geochemical characteristics of deep sea sediments from the arabian sea a high resolution study abstract five deep sea cores from the arabian sea covering a time interval of 170 ky were sampled at high resolution and analysed for major and trace elements the stratigraphy of the cores was obtained by comparing the ba al ratio with the specmap data this method is based on the close correlation between ba concentrations or ba al ratios and oxygen isotope ratios 18o of foraminifera in arabian sea sediments ba al ratios used as a productivity proxy are variable but high in all deep sea cores of the arabian sea indicating a basin wide influence of nutrient rich water masses compared to glacial intervals the warmer periods are characterised by higher element al ratios of proxies which are directly ba ca sr p i e hard parts of organisms or indirectly u 230thex related to biological productivity 230thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the indus fan the mn distribution in a core from the western arabian sea shows enrichments during interglacial periods and may indicate mn export owing to the presence of a stronger oxygen minimum zone several processes limit the applicability of proxies intercalated turbidites in the deep sea cores can be identified by means of si al ti al and zr al ratios versus depth due to the enrichment of quartz ti minerals and zircon in the basal layer these elements are also important for the reconstruction of the history and extent of the eolian dust input diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and normal pelagic sedimentation and may mask or destroy primary signals

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

geochemical signatures of the namibian diatom belt perennial upwelling and intermittent anoxia 1 a gravity core from the nambian diatom belt 83 m water depth was analyzed for major al ca fe k mg p si ti and minor elements ag as ba bi cd co cr cu mn mo ni rb re se sr tl u v zn zr as well as c and s the major element geochemistry is dominated by the contribution of biogenic phases opal and minor amounts of carbonate and phosphate whereas terrigenous components of eolian and fluvial origin form only a minor admixture of 15 examination of the fe s c relationship reveals a completion of pyritization close to the sediment seawater interface higher than average shale fe al ratios reflect the presence of highly reactive iron oxides in terrigenous source material from the namibian desert and suggest little loss of fe within the omz the low total fe content likely represents an important factor responsible for sporadic h2s eruptions the upwelling system off namibia may be regarded as an intermediate system combining high productivity typical for upwelling areas with the sporadic presence of h2s in the water column by examining the source sink relationship of individual trace metals two different trends are discernible mo re and u accumulate following seawater availability whereas ag cd cu ni tl and zn require a biological preconcentration step trace metals like as ba cr se and v show an intermediate behavior metal enrichment is favored by high interstitial water sulfide concentrations close to the sediment seawater interface leading to slightly elevated co contents as opposed to extremely low mn al ratios reflecting the overall reducing character of the namibian system maximum contents of cu ni and zn are significantly lower than those reported previously this may in part be related to the absence of fecal material and a stronger dilution by biosiliceous material at this location compared to the seaward flank of the diatom belt

hydrogen index as reflecting intensity of sulphidic diagenesis in non bioturbated shaly sediments abstract based on 13 published porewater h 2 s and sulphate profiles the amount of h 2 s escaping from non bioturbated shales varies between some few to 45 of the amount of bacterially generated h 2 s this finding permits calculation of the original organic carbon toc or content of immature nonbioturbated shales using toc and sulphur content data in two immature non bioturbated sequences from hungary toarcian and oligocene the first order correlation between hi and toc toc or was found to be stronger than that between hi and toc indicating that sulphate reduction was the leading process both in decrease in toc content and degradation of kerogen source potential

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

recent sedimentation and mass accumulation rates based on 210pb along the peru chile continental margin abstract the concentrations of the natural radionuclide 210 pb were measured in sediment samples collected with a multicorer boxcorer and gravity corers along the chilean continental margin 20 45 s in order to estimate sedimentation rate sr and mass accumulation rate this database was compared with previously obtained results from central peru callao 210 pb activities decreased exponentially with core depth and profiles showed a deepening of the 210 pb in excess unsupported in the sediments from north to south 6 18 cm sediment depth off peru and northern chile vs 8 35 cm off central and southern chile an onshore offshore decrease of surface activities was observed off central peru whereas the reverse was true for surface activities along the chilean margin srs increased from north 0 04 0 17 cm yr 1 to south 0 09 0 29 cm yr 1 across the shelf higher rates were observed at deeper sites especially at those close to submarine canyons except off central peru where srs showed similar values through the slope area off northern chile and central peru srs seem to reflect the export of particles over the shelf and redistribution processes such as slumps and lateral transport in contrast srs in central and southern chile seem to be controlled by several additional factors that modulate the settling particles such as onshore precipitation and river runoff in addition the more complex topography of the southern chile margin allows for settling or winnowing areas

