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

phosphorus cycling in marine sediments from the continental margin off namibia abstract in this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the namibian coastal upwelling area examination of the sediments reveals that organic and biogenic phosphorus are the major p containing phases preserved high corg porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle dissolved phosphate and iron in the pore water are closely coupled showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment water interface the abundant presence of fe iii bound phosphorus in the sediments document the co precipitation of both constituents as p containing iron oxyhydr oxides however highly dissolved phosphate concentrations in pore waters cannot be explained neither by simple mass balance calculations nor by the application of an established computer model under the assumption of steady state conditions phosphate release rates are too high as to be balanced with a solid phase reservoir this discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail we assume that the known active fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron oxyhydro oxides from the oxic surface sediments accordingly a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone

mineralogy and geochemistry of the clay fraction of sediments from the namibian continental margin and the adjacent hinterland the continental shelf off namibia can be divided into two sectors at latitude 18 40 s based on the distinctive morphologies of the two regions and on differences in composition of their surficial sediments the kunene shelf is narrow shallow and covered mainly by terrigenous sediment whereas the walvis shelf is wide deep and supports predominantly biogenic sediment the walvis shelf also possesses a number of distinguishing features its outer region has a concave profile which is attributed to upbuilding of the inner shelf during periods of lowered sea level its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast the seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water and to the influx of aeolian terrigenous detritus especially illite close to shore long belts of terrigenous mud extend southwards from both the kunene and orange rivers the currents responsible for transporting this sediment are poleward directed tongues of the angola benguela front in the north and mixed poleward moving south atlantic central water in the south the kunene river carries more smectite to the sea than the much larger orange the distribution of clay minerals on the continental shelf follows that of the adjacent hinterland with smectite and to a lesser extent kaolinite being significant in the north and illite predominating in the south along the upper slope the concentrations of smectite and kaolinite increase due to their importation from the kunene river by poleward directed antarctic intermediate water the geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins compared with marine sediments these clays are enriched in eight of the fourteen elements investigated namely al fe mg co mn pb rb and zn in contrast the offshore element distribution patterns show little resemblance to the patterns of individual drainage basins clays in the marine sediments are enriched relative to the river sediments in ca k cd ni and sr five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal calcium carbonate phosphorite glauconite and terrigenous components average element concentrations of individual lithofacies are enriched relative to the river clays in cd diatomaceous facies ca cd ni and sr calcareous and facies k and ni slightly glauconitic facies cd and sr terrigenous facies regionally the clay fractions of unconsolidated marine sediments contain high concentrations of al fe mg k mn and rb on the kunene margin due to aluminosilicates being imported by the kunene river ca and sr reach high values on the walvis outer margin in response to upwelling along the coast maximum concentrations of cd are associated with diatomaceous mud on the walvis inner shelf cu ni and zn reach high levels at a few spot locations where organic matter morg has suffered little dilution from other components especially opal and pb is concentrated with glauconitic sediment on the kunene shelf semi consolidated sediments are exposed intermittently on the outer margin they provide evidence that the palaeoclimate became much drier in post miocene times but in the north this picture is complicated by the introduction of sediment from the kunene river during the late tertiary palaeo oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased

geochemistry of peruvian near surface sediments sixteen short sediment cores were recovered from the upper edge ueo within wo and below bo the oxygen minimum zone omz off peru during cruise 147 of r v sonne solids were analyzed for major trace elements total organic carbon total inorganic carbon total sulfur the stable sulfur isotope composition 34 s of pyrite and sulfate reduction rates srr pore waters were analyzed for dissolved sulfate sulfide and 34 s of sulfate in all cores highest srr were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and or diffusion of sulfate from bottom water 34 s of dissolved sulfate showed only minor downcore increases strong 32 s enrichments in sedimentary pyrite to 48 vs v cdt are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction manganese and co are significantly depleted in peruvian upwelling sediments most likely due to mobilization from particles settling through the omz whereas release of both elements from reducing sediments only seems to occur in near coastal sites cadmium mo and re are exceptionally enriched in wo sediments 600 m water depth high re and moderate cd and mo enrichments are seen in bo sediments 600 m water depth re mo ratios indicate anoxic and suboxic conditions for wo and bo sediments respectively cadmium and mo downcore profiles suggest considerable contribution to ueo wo sediments by a biodetrital phase whereas re presumably accumulates via diffusion across the sediment water interface to precipitation depth uranium is distinctly enriched in wo sediments due to sulfidic conditions and in some bo sediments due to phosphorites silver transfer to suboxic bo sediments is likely governed by diatomaceous matter input whereas in anoxic wo sediments ag is presumably trapped due to sulfide precipitation cadmium cu zn ni cr ag and t1 predominantly accumulate via biogenic pre concentration in plankton remains rhenium sb as v u and mo are enriched in accordance with seawater te availability lead and bi enrichment in ueo surface sediments is likely contributed by anthropogenic activity mining accumulation rates of toc cd mo u and v from peruvian and namibian sediments exceed those from the oman margin and gulf of california due to enhanced preservation off peru and namibia copyright 2004 elsevier ltd

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

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

magnetic grain size and viscous remanent magnetization of pelagic clay unfossiliferous pelagic clay red clay cores from the pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface the origin of this instability has been considered to be viscous remanent magnetization vrm we conducted a rock magnetic study of a pelagic clay core from the south pacific which has the unstable to stable transition at about 1 5 m below the surface using the suspension method of yoshida and katsura 1985 we determined downcore variation of complete alignment magnetization cam and magnetic moment distribution geometric mean moment mg and log standard deviation log a assuming a lognormal distribution mean magnetic grain diameter was calculated from the mg using the saturation magnetization of magnetite 4 8 105 a m 1 on the assumption that all the magnetic grains are single domain magnetites of spherical shape this assumption is supported by thermomagnetic analyses isothermal remanent magnetization irm acquisition experiments and the ratio of cam to saturation irm the grain diameter ranges from about 0 02 to 0 15 m and decreases with depth below about 1 5 m the derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite the frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains on the other hand the magnetic viscosity acquisition coefficient normalized by the cam increases with depth we conclude that magnetic grain size can control the magnitude of the secondary magnetization vrm of pelagic clay it has been proposed that the major source of pelagic clay is atmospherically transported dust intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component which can cause the unstable to stable transition of the remanent magnetization

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

diagenetic regimes in arctic ocean sediments implications for sediment geochemistry and core correlation dark brown sediment layers are a potential stratigraphic tool in quaternary arctic ocean sediments they are rich in mn fe and trace metals scavenged from the water column and were most likely deposited during interglacial intervals in this study we combine sediment and pore water data from sediment cores taken in different parts of the arctic ocean to investigate the influence of early diagenetic processes on sediment geochemistry in most studied cores mn co and mo are released into the pore waters from mn oxide dissolution in deeper 1 5 m sediment layers the relationship between sedimentary mn co and mo contents in excess of the lithogenic background elementxs shows thatcoxs moxs values are a diagnostic tool to distinguish between layers with diagenetic metal addition from the pore waters coxs moxs 10 and unaffected layers coxs moxs from 1 to 10 steady state calculations based on current pore water profiles reveal that in the studied cores the diagenetic addition of these metals from the pore water pool alone is not sufficient to produce the sedimentary metal enrichments however it seems evident that dissolution of mn oxides in the mn reduction zone can permanently alter the primary geochemical signature of the dark brown layers therefore pore water data and coxs moxs values should be considered before core correlation when this correlation is solely based on mn contents and dark sediment color in contrast to the mostly nonlithogenic origin of mn in the dark brown layers sedimentary fe consists of a large lithogenic 80 and a small non lithogenic fraction 20 our pore water data show that diagenetic fe remobilization is not currently occurring in the sediment the dominant fe sources are coastal erosion and river input budget calculations show that fe seems to be trapped in the modern arctic ocean and accumulates in shelf and basin sediments the fe isotopic signal 56fe of the solid phase is positive 0 2 to 0 3 in samples defined as the lithogenic background without significant fe enrichments with increasing non lithogenic fe contents in the sediment 56fe becomes more negative which indicates a shelf to basin export of an isotopically lighter fe fraction we assume that the same transport process is true for mn

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

mineral composition control on inter mineral iron isotopic fractionation in granitoids abstract this study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four dabie i type granitoids to evaluate the factors that control inter mineral fe isotopic fractionation in granitoids the order of heavy iron isotope enrichment is feldspar pyrite magnetite biotite hornblende feldspar has heavier iron isotopic compositions than its co existing magnetite 56 fe plagioclase magnetite 0 376 to 1 084 56 fe alkali feldspar magnetite 0 516 to 0 846 which can be attributed to its high fe 3 fe tot ratio and low coordination number tetrahedrally coordinated of fe 3 56 fe magnetite biotite of coexisting magnetite and biotite ranges from 0 090 to 0 246 based on homogeneous major and iron isotopic compositions of mineral replicates the inter mineral fractionation in this study should reflect equilibrium fractionation the large variations of inter mineral fractionation among feldspar magnetite and biotite cannot be simply explained by temperature variation but strongly depend on mineral compositions the 56 fe plagioclase magnetite and 56 fe alkali feldspar magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali feldspar respectively this could be explained by different fe o bond strength in feldspar due to different fe 3 fe or different crystal parameters the 56 fe magnetite biotite increases with decreasing fe 3 fe biotite and increasing mole na k mg biotite indicating a decrease of factor in low fe 3 fe and high na k mg biotite high silica leucosomes from dabie migmatites with a feldspar accumulation petrogenesis have higher 56 fe values 56 fe 0 42 0 567 than leucosome that represents pristine partial melt 56 fe 0 117 0 016 indicating that accumulation of feldspar could account for high 56 fe values of these rocks high 56 fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals e g anorthosites feldspar accumulation however cannot explain high 56 fe values of most high silica granitoids reported in the literature based on their low sr ba contents and negative eu anomalies

origin of stable remanent magnetization of siliceous sediments in the central equatorial pacific abstract siliceous sediments distributed in the central equatorial pacific have a stable remanent magnetization regardless of age or depth this is in contrast with the unstable remanence of unfossiliferous red clay defined as pelagic clay here which accumulates in the middle latitudes of the pacific we have conducted a rock magnetic study of the siliceous sediments to clarify the magnetic carriers of the stable remanence for this purpose we combined magnetic granulometry by the suspension method and observation of magnetic grains with a transmission electron microprobe tem the advantage of the former is that the magnetic extraction procedure is not required for obtaining size distribution of the magnetic grains in sediments the tem observations revealed that most of the magnetic extracts are identical in size and shape to bacterial magnetosomes the suspension method proved that the magnetic assemblages of the siliceous sediments of both quaternary and early miocene age have mean diameters of about 0 05 m which is within the single domain range of magnetite and have narrow size distributions compared with pelagic clay these characteristics can be explained if the magnetofossils observed by tem are the major constituent of the magnetic grains in the sediments and have been preserved for a long period of time the difference in the stability of the remanent magnetization between the siliceous sediments and the pelagic clay can be explained by the difference in the size distribution of magnetic grains which would reflect differences in their sources i e biogenic vs detrital eolian

on the isotope composition of reactive iron in marine sediments redox shuttle versus early diagenesis the isotope composition of reactive iron fe in marine sediments and sedimentary rocks is a promising tool for identifying fe sources and sinks across ocean basins in addition to cross basinal fe redistribution which can modify fe isotope signatures fe minerals also undergo diagenetic redistribution during burial the isotope fractionation associated with this redistribution does not affect the bulk isotope composition but complicates the identification of mineral specific isotope signatures here we present new fe isotope data for peru margin sediments and revisit previously published data for sediments from the california margin to unravel the impact of early diagenesis on fe isotope compositions of individual fe pools sediments from oxic california margin sites are dominated by terrigenous fe supply with fe release from sediments having a negligible influence on the solid phase fe isotope composition the highly reactive fe pool sum of fe bound to oxyhydr oxide carbonate monosulfide and pyrite of these sediments has a light isotope composition relative to the bulk crust which is consistent with earlier studies showing that continental weathering shifts the isotope composition of fe oxyhydr oxides to lighter values ferruginous sedimentswithin the peruvian oxygen minimumzone are depleted in fe relative to the lithogenic background which we attribute to extensive fe release to the water column the remaining highly reactive fe pool has a heavier isotope composition compared to california margin sediments this observation is in agreement with the general notion of an isotopically light benthic fe efflux most of the reactive fe delivered and retained in the sediment is transferred into authigenic mineral phases within the topmost 10 to 20 cm of the sediments we observe a first order relationship between the extent of pyritization of fe monosulfide and the isotope composition of authigenic pyrite with increasing pyritization the isotope composition of authigenic pyrite approaches the isotope composition of the highly reactive fe pool we argue that the isotope composition of authigenic pyrite or other fe minerals that may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is separately evaluated and either close to 100 or 0 alternatively one may calculate the isotope composition of the highly reactive fe pool thereby avoiding isotope effects due to internal diagenetic redistribution in depositional settings with high fe but lowsulfide concentrations source and sink signatures in the isotope composition of the highly reactive fe pool may be compromised by sequestration of fe within authigenic silicate minerals authigenic silicate minerals appear to be an important burial phase for reactive fe below the peruvian oxygen minimum zone

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

lateral transport controls distribution quality and burial of organic matter along continental slopes in high productivity areas in this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon oc accumulation and burial across high productive continental margins we present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer bnl from the most productive coastal upwelling area of the modern ocean the benguela upwelling system offshore southwest africa interpretation of depositional patterns and comparison of downslope trends in oc content organic matter composition and 14 c age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of oc we propose that effective seaward particle transport primarily along the bnl is a key process that promotes and maintains local high sedimentation rates ultimately causing high preservation of oc in a depocenter on the upper slope offshore namibia as lateral transport efficiently displaces areas of enhanced oc burial from maximum production at highly productive continental margins vertical particle flux models do not sufficiently explain the relationship between primary production and shallow marine oc burial on geologic time scales the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea level change at times in the geologic past widespread downslope lateral transport of oc thus may have been a primary driver of enhanced oc burial at deeper continental slopes and abyssal basins

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

routine isotopic analysis of iron by hr mc icpms how precise and how accurate abstract high temperature isotopic variations documented in natural materials span a narrow range and call for precise and accurate isotopic analyses precisions better than 0 03 95 confidence interval for 56 fe have been obtained by high resolution multi collector inductively coupled plasma mass spectrometry hr mc icpms whether these uncertainties encompass all sources of error has not been carefully evaluated the current study examines all parameters that can affect accuracy and precision of fe isotopic measurements by hr mc icpms it is demonstrated that accurate 56 fe measurements can be routinely achieved within the quoted uncertainties

ultra long distance littoral transport of orange sand and provenance of the skeleton coast erg namibia abstract quantitative provenance analysis based on high resolution bulk petrography and heavy mineral data on beach and dune sands integrated with detrital zircon geochronology and chemical analyses of pyroxene garnet and staurolite demonstrates that sand derived from the orange river is carried by powerful and persistent longshore currents as far as northern namibia and southern angola 1750 km north of its mouth this is the longest cell of littoral sand transport documented so far compositional forward modeling indicates that 80 of dune sand in the skeleton coast is orange derived the remaining 20 being supplied by slow erosion of the damara orogen chiefly via the swakop river a decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet staurolite tourmaline and other metamorphic minerals north of walvis bay indicates that only one third of beach sand in the 350 km dune free gap between the namib and skeleton coast ergs is orange derived the remaining two thirds being supplied largely by the swakop river draining the damara orogen although volcanic gravel becomes dominant in beaches of the skeleton coast north of the uniab mouth detritus from cretaceous etendeka lavas accounts for only 4 of beach sand reflecting limited sand generation in the arid catchment contributions from the kunene river to either dune or beach sands is ruled out indicating that gem diamonds found in placer deposits along the coast of namibia are all derived from the orange river

sulfate reduction in surface sediments of the southeast atlantic continental margin between 15 38 s and 27 57 s angola and namibia sulfate reduction rates in the surface sediments from 17 stations from an along slope transect 1 300 m and from a cross slope transect 855 4 766 m were determined in the continental margin sediments of the benguela upwelling system profiles at all sites in the upwelling area showed increasing sulfate reduction rates from near zero at the surface to a peak at 2 5 cm up to 29 nmol cm 23 d 21 and then decreasing exponentially with depth to near background rates at 10 20 cm depth 2 nmol cm 23 d 21 depth integrated sulfate reduction rates were greatest at 1 300 m and decreased exponentially with water depth along the transect following the 1 300 m isobath depthintegrated sulfate reduction rates were highest in the north cape basin 1 16 6 0 23 mmol m 22 d 21 decreased over the walvis ridge 0 67 6 0 02 mmol m 22 d 21 and were lowest in the south angola basin 0 31 6 0 23 mmol m 22 d 21 depth integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and were also strongly correlated with surface organic carbon concentrations sulfate reduction rates both as a function of depth and in comparison with sediment trap data indicated that lateral downslope transport of organic carbon occurs sulfate reduction was estimated to account for 20 90 of the published rates of total oxygen consumption for the sediments at 1 300 m depth and 3 16 of sediments from 2 000 to 3 000 m depth comparison of the sulfate reduction rate profiles with the published diffusive oxygen uptake rates showed that the kinetics of oxygen utilization in the surface sediments are much faster than those for anaerobic organic carbon remineralization although the underlying cause of the difference was not clear

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

the paleomagnetic field from equatorial deep sea sediments axial symmetry and polarity asymmetry paleomagnetic data from 89 equatorial deep sea sediment cores indicate that the configuration of the time averaged geomagnetic field depends strongly on polarity state but that it remains within 1 degree of axial symmery throughout the pliocene and pleistocene last 5 million years the relative magnitude of the nondipole field was greater by almost a factor of 2 during reverse than during normal polarity intervals these results thus support earlier suggestions that there may be a standing nonreversing component of the geomagnetic dynamo

nickel as indicator of fresh organic matter in upwelling sediments abstract trace metals involved in biological cycling e g cd cu ni zn typically accumulate in upwelling sediments due to a high productivity related particle flux and an enhanced preservation at depth however poor constraint on the contribution of lithogenic metal fraction early diagenetic transformation processes and anthropogenic metal inputs may complicate sediment metal signatures the identification of source and accumulation mechanisms is essential to the validation of these metals as productivity proxies here we combine data from various short cores upper 50 cm and two longer cores of organic rich upwelling sediments peru namibia chile and gulf of california which suggest a highly significant linear and uniform relationship between ni and total organic carbon toc the overall high ni enrichment may be explained by the occurrence of diatoms which dominate productivity in these systems the peru surface sediments upper 2 cm show a less pronounced ni toc relationship and support a transition between lower ni toc ratio of east pacific water column particles and the higher ni toc ratio observed in deeper sediments in peru surface sediments the process is confirmed as a stoichiometric relation between ni and total chlorins the immediate degradation products of chlorophyll pigments which is not observed for cu or zn our data strongly support previous findings that ni is a clear if not the best indicator of the organic sinking flux this is also due to the fact that ni signatures undergo less alteration associated with sulfur and manganese cycling and low contribution from anthropogenic sources the apparently exclusive ni chlorin stoichiometry suggests that ni may be associated with enzymes that are involved in photoautotrophic production which underlines the previous finding from laboratory experiments and field work that diatoms have a dominant role in marine ni cycling the ni chlorin ratio increases with increasing sediment depth suggesting that chlorins are effected by on going diagenesis therefore ni may serve as a reliable indicator of the original chlorophyll flux rather than chlorins the very good correlation between ni and toc and the preferential preservation of ni over toc justify previous paleo productivity estimates based on ni accumulation

beyond the black sea paradigm the sedimentary fingerprint of an open marine iron shuttle we present iron fe concentration and fe isotope data for a sediment core transect across the peru upwelling area which hosts one of the ocean s most pronounced oxygen minimum zones omzs the lateral progression of total fe to aluminum ratios fet al across the continental margin indicates that sediments within the omz are depleted in fe whereas sediments below the omz are enriched in fe relative to the lithogenic background rates of fe loss within the omz as inferred from fet al ratios and sedimentation rates are in agreement with benthic flux data that were calculated from pore water concentration gradients the mass of fe lost from sediments within the omz is within the same order of magnitude as the mass of fe accumulating below the omz taken together our data are in agreement with a shuttle scenario where fe is reductively remobilized from sediments within the omz laterally transported within the anoxic water column and re precipitated within the more oxic water below the omz sediments within the omz have increased 56fe 54fe isotope ratios relative to the lithogenic background which is consistent with the general notion of benthic release of dissolved fe with a relatively low 56fe 54fe isotope ratio the fe isotope ratios increase across the margin and the highest values coincide with the greatest fe enrichment in sediments below the omz the apparent mismatch in isotope composition between the fe that is released within the omz and fe that is re precipitated below the omz implies that only a fraction of the sediment derived fe is retained near shore whereas another fraction is transported further offshore we suggest that a similar open marine shuttle is likely to operate along many ocean margins the observed sedimentary fingerprint of the open marine fe shuttle differs from a related transport mechanism in isolated euxinic basins e g the black sea where the laterally supplied reactive fe is quantitatively captured within the basin sediments we suggest that our findings are useful to identify omz type fe cycling in the geological record

sedimentary iron cycling in the benguela upwelling system off namibia abstract iron fe is an essential micronutrient and its availability controls primary production in large parts of the ocean further the variability of oxygen minimum zones omz exerts a major control on the availability of fe concentrations of which are elevated off namibia therefore to understand the sources and processes involved in the cycling of fe off namibia we determined major and trace element contents fe al si k p organic carbon mn co zr and the isotopic composition of fe 56 fe in seven sediment cores along a transect from the shelf to the abyssal plain at 5 km water depth meteor cruise m76 1 as well as in the local lithogenic background namibian river bed sediments and dust the depletion of mn vs the local lithogenic background llb reflects the reducing character of sediments from the shelf to the lower slope while depletion of mn and enrichment of co indicates the anoxic sulfidic environment on the shelf the enrichment of both elements in abyssal sediments documents highly oxidizing conditions there iron al ratios are elevated 0 6 to 1 6 and therefore equal to or higher than the llb the highest fe enrichment is seen on the shelf core 1 and the shelf break core 2 with a clear decrease towards the abyss the bulk 56 fe values range from 0 30 to 0 05 in the marine sediments with an average of 0 08 which is slightly lighter than the 56 fe of the llb 0 16 to 0 21 further combined enrichments of k and fe suggest the presence of detrital biotite core 1 and authigenic glauconite core 2 both of which are perfectly correlated with zr al ratios reflecting the highly dynamic conditions of the shelf environment 0 300 m water depth the absent large sedimentary fe isotope fractionation in the sediments of this study compared to peru sediments is likely due to a combination of the following i a highly variable omz leading to random re oxidation of any sediment sourced fe which prevents the export of larger amounts of dissolved fe to the interior of the ocean and which also prevents focused enrichments of fe and ii an enhanced dust flux which frequently resupplies terrestrial fe dampening any fe isotope fractionation this study is a snapshot on sedimentary fe cycling in one area from the vast namibian margin compared to the peruvian margin the data suggest little sedimentary fe release transport and redeposition however previous water column data suggest substantial fe release hence for a more subtle investigation of the fe loss and re supply a higher sampling resolution of multiple cores across the shelf and slope will be required

nitrate dependent iron oxidation limits iron transport in anoxic ocean regions abstract iron is an essential element for life on earth and limits primary production in large parts of the ocean oxygen free continental margin sediments represent an important source of bioavailable iron to the ocean yet little of the iron released from the seabed reaches the productive sea surface even in the anoxic water of oxygen minimum zones where iron solubility should be enhanced most of the iron is rapidly re precipitated to constrain the mechanism s of iron removal in anoxic ocean regions we explored the sediment and water in the oxygen minimum zone off peru during our sampling campaign the water column featured two distinct redox boundaries separating oxic from nitrate reducing i e nitrogenous water and nitrogenous from weakly sulfidic water the sulfidic water mass in contact with the shelf sediment contained elevated iron concentrations 300 nm at the boundary between sulfidic and nitrogenous conditions iron concentrations dropped sharply to

electron optical characterization of bacterial magnetite abstract magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy electron diffraction and electron microprobe analysis observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although chains are also seen electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure electron microprobe analysis shows the magnetite to be slightly titaniferous electron microscopy indicates that the grains rather than being flake shaped are parallelepiped crystals with a mean length of 99 3 8 7nm a mean width of 62 3 6 1nm yielding a width to length ratio of 0 63 these data support the contention that the magnetic bacterial grains are single domain crystals capable of producing a natural remanent magnetization in sediments

underestimation of the authigenic fraction of cu and ni in organic rich sediments abstract there are two possible techniques to estimate the degree of authigenic or non lithogenic metal enrichment in sediments first the authigenic metal fraction can be calculated according to m auth m total m al background al total while m is the metal of interest and al is aluminum second a metal enrichment factor ef is calculated according to ef m al total m al background both techniques rely on the appropriate use of the m al ratio of the lithogenic background for the latter the m al ratio of average shale or crust is widely applied as lithogenic background in marine geochemistry however as we show here the lithogenic background based on average shale or crust is too high for cu and ni as a consequence the authigenic fraction of cu and ni in sediments and suspended particulate matter spm is significantly underestimated as shown for examples from various environments upwelling areas of peru chile and the gulf of california the norwegian kyllaren fjord and the german wadden sea by correlating cu al and ni al with toc in the samples the apparent m al ratio of the background can be estimated a prerequisite for this technique is high linearity and high quality of correlation r 2 0 8 otherwise the lithogenic background cannot be exactly determined the resulting higher authigenic level of cu and ni may promote their use as productivity indicators while the status of cu and ni may shift from moderately to highly enriched when the enrichment is assessed via efs

olduvai onset polarity transition two detailed paleomagnetic records from north central pacific sediments abstract paleomagnetic analyses of two azimuthally oriented deep sea sediment cores from the north central pacific ocean which contain a record of the onset of the olduvai subchron polarity transition show 1 intermediate directions of magnetization with transitional inclinations reaching values up to 70 2 up to three sequential 180 declination changes two of which imply rapid oscillations of the geomagnetic field before the main reversal and 3 a pronounced drop in intensity of magnetization within the transitional interval the data further indicate that the intensity decay and recovery cycle took longer than the directional changes of the reversal whereas the directional changes lasted about 5000 years the duration of the recorded intensity cycles suggest that the complete onset of the olduvai reversal may have lasted as much as 28 000 years anhysteretic remanent magnetization studies and thermomagnetic analyses indicate that uniform magnetic mineralogy characterizes the transition zones of the cores this further suggests that the j 10 arm 70 ratio may represent a relative paleointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the onset of the olduvai reversal virtual geomagnetic pole paths of the two records are near sided directions in projected d i space indicate that the olduvai onset transitional field geometry possessed a contribution from non axisymmetric terms in addition to the dominating zonal harmonics the analyses also suggest that the transitional field during the onset of the olduvai reversal was dominated by harmonics different from those of the matuyama brunhes boundary jaramillo transitions and upper olduvai reversal

early diagenesis of redox sensitive trace metals in the peru upwelling area response to enso related oxygen fluctuations in the water column pore water and solid phase data for redox sensitive metals mn fe v mo and u were collected on a transect across the peru upwelling area 11 s at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from 0 to 93 m by comparing authigenic mass accumulation rates and diffusive benthic fluxes we evaluate the respective mechanisms of trace metal accumulation retention and remobilization across the oxygen minimum zone omz and with respect to oxygen fluctuations in the water column related to the el nino southern oscillation enso sediments within the permanent omz are characterized by diffusive uptake and authigenic fixation of u v and mo as well as diffusive loss of mn and fe across the benthic boundary some of the dissolved mn and fe in the water column re precipitate at the oxycline and shuttle particle reactive trace metals to the sediment surface at the lower and upper boundary of the omz at the lower boundary pore waters are not sufficiently sulfidic as to enable an efficient authigenic v and mo fixation as a consequence sediments below the omz are preferentially enriched in u which is delivered via both in situ pre cipitation and lateral supply of u rich phosphorites from further upslope trace metal cycling on the peruvian shelf is strongly affected by enso related oxygen fluctuations in bottom water during periods of shelf oxygenation surface sediments receive particulate v and mo with metal oxyhydr oxides that derive from both terrigenous sources and precipitation at the retreating oxycline after the recurrence of anoxic conditions metal oxyhydr oxides are reductively dissolved and the hereby liberated v and mo are authigenically removed this alternation between supply of particle reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of v and mo compared to u on the peruvian shelf the decoupling of v mo and u accumulation is further accentuated by the varying susceptibility to re oxidation of the different authigenic metal phases while authigenic u and v are readily re oxidized and recycled during periods of shelf oxygenation the sequestration of mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions our findings reveal that redox sensitive trace metals respond in specific manner to short term oxygen fluctuations in the water column the relative enrichment patterns identified might be useful for the reconstruction of past omz extension and large scale redox oscillations in the geological record

modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans the increase in atmospheric oxygen ca 2 4 ga had a significant impact on the geochemical cycling of fe the history of environmental oxygenation may be recorded in the fe isotope composition of archean and proterozoic sediments but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations here we present fe isotope data and iron aluminum ratios from the black sea oxic shelf and euxinic basin the isotope data demonstrate that shelf fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin we propose that there is net transport of isotopically light fe from sediments of the shelf to those of the distal anoxic basin consistent with enrichments in reactive fe seen in the deep basin the low 56 fe benthic fe flux is generated during the coupling of microbial fe iii reduction or sulfidization with fe 2 aq oxidation low 56 fe values reported previously from late archean sedimentary pyrites may be an isotopic fingerprint of analogous fe redox cycling in the late archean oceans this interpretation implies shallow water fe redox recycling in the late archean we predict that the light isotopic compositions of the late archean will prove to be distinct from those of the early archean before fe redox cycling became an important process and we infer that this difference may be related to the presence of oxygen in the surface ocean

phosphorite deposits on the namibian shelf abstract phosphorite deposits provide a source of fertilizer to feed an increasingly populated world and they provide a record of paleoceanographic changes in upwelling systems linked to climate the benguela upwelling system bus is among the most productive today and is associated with major phosphorite deposits exposed over an area of 24 700 km 2 on the namibian shelf analyses of cores associated with recent offshore mineral exploration provide new insights into the age and origin of these phosphorite deposits the deposit consists of coarsening upward muddy to gravelly pelletal phosphorite sand up to several meters thick on the middle to outer shelf 190 and 350 m water depth offshore of luderitz and walvis bay less extensive and less continuous deposits occur offshore of walvis bay as far north as the kunene river mouth on the inner to middle shelf 50 and 250 m water depth pelletal phosphorite sand some concentrically banded and concretionary phosphorite pebbles are the dominant grain types consisting of up to 90 wt carbonate fluorapatite francolite cement and inclusions of organic matter pyrite and terrigenous mud strontium isotope stratigraphy and foraminiferal biostratigraphy indicate that phosphogenesis was initiated in the latest miocene but that most phosphorite formed in the plio pleistocene during early burial diagenesis of organic rich mud as it does today in the holocene diatomaceous mud belt the highly condensed coarsening upward succession reflects increasingly high amplitude pleistocene sea level fluctuations phosphorite formation correlates to terrestrial aridification as well as to marine proxies of intensified coastal upwelling in the 600 m thick equivalent successions on the upper slope repeated phosphorite formation and reworking over pleistocene glacial to interglacial cycles resulted in the economic concentration of phosphorite with an estimated total resource of 7800 million tons of phosphate rock at an average grade of 19 wt p 2 o 5 on the namibian shelf

magnetofossil dissolution in a palaeomagnetically unstable deep sea sediment magnetofossils1 the fossil remains of bacterial magneto somes2 are found in various deep sea sediments and have been linked to the preservation of stable natural remanent magnetization in many of them1 3 5 they have also been extracted and identified from lithified carbonates of jurassic5 and precambrian6 age showing that magnetotactic bacteria have been a sedimentary source of fine grained magnetite for much of geological time some clay rich deep sea sediments however do not record a stable remanent magnetization for unknown reasons here we report results from a high resolution transmission electron microscope study on samples of this type collected by the deep sea drilling project dsdp the material contains a complex mixture of single domain magnetic minerals well preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria7 10 whereas others in these same preparations display a wide range of dissolution corrosion and aggregation effects rock magnetic measurements are consistent with the presence of these alterations in many dsdp sediments including those that preserve reliable palaeomagnetic directions as we were unable to fina authigenic magnetic minerals in our sample and as the magnetic fraction is dominated by well preserved magnetofossils we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

benthic iron and phosphorus fluxes across the peruvian oxygen minimum zone benthic fluxes of dissolved ferrous iron fe2 and phosphate tpo4 were quantified by in situ benthic chamber incubations and pore water profiles along a depth transect 11 s 80 1000 m across the peruvian oxygen minimum zone omz bottom water o2 levels were 2 mol l 1 down to 500 m water depth and increased to 40 mol l 1 at 1000 m fe2 fluxes were highest on the shallow shelf maximum 316 mmol m 2 yr 1 moderate 15 4 mmol m 2 yr 1 between 250 m and 600 m and negligible at deeper stations in the persistent omz core continuous reduction of fe oxyhydroxides results in depletion of sedimentary fe al ratios tpo4 fluxes were high maximum 292 mmol m 2 yr 1 throughout the shelf and the omz core in association with high organic carbon degradation rates ratios between organic carbon degradation and tpo4 flux indicate excess release of p over c when compared to redfield stoichiometry most likely this is caused by preferential p release from organic matter dissolution of fish debris and or p release from microbial mat communities while fe oxyhydroxides can only be inferred as a major p source on the shallow shelf the benthic fluxes presented here are among the highest reported from similar oxygen depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean the shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of fe2 and tpo4

a rock magnetic study of giant piston core ll44 gpc3 from the central north pacific and its paleoceanographic implications previous magnetostratigraphic studies of nonfossiliferous pre pliocene age pelagic clay cores from the central north pacific have proven unsuccessful because of the unstable behavior of the natural remanent magnetization the inability to obtain a reliable magnetostratigraphy has severely limited the temporal resolution that can be achieved in paleoceanographic studies of these nonfossiliferous pelagic clays we carried out a rock magnetic study of core ll44 gpc3 which spans the interval 0 70 m y ago to determine whether rock magnetic parameters can be used to construct a high resolution stratigraphic framework for paleoceanographic studies of central north pacific pelagic clays despite the instability of the natural remanence in addition we tested the ability of rock magnetic methods to detect and characterize the paleoceanographic changes that are recorded in the sediments of ll44 gpc3 stratigraphic variations in rapid and nondestructive rock magnetic parameters and related ratios reflect changes in the concentration mineralogy and grain size of the magnetic minerals within the sediments rock magnetic parameters exhibit coherent fluctuations within both the stable and unstable sections of ll44 gpc3 this result suggests the potential use of these parameters for regional correlation and relative dating of the nonfossil bearing pelagic clays of the central north pacific gyre major fluctuations in rock magnetic parameters plotted as accumulation rates correspond to intervals of paleoenvironmental change that have been previously detected by other paleoceanographic methods in addition variations in a parameter proportional to the concentration of goethite hematite correlate remarkably well with the mass accumulation rate of the total eolian component suggesting that in some situations it can be used as a proxy indicator for eolian activity

iron speciation in the eastern tropical south pacific oxygen minimum zone off peru vertical profiles of total dissolved iron dfe fe ii and hydrographic parameters were obtained along three transects across the continental shelf off peru in october 2005 fe ii and dfe concentrations were determined using luminol chemiluminescence and isotope dilution inductively coupled plasma mass spectrometry icp ms respectively the objective was to study the relationships among redox chemistry nitrogen cycle processes bottom topography and the distribution and chemistry of iron in the region the results are the most thorough survey to date of fe geochemistry in this region exceedingly high dfe 50 75 nmol l 1 was measured over the wide northern continental shelf with most of the dissolved fe present as fe ii below the oxycline in southern peru the shelf is narrower and dissolved fe concentrations were 10 fold lower moreover a smaller fraction of the dissolved fe was present as fe ii in the south even below the oxycline at the western ends of the transects fe ii maxima were coincident with deep i e secondary nitrite maxima this suggests a relationship between nitrate reduction and fe ii accumulation in the water column however over the shelf fe ii was also influenced by benthic processes large lateral gradients in dissolved fe across the shelf slope break reflect fe removal by oxidative scavenging and it seems plausible that much of the fe is trapped by redox cycling on the shelf nevertheless the maintenance of dissolved fe ii filaments within the secondary nitrite maxima constitutes an important mechanism for fe transport offshore

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

phosphate oxygen isotopes insights into sedimentary phosphorus cycling from the benguela upwelling system abstract marine sediments are the main sink in the oceanic phosphorus p cycle the activity of benthic microorganisms is decisive for regeneration reflux or burial of inorganic phosphate pi which has a strong impact on marine productivity recent formation of phosphorites on the continental shelf and a succession of different sedimentary environments make the benguela upwelling system a prime region for studying the role of microbes in p biogeochemistry the oxygen isotope signature of pore water phosphate 18op carries characteristic information of microbial p cycling intracellular turnover of phosphorylated biomolecules results in isotopic equilibrium with ambient water while enzymatic regeneration of pi from organic matter produces distinct offsets from equilibrium the balance of these two processes is the major control for 18op our study assesses the importance of microbial p cycling relative to regeneration of pi from organic matter from a transect across the namibian continental shelf and slope by combining pore water chemistry sulfate sulfide ferrous iron pi steady state turnover rate modeling and oxygen isotope geochemistry of pi we found 18op values in a range from 12 8 to 26 6 both in equilibrium as well as pronounced disequilibrium with water our data show a trend towards regeneration signatures disequilibrium under low mineralization activity and low pi concentrations and microbial turnover signatures equilibrium under high mineralization activity and high pi concentrations these findings are opposite to observations from water column studies where regeneration signatures were found to coincide with high mineralization activity and high pi concentrations it appears that preferential pi regeneration in marine sediments does not necessarily coincide with a disequilibrium 18op signature we propose that microbial pi uptake strategies which are controlled by pi availability are decisive for the alteration of the isotope signature this hypothesis is supported by the observation of efficient microbial pi turnover equilibrium signatures in the phosphogenic sediments of the benguela upwelling system