infill depressions created by crustal thinning and seafloor spreading these occurred under the coeval influence of circumpolar deep water circulation the major processes influencing younger units those above reflector c include bottom water circulation of the circumpolar deep water and weddell sea deep water water masses which coursed along bathymetric contours of the seafloor the reflector c discontinuity developed concurrently with middle miocene tectonic changes which led to the opening and deepening of deep gateways in the south scotia ridge these facilitated overflow of weddell sea deep water from the weddell gyre into the scotia sea this overflow in turn forced the circumpolar deep water northwards analysis of the protector and pirie basins shows that their tectonic evolution influenced regional deep water circulation patterns in an area that makes a significant cold water contribution to the global conveyor belt system as a long term factor controlling basin evolution and sedimentary processes tectonics events in this region therefore influenced the present day climate system these results further clarify our understanding of deep isolated oceanic basins in terms their sedimentologic climatologic and oceanographic significance

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings