what is diatomite different types of biogenic remains ranging from siliceous algae to carbonate precipitates accumulate in the sediments of lakes and other aquatic ecosystems unicellular algae called diatoms which form a siliceous test or frustule are an ecologically and biogeochemically important group of organisms in aquatic environments and are often preserved in lake or marine sediments when diatoms accumulate in large numbers in sediments the fossilized remains can form diatomite in sedimentological literature diatomite is defined as a friable light coloured sedimentary rock with a diatom content of at least 50 however in the quaternary science literature diatomite is commonly used as a description of a sediment type that contains a large quantity of diatom frustules without a precise description of diatom abundance here we pose the question what is diatomite what quantity of diatoms define a sediment as diatomite is it an uncompacted sediment or a compacted sediment we provide a short overview of prior practices and suggest that sediment with more than 50 of sediment weight comprised of diatom sio2 and having high 70 porosity is diatomaceous ooze if unconsolidated and diatomite if consolidated greater burial depth and higher temperatures result in porosity loss and recrystallization into porcelanite chert and pure quartz less

balancing rare earth element distributions in the northwestern mediterranean sea abstract elemental concentrations of rare earth elements ree and isotopic compositions of neodymium end have been measured in three water column profiles in the north western mediterranean sea clear enrichments of ree are observed when comparing to adjacent atlantic waters suggesting ree inputs along the circulation in this area for the first time relative proportions of external sources including submarine groundwater discharges sgd have been quantified for the studied area atmospheric deposition is estimated to be the most important external source for all the ree with an average contribution of 44 followed by diffusion from porewaters which provide a 30 dissolved riverine fluxes account for 11 sgd for 10 and dissolution of remobilized surface sediments the remaining 6 mass balances accounting for seawater transport and identified external sources have been delineated for the three main water masses modified atlantic waters levantine intermediate waters and western mediterranean deep waters they show that the balances of ree in this area are dominated by seawater mass mixing superimposed on this hydrography ree vertical profiles are affected by external sources and biogeochemical cycling dissolved ree are correctly balanced in deep waters whereas substantial missing fluxes are identified in the surface and intermediate water masses additional net lree outputs and hree inputs are required in the surface waters while net output fluxes for all the ree are missing at intermediate waters the most likely process suggested here is an active reversible scavenging consistent with a stronger adsorption of lree compared to hree in the particular case of the redox sensitive cerium the most plausible mechanism to explain the net output missing fluxes is ce 3 removal by particle scavenging via oxidation to insoluble ce 4 estimated ce oxidation rates of 0 33 d 1 in surface waters agree well with previously published values exchange fluxes derived from the isotopic nd mass balance indicate higher nd scavenging in surface compared to intermediate waters

exchange of neodymium and its isotopes between seawater and small and large particles in the sargasso sea the concentrations and isotopic ratios of nd were measured in small 1 53 m and large 53 m in situ filtered particles sediment trap material sediment and aerosol samples collected during oligotrophic and productive conditions in the sargasso sea atmospheric input is the main source of exogeneous nd into the sargasso sea based on ndal ratios in trapped material we suggest that up to 50 of atmospheric nd could dissolve at the air sea interface the concentration and isotopic composition of nd vary with both depth and the particle size neodymium concentrations of small and large particles increase with depth from 2 9 12 g g and 5 8 11 8 g g respectively at 40 m the nd signature of large particles is significantly more negative nd 0 13 1 0 2 than that of the small ones nd 0 11 7 0 3 and the particles filtered deeper below 237 m the nd 0 values for small particles match the seawater whereas the nd 0 values of large particles remain less radiogenic these differences are related to the different behaviors of the particulate fractions small particles apparently exchange nd with seawater and then sink rapidly to depth as biologically formed aggregates large particles these aggregates isolate the small particles slowing the exchange with the water to produce different signatures between large particles and seawater since these exchange processes modify the particulate nd signature the shallow radiogenic nd 0 signature observed in the surface and subsurface waters in the nw atlantic is not due to external contributions and is likely of advective origin at depth below 800 m the magnitude of the aggregation disaggregation process varies significantly with season the fraction of small particles repackaged into large aggregates can vary from 0 8 to 80 100 in productive and oligotrophic conditions respectively this seasonal variation may correspond to sinking particles of different zooplanktonic origin vertical fluxes of nd into and out of the sargasso sea as well as exchange fluxes in the water column are examined about 50 of the particulate flux from the surface is remineralized at depth a net scavenging flux of 0 8 10 4 g m2 y is required this value corresponds to a scavenging nd residence time of 1900 y considerably longer than previous estimates of global oceanic residence time of nd on the order of 400 y

the hafnium neodymium isotopic composition of atlantic seawater the first full water column hafnium isotopic compositions of atlantic seawater have been obtained at seven locations from the labrador sea to the drake passage despite subpicomolar concentrations in seawater a precision of the hf isotopic measurements of 03 epsilon hf units was achieved an overall epsilon hf range between 3 1 in the labrador sea and 4 4 in antarctic bottom water was determined the distribution of which broadly reflects continental weathering inputs within particular water column profiles significant differences of up to 4 epsilon hf units occur combined with nd isotope data of the same samples it is evident that the hf isotopic composition of seawater is too radiogenic for a given nd isotopic composition and that the largest difference between expected and measured hf isotopic compositions in seawater occurs near the oldest continental crust in the labrador sea this corroborates the previous proposition which was mainly based on ferromanganese crust data that the hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly to some extent by hydrothermal contributions hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years which is significantly shorter than previously assumed the hf isotopic composition of past seawater can therefore serve as a proxy for short distance basin scale mixing processes and the regime and intensity of nearby continental weathering processes c 2009 elsevier b v all rights reserved

paleoceanography of the east equatorial pacific over the past 16 myr and pacific atlantic comparison high resolution benthic foraminiferal δ18o and δ13c records at iodp site u1337 abstract we present astronomically tuned high resolution 3 4 kyr 15 8 myr long benthic foraminiferal 18 o and 13 c records from iodp site u1337 in the east equatorial pacific which provide a reference standard for stratigraphic correlation of deep sea sediments a long term glaciation history over the past 15 8 myr has been recorded in the 18 o two of the most significant glaciation events include the east antarctic ice sheet expansion eaie at 13 8 ma and the final onset of the northern hemisphere glaciation nhg in 3 3 2 5 ma a series of miocene and pliocene glaciation events exist between the eaie and nhg which are important glaciation attempts before the earth finally entered the full icehouse world in the late pleistocene a long term ocean carbon isotope decrease over the past 15 8 myr has been recorded in the 13 c ocean circulation change was the major cause of the ocean carbon isotope decrease in 9 4 8 1 ma and 4 0 ma whereas strengthened continental chemical weathering controlled the long term ocean carbon isotope decrease in most of the time of the past 15 8 myr the divergence in benthic foraminiferal 13 c and end provides strong evidence of significant ocean circulation change between 12 1 and 9 2 ma which was probably related to the formation of the isthmus of panama the internal feedbacks such as continental weathering and sea surface temperature change have great potential for modulating the weak 100 and 400 kyr signals in astronomical forcing into strong 100 and 400 kyr signals in global ice volume and carbon cycle

coupled isotopic systematics of surface cerium and neodymium in the pacific ocean trace metals are known to be essential elements in marine ecosystems radiogenic isotopes of neodymium nd have been used as tracers in many recent oceanic trace metal studies although among rare earth elements cerium ce isotopes might be an interesting complementary tracer for particle reactive and lithogenic metals such as manganese this study determined the 138ce 142ce ratios in surface waters of the pacific ocean and its surrounding marginal seas the sulu sea the south china sea the east china sea and the south australian basin the 138ce 142ce and 143nd 144nd data are discussed in terms of the sources of rare earth elements and elemental fractionation between ce and nd in the marine environment in the western north pacific central water east china sea and south china sea isotopic compositions of ce ce 0 7 to 1 4 are most affected by radiogenic ce of continental origin in contrast less radiogenic isotopic compositions of ce ce 0 4 to 0 3 in the pacific equatorial water were observed locally near volcanic islands such as new guinea island suggesting the influence of mantle derived ce compared with nd the isotopic composition of ce showed a heterogeneous distribution in a given surface water mass reflecting the importance of local sources variations of isotopic compositions and concentrations of ce in the western equatorial pacific and the east china sea suggest that lithogenic ce is supplied and scavenged by particle dissolved interaction near the margins radiogenic ce in the western north pacific central water which is more continental like than nd isotopes suggests direct input by atmospheric dust into the north pacific ocean the isotopic distribution of ce is sensitive to aeolian supply to the surface waters of the open ocean this unique feature indicates that the 138ce 142ce ratio can be a useful chemical tracer for lithogenic trace elements such as iron and manganese which have short oceanic residence time