some comparative marine chemistries of rhenium gold silver and molybdenum abstract four metals re au ag and mo whose dissolved forms in seawater can potentially be reduced to insoluble states have been measured in a variety of solids depositing under anoxic conditions hydrothermal sulfides coastal sediments and phosphorites for comparative purposes these elements have also been determined in ferromanganese minerals and pelagic sediments which have accumulated under oxidizing conditions rhenium appears to be a unique sentinel for identifying reducing depositional environments where enrichments of greater than three orders of magnitude above crustal concentrations are found molybdenum follows re in these sediments but mo is enriched also in those that accumulate under oxidizing conditions the reported mo crustal concentrations appear low in comparison to our measured sedimentary values gold and ag are concentrated in hydrothermal deposits and sulfides appear to be involved in the precipitation of these elements coastal sediments adjacent to a domestic outfall of los angeles california contain extraordinary high accumulations of anthropogenic au ag and pt the study seeks an understanding of the mobilities and sinks of these metals in the marine environment during the major weathering cycle further anthropogenic contributions to their sedimentary concentrations are identified

anaerobic oxidation of methane and sulfate reduction along the chilean continental margin anaerobic oxidation of methane aom and sulfate reduction sr were investigated in sediments of the chilean upwelling region at three stations between 800 and 3000 m water depth major goals of this study were to quantify and evaluate rates of aom and sr in a coastal marine upwelling system with high organic input to analyze the impact of aom on the methane budget and to determine the contribution of aom to sr within the sulfate methane transition zone smt furthermore we investigated the formation of authigenic carbonates correlated with aom we determined the vertical distribution of aom and sr activity methane sulfate sulfide ph total chlorins and a variety of other geochemical parameters depth integrated rates of aom within the smt were between 7 and 1124 mmol m 2 a 1 effectively removing methane below the sediment water interface single measurements revealed aom peaks of 2 to 51 nmol cm 3 d 1 with highest rates at the shallowest station 800 m the methane turnover was higher than in other diffusive systems of similar ocean depth this higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis sr within the smt was mostly fuelled by methane aom led to the formation of isotopically light dic 13c 24 6 vpdb and of distinct layers of authigenic carbonates 13c 14 6 vpdb

two dimensional mantle convection simulations using an internal state variable model the role of a history dependent rheology on mantle convection summary we apply the bammann inelastic internal state variable model biisv to a mantle convection code terra2d to investigate the influence of a history dependent solid mechanics model on mantle convection we compare and contrast the general purpose biisv model to the commonly used power law model we implemented the biisv model using a radial return algorithm and tested it against previously published mantle convection simulation results for verification model constants for the biisv are used based on experimental stress strain behaviour found in the literature after implementation we give illustrative simulation examples were the biisv produces hardened areas on the cold thermal boundary layer that the power law model cannot produce the hardened boundary layers divert material downward giving a plausible reason for the current subduction zones that are present on the earth

high resolution marine record of climatic change in mid latitude chile during the last 28 000 years based on terrigenous sediment parameters abstract marine sediment cores from the continental slope off mid latitude chile 33 s were studied with regard to grain size distributions and clay mineral composition the data provide a 28 000 yr14c accelerator mass spectrometry dated record of variations in the terrigenous sediment supply reflecting modifications of weathering conditions and sediment source areas in the continental hinterland these variations can be interpreted in terms of the paleoclimatic evolution of mid latitude chile and are compared to existing terrestrial records glacial climates 28 000 18 000 cal yr b p were generally cold humid with a cold semiarid interval between 26 000 and 22 000 cal yr b p the deglaciation was characterized by a trend toward more arid conditions during the middle holocene 8000 4000 cal yr b p comparatively stable climatic conditions prevailed with increased aridity in the coastal range the late holocene 4000 0 cal yr b p was marked by more variable paleoclimates with generally more humid conditions variations of rainfall in mid latitude chile are most likely controlled by shifts of the latitudinal position of the southern westerlies compared to the holocene the southern westerly wind belt was located significantly farther north during the last glacial maximum less important variations of the latitudinal position of the southern westerlies also occurred on shorter time scales

phosphate regeneration from sediments of the peru continental margin by dissolution of fish debris generally oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes stoichiometric modelling of such reactions takes into consideration varying proportions of decomposable organically bound p to account for the ratios among nutrients in depth concentration profiles of near surface sediments new results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound p dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10 of the nutrient pool in the waters of the peru undercurrent partitioning of p among the two sources reveals further that fish debris phosphate is about four times more important than organically bound p in nutrient generation from sediments of the peru continental margin not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area

geochemical characteristics of chilean upwelling sediments at 36 s abstract we examined the cycling of c s fe and the trace elements te ag cd mo mn re and se in chilean upwelling sediments from water depths of 127 2744 m at 36 s the sediments are clay rich with moderate total organic carbon toc and low biogenic caco3 contents for the shallow site within oxygen minimum zone omz which shows some thioploca biomass highest observed toc contents correspond to highest productivity induced toc flux and or enhanced preservation under o2 deficient conditions pyrite formation may be restricted by the concurrent presence of elemental s in thioploca and or the availability of reactive iron in the other sediments 370 m water depth pyrite formation is strongly restricted due to the low availability of metabolizable toc for sulfate reduction calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than toc was remineralized by sulfate reduction than expected from redfield c n relationship manganese is significantly depleted due to reductive dissolution of mn hydr oxides within all sediments mo is only significantly enriched vs the lithogenic background in the shallow core implying anoxic conditions contents of re and mo as well as re mo ratios indicate suboxic conditions for the other cores cd and re contents decrease with increasing water depth reflecting decreasing reducing conditions in contrast authigenic ag contents increase with water depth suggesting enrichment controlled by seawater availability and or opaline input and not redox conditions elevated se contents in all cores may be either due to bio detrital input or diffusion from seawater accumulation rates of toc ts and te off chile are in the same order as those from the gulf of california and lower than those off peru distinct seasonality in upwelling and dilution of chilean sediments with terrigenous material lead to rather low metabolizable toc contents and lower te trapping capacity

surface sediment distribution along the chilean continental slope related to upwelling and productivity abstract the coastal upwelling system of the peru chile current belongs to the most productive regions in the world oceans in spite of this remarkable fact only very little is known about the sediment distribution in its southern part off the coast of chile to increase the knowledge about this region a multiparameter study of the surface sediment distribution along the chilean continental slope between 27 and 42 s has been carried out detailed analyses of sedimentary data toc and carbonate contents 13corg 15n of the species composition of planktic foraminifera and of the stable isotope composition of the two planktic foraminifera species neogloboquadrina pachyderma dex and globigerina bulloides reveal a close relation to environmental conditions in the region the pattern of a southward increase of surface water productivity known from satellite data is closely reflected in the organic carbon and 15n data information about the water mass distribution especially about varying influences of upwelling waters and warmer subtropical surface waters can be derived from the species composition of the planktic foraminifera while the stable isotope data provide additional clues about water mass characteristics high productivity along the chilean margin north of 40 s is sustained by coastal upwelling while further to the south advection of nutrients with the antarctic circumpolar current and a significant riverine input fuel the highest productivity found in the study area this data set will provide the base for future paleoclimatic and paleoceanographic reconstructions of the southeast pacific region

distribution of be al se and bi in the surface waters of the western north atlantic and caribbean abstract the mixed layer distributions of several trace elements have been determined along a detailed transect from rhode island to the panama canal when taken together with a new and existing profile data from the north atlantic and north pacific some general inferences emerge as to the processes controlling their concentrations in the surface waters the large enrichments in be observed in the upper waters of the north atlantic relative to the north pacific appear to be sustained mainly by fluvial inputs those of se are derived from atmospheric transport aluminium and bi also appear to have an aeolian source in the sargasso sea however in the tropical eastern pacific the al input may be fluvial on this cruise be al and the se species were analyzed on board ship within a few hours of collection the demonstration that this is feasible opens up the prospect of the application of large scale chemical hydrography to the study of the processes controlling the distribution and water column variability of trace elements in the oceans

how to obtain earthquake ground motions for engineering design abstract the earthquake ground motions that ultimately are selected for engineering design depend chiefly on the criticality of a site or structure and the engineering analyses that are to be performed several key steps are necessary in this selection process they are 1 a reconnaissance to understand the hazards and obtain preliminary earthquake ground motions 2 decisions on the application of deterministic or probabilistic methods 3 selection of appropriate motions for requirements in design 4 consideration of thresholds at which motions become significant for engineering and 5 decisions on specifying appropriate earthquake ground motions for sizes of earthquakes distances from sources the structures sites and testing to be done this paper presents five tables that show steps for evaluating these factors and for enabling the investigator to specify earthquake ground motions appropriate for engineering design

geochemistry of recent toc rich sediments from the gulf of california and the black sea major and minor elements incl ag as ba cd co cr cu mn mo ni pb sr v zn have been determined in recent sediments from the gulf of california upwelling area and black sea sapropels in order to reinterpret their chemical composition in view of reliable seawater and plankton data the chemistry of the gulf of california sediments reflects regeneration processes which occur in the water column i e only a small fraction 10 of elements like cd which in seawater are coupled to the labile nutrients c n p is buried in the sedimentary column in contrast elements like the more resistant nutrients si undergo a deeper regeneration cycle ba several trace metals which are present in comparatively higher concentrations in seawater as mo u v and at the same time are reactive under reducing conditions and or are able to form stable sulfides are fixed in the sediments during early diagenesis the early diagenetic behavior of ba is closely related to bacterial sulfate reduction the formation of barite concretions is discussed anoxic conditions in the water column act as ideal traps for a number of redox sensitive and or stable sulfide forming elements a simple trace metal balance calculation shows that the chemical composition of black sea sapropels is controlled by fluvial and mediterranean seawater element input and the accumulation rate of terrigenous detrital material

degree of pyritization of iron as a paleoenvironmental indicator of bottom water oxygenation abstract a new method is presented for recognizing the degree of bottom water oxygenation in organic carbon bearing rock the method uses the parameter degree of pyritization dop which is the ratio of pyritic fe to pyritic fe acid soluble fe sediments with ages ranging from devonian to cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions deposited in fully oxygenated bottom waters restricted marine conditions deposited in waters with low oxygen concentrations or inhospitable bottom conditions where little or no oxygen is present and h2s may be continually or intermittently present aerobic dop

individual faults can t produce a gutenberg richter earthquake recurrence abstract individual faults within a region cannot have a gutenberg richter recurrence if the total of earthquakes within the region follow a gutenberg richter recurrence few faults have produced and adequate number of earthquakes to establish the nature of recurrence on them broad seismic regions however for which there is a statistically viable earthquake data set have been shown to obey a gutenberg richter recurrence with a slope of about 1 a gutenberg richter recurrence is often assumed for faults in probabilistic seismic hazard analyses the requirement that individual faults cannot produce earthquakes with this recurrence casts doubt on the validity of such probabilistic methods errors in probabilistic seismic hazard analysis tend to be amplified in the calculation of rare earthquake probabilities and it is these rare earthquakes for which critical facilities must be designed therefore sole use of probabilistic seismic hazard analysis for critical facilities is not recommended

distribution of organic carbon in surface sediments along the eastern arabian sea a revisit abstract one hundred twelve surface sediment samples along the western continental margin of india were analyzed for organic carbon oc elevated oc concentrations are associated with sediments deposited in areas where productivity is moderate to high very high oc concentrations as reported in earlier studies even for regions where productivity is comparatively low could not be confirmed the re analysis of selected samples by coulometry shows that oc concentrations determined previously by the wet oxidation method may be too high by up to more than a factor of two a broad similarity in the distribution of oc and bioproductivity is observed which is in accordance with other productivity proxies such as the abundance of planktonic foraminiferal species g bulloides and hydrogen indices hi our results indicate the importance of bioproductivity for oc accumulation along the western continental margin of india even though low dissolved oxygen concentrations in the water column sedimentation rate sediment texture clay mineral content dilution by quartz and carbonate as well as topography are considered responsible for the general oc distribution in this area

the impact of sediment provenance on barium based productivity estimates abstract biogenic barium in marine sediments has been suggested to be a reliable proxy of export productivity from the surface ocean and algorithms have been developed to link these properties however problems arise when the proposed algorithms are applied to predominantly terrigenous sediments a major source of error is incorrect estimates of the terrigenous ba al ratio in normative calculations of the amount of biogenic barium in the sediment compared to an often used global average ba al ratio much better results can be obtained by estimating the terrigenous ba al ratio from exponential regression of the ba al ratios of surface sediments obtained from continental slope transects this method has been applied to surface sediments from the chilean continental slope the calculated regional terrigenous ba al ratios could be verified with purely terrigenous samples from chilean rivers the resulting accumulation rates of biogenic barium on the chilean continental slope reliably reproduce the regional pattern of primary productivity in the southern peru chile current indicating the potential of biogenic barium as a useful paleo productivity proxy

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

high concentrations of biogenic barium in pacific sediments after termination i a signal of changes in productivity and deep water chemistry abstract high concentrations of barium have been found in sediments deposited on the chilean continental slope after glacial termination i when compared to independent proxies of paleoproductivity the pattern of biogenic barium accumulation does not match the changes in paleoproductivity in this region and cannot be explained by changes in the terrigenous supply of barium alone this leaves an increased concentration of dissolved barium in sea water as the most likely cause of the observed barium anomalies based upon the accumulation rate of biogenic barium and the paleoproductivity index which was calculated from the accumulation rates of organic carbon biogenic opal and carbonate the changes in the concentration of dissolved barium in the pacific ocean in the course of the last glacial and through the holocene could be modelled this model shows that barium concentrations in the pacific during the last glacial were comparable to today 150 nmol kg1 the highest dissolved barium concentrations were calculated for the early and middle holocene 190 nmol kg1 the resulting pattern of variations in the barium concentration in the seawater agrees well with ba ca ratios in benthic foraminifera published in the literature the observed changes are probably driven by the holocene shoaling of the pacific carbonate compensation depth which is assumed to have caused an increased flux of previously carbonate bound barium from the sediment to the deep ocean waters leading to the observed longstanding maximum of dissolved barium in the pacific during the holocene

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

biogenic silica and diatom thanatocoenosis in surface sediments below the peru chile current controlling mechanisms and relationship with productivity of surface waters abstract based on 76 surface sediment samples collected between ca 22 and 44 s along the chilean coast in the southeast pacific ocean we report on the north south distribution of opal content and diatom concentration and the quantitative and qualitative composition of the preserved diatom community latitudinal differences in the siliceous signal reflect the influence of two main mechanisms of surface water production coastal upwelling off northern and central chile till ca 38 s and nutrient enrichment by the input of cold southern non upwelling associated waters south of ca 38 s the poleward increase in biogenic opal and diatom values parallels well the north south increase of pigment concentration in surface waters as recorded by satellites between 22 and 33 s where low pigment concentrations are measured opal content and diatom concentration are mostly low except at 22 23 s in contrast both higher opal and diatom values at 34 38 s and 41 42 s correspond well with higher pigment estimations preserved diatom populations document the present day dominant hydrographical conditions nutrient enrichment of surface waters due to coastal upwelling results in intensive production of diatoms off northern and central chile reflected by the predominance of the richly composed association of chaetoceros spores north of ca 38 s the diatom association shows that the area between 35 and 38 s is transitional the abrupt decrease of chaetoceros spores south of 38 s off chile coupled with the enhanced contribution of thalassiosira spp representatives of high productivity low temperature waters points to the presumed high nutrient supply by the iron limited nutrient rich antarctic circumpolar water the contribution of benthic diatoms reflects some lateral transport from the near shore area into deeper positions south of 38 s

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

mobility of authigenic rhenium silver and selenium during postdepositional oxidation in marine sediments sedimentary records of redox sensitive trace elements hold significant potential as indicators of paleoceanographic environmental conditions records of re can reveal the intensity of past reducing conditions in sediments at the time of deposition whereas records of ag may record the magnitude of past diatom fluxes to the seafloor confidence in paleoenvironmental reconstruction from records of either metal however requires it to have experienced negligible redistribution since deposition this study examines diagenetic rearrangements of re and ag that occur in response to exposure to bottom water o2 in environments of low sedimentation rate including madeira abyssal plain turbidites and eastern mediterranean basin sapropels authigenic re was remobilized quantitatively by oxidation but poorly retained by the underlying sediments all records are consistent with previous work demonstrating that only a limited reimmobilization of re occurs preferentially in corg rich reducing sediments silver was also mobilized quantitatively by oxidation but it was subsequently immobilized more efficiently in all cases as sharp peaks immediately into anoxic conditions below active oxidation fronts and these peaks remain immobile in anoxic conditions during long term burial comparison of ag s and se records from various cores suggests that ag is likely to have been immobilized as a selenide a mechanism previously proposed for hg in similar situations mercone et al 1999 coexisting narrow peaks of ag and hg with se offer a means of assessing whether oxidative burndown has ever occurred at the top of corg and sulfide rich sedimentary units although these results suggest that caution must be used when inferring paleoenvironmental information from records of ag and re in cores with low sediment accumulation rates 5 cm ka 1 they should not affect the promise that authigenic ag and re records hold for paleoenvironmental reconstruction in sediments with higher accumulation rates and where anoxic conditions have been maintained continuously

the geochemical cycle of rhenium a reconnaissance rhenium re is one of a suite of elements including uranium and molybdenum that display conservative behavior in seawater and are enriched in anoxic sediments the decay of187re to187os provides a geochronometer in ancient sedimentary rocks and gives rise to os isotopic variations in nature in order to better characterize its sources to seawater re was measured in three major rivers amazon orinoco ganges brahmaputra and some of their tributaries re concentrations span four orders of magnitude from 0 02 to 400 pmol kg with the highest concentrations found in rivers draining black shales in the venezuelan andes mainstream re levels in the three rivers are between 1 and 10 pmol kg with a flux weighted average of 2 3 pmol kg the residence time for re in the oceans is estimated to be 750 000 yr with respect to river inputs re profiles from the atlantic and pacific oceans confirm that re behaves conservatively in seawater with no significant uptake onto particles and or recycling within the water column this is also true in the anoxic water column of the black sea re removal into anoxic sediments occurs at or below the sediment water interface as demonstrated in sediment pore waters from chesapeake bay in oxic sediments re is not cycled with manganese oxides and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component or in manganese nodules burial of re in anoxic sediments which accumulate on 0 3 of the ocean floor removes approximately 50 of the riverine re flux to the oceans hence oceanic re concentrations may be very sensitive to changes in the area of anoxic sedimentation

sulfate reduction and methanogenesis in a thioploca dominated sediment off the coast of chile continental shelf sediments of the central chile upwelling area are dominated by the presence of dense mats of the filamentous sulfur depositing bacterium thioploca spp we examined rates and pathways of s and methane cycling in these sediments along a transect from the bay of concepcion to the continental slope sulfate reduction rates 170 4670 nmol cm 3 d 1 were equal to or exceeded rates reported for other subtidal marine sediments elemental s and pyrite were the dominant end products of sulfate reduction in thioploca mats on the continental shelf whereas in the highly reducing beggiatoa dominated sediments of the nearby bay of concepcion acid volatile s was the principal end product dissolved organic c values were lowest at the stations with the highest sulfate reduction rates and increased offshore sediment porewater methane concentrations in all surface sediments were low 12 nmol cm 3 and methane production rates at the station most dominated by thioploca were extremely low 0 5 nmol cm 3 d 1 low methane production rates and concentrations were matched by low methane oxidation rates 0 1 nmol cm 3 d 1 radio tracer studies showed that methane production was almost exclusively from methylamines substrates which are noncompetitive with sulfate reduction rather than from acetate or co2 h2 bacterial mpn most probable number counts also indicated the presence of a methylotrophic population of methanogens surprisingly high numbers of autotrophic acetogenic bacteria were found suggesting that the bacterial population involved in anaerobic doc degradation is more complex than expected in spite of the high sulfate reduction rates sulfide concentrations in the shelf and slope were low or undetectable 0 5 m and sulfate concentrations were never depleted below bottom water levels down to depths of 25 30 cm calculations suggest that thioploca were oxidizing a maximum of 35 of sulfide production not enough to prevent sulfate depletion either other sulfide oxidizers were also present or transient hydrodynamic conditions coupled with bioturbation resulted in oxidation of the sediments

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity