the present day formation of apatite in mexican continental margin sediments abstract results of pore water and sediment analyses from the western mexican continental margin strongly suggest the present day formation of apatite the interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments apatite is identified within this maximum via x ray diffraction but is elsewhere undetectable in the core radioisotopic thorium uranium and radium data support the conclusion that this deposit is modern the present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value

sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

monsoon related variations in zaire congo sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial atlantic during the last 200 000 years two sediment cores from the zaire fan and the angola margin have been investigated for their composition of terrigenous and biogenic constituents respectively for the late quaternary kaolinite feldspar ratios and variations of terrigenous element ratios of zr ti k rb and al reveal that the composition of zaire river sediment load has fluctuated in tune with precessional variations of boreal summer insolation in particular the correspondence of high kaolinite feldspar and al k ratios with low latitude insolation maxima strongly corroborates the assumption that west african monsoonal precipitation and chemical weathering was enhanced during periods of increased central african heating the most striking feauture observed is that opal accumulation has been 2 to 10 times higher in zaire fan sediments than outside in continental margin sediments off angola although biogenic ba and corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the zaire river plume and off angola from this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the zaire river plume than in the oceanic upwelling regime farther to the south off angola over the last 200 000 years the pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the zaire river to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold more arid glacial climate conditions we presume that paleoproductivity off the zaire was controlled by the following two sources of nutrients 1 marine nutrients including nitrate and phosphate as well as the uptake of ba on particulate corg caused by upwelling and 2 silica mainly delivered by the zaire river dissolved silicate hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical atlantic at a millenial timescale

zone of metal driven anaerobic oxidation of methane is an important sink for phosphorus in the taixinan basin south china sea abstract phosphorus p is an important nutrient and has an important impact on global primary productivity presently the dynamics of the p cycle in low sulfate anoxic settings are largely unknown in this study the adsorption desorption behavior of fe bound p organic p loosely bund p detrital apatite p and refractory p was described in the sulfate and methane transition zone smtz at site 973 4 in the cold seep area of the taixinan basin south china sea results showed that the fe bound p and organic p were influenced by sulfate driven anaerobic oxidation of methane sd aom and metal driven anaerobic oxidation of methane md aom and subsequently the fe bound p and organic p were released as hpo42 into the pore water meanwhile significant increases were detected in authigenic carbonate fluorapatite pcfa and vivianite within the md aom zone which indicate that this zone is an important p sink area the md aom plays an important role in the formation of pcfa and vivianite because md aom provide the necessary hpo42 and fe2 for the formation of these minerals our findings have major implications for understanding the p cycle in low sulfate anoxic settings and are important for understanding the influence of md aom on the phosphate cycle

cyclic 100 ka glacial interglacial migration of subseafloor redox zonation on the peruvian shelf the coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood we examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial interglacial cycles in a sedimentary section drilled on the peruvian shelf ocean drilling program leg 201 site 1229 multiple and distinct layers of diagenetic barite and dolomite i e minerals that typically form at the sulfate methane transition smt occur at much shallower burial depth than the present smt around 30 meters below seafloor these shallow layers co occur with peaks of 13c depleted archaeol a molecular fossil of anaerobic methane oxidizing archaea present day non steady state distributions of dissolved sulfate also suggest that the smt is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the smt reaction transport modeling substantiates our hypothesis that shallow smts occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales e g glacial interglacial periods however our study demonstrates how dynamically sediment biogeochemistry of the peru margin has responded to glacial interglacial change and how these changes are now preserved in the geological record such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling

controls on phosphorus concentration and accumulation in oceanic sediments understanding phosphorus p sedimentation in the ocean is important both for quantifying outputs in the marine phosphorus cycle and for determining the controls on phosphogenesis the authigenic formation of carbonate fluorapatite and phosphorite deposition large accumulations of p rich sediments on continental margins in this study i have compiled literature data on sedimentary p concentrations and sedimentation rates from continental margins phosphogenic continental margins and open ocean environments i have also calculated p accumulation rates the product of p concentration sediment density and sedimentation rate in these settings sedimentary p concentrations exhibit a similar range in a wide variety of oceanic environments and sedimentation rate conditions for example p concentrations in continental margin sediments range from 8 to 108 mol p g 1 with sedimentation rates from 80 to 8 000 m my 1 in open ocean sediments the p concentration range is similar 7 307 mol p g 1 even though sedimentation rates are generally about 100 times lower 2 50 m my 1 the exception to this are phosphogenic environments where p concentrations are exceptionally high 580 3 700 mol p g 1 while sedimentation rates are relatively low 20 310 m my 1 phosphorus accumulation rates appear to be controlled by sedimentation rate in ocean sediments although p concentrations generally exhibit similar ranges the large difference in sedimentation rates between various environments results in p accumulation rates that are several orders of magnitude higher in continental margins 90 8 000 mol p cm 2 kyr 1 compared to open ocean sediments 2 75 mol p cm 2 kyr 1 in the case of phosphogenic environments low sedimentation rates result in p accumulation rates 80 6 300 mol p cm 2 kyr 1 that are surprisingly similar to those of non phosphogenic continental margin sediments

early diagenetic processes in recent sediments of the gulf of st lawrence phosphorus carbon and iron burial rates abstract selective extraction procedures were used to quantify different forms of solid phase phosphorus carbon and iron in marine sediments and to evaluate the impact of authigenic formation of mineral forms such as carbonate fluorapatite cfa calcium carbonate caco3 and pyrite fes2 on major elemental cycles during early diagenesis detrital p and fe phases were successfully used as indicators of the constancy or variability of detrital inputs to several sedimentary environments from the deep channels of the gulf of st lawrence in cores characterized by near steady state influx rates solid phase p c and fe data and sediment burial rates indicate that cfa caco3 and probably fes2 are currently forming in the sediments of the gulf however high concentrations and or formation of caco3 in marine sediments appear to inhibit the formation of authigenic cfa on the other hand the formation of fes2 does not influence authigenic cfa precipitation in the deep troughs of the gulf of st lawrence total p burial rates range from 50 to 500 mgp m2 yr truly authigenic precipitation of cfa when observed may represent up to 25 of the total burial rate of p bioturbation of sub surface sediments reduces the potential for authigenic precipitation of cfa and caco3 thus affecting immobilization reactions that have a strong impact on the global oceanic cycles of c and p the spatial heterogeneity of diagenetic reactions precludes the establishment of an accurate quantification of p removal on the scale of a continental shelf such as the gulf of st lawrence

carbon isotopic evidence for coupled sulfate reduction methane oxidation in amazon fan sediments abstract carbon isotope ratios of dissolved inorganic carbon were measured on porewaters from amazon fan sediments collected on ocean drilling program leg 155 values as low as 49 6 vs pdb were recorded with the lowest values coinciding with the base of the sulfate reduction zone at each site the values indicate that on the order of 50 85 of sulfate reduction in amazon fan sediments takes place by methane oxidation estimated rates of methane oxidation range from 0 3 1 0x10 5 moles l yr the reasons for the high percentage of sulfate reduction taking place by methane oxidation are thought to be the large decrease in fan sedimentation rates in the early holocene and the terrestrial source of organic matter in fan sediments

apparent correlation of palaeomagnetic intensity and climatic records in deep sea sediments most reports of a correlation between pleistocene climate and geomagnetic field intensity rely strongly on the assumption that sediment natural remanent magnetic nrm intensity provides a record of geomagnetic field strength and is not sensitive to local changes in properties of the sediment critical assessment of relevant data presented here and elsewhere1 2 from deep sea sediment cores shows that a pronounced dependence of nrm intensity on sediment composition can occur which implies that this assumption is unlikely to be generally valid as sediment composition often reflects varying depositional conditions induced by climatic change the significance of correlations proposed between pleistocene palaeomagnetism and climatic indicators in deep sea sediments may be less dramatic than sometimes supposed

controls on phosphorus regeneration and burial during formation of eastern mediterranean sapropels the carbon c and phosphorus p geochemistry of sapropels from four sites in the eastern mediterranean sea was determined to obtain more insight into the role of differences in sediment accumulation rates and bottom water anoxia on p regeneration and burial in sediments sediment corg porg with org organic ratios above redfield indicate enhanced regeneration of p relative to c from organic matter during formation of the most recent sapropel s1 and a pliocene sapropel odp site 969e i 282c release of p from fe oxides was relatively unimportant increased burial of ca p authigenic carbonate fluorapatite and or biogenic hydroxyapatite occurred but was of minor importance compared to the enhanced p release from organic matter for sapropel s1 corg porg and corg preac with reac reactive defined as the sum of organic fe bound authigenic and biogenic ca p ratios decreased with increasing sedimentation rate and oxygen exposure and decreasing water depth whether the water column in the deep basin was dysoxic semi euxinic s1 or euxinic pliocene does not appear to affect the estimated burial efficiencies of porg and preac enhanced corg burial under the euxinic water column explains the much higher corg porg and corg preac ratios in the pliocene sapropel on average 2860 and 760 respectively compared to sapropel s1 on average 530 and 160 respectively

manganese authigenesis in the landsort deep baltic sea abstract manganese mineralogy because of its redox sensitivity is a valuable parameter for interpreting water column anoxia and sediment diagenesis in the baltic and especially the landsort deep the relatively widespread occurrence of mn carbonates and the unique observations of authigenic mn sulphides in sedimentary environments need to be accounted for within the models proposed for mn authigenesis in diverse settings in this study stratigraphic changes in the geochemistry and mineralogy are documented in two gravity cores using petrographic microscopy xrd sem eds analyses of c and s content 13 c isotopes icp ms cation determinations and electron microprobe element profiling the lamination in the bottom sediments of the landsort deep suggests essentially continuous anoxic conditions at the sediment surface during the last ca 4500 14 c years the most striking variations in the sediments are related to the occurrence of two mn ii precipitate rich sapropel intervals recent and 1390 1990 14 c years bp sapropel deposition is associated with relatively slow detrital sedimentation allowing the concentration of organic matter the sediment c org content is influenced by the variations in the primary production as well as the supply of terrestrial c org but a major control appears to be detrital dilution the precipitation of mn carbonate as individual aggregates of spherical particles dispersed within clayey sediment matrix and as laminae with semi seasonal cyclicity is believed to be largely induced by the high alkalinity in connection with organic matter deposition and degradation where h 2 s has exceeded fe availability mns has also precipitated most commonly as well developed hexagonal crystals but also as the cubic variety alabandite which has not previously been reported from a sedimentary environment an expansion of the anoxic bottom water conditions during sapropel deposition is interpreted by correlation with laminated intervals on the slope of the deep basin budget calculations for the supply of mn to the basin during mn rich sapropel formation can be better balanced by the addition of mn from dissolution of oxides formed prior to the anoxic expansion the importance of alkalinity for the precipitation of mn carbonates and sulphides does not exclude the need for considerable mn concentrations but the model argued in several recently published studies stressing mn availability as the major control does not seem appropriate for the landsort deep and may be too restrictive in many other settings as well

using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions abstract seepage of hydrocarbon rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time in this study we investigated the potential of mossbuaer spectroscopy and fe contents of seep related authigenic carbonates from the congo fan the gulf of mexico and the black sea for reconstructing past redox conditions and fluid seepage activity at cold seeps the fe speciation observed by mossbauer spectroscopy and fe contents suggest that 1 the congo fan carbonates precipitated in a sulfidic environment 2 the formation conditions of seep carbonates were variable at the gulf of mexico seep site ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone and 3 the stratified water column of the black sea or suboxic condition resulted in low fe contents of black sea carbonates the study reveals that fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage similarly mossbauer spectroscopy particularly when used in combination with the analysis of redox sensitive elements is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps

terrestrial organic carbon accumulation on the amazon deep sea fan during the last glacial sea level low stand abstract sediment cores from the amazon deep sea fan recovered during r v meteor cruise 16 2 show in detail the modern areal distribution of sedimentary organic carbon stable organic carbon isotopes of the organic matter om as well as variations in the depositional processes in addition we studied up to 300 m long drilled sediment records recovered during odp leg 155 which allow evaluation of temporal variations on the amazon fan our results reveal new evidence for a very rapid change of fan depositional processes and organic carbon source at times of sea level change over the middle and lower amazon fan to estimate the amount of terrestrial organic carbon stored in sediments from the last glacial in the amazon fan we used stable organic carbon isotopes of the om 13 c org organic carbon content corg and age models based on oxygen isotopes faunal data and magnetic excursions following our results the organic carbon accumulation on the amazon deep sea fan is controlled by glacio eustatic sea level oscillations interglacial sea level high stand sediments are dominated by marine om whereas during glacial sea level low stands terrestrial organic carbon is transported beyond the continental shelf through the amazon canyon and deposited directly onto the amazon deep sea fan glacial sediments of the amazon fan stored approximately 73 1015 g terrestrial corg in 20 000 years or 3 7 1012 g terrestrial corg yr 1 equivalent to 7 12 of the riverine organic carbon discharge assuming constant paleo discharge which is about the same amount of terrestrial organic carbon as deposited on the amazon shelf today 3 1 1012 g terrestrial corg yr 1 or 6 10 of the modern riverine organic carbon discharge

carbon and phosphorus burial associated with modern phosphorite formation phosphorite formation plays a significant but poorly constrained role in the marine c and p cycles this report concerns a site of modern phosphorite formation on the upper continental slope off southern baja california mexico sta 6 380 m water depth 23 41 n 111 05 w at this site a series of six narrow 1 3 cm partially indurated phosphorite layers have formed authigenically during the last 15 000 years within a surficial section of slowly but steadily accumulating 3 8 10 3 cm year 1 organic rich mud a poorly oxygenated benthic environment has prevailed at the study site since approximately 8100 year b p probably helping to produce and preserve the pristine phosphorite layers the mean p burial flux 1 1 mol cm 2 year 1 recorded by these deposits agrees closely with the downward diffusive p flux 1 3 mol cm 2 year 1 attributed to active francolite growth such a coincidence implies that the phosphorite layers could have formed entirely under present conditions however a much larger upward diffusive p flux 4 2 mol cm 2 year 1 directed out of the sediment limits the efficiency of ongoing phosphorite formation and the most recently deposited sediment does not contain enough labile p to maintain the overall p regeneration rate indefinitely attendant burial fluxes of inorganic and especially organic c 7 7 and 6 6 mol cm 2 year 1 respectively do not accord with a depositional model of coastal upwelling suggesting that an alternative mechanism possibly fe redox cycling controls porewater p concentrations and hence phosphorite formation a comparison with previously published p burial fluxes for a wide variety of other modern marine settings reveals that phosphorite formation in general does not disproportionately affect the global p burial flux from these data we tentatively estimate a minimum global p burial flux of approximately 70 109 mol year 1 with almost 80 occurring on continental margins

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

organic carbon cycling and modern phosphorite formation on the east australian continental margin an overview during 1987 the australian bureau of mineral resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern australia between 28 and 32 s the objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements nitrogen phosphorus sulphur and iron in the sediments and to investigate the roles which these processes played in the formation of the modern phosphorites bacterial productivities sulphate reduction rates sedimentary oxygen and pore water concentrations of nitrate ammonia phosphate iron sulphate and fluoride were measured at sea the highest rates of microbial productivity were found in the surficial 0 20 mm sediments of the modern phosphorite zone in 350 460 m water depth these rates were about double those in shallower shelf 300 m sediments and 3 4 fold those rates in mid slope 600 1000 m sediments aerobic and anaerobic oxidation rates of organic matter calculated from sediment oxygen profiles and sulphate reduction rates were highest in the surface sediments in the modern phosphorite zone the recycling of sedimentary iron via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic anoxic boundary results in a near surface sedimentary trap for iron in the phosphorite zone sediments phosphate released from organic matter in the interfacial sediments and fluoride from seawater are scavenged by iron oxyhydroxides in the top few centimetres of sediment phosphorus in this way is decoupled from organic carbon in the near surface sediments and linked to the redox cycling of iron phosphate and fluoride scavenged onto iron oxyhydroxides and concentrated in the surficial sediments are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve the recycling process releases phosphate and fluoride for incorporation into apatite fluoride is depleted from pore waters at depths 18 cm phosphorite nodules form within anoxic sediments at depths 18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer combinations of rapid sediment mixing rates a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten fold that of the mid slope sediments long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules phosphorite nodules are not found in mid slope sediments probably because of combinations of relatively rapid sedimentation rates ineffective iron phosphorus and fluoride recycling and trapping mechanisms plus dilution and dissemination of any incipient apatite

late glacial to post glacial sea levels in the western indian ocean late glacial to post glacial sea level changes provide direct evidence of the progress of melting of large ice sheets during the last deglaciation but although the correlation between ice and ocean volumes is incontrovertible the causal link is commonly obscured local effects including tectonics isostatic and hydroisostatic responses and equatorial ocean syphoning impose additional signals that hide the true picture a detailed regional study of the western indian ocean based on the analysis of drill cores carried out through modern reefs in combination with observations and sampling of reef foreslopes and investigations of outcrops provides a comprehensive data base sites from a range of tectonic settings include the microcontinental margins of madagascar the granitic seychelles and the isolated volcanic islands of reunion mauritius and the comoros in which the effects of subsidence can be shown to be small these cover a range of latitudes and comparisons with adjacent sites on continental margins allow the construction of sea level curves that closely reflect the eustatic response and disengage this from the effects of other mechanisms the mayotte foreslope in the comoro islands provides the first coral reef record of sea level change during the early deglaciation in the indian ocean 110 115 m below present sea level between 18 000 and 17 000 yr bp two distinctive reef terraces at 90 and 60 m water depth are dated at 13 600 yr bp and partly attributed to the younger dryas period 12 700 11 600 cal yr bp reef drowning at around 13 500 yr bp may correspond to meltwater pulse 1a and although there were surges in the rate of sea level rise most notably between 11 950 and 11 350 yr bp there is little evidence to support a well defined meltwater pulse 1b reconstructed holocene sea level curves are in good agreement and reflect a rapid sea level rise of about 6 mm yr 1 between 10 000 and 7500 yr bp followed by a clear inflection around 7500 yr bp when the rate fell to 1 1 mm yr 1 modern reefs started to grow 8000 9000 years ago in the post glacial period the rate of sea level rise was 1 1 5 mm yr 1 before stabilization at its present level 3000 2500 years ago curves for the 10 000 6000 yr 1 bp interval correspond closely with those predicted by theoretical models but lie below these in the subsequent period in particular and with the exception of the margins of the madagascar microcontinent influenced by hydroisostatic processes they do not reflect predicted higher sea level stands during the late holocene

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

detrital sediment fluxes from continents to oceans abstract detrital sediment is carried from land to the sea by three agents rivers glaciers and winds the shoreline is an arbitrary boundary within the detrital sediment transport system which extends from a site of origin across areas of temporary storage to a site of long term deposition the most important of the agents moving sediment across the land is river transport estimated to be in the order of 20 10 12 kg of sediment annually at present analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers the competence of rivers to transport sediment is governed by the volume flow gradient and the sediment load itself today most large rivers are fed by snowmelt in highland areas runoff from rainfall in the drainage basin and groundwater inflow along the river course water is lost to evaporation and groundwater infiltration river courses can often be divided into two segments a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place and an aggradational section where the gradient is sharply reduced through meandering and where large scale temporary sediment storage forms a flood plain lakes trap sediment inland and prevent its transport to the sea today many high and mid latitude rivers are interrupted by lakes of glacial origin there are also some large areas of internal drainage that deliver no sediment to the sea the load carried by rivers has been markedly altered by human activity and may have doubled over the past few thousand years only to be reduced in the past century by the widespread construction of dams the ancient use of fire in hunting and its subsequent use in clearing land has increased erosion extensive deforestation and cultivation processes have also increased the sediment supply dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea the resulting lower sediment supply from rivers is at least in part compensated by increased coastal erosion glacial erosion is difficult to estimate there is an ongoing controversy whether ice sheets are effective erosive agents or not estimates of the present global flux of glacial detritus range from 0 8 50 10 12 kg annually with the lower value most probable the dust flux is in the order of 0 5 to 0 9 10 12 kg annually but may vary greatly with time

early diagenetic processes fluxes and reaction rates in sediments of the south atlantic abstract porewaters recovered from sediment cores gravity corers box corers and multicorers from various subrogions of the south atlantic amazon river mouth equatorial upwelling congo river mouth benguela coastal upwelling area and angola basin were investigated geochemically objectives included determination of eh ph oxygen nitrate sulfate alkalinity phosphate ammonium fluoride sulfide cal mg sr fe mn and si in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes porewater profiles from the eastern upwelling areas of the south atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation whereas organic matter degradation by sulfate reduction is restricted to the near surface sediments further a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m far below the zone of bioturbation or bioirrigation because other sedimentological reasons seem to fail as explanations an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed porewater profiles from the sediments of the amazon fan area are mainly influenced by reactions with fe iii phases the remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments in contrast to the upwelling sediments the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5 3 m probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals a comparison of organic matter degradation rates from the different areas of the south atlantic show the expected relationship to primary productivity oxygen is the dominating oxidant whereas organic matter degradation by nitrate mn iv and probably also fe iii reduction is several orders of magnitude lower sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic rich sediments but may also include methane oxidation reoxidation of ammonium and mn2 by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone but may contribute to the nitrate reduction in suboxic layers

simultaneous formation of iron rich layers at different redox boundaries in sediments of the amazon deep sea fan high resolution examinations of the solid phase have been performed on a gravity core that shows the typical amazon fan stratigraphy of the last 20 000 years the sedimentary record is characterized by two pronounced enrichments of solid phase fe the first fe peak from 0 34 to 0 37 m sediment depth consists of an indurated fe oxyhydroxide crust located close to the pleistocene holocene boundary this peak represents a widespread nonsteady state diagenetic feature on the fan the second fe peak is a pronounced enrichment of authigenic fe sulfides located between 6 15 and 6 45 m in the sediment the formation of this maximum is attributed to sulfate reduction that is mainly driven by anaerobic methane oxidation based on so42 pore water profiles and calculations of the time of formation of the fe sulfide enrichment we conclude that both fe rich layers formed more or less simultaneously the condition that caused both fe enrichments is a period of nonsteady state diagenesis induced by a pronounced decrease in sedimentation and organic carbon accumulation rates between the pleistocene and the holocene during this depositional change the redox boundaries and reaction fronts were fixed at particular sediment levels for a prolonged time thus producing higher concentrations of the authigenic minerals than possible under steady state diagenetic conditions

rock magnetic signatures in diagenetically altered sediments from the niger deep sea fan 1 diagenesis has extensively affected the magnetic mineral inventory of organic rich late quaternary sediments in the niger deep sea fan changes in concentration grain size and coercivity document modifications of the primary magnetic mineral assemblages at two horizons the first front the modern iron redox boundary is characterized by a drastic decline in magnetic mineral content coarsening of the grain size spectrum and reduction in coercivity beneath a second front the transition from the suboxic to the sulfidic anoxic domain a further but less pronounced decrease in concentration and bulk grain size occurs finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment low and high temperature experiments were performed on bulk sediments and on extracts which have also been examined by x ray diffraction thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components their broad range of titanium contents reflects the volcanogenic traits of the niger river drainage areas diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition low titanium compounds are less resistant to diagenetic dissolution intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments at the fe redox boundary precipitation of authigenic possibly bacterial magnetite is documented the presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain grain size specific data argue for a gradual development of a superparamagnetic and single domain iron sulfide phase in this milieu most likely greigite

distribution of various extracted phosphorus compounds in the interbedded turbiditic pelagic sediments of the madeira abyssal plain eastern north atlantic abstract a sequential extraction scheme was used to differentiate different p fractions in the interbedded turbiditic pelagic sediments of the madeira abyssal plain these fractions were carbonate associated exchangeable phosphate pex iron and aluminium associated phosphate poh calcium associated phosphate apatite phcl organic phosphorus porg and remaining phosphorus pr the pex forms a minor fraction of the total sedimentary phosphorus ptot whereas for most samples phcl represents near 80 of the ptot the amount of p in most fractions appears to be determined by the composition and provenance of the various turbidites the relatively high pex and the moderately high apatite p phcl for the sediment of turbidite e organicrich turbidite provenance group may point to a source area that is possibly upwelling related on the other hand the high phcl for the sediments of the turbidites b g n o p corresponds nicely to their volcanic and near coastal provenance area in contrast to this major impact of inherited p only a small contribution comes from p that is associated to post depositional re allocation of fe poh this results in a poh concentration that is slightly higher in the upper post depositionally oxidized layer of a turbidite than it is in the lower still un oxidized layer of the same turbidite in addition this postdepositional oxidation of the upper layer of a turbidite may also result in enhanced fe and p levels in the overlying pelagic interval the c p molar ratio for organic matter in these sediments is much higher than that reported for more recent sediments and for average marine plankton 1200 versus 500 and 106 respectively which may be caused by postdepositional differential decomposition of organic matter

authigenic p formation and reactive p burial in sediments of the nazare canyon on the iberian margin ne atlantic profiles of different forms of sedimentary phosphorus were measured at four sites at the iberian margin ne atlantic which were chosen on the basis of differences in depositional environment a shelf site 113 m a mid slope station 1387 m and two stations at the head 396 m and base 3097 m of the nazarecanyon the sediment was sequentially extracted for fe bound p carbonate fluorapatite cfa biogenic p caco3 bound p and detrital ca p and non sequentially for total p and inorganic p where the difference between total and inorganic p was assumed to be organic p measurements of organic carbon and nitrogen citrate dithionite bicarbonate and citrate ascorbate bicarbonate extractable fe carbon oxidation rates fe reduction rates phosphate effluxes and sedimentation rates were used to quantify the cycling and burial of phosphorus at each site cfa formation was observed only in sediments of the nazarecanyon where enhanced rates of organic matter decomposition and fe reduction provided the necessary conditions the concentrations of reactive p were similar at all sites but the sediment accumulation rates differed greatly the high bulk accumulation rate at the base of the canyon results in a reactive p burial rate exceeding those estimated for continental margins including the iberian margin by an order of magnitude the estimated p burial efficiency is smallest at the slope station 3 24 reasonably high 63 86 at the shelf and head of the canyon stations and extremely high s 97 at the base of the canyon we propose that local depositories such as those at the base of canyons may be key sites for reactive p burial 2002 elsevier science b all rights reserved

a comparison of benthic nutrient fluxes from deep sea sediments off namibia and argentina abstract benthic fluxes of nitrate phosphate silicate and oxygen estimated c org input to the sea floor and c org content of surface sediments at various stations in the argentine basin and the cape basin south atlantic were used to characterize the benthic mineralization pattern in these areas diffusive benthic fluxes across the sediment water interface reveal very intense subsurface biodegradation of organic matter and dissolution of biogenic silica across the namibian continental margin diffusive benthic fluxes generally decrease with increasing water depth but a considerable site to site variation exists along the 1300 m isobath mineralization patterns indicate that the input flux of organic matter seems to be significantly controlled by vertical transport through the water column a nearly reverse pattern can be observed in the western argentine basin intense nutrient release is observed even at locations on the deeper slope for this whole area a dependence of nutrient release and water depth is not observed an exception is the rio de la plata region where stations between 1800 and 3500 m water depth reveal the highest nitrate and phosphate fluxes decreasing towards the shelf and into the deeper basin silicate fluxes off the rio de la plata increase continuously with water depth below 4000 m water depth this is probably due to the input of antarctic diatoms by the antarctic bottom water mineralization processes on the continental margin of the argentine basin do not correlate with the estimated organic matter input from surface waters in accordance with previous sedimentological studies we conclude that intense sediment mass flows from the shelf are the main source for the input of reactive organic matter to the deeper parts of the continental margin

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

reduced magnetization produced by increased methane flux at a gas hydrate vent abstract magnetic susceptibility measurements on near surface sediment cores from the north cascadia accretionary sedimentary prism show that seismic blanking or wipe out zones in the upper few hundred metres of sediments are associated with a prominent low magnetic susceptibility signature seismic blanking and low magnetization are both attributed to high upward methane flux within a vent zone as evidenced by the presence of massive gas hydrate within the cores sedimentological analysis of these cores also reveals the presence of authigenic pyrite within the areas of magnetic susceptibility lows this phenomenon is suspected to be produced by the reducing environment associated with the high upward methane flux and increased bacterial activity within the topmost sediments resulting in diagenesis of highly magnetic detrital minerals such as magnetite into nearly non magnetic pyrite these low magnetic susceptibility zones may produce magnetic anomalies with a magnitude of 10 35 nt near the seafloor such anomalies might be detected using high resolution near bottom magnetometers to provide a means of mapping zones of methane venting

diagenetic alteration of magnetic signals by anaerobic oxidation of methane related to a change in sedimentation rate geochemical and rock magnetic investigations of sediments from three sites on the continental margin off argentina and uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane aom the western argentine basin represents a suitable sedimentary environment to study nonsteady state processes because it is characterized by highly dynamic depositional conditions mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals as a typical feature of these deposits distinct minima in magnetic susceptibility are observed pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate methane transition smt where hs is generated by the process of aom the released hs reacts with the abundant iron oxyhydr oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate sr which occurred during the pleistocene holocene transition we assume that the strong decrease in mean sr encountered during this glacial interglacial transition induced a fixation of the smt at a specific depth the stagnation has obviously enhanced diagenetic dissolution of iron oxyhydr oxides within a distinct sediment interval this assumption was further substantiated by numerical modeling in which the mean sr was decreased from 100 cm kyr 1 during glacial times to 5 cm kyr 1 in the holocene and the methane flux from below was fixed to a constant value to obtain the observed geochemical and magnetic patterns the smt must remain at a fixed position for 9000 yrs this calculated value closely correlates to the timing of the pleistocene holocene transition the results of the model show additionally that a constant high mean sr would cause a concave up profile of pore water sulfate under steady state conditions copyright 2005 elsevier ltd

orbital forcing of freshwater input in the zaire fan area clay mineral evidence from the last 200 kyr abstract late quaternary sediments recovered in a core from the area of the zaire fan central africa were analyzed for clay mineral composition in order to reconstruct fluctuations in the sediment input and freshwater discharge of the zaire river clay mineral assemblages are dominated by kaolinite and smectite which both originate mainly from the zaire river and contain only minor contributions of eolian dust smectite crystallinity and chemical character of illites fe mg or al rich are used to track sediment input from the zaire river and assess fluctuations in the freshwater discharge both parameters record a high latitude forcing of river runoff at 100 ka periodicities reflecting glacial aridity and increased runoff during interglacials 1 5 and 7 this signal is also observed in kaolinite smectite ratios which represent the extension and intensity of the freshwater plume of the zaire river clay mineral proxies reveal that river discharge and associated sediment input fluctuated in tune with precessional cycles of african monsoon intensity increased eolian input of kaolinite rich dust with intensified northeast trades during glacials flattens the precessional signal in kaolinite smectite ratios

formation of carbonate pipes in the northern okinawa trough linked to strong sulfate exhaustion and iron supply abstract the microbial anaerobic oxidation of methane aom a key biogeochemical process that consumes substantial amounts of methane produced in seafloor sediments can lead to the formation of carbonate deposits at or beneath the sea floor although fe oxide driven aom has been identified in cold seep sediments the exact mode by which it may influence the formation of carbonate deposits remains poorly understood here we characterize the morphology petrology and geochemistry of a methane derived fe rich carbonate pipe in the northern okinawa trough ot we detect abundant authigenic pyrites as well as widespread trace fe within microbial mat like carbonate veins in the pipe the in situ 34 s values of these pyrites range from 3 9 to 31 6 vcdt suggesting a strong consumption of seawater sulfate by sulfate driven aom at the bottom of sulfate reduction zone the positive 56 fe values of pyrite and notable enrichment of fe in the ot pipe concurrently indicate that the pyrites are primarily derived from fe oxides in deep sediments we propose that the fe rich carbonate pipe formed at the bottom of sulfate reduction zone below which fe driven aom rather than fe oxide reduction coupled to organic matter degradation might be responsible for the abundantly available fe 2 in the fluids from which pyrites precipitated the fe rich carbonate pipe described in this study probably represents the first fossil example of carbonate deposits linked to fe driven aom because fe rich carbonate deposits have also been found at other cold seeps worldwide we infer that similar processes may play an essential role in biogeochemical cycling of sub seafloor methane and fe at continental margins

deep sulfate reduction completely mediated by anaerobic methane oxidation in sediments of the upwelling area off namibia abstract porewater concentrations of sulfate methane and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off namibia which were recovered from the continental slope at water depths of 1300 and 2000 m at all four stations a distinct sulfate methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100 of deep sulfate reduction within the sulfate methane transition zone and consumes the total net diffusive sulfate flux a significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could therefore be excluded we state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction

iron oxide formation in the active oxidation front above sapropel s1 in the eastern mediterranean sea as derived from low temperature magnetism low temperature magnetic properties of eastern mediterranean sediments from a box core have been investigated this box core contains the present day oxic suboxic boundary that is situated at the top of the relic of the youngest sapropel sl the upper half of sapropel s1 has been oxidized and fe oxides have precipitated in the oxidized sapropel zero field cooling zfc and field cooling fc saturation remanent magnetization m r induced in a field of 2 5 t at 20 k after cooling from 300 k with or without the 2 5 t field was measured during warming to 300 k an m r imparted at room temperature rtsirm was cycled to 20 k as well the difference between zfc and fc curves around the verwey transition as estimated by the parameter f c z f c suggests the presence of magnetosomes in the oxidized part of the sapropel we propose a new parameter d m 1 5 0 3 0 0 k r f c m 1 5 0 1 3 0 0 k r z f c m 3 0 0 k r f c where m 1 5 0 3 0 0 k r z f c represents the average remanence between 150 and 300 k to estimate the difference between zfc and fc curves above the verwey transition this is interpreted as being indicative of the number of defects and the extent of partial maghemitization at the oxic suboxic boundary where fe oxides actively precipitate the magnetite appears to be least maghemitized as inferred from low values for d further upward in the oxidized sapropel in older precipitates the magnetite is slightly more maghemitized just below the sapropel maghemitization is most pronounced in addition the initial slope of zfc and fc curves indicates that small grains or grain coatings that are superparamagnetic at room temperature sp are enriched at the oxic suboxic boundary higher in the oxidized sapropel the relative contribution of sp grains decreases presumably because they age to larger grains

a probabilistic method of assessing carbon accumulation rate at imnavait creek peatland arctic long term ecological research station alaska arctic peatlands are an important part of the global carbon cycle accumulating atmospheric carbon as organic matter since the late glacial current methods for understanding the changing efficiency of the peatland carbon sink rely on peatlands with an undisturbed stratigraphy here we present a method of estimating primary carbon accumulation rate from a site where permafrost processes have either vertically or horizontally translocated nearby carbon rich sediment out of stratigraphic order briefly our new algorithm estimates the probability of the age of deposition of a random increment of sediment in the core the method assumes that if sediment age is measured at even depth increments dates are more likely to occur during intervals of higher accumulation rate and vice versa multiplying estimated sedimentation rate by measured carbon density yields carbon accumulation rate we perform this analysis at the imnavait creek peatland near the arctic long term ecological research network site at toolik lake alaska using classical radiocarbon age modeling we find unreasonably high rates of carbon accumulation at various holocene intervals with our new method we find accumulation rate changes that are in improved agreement within the context of other sites throughout alaska and the rest of the circum arctic region

the low temperature geochemical cycle of iron from continental fluxes to marine sediment deposition suspended sediments from 34 major rivers geographically wide spread and 36 glacial meltwater streams have been examined for their variations in different operationally defined iron fractions fehr iron oxides soluble in dithionite fepr iron soluble in boiling hcl but not in dithionite and feu total iron less that soluble in boiling hcl river particulates show a close association between fehr and total iron fet reflecting the effects of chemical weathering which derive oxide iron from and retain it in close association with total iron consistent with this continental scale average fehr fet ratios vary with runoff ratios average river runoff per unit area average precipitation per unit area by contrast the diminished effects of chemical weathering produce no recognizable association of fehr with fet in glacial particulates and instead both fepr and feu are closely correlated with fet reflecting essentially pristine mineralogy a comparison of the globally averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in fehr fepr and fet but enriched in feu the river and glacial particulate data are combined with estimates of authigenic hydrothermal atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for fehr f e pr f e u and fet this budget suggests that the differences between riverine particulates and marine sediments can be explained by i preferentially removing fehr from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains salt marshes and estuaries and ii mixing the resulting riverine particulates with fehr depleted glacial particulates preliminary measurements of inner shore sediments are consistent with i above phanerozoic and modern normal marine sediments have similar iron speciation characteristics which implies the existence of a long term steady state for the iron cycle this steady state could be maintained by a glacioeustatic feedback where fehr enriched riverine particulates are either more effectively trapped when sealevel is high small ice masses diminished glacial erosion or are mixed with greater masses of fehr depleted glacial particulates when sealevel is low large ice masses enhanced glacial erosion further important controls on the steady state for fehr operate through the formation of euxinic sediments and ironstones which also provide sealevel dependent sinks for fehr enriched sediment

alteration of magnetic mineralogy at the sulfate methane transition analysis of sediments from the argentine continental slope on the argentine continental slope off the rio de la plata estuary the sulfate methane transition smt has been encountered at shallow depths of a few meters below the seafloor at around this horizon where sulfate diffusing downward from the bottom water is met and reduced by methane rising from deeper in the sediment column intense alteration affects the detrital magnetic mineral assemblage less than 10 of the dominant primary low coercivity ferrimagnetic titano magnetite remains after alteration in the upper part of the suboxic environment underlying the iron redox boundary which is located at a depth of 0 1 m approximately 60 of the finer grained detrital fraction is already dissolved while the high coercivity minerals are relatively unaffected in the suboxic environment large portions 40 are diagenetically dissolved in the sulfidic smt zone nevertheless the characteristics of the magnetic residue are entirely controlled by a high coercivity mineral assemblage unlike common observations that diagenetic alteration produces coarser magnetic grain sizes in suboxic milieus a distinct overall fining is found in the sulfidic zone different factors should contribute to this effect scanning electron microscope analysis combined with x ray microanalysis identified fine grained titano magnetite preserved as inclusions in silicates and between high ti titanohematite lamellae and possibly of prime importance a comprehensive fragmentation of larger grains in the course of maghemitization the only secondary iron sulfide mineral detected is pyrite which is present as clusters of euhedral crystals or directly replaces titano magnetite the thermomagnetic measurements did not provide evidence for the presence of ferrimagnetic sulfides such as greigite different from other studies reporting a marked magnetic enhancement at around the smt due to the precipitation and preservation of such metastable ferrimagnetic sulfides a complete pyritization process will cause a distinct magnetic depletion like in the present case 2005 elsevier b v all rights reserved

solid sediment load history of the zambezi delta abstract we calculate the solid sediment load history of the zambezi river for the last 120 ma i e middle cretaceous recent times our starting point is a detailed grid of two dimensional seismic reflection profiles calibrated by well log information after conversion from two way travel time to depth we made simple assumptions about the compaction history in order to construct isochore maps of solid sediment load and yield as a function of geological time our results show that solid sediment load has varied by 1 order of magnitude over the last 120 ma there have been three periods of elevated flux the first period occurred in late cretaceous times 90 65 ma and was synchronous with the rapid denudation of southern africa recognised by for example apatite fission track modelling the second period occurred in oligocene times 34 24 ma during proposed rapid regional uplift of southern africa the youngest phase of elevated flux started in late miocene times 10 ma and has continued to increase rapidly through to the present day a large proportion of this increase can be attributed to a doubling of the size of the zambezi catchment during the pliocene at other times we suggest that load variations can be used to bound the uplift history of the region which encompasses the catchment finally changes in the shape of the zambezi delta through time suggest that the mozambique current which sweeps through the mozambique channel from northeast to southwest initiated at the start of the miocene

first occurrence and stable isotope composition of authigenic γ mns in the central gotland deep baltic sea the occurrence of authigenic mns in organic matter rich sediments of the central gotland deep is reported for the first time the hexagonal manganese sulfide was found in a thin sediment layer with an approximate age of 1130 years and has an isotopic composition of 34s 13 vs v cdt this indicates that the bonded sulfur is derived from the anaerobic microbial reduction of pore water sulfate sem analysis reveals that mns occurs as idiomorphic crystals of about 200 m length and 150 m width much larger than the diameters of co existing calcian rhodochrosite globules 6 m it is suggested that formation of mns took place in a diagenetic environment which was locally free from reactive iron

authigenic apatite formation and burial in sediments from non upwelling continental margin environments abstract evidence for precipitation of authigenic carbonate fluorapatite cfa in long island sound and mississippi delta sediments suggests that formation of cfa is not restricted to environments of active coastal upwelling we present porewater data suggestive of cfa formation in both these areas application of a sequential leaching procedure designed specifically to separate authigenic carbonate fluorapatite from other phosphorus containing phases including detrital apatite of igneous or metamorphic origin provides strong supporting evidence for authigenic apatite formation in these sediments the size of the authigenic apatite reservoir increases with depth indicating continued formation of cfa during early diagenesis this depth increase is mirrored by a decrease in solid phase organic p at both sites suggesting that cfa is forming at the expense of organic p mass balance considerations application of diagenetic models to interstitial water nutrient data and the saturation state of the interstitial water are consistent with this interpretation diagenetic redistribution of phosphorus among the different solid phase reservoirs is observed at both sites and results in near perfect retention of p by these sediments over the depth intervals sampled formation of cfa in continental margins which do not conform to the classically defined regions of phosphorite formation renders cfa a quantitatively more important sink than has previously been recognized including this reservoir as a newly identified sink for reactive p in the ocean the residence time of p in the modern ocean must be revised downward the implication for ancient oceans of cfa formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive p from the oceans than of localized concentration of cfa in phosphatic sediments by secondary physical processes

diagenetic changes of magnetic and geochemical signals by anaerobic methane oxidation in sediments of the zambezi deep sea fan sw indian ocean abstract the zambezi deep sea fan the largest of its kind along the east african continental margin is poorly studied to date despite its potential to record marine and terrestrial climate signals in the southwest indian ocean therefore gravity core geob 9309 1 retrieved from 1219 m water depth was investigated for various geophysical magnetic susceptibility porosity colour reflectance and geochemical pore water and sediment geochemistry fe and p speciation properties onboard and onshore data documented a sulphate methane transition smt zone at 450 530 cm sediment depth where the simultaneous consumption of pore water sulphate and methane liberates hydrogen sulphide and bi carbonate into the pore space this leads to characteristic changes in the sediment and pore water chemistry as the reduction of primary fe oxyhydr oxides the precipitation of fe sulphides and the mobilization of fe oxyhydr oxide bound p these chemical processes also lead to a marked decrease in magnetic susceptibility below the smt we find a reduction of porosity possibly due to pore space cementation by authigenic minerals formation of the observed geochemical magnetic and mineralogical patterns requires a fixation of the smt at this distinct sediment depth for a considerable time which we calculated to be 10 000 years assuming steady state conditions following a period of rapid upward migration towards this interval we postulate that the worldwide sea level rise at the last glacial interglacial transition 10 000 years b p most probably caused the fixation of the smt at its present position through drastically reduced sediment delivery to the deep sea fan in addition we report an internal redistribution of p occurring around the smt closely linked to the de coupling of sedimentary fe and p and leaving a characteristic pattern in the solid p record by phosphate re adsorption onto fe oxyhydr oxides above and formation of authigenic p minerals e g vivianite below the smt deep sea fan deposits may potentially act as long term sinks for p

francolite authigenesis in california continental slope sediments and its implications for the marine p cycle abstract authigenesis of francolite carbonate fluorapatite in continental slope sediments may represent the most important oceanic sink for reactive p we infer from sedimentary p speciation and pore fluid fluoride concentrations that francolite currently forms in holocene sediments on the california continental slope authigenic p accumulation rates range from 0 006 to 0 089 mol cm 2 yr and dissolved fluoride uptake rates range from 0 004 to 0 039 mol cm 2 yr these two independent parameters correlate positively with each other among all sites r 2 0 77 and the slope of their linear relationship gives a p f molar ratio of 2 4 a typical value for authigenic francolite our results imply that active francolite authigenesis sequesters nearly 30 of the remobilized organic and fe bound p at shallow burial depths in california continental slope sediments assuming that similar processes occur globally we estimate that francolite authigenesis in near surface continental slope sediments may account for a burial flux of 1 4 1 0 10 10 mol p yr or 8 to 48 of the global input flux of reactive p to the ocean

phosphogenesis and active phosphorite formation in sediments from the arabian sea oxygen minimum zone in this study porewater chemistry solid phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the arabian sea three parameters namely a decrease of the dissolved phosphate and fluoride concentrations with depth saturation with respect to carbonate fluorapatite and the presence of a solid phase ca phosphate mineral all indicate that phosphogenesis is currently taking place at all three sites authigenic apatite precipitation rates vary between 0 076 and 1 04 molp cm 2 yr 1 and are of the same order of magnitude as reported for other high productivity areas precipitation of an intermediate precursor precedes francolite formation in the continental slope sediments on the karachi margin results of a diagenetic p model indicate that phosphogenesis is induced by high rates of organic matter degradation dissolution of fish debris is likely to provide a substantial additional source of phosphate redox iron cycling does not influence phosphogenesis in these environments model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis the highest rate of francolite formation was observed in a boxcore taken on the oman margin where it contributes to the formation of a holocene phosphorite deposit this observation contrasts with previous reports of only old phosphorites in this area phosphorites are presently forming on the oman margin as a result of a deposition of older reworked material from the continental shelf which has undergone an earlier phase of phosphogenesis b a high input of reactive p fish debris and degradable organic matter c a relatively low sediment accumulation rate and d the absence of winnowing on this location holocene phosphorite deposits may be less common on the oman margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope which limit the area where phosphorite formation may occur

a novel chemical method to quantify fish debris in marine sediments burial of fish bones which consist primarily of hydroxyapatite has been recognized as a mechanism to remove reactive phosphorus from the oceans in this study a new method is presented which differentiates p associated with biogenous apatite pfish from other p fractions the method consisting of a sequential chemical extraction with 2 m nh4cl has been successfully tested on standard materials it enables us for the first time to quantify fish debris in sediment records and to assess their importance for the marine phosphorus cycle the nh4cl extraction has been applied to sediment samples from the arabian sea preservation of fish debris is significantly higher in sediments located above 1 200 m water depth than it is for deeper sediments the distribution of pfish contents in surface sediments is predominantly governed by the extent of fish debris regeneration which is related to differences in water depth and sedimentation rates in addition a good correlation between high sedimentary pfish contents and low oxygen bottom water concentrations suggests that the presence of the intense oxygen minimum zone may account for the enhanced preservation of fish debris in continental slope sediments

phosphorus accumulation in marine sediments and the oceanic phosphorus cycle ideas about key factors in the oceanic mass balance of dissolved reactive phosphate have changed substantially i present an integrated overview of these here with an emphasis on evaluating the burial sinks for p and defining areas needing further research the major source of reactive p to the ocean is river input reactive p is delivered to the oceanic sediment water interface primarily in particulate organic matter p scavenged by hydrothermal iron rich oxyhydroxide particles with uptake in proportion to deep water phosphate concentrations represents a substantially smaller flux to the sediment water interface diagenetic transformations are important influences on the form of reactive p burial in marine sediments p burial occurs with organic carbon burial and as p associated with iron rich oxyhydroxide particles and coatings formation of authigenic p rich phases presumably apatite at the expense of organic p and oxide associated p is significant in open ocean marine sediments the authigenic p sink may represent a substantially larger portion of the sedimentary burial than indicated by previous estimates focused on p burial in organic rich continental margin sediments

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

natural controls of fluvial denudation rates in major world drainage basins we present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5 105 km2 in area these estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible chemical denudation rates have been calculated by deducting the nondenudational component of solute load mechanical denudation rates range from 1 mm kyr 1 for the st lawrence and dnepr basins to 670 mm kyr 1 for the brahmaputra basin chemical denudation rates vary from 1 mm kyr 1 kolyma niger nile and rio grande basins to 27 mm kyr 1 chiang jiang basin the kolyma basin has the lowest 4 mm kyr 1 and the brahmaputra basin the highest overall rate of denudation 688 mm kyr 1 relationships between denudation rates and a range of morphometric hydrologic and climatic variables are investigated through correlation and regression analysis morphometric variables such as mean local relief are accurately calculated for large basins for the first time by using the national geophysical data center 10 minute topographic database variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates with more than 60 of the variance in total denudation being accounted for by basin relief ratio and runoff basin area runoff variability and mean temperature however are only weakly associated with rates of denudation although direct comparisons cannot be made it appears that rates of basin denudation derived from present day mass flux estimates are not overall significantly different from estimates of long term rates based on sediment volume and thermochronologic data it therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes

nutrient leakage from the north pacific to the bering sea iodp site u1341 following the onset of northern hemispheric glaciation 1 intergrated ocean drilling program expedition 323 recovered a sediment record covering the last 4 3 ma from the bering sea integrated ocean drilling program site u1341 bowers ridge 2177 m water depth to resolve pliocene pleistocene paleoenvironmental changes in this marginal basin 190 sediment samples were analyzed for their bulk element composition aluminium contents in bowers ridge sediments are variable but overall higher toward younger sediments probably related to the intensification of the northern hemispheric glaciation and increasing sea ice transport in the bering sea the gradual increase of terrigenous input is mirrored by decreasing sio2 and excess si sixs contents but the overall si enrichment of the deposits reflects continuous opal deposition since the pliocene at bowers ridge unlike in the north pacific the sixs record at site u1341 does not support a dramatic decrease in opal export following the onset of the northern hemispheric glaciation around 2 7 ma but sio2xs have higher accumulation rates up to 8 g cm2 ka between 2 6 and 1 8 ma bp during this period the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas we here discuss how the onset of north pacific stratification at 2 7 ma bp may have caused leakage of nutrient rich deep intermediate north pacific water into the bering sea via the deep kamtchatka strait leading to increased opal deposition and likely reactive organic matter export at bowers ridge as a result magnetic and geochemical records were overprinted by intensified diagenesis significantly affecting their applications as paleoceanographic proxies

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

a comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off uruguay sw atlantic pore water and solid phase from surface sediments of the continental slope off uruguay and from the argentine basin southwestern atlantic were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese solid phase iron speciation was determined by extractions as well as by mossbauer spectroscopy both methods showed good agreement 6 deviation for total fe speciation the proportion of easy reducible iron oxyhydroxide relative to total fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization the product of iron reoxidation is fe oxyhydroxide which made up less than 5 of total fe in addition to this fraction a proportion of smectite bound iron was found to be redox reactive this fraction made up to 10 of total fe in sediments of the argentine basin and was quantitatively extracted by 1 n hcl the redox reactive fe ii fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs in the case of the slope sediments we found concurrent iron and manganese release to pore water it is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing mn iv reduction by abiotic reaction with released fe2 the argentine basin sediment showed a significant manganese solid phase enrichment above the denitrification depth despite the absence of a distinct pore water gradient of mn this implies a recent termination of manganese mobilization and thus a non steady state situation with respect to sedimentation or to organic carbon burial rate

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

phosphate removal by oceanic hydrothermal processes an update of the phosphorus budget in the oceans abstract we present a compilation of dissolved phosphate and solid phase p data from the oceanic crust to evaluate the effects of hydrothermal processes on the oceanic budget of p concentrations of phosphate in fluids that emanate from ridge axis hydrothermal systems are less than that in bottom seawater the extent of removal in these fluids is at least 30 and in some hydrothermal systems dissolved phosphate is removed completely from the circulating fluid evidence for the removal of phosphate in each of six ridge flank hydrothermal systems is based on systematic variations in porewater profiles of phosphate and speeds of porewater flow the extent of removal is 80 in these ridge flank systems these removal processes are recorded in the basaltic crust as an increase in p concentration that coincides with an increase in extent of alteration and content of ferric iron phosphate also is removed in hydrothermal plumes by coprecipitation with fe oxyhydroxide particles which eventually deposit on the seafloor each of these hydrothermal processes results in a flux of p into the oceanic crust bottom seawater flow through ridge axis hydrothermal systems removes at most 0 4 of the preindustrial dissolved riverine flux of p while ridge flanks remove at least 5 but less than 50 of the dissolved riverine flux consistent with p data from deep sea drilling project dsdp sites 417 and 418 removal of phosphate by coprecipitation with fe rich particles in hydrothermal plumes along ridge axes accounts for 18 33 of the dissolved riverine flux thus hydrothermal systems remove about 50 of the preindustrial dissolved riverine flux of phosphate we have included these new estimates in a revised budget for oceanic p

magnetic mineral diagenesis in suboxic sediments at bettis site w n ne pacific ocean the effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from bettis site w n in the ne pacific ocean as part of the oxidative decomposition of organic matter ultrafine grained authigenic magnetites are produced immediately above the iron reduction zone this zone is commonly observed in deep sea sediments as a prominent color change from brown to greenish grey the magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties the magnetites are composed of relatively pure fe3o4 and fall within a restricted size range of 0 06 0 12 m which is nominally single domain some of these finest grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction leaving a relatively coarser population magnetite dissolution appears to continue with depth according to a first order surface area reaction process when at steady state iron reduction is independent of sulfide formation in these sediments because solid sulfur profiles show no increase at depth and most of the sulfur occurs as barite despite a well constrained carbonate stratigraphy comparisons of paleomagnetic directions between nearby cores show little agreement suggesting that the directions are composed of a complex mixture of remanences acquired at different depths subsurface magnetic peaks may represent paleoredox markers preserved by non steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates

sedimentation on the inner shelf of the east china sea magnetic properties diagenesis and paleoclimate implications abstract high resolution magnetic analysis was performed on the 19 22 m core md06 3040 27 43 3663 n 121 46 8822 e 46 m water depth from the east china sea ecs inner continental shelf which was recovered during the images xiv md155 marco polo 2 cruise of the r v marion dufresne ipev magnetic grains are mainly transported to the inner shelf from erosion on mainland china delivered by the yangtze river nine 14 c ams dates indicate that the core spans about the last 10 4 ka with an average sedimentation rate of nearly 2 mm yr the primary magnetic mineral assemblage of the uppermost sediments contains magnetite with some contributions from hematite and greigite two main stepwise changes are observed in the magnetic mineral content due to post depositional reductive diagenesis the first occurs from the top of the core to 3 5 m and is characterized by reduction of both the magnetite and hematite contents the second occurs at 8 45 9 m and is characterized by a loss of fine grained ferrimagnetic material variations in the magnetic mineral content of this core are related to geochemical and redox environment changes in the ecs through the oxic suboxic sulfidic methanic zones from top to bottom despite significant reduction of a large amount of the ferrimagnetic mineral content due to diagenesis intervals characterized by lower magnetic mineral concentrations and higher coercivities are still identified at 0 45 0 6 1 5 1 7 2 9 3 1 4 4 4 7 5 3 5 7 5 9 6 1 and 8 2 8 4 ka these intervals are in phase within dating errors these intervals are in phase with weak summer monsoon events which indicate that a relationship between magnetic results and paleoclimatic events is partially preserved

cycling of trace metals and rare earth elements ree in acid sulfate soils in the plain of reeds vietnam abstract trace metals and rare earth elements ree are subject to intensive chemical cycling in acid sulfate soils the oxidation of pyrite bearing deposits and the subsequent acidification mobilizes a variety of trace metals and ree in this study solid phase and pore water data of three cores from the plain of reeds in the mekong river delta vietnam were analyzed distinct authigenic enrichments of trace metals and ree were observed in the solid phase of these cores thermodynamic modeling was employed to elucidate the controls on trace metal and ree mobility and the governing factors for the formation of these authigenic enrichments the formation of ree enrichments in these settings is clearly controlled by ph and the availability of potent sorbents under suitable conditions trace metals such as e g zinc nickel and cobalt are likely bound as trace metal monosulfides in the enrichment zones the solid phase and pore water data and the results of a thermodynamic model suggest a possible sequential precipitation of trace metal sulfides however the incorporation of trace metals into iron sulfides cannot be ruled out in the case of lead and arsenic the formation of pb as jarosite proved to be a further major sink these insights provide a better understanding of the controls and governing factors for trace metal and ree mobility in acid sulfate soils and will therefore help to assess impacts on soil and water