neodymium isotopes in seawater from the barents sea and fram strait arctic atlantic gateways abstract the neodymium concentration c nd and isotopic composition e nd in seawater have been determined in the water column at five sites in the barents sea fram strait area where most of the water exchange between the arctic ocean and the north atlantic takes place in the main arctic ocean inflow branch across the barents sea the concentration and isotopic composition c nd 15 5 pmol kg and e nd 10 8 are similar to those reported for the northeastern nordic seas which is consistent with this region being a source area for the arctic inflow due to the addition of nd from svalbard shelf sediments the c nd in the surface waters above 150 m in the fram strait inflow branch is higher by a factor of 2 and the e nd is shifted to lower values 11 8 in the stratified nansen basin where cold low salinity water overlies warmer atlantic water the c nd and e nd do not vary with the vertical temperature salinity structure but are essentially constant and similar to those of the atlantic inflow throughout the entire water column down to 3700 m depth which indicates that the nd is to a large extent of atlantic origin compared to the atlantic inflow water the nd in the major arctic ocean outflow the fram strait show higher c nd in the surface waters above 150 m and a higher e nd 9 8 throughout the entire water column down to 1300 m depth sources for the more radiogenic nd isotopic composition in deep water of the fram strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the canadian archipelago river water is a possible source in the surface water but it also seems likely that pacific water nd modified by interactions on the shelf is an important component in the fram strait surface outflow changes in the relative proportions of inflow of river water and flow of pacific water through the arctic ocean could thus influence the isotopic composition of nd in the north atlantic

tracing past ocean circulation to understand how ocean circulation and climate may change in the future we must first understand how the system operated in the past without man9s influence new inorganic isotopic tracers are now helping to elucidate past ocean circulation patterns but controversies remain regarding the relative influence of ocean circulation and weathering on the isotopic signals

reduced north atlantic deep water flux to the glacial southern ocean inferred from neodymium isotope ratios the global circulation of the oceans and the atmosphere transports heat around the earth broecker and denton1 suggested that changes in the global ocean circulation might have triggered or enhanced the glacial interglacial cycles but proxy data for past circulation taken from sediment cores in the south atlantic ocean have yielded conflicting interpretations of ocean circulation in glacial times 13c variations in benthic foraminifera2 3 4 5 6 support the idea of a glacial weakening or shutdown of north atlantic deep water production whereas other proxies such as cd ca ba ca and 231pa 230th ratios show little change from the last glacial maximum to the holocene epoch7 8 9 here we report neodymium isotope ratios from the dispersed fe mn oxide component of two southeast atlantic sediment cores both cores show variations that tend towards north atlantic signatures during the warm marine isotope stages 1 and 3 whereas for the full glacial stages 2 and 4 they are closer to pacific ocean signatures we conclude that the export of north atlantic deep water to the southern ocean has resembled present day conditions during the warm climate intervals but was reduced during the cold stages an increase in biological productivity may explain the various proxy data during the times of reduced north atlantic deep water export

a second look at low frequency marine vertical seismometer data quality at the osn 1 site off hawaii for seafloor buried and borehole emplacements we improve marine low frequency 1 100 mhz vertical seismometer data by subtracting noise generated by tilting under fluid flow and by seafloor deformation under ocean surface gravity waves we model the noise from the coherency and transfer functions between the vertical channel and other data channels that are more sensitive to the noise sources the horizontal seismometer components for tilting and a differential pressure gauge for ocean waves we subtract noise from three adjacent seafloor broadband seismometer stations at the osn 1 deep ocean test site one sitting on the seafloor another buried 1 m deep in sediments and the third clamped in a borehole 248 m beneath the seafloor seafloor currents generate the seafloor sensor tilt noise whereas tidally driven fluid pumping generates the borehole sensor tilt noise subtracting the tilt noise reduces the vertical channel noise levels by 35 40 db between 1 and 60 mhz on the seafloor sensor and by 15 20 db between 1 and 10 mhz on the borehole sensor subtracting the ocean wave noise further reduces the noise level on all instruments by 5 15 db between 4 and 20 mhz after subtracting tilt and ocean wave noise the seafloor vertical channel is 5 10 db quieter than the buried sensor vertical channel at frequencies below 30 mhz the corrected borehole vertical channel has a similar noise level to the seafloor and buried sites above 10 mhz but noise increases rapidly at lower frequencies probably because of vertical strumming under tidally driven fluid flow

is ambient noise tomography across ocean basins possible 1 based on year long cross correlations of broad band seismic records obtained at sixty six stations within or adjacent to the pacific basin we show that broad band ambient noise is observed to propagate coherently between island stations and between island and continent stations for many station pairs high signal to noise ratio snr fundamental mode rayleigh wave green functions are observed which establishes the physical basis for ambient noise tomography across the pacific similar trends for continental and oceanic stations are observed in the relationship between the ambient noise level at a station and the noise coherence distance the longest distance at which a high snr cross correlation signal is observed for a station because locally generated noise obscures long distance coherent noise situating stations at quiet locations on islands is necessary for the success of ambient noise tomography local noise poses a particular challenge at atoll sites and on the basis of analysis of data from station h2o at ocean bottom sites at periods above 25 sec citation lin f c m h ritzwoller and n m shapiro 2006 is ambient noise tomography across ocean basins possible geophys res lett 33 l14304 doi 10 1029 2006gl026610

influence of the mediterranean outflow on the isotopic composition of neodymium in waters of the north atlantic the isotopic composition of neodymium in the water column of the eastern north atlantic near the strait of gibraltar has been determined for several depths the data show that the mediterranean outflow results in a significant shift in e nd 0 toward more radiogenic values of 143 nd 144 nd in the water column at a 1000 m depth this corresponds to a depth in the neighborhood of the salinity maximum associated with the mediterranean outflow the core of the mediterranean outflow gives e nd 0 9 8 as compared to e nd 0 12 in overlying and underlying waters demonstrating that the mediterranean waters are distinct from the atlantic from mixing considerations we estimate that pure mediterranean waters have e nd 0 6 possible sources of this relatively radiogenic nd could be from drainage of young continental terranes or the injection of remobilized nd from deep sea sediments that have a young radiogenic volcanic component new data from a depth profile in the western atlantic is presented comparison of nd data for the eastern north atlantic with that for the western north atlantic shows fundamental differences in the water column structures for e nd 0 while both regions show a pronounced maximum in e nd 0 the western basin maximum occurs at the near surface rather than at 1000 m in addition deep waters of the eastern basin are found to be more radiogenic than the western basin these differences indicate several sources of isotopically distinct nd in the north atlantic the deep waters of the north atlantic 1000 m have the lowest values of e nd 0 measured in the oceans we believe that the source of these low e nd 0 values which we associate with north atlantic deep water is either from freshwater drainage off the precambrian shields of north america and asia into the arctic ocean or from the injection of older continentally derived ree from deep sea sediments sm and nd concentrations are found to increase with depth and e nd 0 changes with depth indicating both vertical and lateral transport processes from different sources this suggests a surface source of nd and injection of ree into the water column from deep sea sediments or large scale bottom currents with high ree concentrations

the hafnium isotope composition of pacific ocean water the first hf isotope data for seawater are reported for a series of stations in the northwestern pacific and define a range from epsilon hf 3 5 1 4 to 8 6 1 6 most samples have values within error of the average of epsilon hf 5 9 but significant variations are found in intermediate waters at a depth of 600 m as well as in deep waters the nd and hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region confirming that hf in the fe mn crusts has been derived from the overlying water column which thus provide an archive of past seawater compositions although the seawater samples are generally close to the global epsilon nd epsilon hf correlation obtained from ferromanganese crusts there are significant deviations from this correlation indicating that there is some additional decoupling between nd and hf isotope signals most likely caused by local water mass mixing and differences in residence times this is not resolved in the crust samples which integrate seawater signals over 10 4 years the combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans studies of the distribution of oceanic hf isotope compositions that have been confined to deep water and boundary waters as recorded in seafloor ferromanganese crusts can now be extended and aimed at characterising the entire present day water column average hf concentrations measured in this study are somewhat lower than previously reported suggesting a shorter residence time for hf in the global oceans although the uncertainty in the extent of hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined c 2008 elsevier ltd all rights reserved

a new approach to the nd residence time in the ocean the role of atmospheric inputs concentrations of rare earth elements ree and nd isotopic ratios were analyzed for seawater filtered suspension and sediment trap samples collected in the tropical atlantic ocean eumeli program eutrophic mesotrophic and oligotrophic sites 20 n 18 21 w this is the first ree nd dataset on solution and different sized particles collected at the same site we present direct evidence of the nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer this exchange is probably one of the main factors that simultaneously constrains the nd concentration and isotopic ratio budget we propose a new approach to estimate the residence time of nd in the ocean nd based on isotopic exchange 200 yr nd 1000 yr the exchange requires a partial dissolution of lithogenic nd we estimate that the fraction of soluble nd proportion in atmospheric dust is of the order of 20 based on the isotopic ratios we suggest that the partial dissolution of atmospheric fallout is probably one of the main ree sources of the ocean

the isotopic composition of neodymium in the north pacific abstract we have determined the isotopic composition of neodymium in two water column profiles in the western north pacific and in bottom waters from three other locations in the north pacific the vertical profiles reveal that the water column is stratified with respect to isotopic composition mid depths associated with pacific deep water are uniform having nd 0 3 shallow waters are variable but deep waters are less radiogenic having md 0 4 4 to 5 in the western basin the more negative bottom waters in the western north pacific provide evidence for the advective transport of rare earth elements from antarctic sources to this region bottom waters in the eastern north pacific are more radiogenic nd 0 3 3 to 3 8 indicating very little transport of ree from antarctic sources to this basin overall the isotopic composition of nd in pacific bottom waters appears to be conservative when compared to the silica distribution in spite of evidence for nd removal the findings are generally compatible with other parameters such as temperature salinity and nutrient distributions the presence of nd isotopic signatures associated with antarctic sources in these waters provides evidence for a small component of rare earth elements of north atlantic origin in the bottom waters of the north pacific this is the first tracer evidence to corroborate the suggestion of reid and lynn 1971 that some of the temperature and salinity properties of the deep waters of the north pacific originated in the norwegian and greenland seas

geochemistry of holocene cryptotephras from the north iceland shelf md99 2269 intercalibration with radiocarbon and palaeomagnetic chronostratigraphies tephra grain counting morphology and geochemistry in the 150 1000 m size fraction were used to identify 22 individual tephra horizons and one visible tephra layer saksunarvatn from postglacial sediment of the north iceland shelf core md99 2269 66 37 53 n 20 51 16 w water depth 365 m length 2533 cm from the reykjafjardarall basin is constrained at the base by the vedde tephra c 12 000 cal yr bp geochemical analyses and stratigraphical position of the 22 tephra horizons were used to identify the following cryptotephras kol 1372 ad 1372 hekla 1104 ad 1104 snaefellsnes 1 c 1790 cal yr bp hekla 3 c 2970 cal yr bp hekla 4 c 4230 cal yr bp tv 5 c 6860 cal yr bp and suduroy c 8070 cal yr bp a tephra of the same geochemistry as kol 1372 was found near the base of the core and is here named kol3 2269 11 000 cal yr bp eleven additional cryptotephras were geochemically typed to their respective icelandic volcanic centres of origin and their ages constrained four teph

a new high resolution holocene tephra stratigraphy in eastern iceland improving the icelandic and north atlantic tephrochronology a new and improved holocene tephra stratigraphy and tephrochronological framework for eastern and northern iceland is presented investigations of a sediment sequence from lake logurinn have revealed a comprehensive tephra record spanning the last 10 200 years a total of 157 tephra layers have been identified whereof 149 tephra layers have been correlated to its source volcanic system using geochemistry stratigraphy and age fifteen layers have chemical composition of two affinities that possibly represent two very closely spaced eruptions thus these 157 tephra layers are believed to represent 172 explosive eruptions nineteen tephra marker layers have been identified in the lake logurinn record g1922 a1875 v1477 v1410 h1636 k1625 o1362 g1354 k1262 v874 hrafnkatla sn 1 grakolla hy h3 h4 ho ll1755 and reitsvik 8 tephra markers new potential tephra markers are the silicic askja l 9400 cal bp the low titanium basalt layers ll 1774 10 150 cal bp and ll 1755 9990 cal bp assigned to veidivotn bardarbunga and the tephra layers ll 1527 8 7850 cal bp ll 911 2 2370 cal bp ll 908 4 2350 cal bp ll 781 9 1930 cal bp ll 644 4 1480 cal bp not yet correlated to a source volcanic system a silicic tephra marker layer reitsvik 8 correlated to the fosen tephra in norway has been identified in lake logurinn the lake logurinn tephra record has been connected and integrated with the icelandic terrestrial tephrochronology and stratigraphy through 102 tephra layers the marine tephra stratigraphy through 39 layers and overseas through 9 tephra layers this record is the first high resolution tephra stratigraphical and chronological framework for the holocene in eastern iceland as well as the most detailed and continuous record and has considerable potential to serve as a key section or a stratotype for the holocene in eastern iceland and the north atlantic

ambient noise rayleigh wave tomography of new zealand summary we present the first new zealand wide study of surface wave dispersion using ambient noise observed at 42 broad band stations in the national seismic network geonet and the global seismic network gsn year long vertical component time series recorded between 2005 april 1 and 2006 march 31 have been correlated with one another to yield estimated fundamental mode rayleigh wave green s functions we filter these green s functions to compute rayleigh wave group dispersion curves at periods of 5 50 s using a phase matched filter frequency time analysis technique the uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3 month stacks after selecting the highest quality dispersion curve measurements we compute group velocity maps from 7 to 25 s period these maps and 1 d shear wave velocity models at four selected locations exhibit clear correlations with major geological structures including the taranaki and canterbury basins the hikurangi accretionary prism and previously reported basement terrane boundaries

a precisely dated lake level rise marked by diatomite formation in northeastern ireland the intercorrelation of palaeoclimate events from various studies is often hindered by a lack of precise chronological control tephra isochrons can overcome this problem by providing direct site linkages this paper outlines a study of holocene peat and diatomite deposits that accumulated within the floodplain of lough neagh northern ireland the icelandic hekla 4 tephra has been identified at the base of diatomite deposits at a number of sites and provides firm dating evidence for a widespread flooding event in the area at ca 2300 bc the evidence is consistent with other studies in ireland and elsewhere for increased wetness at this time the results demonstrate that the terrestrial deposits around lough neagh contain an important record of holocene lake level change dendrochronological evidence from the lough neagh area provides additional information about lake level fluctuations over the past two millennia copyright 2004 john wiley sons ltd

isotopic composition of neodymium in waters from the drake passage the isotopic composition of neodymium has been determined in seawaters from the drake passage the antarctic circumpolar current which controls interocean mixing flows through this passage the parameter e nd 0 which is a function of the ratio of neodymium 143 to neodymium 144 is found to be uniform with depth at two stations with a value which is intermediate between the values for the atlantic and the pacific and indicates that the antarctic circumpolar current consists of about 70 percent atlantic water cold bottom water from a site in the south central pacific has the neodymium isotopic signature of the waters in the drake passage by using a box model to describe the exchange of water between the southern ocean and the ocean basins to the north together with the isotopic results an upper limit of approximately 33 million cubic meters per second is calculated for the rate of exchange between the pacific and the southern ocean concentrations of samarium and neodymium were also determined and found to increase approximately linearly with depth these results suggest that neodymium may be a valuable tracer in oceanography and may be useful in paleo oceanographic studies

river response to rapid holocene environmental change evidence and explanation in british catchments macklin m g jones a f lewin j 2010 river response to rapid holocene environmental change evidence and explanation in british catchments quaternary science reviews 29 13 14 1555 1576 sponsorship welsh assembly government and the higher education funding council for wales for funding this study through its support of the centre for catchment and coastal research at aberystwyth university

neodymium isotopic variations in northwest pacific waters abstract four vertical profiles of the concentration and isotopic composition of nd in seawater were obtained in the western north pacific two profiles from the kuroshio current regime showed congruently that although the nd concentration increases gradually with depth its isotopic composition varies significantly with depth depending upon the water mass occupying the water column the high salinity kuroshio waters originating from the north pacific tropical water nptw carry the least radiogenic nd nd 7 4 to 8 7 to this region at 250 m from the western margin continental shelves most likely from the east china sea the nd isotopic compositions in the north pacific intermediate water npiw that occurs at 600 to 1000 m in the subtropical region are fairly uniform at nd 3 7 the profile data from the 38 to 40 n kuroshio oyashio mixed water region off sanriku of honshu japan also suggest that the newest npiw with nd 3 2 is formed there by the mixing of various source waters and the radiogenic component of nd is derived mainly from the oyashio waters in the pacific deep water pdw below 1000 m the nd isotopic composition is neither vertically nor horizontally homogeneous suggesting that it serves as a useful tracer for sluggish deep water circulation as well two profiles from the izu ogasawara trench showed a minimum nd value at 2000 m suggesting that there exists a horizontal advective flow in the vicinity of honshu japan there is some evidence from other chemical properties to support this observation the waters below 4000 m including those within the trench in the subtropical region have nd values of around 5 suggesting that the deep waters are fed from the south along the western boundary ultimately from the antarctic bottom water aabw in the south pacific this extends up to 40 n along the japanese islands in the subarctic region 42 n the waters have more radiogenic nd with nd 4 0 throughout the water column presumably due to the supply of nd by weathering in such igneous provinces as the kuril kamchatska aleutian island chain the lateral inhomogeneity of the nd isotopic composition in pdw suggests that there may be different circulation and mixing regimes in the north pacific basin

the sedimentary flux of dissolved rare earth elements to the ocean abstract we determined pore fluid rare earth element ree concentrations in near surface sediments retrieved from the continental margin off oregon and california usa these sites represent shelf to slope settings which lie above within and below the oxygen minimum zone of the northeast pacific the sediments are characterized by varying degrees of net iron reduction with pore fluids from the shelf sites being generally ferruginous and the slope sediments having less pronounced iron reduction zones that originate deeper in the sediment package ree concentrations show maxima in shallow upper 2 10 cm subsurface pore fluids across all sites with concentrations that rise more than two orders of magnitude higher than seawater these pore fluid enrichments highlight the importance of a sedimentary source of rees to the ocean s water column here we use our measurements to estimate the diffusive flux of nd out of ocean sediments resulting in a global flux between 18 and 110 10 6 mol nd yr 1 while we do assume that our pore fluid profiles as well as the very limited data previously published are representative of a wide array of ocean environments this calculated flux can account for the modeled missing nd source flux 76 10 6 mol nd yr 1 in global budgets arsouze et al 2009 pore fluid normalized ree patterns show distinct variation in the middle ree and heavy ree enrichments with sediment depth and amongst sites these patterns show that the heavy ree enrichment of pore fluids at our deep slope site 3000 m water depth is closest to the heavy ree enrichment of seawater this observation supports the view that ree cycling within the upper ten centimeters of deep sea marine sediments as opposed to shallower continental shelf and slope sediments plays a significant role in controlling the integrated global ree flux from the pore fluids and consequently the broad scale ree pattern in seawater

the distribution of neodymium isotopes in arctic ocean basins nd concentration and isotope data have been obtained for the canada amundsen and makarov basins of the arctic ocean a pattern of high nd concentrations up to 58 pm at shallow depths is seen throughout the arctic and is distinct from that generally seen in other oceans where surface waters are relatively depleted a range of isotopic variations across the arctic and within individual depth profiles reflects the different sources of waters the dominant source of water and so nd is the atlantic ocean with lesser contributions from the pacific and arctic rivers radiogenic isotope nd signatures up to epsilon nd 6 5 can be traced in pacific water flowing into the canada basin waters from rivers draining older terrains provide very unradiogenic nd down to epsilon nd 14 2 that can be traced in surface waters across much of the eurasian basin a distinct feature of the arctic is the general influence of the shelves on the nd concentrations of waters flowing into the basins either from the pacific across the chukchi sea or from across the extensive siberian shelves water shelf interaction results in an increase in nd concentration without significant changes in salinity in essentially all waters in the arctic through processes that are not yet well understood in estuarine regions other processes modify the nd signal of freshwater components supplied into the arctic basin and possibly also contribute to sedimentary nd that may be subsequently involved in sediment water interactions mixing relationships indicate that in estuaries nd is removed from major river waters to different degrees deep waters in the arctic are higher in nd than the inflowing atlantic waters apparently through enrichments of waters on the shelves that are involved in ventilating the deep basins these enrichments generally have not resulted in major shifts in the isotopic compositions of the deep waters in the makarov basin epsilon nd similar to 10 5 but have created distinctive nd isotope signatures that were found near the margin of the canada basin with epsilon nd similar to 9 0 the deep waters of the amundsen basin are also distinct from the atlantic waters with epsilon nd 12 3 indicating that there has been limited inflow from the adjacent makarov basin through the lomonosov ridge c 2009 elsevier ltd all rights reserved

neodymium isotopes and concentrations in caribbean seawater tracing water mass mixing and continental input in a semi enclosed ocean basin we present the first full water column nd isotope end and concentration data for caribbean seawater as well as for stations close to the orinoco river mouth and in the florida straits the surface inflow into the southeastern caribbean via the guyana current is characterized by an endsignature of 10 9 which is a consequence of the mixing of relatively unradiogenic endsignatures 13 6 supplied by the orinoco river with contributions from the amazon river 10 despite the proximity to land sub surface and intermediate waters within the caribbean largely retain the endsignatures of their source water masses in the atlantic in contrast the deep waters of the caribbean show endsignatures at least 3 endunits more radiogenic than the inflowing upper north atlantic deep water unadw a endshift from 13 to 9 7 can be explained by addition of radiogenic nd to the deep caribbean through weathering inputs from land however in order to balance such large shifts in endwith at the same time modest increases in nd concentrations nd must also be removed from seawater within the basin it is suggested that the long residence time of deep waters in the caribbean allows significant interaction of seawater with sinking particles and seafloor sediments resulting in more radiogenic values these findings have implications for the use of endas a proxy for paleocirculation in restricted basins in which the residence times of the deep waters are long

evolution of the lithosphere in the indian ocean from combined earthquake and ambient noise tomography rayleigh wave dispersion extracted from ambient seismic noise has been widely used to image crustal and uppermost mantle structure applications of this approach in continental settings are abundant but there have been relatively few studies within ocean basins by cross correlating ambient noise recorded at broadband seismic stations around the indian ocean we demonstrate the feasibility of extracting high quality long period 10 30 mhz rayleigh waves that traverse the entire ocean basin high quality rayleigh wave cross correlation functions can be obtained from stacking waveforms over less than two years at land stations and less than four years at island stations we show that adding the dispersion information extracted from ambient noise to a global earthquake data set can improve the resolution of phase velocity maps by about 20 in the northern indian ocean where the station distribution is the best we find that a plate cooling model with a potential temperature of 1450 c and plate thickness of 125 km can fit both the seismic observations and seafloor topography the seychelles mascarene plateau is characterized by anomalously slow velocity at 30 mhz the inclusion of ambient noise data in the tomographic inversion shifts the slow velocity anomaly into better agreement with the topographic relief allowing us to estimate its crustal thickness and confirm that the plateau s elevation is supported by thick crust the 10 and 20 mhz phase velocity maps show a strong asymmetry across the central indian ridge that is best explained by eastward asthenospheric flow emanating from nearby hotspots

radiogenic isotopic compositions of low concentration dust and aerosol from the gisp2 ice core abstract we have produced a record of radiogenic isotope compositions of particulate and soluble dust fractions from the gisp2 ice core from basal ice to the surface for each respective dust fraction we measured sr and nd isotope values to constrain the provenance of mineral dust and sea salt aerosol transported to the greenland ice sheet we have produced high precision measurements of end values on sub ng quantities of nd in ice core samples via tims with 1013 ohm resistors the range of measured end values of soluble material 12 9 2 6 to 7 3 0 9 is interpreted to represent sea salt aerosol based on sr isotope compositions that resemble seawater values 0 711007 0 000030 to 0 709416 0 000072 therefore the end potentially provides important insights to the atmospheric circulation patterns at high latitudes at least through mis 5 we compare end values of surface seawater from ocean basins and deltas at high latitudes to alternating relatively radiogenic and unradiogenic end at gisp2 we suggest these changes represent transitions between a climate system dominated by enhanced and persistent arctic oscillation positive anomalies as determined from sea level and ocean bottom pressure measurements and a state representative of modern atmospheric circulation which typically exhibits either negative anomalies or none altogether

on the coherent components of low frequency ambient noise in the indian ocean this letter demonstrates that the dominant coherent component of low frequency 1 hz f 20 hz ambient noise propagating between hydrophone pairs of the same hydroacoustic station deployed in the deep sound channel of the indian ocean is directional and mainly originates from antarctica however the amplitude of the peak coherent noise arrivals obtained using a 4 month long averaging interval was relatively low given the small hydrophones spacing hydrophones 2 km hence extracting similar coherent arrivals between two distinct hydroacoustic stations separated instead by thousands of kilometers for noise based acoustic thermometry purposes seems unlikely even using a year long averaging

acquisition of the neodymium isotopic composition of the north atlantic deep water the north atlantic deep water nadw neodymium isotopic composition nd ic is increasingly used in oceanography and paleoceanography to trace large scale circulation and weathering processes notably to investigate past variations of the global thermohaline circulation although the present day nadw nd ic is well characterized at nd 13 5 the acquisition of this isotopic signature in other words the causes of this value has so far been very sparsely documented such an understanding is however fundamental to the interpretation of paleo records nd ic and rare earth element concentrations were measured at 9 stations within the north atlantic subpolar gyre signature cruise summer 1999 the comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the nadw the labrador sea water lsw nd 13 9 0 4 north east atlantic deep water neadw nd 13 2 0 4 and north west atlantic bottom water nwabw nd 14 5 0 4 acquire their nd ic through distinct water mass mixings and lithogenic inputs these different mechanisms acting upon water masses from very diverse sources seem to bring the nd ic of the three nadw layers to values close together and similar to that of the nadw it is suggested that sediment seawater interactions significantly lower the neadw and nwabw nd ic along the south east greenland margin since these interactions do not significantly modify the nd content of these water masses sediment remobilizations leading to the nd ic variations are probably associated with nd removal fluxes from the water mass toward the sediment a process called boundary exchange on the other hand lsw seems to acquire its nd ic from the subpolar mode waters from which it is formed by deep convection and no other mechanism needs to be invoked its unradiogenic signature could ultimately be linked to fresh water runoff from the canadian shield these conclusions should allow more precise interpretations of paleoceanographic nd ic records taking into account the distinct histories of the three nadw layers including distinct water mass mixings and distinct lithogenic inputs

towards explaining the nd paradox using reversible scavenging in an ocean general circulation model abstract the isotopic composition of the rare earth element neodymium nd has the potential to serve as water mass tracer because it is naturally tagged by continental sources with distinct ages and lithologies however in order to understand the limitations of this approach we need to know more about the physical and biogeochemical processes controlling the distribution of nd in the modern ocean for example nd isotope ratios behave quasi conservatively while concentrations in the water column generally increase with depth showing a broadly nutrient like behaviour we define this decoupling of nd concentrations and isotopic compositions as the nd paradox for the first time we model nd concentrations and isotopic compositions simultaneously and address the hypothesis that the nd paradox can be explained by a combination of lateral advection and reversible scavenging we impose a reversible scavenging model of nd removal from the ocean on the ocean circulation fields from the mit general circulation model using the transport matrix method we conclude that reversible scavenging is an active and important component in the cycling of nd in the ocean in the absence of an adequate alternative explanation reversible scavenging should be considered a necessary component in explaining the nd paradox

nd isotopic composition in the central north pacific three vertical profiles of seawater concentration and isotopic composition of nd were determined for the western to central north pacific ocean in the subarctic oceanic region at depths greater than 500 m one vertical profile of nd isotopic composition was indistinguishable from most previously reported profiles from here the data indicate a rather homogeneous nd isotopic composition in the subarctic oceanic region at middle to deep depths 500 m two stations in the subtropical oceanic region exhibited similar nd isotopic composition profiles to those previously reported the maxima end values at depths of 800 1000 m end 3 4 to 2 7 which correspond to the north pacific intermediate water npiw are found at both subtropical stations this implies a ubiquitous distribution of npiw showing a radiogenic end value in the north pacific the subsurface minimum at a depth of 200 m which indicates the penetration of the north pacific tropical water nptw with an unradiogenic nd isotopic signal was observed at one station in the western pacific this station had much lower end than the central station at depths around 5000 m suggesting the greater prominence of antarctic bottom water aabw in the western subtropical pacific than in the central to eastern subtropical pacific results of a model calculation assuming boundary exchange indicate that the hawaiian islands play an important role in supplying radiogenic nd to the central pacific similar to some continental margins we show that nd isotopic composition is a versatile tracer for ocean circulation and the geochemical cycle of nd in the north pacific further studies on the distribution of nd isotopic composition in the pacific ocean including the southern pacific will better elucidate the circulation and geochemical cycle of nd in the pacific

an improved method to extract very broadband empirical green s functions from ambient seismic noise the extraction of green s functions by cross correlation of continuous seismic records at station pairs is best achieved in a diffuse wave field where energies radiated by random sources have equal power to partially satisfy the diffuse wave field condition original seismic records must be normalized as they are highly nonstationary and may have amplitude variations of orders of magnitude within the cross correlation time window we adapt a frequency time normalization method to obtain seismograms with an even spectrum at all times within the data processing unit compared with the commonly used one bit normalization the new method improves the signal to noise ratio of empirical green s functions by a factor of 2 and increases the effective data recording duration by a factor of 4 assuming random local and instrument noise it yields useful rayleigh waves at periods up to 300 s for program for array seismic studies of the continental lithosphere passcal type deployments and 600 s for permanent stations with very broadband sensors thus the new method makes it possible to extend surface wave signals to frequencies beyond those in traditional earthquake based surface wave tomography at both the high and low frequency ends

neodymium budget in the modern ocean and paleo oceanographic implications the oceanic nd budget is calculated using a steady state 10 box model and a compilation of field data this is the first attempt to propose consistent estimates of the nd fluxes entering the ocean as well as indicating possible nd sources and the proportion of nd fluxes exchanged between dissolved and particulate fractions with presently available nd data the best estimates give a total nd influx of 9 x 10 9 g yr which leads to an oceanic nd residence time of 500 years from modeling tests we suggest that the authigenic nd scavenged by particulates is 100 remineralized in the deep ocean the total exchanged nd flux may be as high as 2 x 10 10 g yr the epsilon nd 0 values of the influxes are 22 11 1 and 4 for the north atlantic surface atlantic north pacific and surface indo pacific regions respectively atmospheric and riverine nd fluxes are insufficient to explain the magnitude and regional variability of calculated nd influxes and epsilon nd 0 we propose continental margins as an additional source supplying nd to the ocean using the model calibrated for nd we examine the sensitivity of deep water epsilon nd 0 to variations of nd inputs to the ocean deep water nd concentrations and epsilon nd 0 vary with the changes in nd influxes and their epsilon nd 0 as nd sources to the ocean may change during glacial interglacial periods the epsilon nd 0 shifts recorded in ferromanganese nodules and crusts do not necessarily reflect changes in paleoceanic circulation the effects of continental erosion should be considered in reconstructing patterns of ocean circulation using nd isotopes

antarctic dome c ice core dust at 18 k y b p isotopic constraints on origins we have determined the source area from which dusts from the last glacial maximum lgm section of the dome c ice core were derived by comparing their strontium and neodymium isotopic ratios with those of samples from potential source areas the87sr 86sr and143nd 144nd isotope ratios of the dusts deposited 18 k y b p at the east antarctic dome c site were compared with potential antarctic australian southern african and south american sources the isotope ratios clearly define the patagonian provenance of the dust with the other potential source areas being at most minor contributors contributions by volcanic ash and tephra to the dust sample were also determined to be minimal based on the patterns of rare earth elements knowing the source of the dusts places constraints on the aeolian trajectory by which it was transported to antarctica and this serves as a test of the simulation of southern hemispheric circulation by atmospheric global circulation models during the lgm

from the subtropics to the central equatorial pacific ocean neodymium isotopic composition and rare earth element concentration variations 1 neodymium isotopic compositions end and rare earth element ree concentrations were measured for filtered surface to deep waters 112 samples in the southern tropical pacific the relatively detailed picture of these tracer distributions allowed us to refine the areas where oceanic end variations occur end values increase for most of the water masses flowing from samoa to the solomon sea and in the papua new guinea png area as already observed furthermore water masses arriving from the eastern equatorial pacific 200 550 m depth also revealed radiogenic values possibly acquired in the vicinity of the south american coasts and galapagos islands these end variations affect the whole water column the most likely process causing such variations is boundary exchange between the numerous radiogenic slopes margins located in this area and seawater flowing past dissolution of atmospheric deposition and or diffuse streaming of volcanic ash are also suggested to explain the radiogenic end observed at the surface in the png area interestingly a positive europium eu anomaly characterizes the normalized ree patterns of most of the studied water masses this anomaly is consistent with the ree patterns of sediment and rock samples that are potential sources for the local waters such consistency reinforces the hypothesis that lithogenic sources play a major role in the oceanic ree budget thanks to boundary exchange the data set presented here is a good basis for further sampling that will be realized in the framework of the ongoing geotraces program www geotraces org

the use of foraminifera as a record of the past neodymium isotope composition of seawater 1 we present new isotopic data for sedimentary planktonic foraminifera as well as for potential water column and sedimentary sources of neodymium nd which confirm that the isotopic composition of the foraminifera is the same as surface seawater and very different from deep water and sedimentary nd the faithfulness with which sedimentary foraminifera record the isotopic signature of surface seawater nd is difficult to explain given their variable and high nd ca ratios ratios that are often sedimentary foraminifera ratios that are often much higher than is plausible for direct incorporation within the calcite structure we present further data that demonstrate a similarly large range in nd ca ratios in plankton tow foraminifera a range that may be controlled by redox conditions in the water column cleaning experiments reveal in common with earlier work that large amounts of nd are released by cleaning with both hydrazine and diethylene triamine penta acetic acid but that the nd released at each step is of surface origin while further detailed studies are required to verify the exact location of the surface isotopic signature and the key controls on foraminiferal nd isotope systematics these new data place the use of planktonic foraminifera as recorders of surface water nd isotope ratios and thus of variations in the past supply of nd to the oceans from the continents via weathering and erosion on a reasonably sure footing

multi proxy evidence for holocene lake level and salinity changes at an loch mor a coastal lake on the aran islands western ireland abstract curves for holocene lake levels and salinity changes are presented for an loch mor a small oligohaline lake on the aran islands off the west coast of ireland based on palaeoecological investigations of a 12 m long lake sediment core new insights are also provided into holocene sea level change in the galway bay region particular emphasis has been placed on the ostracod fauna both past and present salinity and lake level changes were reconstructed from the fossil ostracod assemblages based on the known tolerances of individual species and on the assemblages as a whole additional evidence was provided by other proxies including strontium isotope ratios derived from ostracod shells and other carbonates plant macrofossil and pollen analyses and sedimentological changes the early holocene pre boreal i e 11 5 10 ka was characterised by low lake levels and slightly elevated salinity values probably the result of high evapotranspiration and low precipitation rather than elevated sea levels early holocene plant and animal migration to the island does not seem to have been impeded but relative sea levels were not necessarily so low below 40 m a s l that landbridges were present to the mainland between ca 10 and 8 5 ka relatively high lake levels prevailed at 8 3 and 7 5 ka minor fluctuations lowering of the lake level occurred that are assumed to relate to early holocene abrupt events beginning at 7 05 ka lake levels declined sharply a general trend towards rising lake levels started at ca 6 4 ka and accelerated at ca 5 6 ka as runoff increased as a result of neolithic clearances at ca 4 8 ka lake levels began to rise once again probably in response to changes in rainfall and or evapotranspiration and runoff lower lake levels during the first half of the 1st millennium ad were probably a response to decreased runoff as a result of a drier climate coupled with regeneration of woody vegetation the sharpest rise in both lake levels and salinity began during the ninth century ad which is attributed to a rapid rise in relative sea level

rare earth elements in pore waters of marine sediments abstract the rare earth elements rees were measured in pore waters of the upper 25 cm of sediment from one site off peru and three sites on the california margin the pore water ree concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern from these analyses and from comparison to other chemical species measured dissolved fe mn ba oxygen nitrate phosphate it is suggested that pore water rees can be grouped into three categories those that are from an fe source those that are from a poc source and cerium oxide rees from the fe source appear where anoxia is reached they have a distinctive middle ree mree enriched pattern the concentrations in this source are so elevated that they dominate ree trends in the fe oxide reduction zone the net result of flux from the poc source is relative enrichment of heavy rees hrees over light rees lrees reflecting remineralizing poc and complexation with doc a common linear ree pattern seen in both oxic and anoxic sediments is associated with this poc source as well as a hree enriched pattern that is seen in surficial sediments at the peru site overall the pore water results indicate that mn oxides are not an important carrier of rees in the oceans a ree biogeochemical model is presented which attempts to reconcile ree behavior in the water and sediment columns of the oceans the model proposes that poc fe oxide and ce oxide sources can explain the ree concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water the model is also consistent with our observation that the ce anomaly of pore water does not exceed unity under any redox condition

oscillating glacial northern and southern deep water formation from combined neodymium and carbon isotopes abstract while ocean circulation is driven by the formation of deep water in the north atlantic and the circum antarctic the role of southern sourced deep water formation in climate change is poorly understood here we address the balance of northern and southern sourced waters in the south atlantic through the last glacial period using neodymium isotope ratios of authigenic ferromanganese oxides in thirteen deep sea cores from throughout the south atlantic the data indicate that northern sourced water did not reach the southern ocean during the late glacial and was replaced by southern derived intermediate and deep waters the high resolution neodymium isotope record 300 yr sample spacing from two spliced deep cape basin sites indicates that over the last glacial period northern sourced water mass export to the southern ocean was stronger during the major greenland millennial warming intervals and southern hemisphere cool periods and particularly during the major interstadials 8 12 and 14 northern sourced water mass export was weaker during greenland stadials and reached minima during heinrich events the benthic foraminiferal carbon isotopes in the same cape basin core reflect a partial control by southern hemisphere climate changes and indicate that deep water formation and ventilation occurred in the southern ocean during major greenland cooling intervals stadials together neodymium isotopes and benthic carbon isotopes provide new information about water mass sourcing and circulation in deep southern ocean waters during rapid glacial climate changes combining carbon and neodymium isotopes can be used to monitor the relative proportion of northern and southern sourced waters in the cape basin to gain insight into the processes which control the carbon isotopic composition of deep waters in this study we show that deep water formation and circulation was more important than biological productivity and nutrient regeneration changes for controlling the carbon isotope chemistry of antarctic bottom water during millennial scale glacial climate cycles this observation also lends support to the hypothesis that ocean circulation is linked to interhemispheric climate changes on short timescales and that ventilation in the glacial ocean rapidly switched between the northern and southern hemisphere on millennial timescales

neodymium isotopic composition and rare earth element concentrations in the deep and intermediate nordic seas constraints on the iceland scotland overflow water signature neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the nordic seas these data allow us to study how the iceland scotland overflow water isow acquires its neodymium signature in the modern ocean the waters overflowing the faroe shetland channel are characterized by nd 8 2 0 6 in good agreement with the only other data point published 19 years ago in the greenland and iceland seas the water masses leading to the formation of the isow display lower neodymium isotopic composition with nd around 11 and 9 respectively since no water masses in the nordic seas are characterized by nd 8 the radiogenic signature of the isow likely reflects inputs from the highly radiogenic norwegian basin basaltic margins jan mayen iceland faroe with nd 7 in addition to the neodymium isotopic composition the rare earth element patterns suggest that these inputs occur via the remobilization which includes resuspension and dissolution of sediments deposited on the margins whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs and can be used as a water mass tracer these results emphasize the role of interactions between sediments deposited on margins and seawater in the acquisition of the neodymium isotopic composition of water masses these results should allow a better use of this parameter to trace the present and the past circulation in the north atlantic

tracing papua new guinea imprint on the central equatorial pacific ocean using neodymium isotopic compositions and rare earth element patterns abstract the nd isotopic composition ic and rare earth patterns of hydrodynamic structures of the equatorial pacific ocean were characterized along 140 w the nd ic of antarctic intermediate water aaiw and of the lower layer of the equatorial undercurrent euc at 140 w 13 c water are much more radiogenic at the equator than at their origin in the south equatorial current 12 s revealing that these water masses have been in contact with the highly radiogenic papua new guinea png slope in both cases only a small fraction less than 9 of the sediment deposited on the png slope is required to be exchanged or dissolved to explain these nd ic variations whereas the hydrographic properties of the same water masses remain unchanged this confirms the usefulness of this tracer to identify pathways of water masses these results emphasize the importance of jets in transporting lithogenic material into the subsurface layers of remote areas where aeolian inputs are particularly weak and corroborate the previous results on fe and al maximum in this area m l wells g k vallis e a silver nature 398 1999 601 604 the nd ic of the upper layer of the euc contrasts strongly to that of the subpycnocline layer indicating that the equatorial upwelling only affects the surface waters and is not effective between 120 and 150 m we calculate that the nd imprint of the png input is likely to vanish from this surface layer as it traverses the basin due to the replacement of upwelled waters by non radiogenic ones

rayleigh wave tomography of the british isles from ambient seismic noise we present the first rayleigh wave group speed maps of the british isles constructed from ambient seismic noise the maps also constitute the first surface wave tomography study of the crust under the british isles at a relatively high resolution we computed interferometric interstation rayleigh waves from vertical component records of ambient seismic noise recorded on 63 broad band and short period stations across the uk and ireland group velocity measurements were made from the resulting surface wave dispersion curves between 5 and 25 s using a multiple phase matched filter method uncertainties in the group velocities were computed by calculating the standard deviation of four dispersion curves constructed by stacking a random selection of daily cross correlations where an uncertainty could not be obtained for a ray path using this method we estimated it as a function of the interreceiver distance group velocity maps were computed for 5 25 s period using the fast marching forward solution of the eikonal equation and iterative linearized inversion at short and intermediate periods the maps show remarkable agreement with the major geological features of the british isles including terrane boundaries in scotland regions of late palaeozoic basement uplift areas of exposed late proterozoic early palaeozoic rocks in southwest scotland northern england and northwest wales and sedimentary basins formed during the mesozoic such as the irish sea basin the chester basin the worcester graben and the wessex basin the maps also show a consistent low velocity anomaly in the region of the midlands platform a proterozoic crustal block in the english midlands at longer periods which are sensitive velocities in the lower crustal upper mantle the maps suggest that the depth of moho beneath the british isles decreases towards the north and west areas of fast velocity in the lower crust also coincide with areas thought to be associated with underplating of the lower crust such as northern ireland the eastern irish sea and northwest wales

defining the potential source region of volcanic ash in northwest europe during the mid to late holocene abstract far travelled volcanic ash fine grained tephra clouds pose economic and societal risks largely through their potential impact on the aviation industry a long view of ash clouds extending over northwest europe assembled from the study and geochemical characterisation of cryptotephra i e tephra invisible to the naked eye in sedimentary records can help inform the understanding of the frequency of such events and the nature of eruptions that generate them here we review the records of cryptotephra deposition in northwest europe during the mid to late holocene the last eight millennia to gain insights into the sources of volcanic ash reaching this region we consider almost 500 published and unpublished datasets for cryptotephras recorded in sedimentary sequences ranging from ireland to latvia and on the basis of their major element geochemical composition we identify 90 unique ash horizons and their likely provenance the majority 72 of the cryptotephras derive from icelandic eruptions but a large proportion 24 originates from other volcanic regions including alaska the cascades kamchatka mexico the azores and possibly the mediterranean although moderate magnitude eruptions in iceland may potentially disperse ash clouds over europe it appears that only large magnitude events in more distant regions have generally had this capacity and then only when suitable meteorological conditions prevailed nevertheless the cryptotephra records demonstrate that ash clouds from most volcanic regions capable of producing large explosive eruptions in the northern mid to high latitudes particularly those to the west of europe as well as in tropical north america have the potential to extend to northwest europe volcanic ash forecasts for this region need therefore to take into account possible risks posed not only by icelandic eruptions but also by any large magnitude eruptions in north america or the north pacific area

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

nd isotopic composition of water masses and dilution of the mediterranean outflow along the southwest european margin nd isotopic compositions epsilon nd of seawater profiles and deep sea corals collected off the coast of iberia and from the bay of biscay were measured 1 to constrain the nd isotopic composition of water masses along the southwest european margin 2 to track the mediterranean outflow water mow during its northward propagation and 3 to establish hydrological changes during the last 1500 years the eastern north atlantic central water enacw is characterized by nd isotopic composition of around 12 0 mediterranean sea water msw is collected from 800 and 1200 m depth and is characterized by epsilon nd values ranging from 10 9 off the coast of iberia to 11 6 in the bay of biscay these epsilon nd results suggest a strong dilution of the pure mow at the strait of gibraltar epsilon nd 9 4 of approximately 40 and 30 along its northward circulation pathway essentially with a contribution from enacw at around 2000 m depth epsilon nd water profiles display the occurrence of a nonradiogenic water mass epsilon nd 13 originating from the labrador sea labrador sea water fossil deep sea corals dated between 84 and 1500 years display nd isotopic compositions that vary moderately from present day seawater values suggesting a weaker influence of mow in the formation of msw during the dark ages and the little ice age these recent cold events seem to be associated with a reduction in the northward penetration of msw which may result from a greater eastward extension of the middepth subpolar gyre and or a reduction of msw formation likely tied to a variation in deep mediterranean water production

the geochemical balance of the rare earth elements and neodymium isotopes in the oceans profiles of ree concentrations and neodymium isotope measurements in the western indian ocean are consistent with published data from the other major oceans ree concentrations show a strong hydrographic control the influence of intermediate water masses is particularly significant surface water concentrations vary with salinity paniculate rees are less than 5 of dissolved concentration except for ce where up to 20 of total rees are present as 0 4 m particles in contrast to dissolved profiles concentration profiles of paniculate rees including ce all increase with depth ree concentrations correlate with silica over all but the deepest waters although in detail correlations are complex the neodymium isotopic composition of indian ocean seawater avg nd 0 8 3 is intermediate between that for the atlantic and pacific oceans comparison of ree data from the major oceans shows that the hrees behave more like si than do the lrees sm nd ratios are near constant for all oceans whereas ernd ratios vary between the atlantic indian and pacific oceans the fractionation of the rees in surface seawaters can be explained by fractionation factors relative to silica from 0 12 for la to 0 05 for er use of a seven box model shows that the interoceanic variations in neodymium isotopic compositions and nd concentrations cannot be reconciled unless particle water exchange is invoked the degree of exchange required is consistent with particle residence times of 1 10 yr

hafnium isotopes in arctic ocean water the first isotopic compositions of dissolved hafnium in seawater from across the arctic ocean are reported most samples from the four sub basins of the arctic ocean have values within error of an average of ehf 0 8 combined hf nd isotope compositions do not fall on the well established positive correlation for mantle and crustal rocks instead arctic waters have hf that is more radiogenic than that typically found in rocks with similar nd isotope compositions a feature previously found in ferromanganese crusts and waters from the pacific ocean arctic seawater samples generally fall on the lower part of the ferromanganese crust array reflecting influences of inputs from arctic rivers and interactions of shelf waters with underlying sediments arctic rivers have much higher hf concentrations 7 30 pm than arctic seawater 0 36 4 2 pm water from the mackenzie river has the least radiogenic hf with ehf 7 1 1 7 and plots furthest away from the ferromanganese crust array while waters from the ob yenisey and lena rivers have values that are indistinguishable from most arctic waters in the amundsen makarov and canada basins hf concentrations are highest at the surface and lowest in the deeper waters reflecting the influences of riverine inputs and of waters that have flowed over the extensive siberian continental shelves and have nd and hf characteristics that reflect water sediment interactions this is in contrast to the relatively low near surface hf concentrations reported for locations elsewhere the pacific water layer in the canada basin exhibits the highest value of ehf 6 8 1 8 reflecting the hf isotopic composition of waters entering the arctic from the pacific ocean mixing relationships indicate that a substantial fraction of the hf in the mackenzie river is lost during estuarine mixing the behaviour of hf from other rivers is less constrained

neodymium isotopes in the mediterranean sea comparison between seawater and sediment signals nine depth profiles of dissolved nd concentrations and isotopic ratios epsilon nd were obtained in the levantine basin the ionian the aegean the alboran seas and the strait of gibraltar thirteen core top sediments and nile river particle samples were also analyzed leached with 1 n hcl acetic acid or hydroxylamine hydrochloride the seawater epsilon nd values become more radiogenic during the eastward circulation in the mediterranean sea the relationship between salinity and the seawater epsilon nd shows that the nd isotopic signature is more conservative than salinity in the mediterranean sea the water mass with the highest epsilon nd 4 8 is found at about 200 m in the easternmost levantine basin the average epsilon nd value for deep waters is 7 0 in the eastern basin 2 5 epsilon units higher than in the western basin by examining the sensitivity of seawater epsilon nd to nd inputs from the nile we conclude that the most significant radiogenic nd source is partially dissolved nile river particles the nd flux from the nile river water has a minor influence on the mediterranean seawater end except for the easternmost levantine basin the leachate epsilon nd values are consistent with the seawater values in the easternmost levantine basin the leachate end values obtained with hcl leaching are systematically higher than the seawater values the relationship between leachate and residual epsilon nd values indicates that the hcl leaching partially dissolves lithogenic nd so the dissolution of nile river particles is the cause of the observed shift some end values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with hcl leaching although the reason for this shift is not clear sr 87 sr 86 successfully detects the presence of a nonmarine component in the leachate our results suggest that leaching performance may vary with the mineralogy of marine sediments at least in the case of the mediterranean sea copyright c 2004 elsevier ltd

abrupt climatic changes and an unstable transition into a late holocene thermal decline a multiproxy lacustrine record from southern sweden the transition from a middle holocene relatively warm and stable climate to a cooler and unstable late holocene climate is reconstructed using sediments from lake igelsjon south central sweden this multiproxy study illustrates local regional and global scale responses to climatic change by focusing on a previously identified abrupt hydrological shift to cooler and or wetter conditions around 4000 cal yr bp the results suggest that between ca 4600 and ca 3400 cal yr bp the environment around and within the lake responded in two major well defined steps the first between 4450 and 4350 cal yr bp and the second between 4000 and 3800 cal yr bp a series of rapid fluctuations of short duration were superimposed on the general cooling trend with the most severe aquatic response peaking at ca 3800 cal yr bp pollen percentage and influx values show forest composition and pollen productivity changes and a distinct decline in total and corylus pollen influx in the period of 4000 3500 cal yr bp stomatal based reconstruction of atmospheric co2 concentration produced a tenuous decrease with a minimum between 3650 and 3500 cal yr bp copyright 2005 john wiley sons ltd

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

low frequency and high frequency changes in temperature and effective humidity during the holocene in south central sweden implications for atmospheric and oceanic forcings of climate an integrated use of independent palaeoclimatological proxy techniques that reflect different components of the climate system provides a potential key for functional analysis of past climate changes here we report a 10 000 year quantitative record of annual mean temperature tann based on pollen climate transfer functions and pollen stratigraphical data from lake flarken south central sweden the pollen based temperature reconstruction is compared with a reconstruction of effective humidity as reflected by a 18o record obtained on stratigraphy of lacustrine carbonates from lake igelsjon c 10 km from lake flarken which gives evidence of pronounced changes in effective humidity the relatively low tann and high effective humidity as reflected by a low evaporation inflow ratio suggest a maritime early holocene climate 10 000 8 300 cal year bp seemingly incompatible with the highly seasonal solar insolation configuration we argue that the maritime climate was due to the stronger than present zonal flow enhanced by the high early holocene sea surface temperatures in the north atlantic the maritime climate mode was disrupted by the abrupt cold event at 8 200 cal year bp followed at 8 000 cal year bp by a stable holocene thermal maximum the latter was characterized by tann values about 2 5 c higher than at present and markedly dry conditions indicative of stable summer time anti cyclonic circulation possibly corresponding with modern blocking anticyclonic conditions the last 4 300 year period is characterized by an increasingly cold moist and unstable climate the results demonstrate the value of combining two independent palaeoclimatic proxies in enhancing the reliability generality and interpretability of the palaeoclimatic results further methodological refinements especially in resolving past seasonal climatic contrasts are needed to better understand the role of different forcing factors in driving millennial scale climate dynamics

subpolar mode water formation traced by neodymium isotopic composition 1 the formation of subpolar mode water spmw is documented by combining neodymium isotopic composition nd ic as a conservative tracer with salinity temperature and o2 concentration nd ic reveals a more pronounced eastward extension of the labrador current particularly its subarctic intermediate water component than the other parameters nd ic also suggests that waters flowing through the denmark strait do not significantly contribute to the spmw formation a two end member mixing involving subarctic intermediate water and maybe ultimately labrador current water and north atlantic central water is proposed to explain spmw formation these results provide important insight into the processes controlling the nd ic in the north atlantic subpolar gyre which could provide a means for reconstructing the northward extension of the gulf stream in the past

neodymium isotopic study of baffin bay water sources of ree from very old terranes water samples were collected from baffin bay and surrounding areas in order to evaluate this region as a potential source of nd from old continental material to atlantic water the isotopic data ranged from e nd 0 9 0 to 26 with most of the data around e nd 0 20 compared with values of north atlantic deep water nadw with e nd 0 13 5 the concentration of nd in baffin bay waters was as high as 6 10 12 g g compared with 2 5 10 12 g g for nadw the combination of low e nd and high nd concentration indicates that baffin bay may be a significant source of nd from very old crustal material a simple box model was used to evaluate the contribution to the nd budget of nadw and it was concluded that a substantial fraction of the nd from ancient crustal sources that is required to maintain the isotopic composition of nadw could be supplied by baffin bay outflow

neodymium isotopic composition of the oceans a compilation of seawater data a global compilation of the neodymium isotopic composition of seawater is presented with 880 data points it confirms the gradual end increase for intermediate deep water masses from the northwest north atlantic via the austral and indian oceans to the pacific this confirms the usefulness of this tracer for studying large scale oceanic circulation the compilation stresses the need for documenting the oceans south of 30 s from which less than 4 of the data are derived the associated neodymium concentrations display heterogeneous vertical gradients among major oceanic basins in addition to particle remineralization along the global thermohaline circulation the database suggests that basin size differences also contribute to that heterogeneity

identifying and removing tilt noise from low frequency 0 1 hz seafloor vertical seismic data low frequency 0 1 hz vertical component seismic noise can be reduced by 25 db or more at seafloor seismic stations by subtracting the coherent signals derived from 1 horizontal seismic observations associated with tilt noise and 2 pressure measurements related to infragravity waves the reduction in effective noise levels is largest for the poorest stations sites with soft sediments high currents shallow water or a poorly leveled seismometer the importance of precise leveling is evident in our measurements low frequency background vertical seismic spectra measured on a seafloor seismometer leveled to within 1 10 4 radians 0 006 degrees are up to 20 db quieter than on a nearby seismometer leveled to within 3 10 3 radians 0 2 degrees the noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides indicating that it is caused by tilting due to seafloor currents flowing across the instrument at low frequencies this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect to the earth s gravitational field the effect is much stronger on the horizontal components than on the vertical allowing significant reduction in vertical component noise by subtracting the coherent horizontal component noise this technique reduces the low frequency vertical noise on the less precisely leveled seismometer to below the noise level on the precisely leveled seismometer the same technique can also be used to remove background noise due to the seafloor pressure field up to 25 db noise reduction near 0 02 hz and possibly due to other parameters such as temperature variations

the concentration and isotopic composition of diffusible nd in fresh and marine waters abstract variations in the nd isotopic composition of ocean water through time recorded in marine deposits are suggested to document changes in erosional input and ocean circulation thus characterization of the dissolved nd component in seawater is important but few attempts have been made to directly measure truly dissolved nd here we report nd data obtained using the technique of diffusive gradients in thin films dgt designed to measure in situ diffusible labile trace metals in aqueous solutions the method samples free ions and possibly very small complexes the concentration and isotopic composition of nd in fresh brackish and seawater have been determined with dgt analyzed with thermal ionization mass spectrometry and compared with filtered and bulk water samples concentrations measured in water samples and with dgt show that the relative amount of diffusible nd increases with salinity from being about 10 in the fresh water to 42 in seawater at each sampled site the isotopic composition of nd in the water shows a similar isotopic composition within errors with nd sampled using the dgt method these results indicate that there is a complete exchange between the particulate colloidal fraction and the truly dissolved phase therefore our findings suggest that solute nd reflects both the isotopic composition of the marine sediments and the bulk water

modeling the neodymium isotopic composition with a global ocean circulation model abstract for the first time modeling of the neodymium nd isotopic composition expressed as end is performed in an ocean global circulation model opa orca2 0 model in the interest of simplicity nd concentration is not explicitly included and end is modeled as a passive tracer recent studies suggest that dissolved particulate exchanges between continental margin sediments and seawater termed boundary exchange be could be the dominant source sink terms that determine the distribution of neodymium isotopes in the global ocean to test this hypothesis we only included this be source sink term in the model be was parameterized as a relaxing term that forces end of seawater to the isotopic composition of the continental margin over a characteristic exchange time sensitivity tests were made using different exchange times and parameterizations the model reproduces most of the general trends of end composition in seawater at global scale inter basin gradient composition of some of the main water masses these results suggest that be process may account for the major sources and sinks of oceanic nd while indicating that the characteristic time of interaction between seawater and margins varies from about half a year in surface waters to several years at depth the disparities between the model and observations might reflect the typical shortcomings of utilizing a coarse resolution ocean model circulation fields or neglected sources of neodymium such as dissolution of aeolian inputs or dissolved river loads

hafnium and neodymium isotopes in surface waters of the eastern atlantic ocean implications for sources and inputs of trace metals to the ocean we present hafnium hf and neodymium nd isotopic compositions and concentrations in surface waters of the eastern atlantic ocean between the coast of spain and south africa these data are complemented by hf and nd isotopic and concentration data as well as rare earth element ree concentrations in saharan dust hafnium concentrations range between a maximum of 0 52 pmol kg in the area of the canary islands and a minimum value of 0 08 pmol kg in the southern angola basin neodymium concentrations also show a local maximum in the area of the canary islands 26 pmol kg but are even higher between 20n and 4n reaching maximum concentrations of 35 pmol kg these elevated concentrations provide evidence of inputs from weathering of the canary islands and from the partial dissolution of dust from the sahara sahel region the inputs from ocean island weathering are also reflected in radiogenic hf and nd isotopes the hf isotopic compositions of dust samples themselves are highly variable ranging between ehf 20 and 0 6 the combined hf and nd isotopic compositions of dust plot close to the terrestrial array during periods of appreciable dust load in the atmosphere during low atmospheric dust loading combined hf and nd isotopic compositions similar to seawater are observed most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples which in turn is linked to sorting during atmospheric transport to the eastern atlantic ocean and possibly presorting by sedimentary redistribution on the continent in addition increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic hf isotopic compositions observed while the nd isotopic composition in the surface ocean reflects the nd isotopic composition of the saharan dust adjacent to the sahara sahel region the release of hf from that dust appears to be incongruent and results in surface ocean hf isotopic compositions which are 10 ehf more radiogenic than the bulk dust radiogenic hf appears to be released from clays and possibly from trace apatite rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings suggesting that such coatings are insignificant in the release of hf and nd from saharan dust to the surface ocean the nd isotopic composition of the surface waters becomes less radiogenic south of the equator most likely reflecting the release of nd from congo river sediments the release of hf from saharan dust and the congo river sediments however does not produce distinct hf isotopic signatures in the surface ocean implying that the mobile fraction of hf integrated over large continental areas is isotopically uniform the hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization

neodymium isotopic composition of the mediterranean outflow and the eastern north atlantic the neodymium concentration and isotopic composition of the mediterranean outflow in the strait of gibraltar has been directly determined the outflow has a dissolved nd concentration of 23 pmoles kg with nd 0 9 4 these results are in clear disagreement with previous values inferred for the mediterranean this concentration and nd 0 do not sharply contrast with north atlantic deep water nadw values and can produce isotopic anomalies no larger than 1 unit in the north atlantic a new profile in the eastern north atlantic does not exhibit an nd 0 anomaly at the depth of the mediterranean water core in accordance with the above inferences while concentrations and nd 0 at this site indicate that nd is conservative in the deep eastern north atlantic a previously reported nd 0 anomaly in the eastern north atlantic originally associated with the mediterranean outflow has a different source the direct contribution of the mediterranean outflow to the nadw nd budget is small 10 of the high latitude sources however the outflow plus entrained water supply 30 of the total nd associated with northern latitude deep water sources concentrations of dissolved nd in the outflow are enriched compared to the atlantic inflow which has 16 pmoles kg with nd 0 11 8 the principal source of the excess dissolved nd is inferred to be partial dissolution of detrital particles simple mass balance calculations suggest that 0 6 of the nd associated with detrital river transported sediments and or 3 5 of the nd associated with eolian particles delivered to the mediterranean is mobilized during passage into the marine environment if general these mobilization efficiencies imply that the dissolution of detrital particles is a significant source of dissolved nd and other rare earth elements in seawater

ambient noise tomography of the east african rift in mozambique seismic ambient noise tomography is applied to central and southern mozambique located in the tip of the east african rift ear the deployment of mozart seismic network with a total of 30 broad band stations continuously recording for 26 months allowed us to carry out the first tomographic study of the crust under this region which until now remained largely unexplored at this scale from cross correlations extracted from coherent noise we obtained rayleigh wave group velocity dispersion curves for the period range 5 40 s these dispersion relations were inverted to produce group velocity maps and 1 d shear wave velocity profiles at selected points high group velocities are observed at all periods on the eastern edge of the kaapvaal and zimbabwe cratons in agreement with the findings of previous studies further east a pronounced slow anomaly is observed in central and southern mozambique where the rifting between southern africa and antarctica created a passive margin in the mesozoic and further rifting is currently happening as a result of the southward propagation of the ear in this study we also addressed the question concerning the nature of the crust continental versus oceanic in the mozambique coastal plains mcp still in debate our data do not support previous suggestions that the mcp are floored by oceanic crust since a shallow moho could not be detected and we discuss an alternative explanation for its ocean like magnetic signature our velocity maps suggest that the crystalline basement of the zimbabwe craton may extend further east well into mozambique underneath the sediment cover contrary to what is usually assumed while further south the kaapval craton passes into slow rifted crust at the lebombo monocline as expected the sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active nne ssw urema rift while further south the mazenga graben adopts an n s direction parallel to the eastern limit of the kaapvaal craton we conclude that these two extensional structures herald the southward continuation of the ear and infer a structural control of the transition between the two types of crust on the ongoing deformation

rare earth element transport in the western north atlantic inferred from nd isotopic observations the isotopic composition of nd in the water column from several western north atlantic sites and formational areas for north atlantic deep water shows extensive vertical structure at all locations in regions where a thermocline is well developed large isotopic shifts 2 to 3 units are observed across the base of the thermocline regions without a thermocline are characterized by much more gradual shifts in isotopic composition with depth in general the data reveal an excellent correlation between the nd isotopic distribution in the western north atlantic water column and the distribution of water masses identified from temperature and salinity characteristics nadw as identified from t s properties is also characterized by a well defined isotopic composition having nd 0 13 5 0 5 this signature is associated with waters identified as nadw from high latitudes near formational areas in the labrador sea down to the equatorial region the isotopic signature of nadw would appear to be formed by a blend of more negative waters originating in the labrador sea nd 0 18 and more positive waters originating in the overflows from the norwegian and greenland seas nd 0 8 to 10 and is consistent with classical theories on the formation of nadw the isotopic signature of nadw is propagated southward to the equator where it is gradually being thinned out by mixing from above and below with more radiogenic nd associated with northward spreading antarctic intermediate and bottom waters the preservation of the isotopic signature of nadw over these large distances indicate that the ree undergo extensive lateral transport the isotopic composition of nd is largely conservative over the time scales of mixing within the atlantic in spite of the intrinsic nonconservative behavior of neodymium nd concentration gradients generally show surface waters to be depleted in nd relative to deep waters which must require vertical transport processes however isotopic differences in the water column preclude the local downward transport of ree from the surface into underlying deep waters as a simple explanation of the concentration gradient the apparent decoupling of ree in nadw from overlying local surface waters and the increasing concentration with depth provide a conflict with simple vertical transport mechanisms that is not yet resolved

crustal and upper mantle s wave velocity structures across the taiwan strait from ambient seismic noise and teleseismic rayleigh wave analyses abstract although orogeny tapers off in western taiwan large and small earthquakes do occur in the taiwan strait a region largely untouched in previous studies owing mostly to logistical reasons but the overall crustal structure of this region is of particular interest as it may provide a hint of the proto taiwan before the orogeny by combining time domain empirical green s function tdegf from ambient seismic noise using station pairs and traditional surface wave two station method ts we are able to construct rayleigh wave phase velocity dispersion curves between 5 and 120 s using broadband array in taiwan for seismology bats stations in taiwan and in and across the strait we are able to derive average 1 d vs structures in different parts of this region the results show significant shear velocity differences in the upper 15 km crust as expected in general the highest vs in the upper crust observed in the coastal area of mainland china and the lowest vs appears along the southwest offshore of the taiwan island they differ by about 0 6 1 1 km s for different parts of the strait the upper crust vs structures are lower in the middle by about 0 1 0 2 km s relative to those in the northern and southern parts the upper mantle vs structure moho 150 km beneath the taiwan strait is about 0 1 0 3 km s lower than the ak135 model the overall crustal thickness is approximately 30 km much thinner and less variable than under the taiwan island the inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves