magnetic petrology of equatorial atlantic sediments electron microscopy results and their implications for environmental magnetic interpretation 1 the magnetic microparticle and nanoparticle inventories of marine sediments from equatorial atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution origin transport and preservation this information is used to establish source to sink relations and to constrain environmental magnetic proxy interpretations for this area magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the ceara rise geob 1523 1 3 49 9 n 41 37 3 w the mid atlantic ridge geob 4313 2 4 02 8 n 33 26 3 w and the sierra leone rise geob 2910 1 4 50 7 n 21 03 2 w sediments from two depths corresponding to marine isotope stages 4 and 5 5 were processed this selection represents glacial and interglacial conditions of sedimentation for the western central and eastern equatorial atlantic and avoids interferences from subsurface and anoxic processes crystallographic elemental morphological and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy on basis of these properties nine particle classes could be defined detrital magnetite titanomagnetite fragmental and euhedral titanomagnetite hemoilmentite intergrowths silicates with magnetic inclusions microcrystalline hematite magnetite spherules bacterial magnetite goethite needles and nanoparticle clusters each class can be associated with fluvial eolian subaeric and submarine volcanic biogenic or chemogenic sources large scale sedimentation patterns are delineated as well detrital magnetite is typical of amazon discharge fragmental titanomagnetite is a submarine weathering product of mid ocean ridge basalts and titanomagnetite hemoilmenite intergrowths are common magnetic particles in west african dust this clear regionalization underlines that magnetic petrology is an excellent indicator of source to sink relations hematite encrustations magnetic spherules and nanoparticle clusters were found at all investigated sites while bacterial magnetite and authigenic hematite were only detected at the more oxic western site at the eastern site surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution it was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate relatively narrow grain size ranges western site or by variable sorting of a single source with a broad grain size distribution eastern site hematite goethite and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high coercive or superparamagnetic properties depending on their partly ultrafine grain sizes these two magnetic fractions are generally discussed as separate fractions but we suggest that they could actually be genetically linked

magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

al substituted goethite an indicator of pedogenic and other weathering environments in south africa abstract over 200 goethite containing samples from widely diverse environments along the eastern seaboard of south africa were examined and described mainly by x ray powder diffraction xrd aluminium substituted goethites ranging from 0 to about 32 mole alooh were identified mainly by xrd line shift and chemical extraction they occur in every soil zone and major soil type in one or other form from sources such as 1 non hydromorphic highly weathered strongly acid soils ferruginous bauxites saprolites and moderately weathered moderately acid soils environments 2 hydromorphic environments mottles concentrations pipe stems crusts ferricretes geodes and 3 crusts in calcareous environments these goethites formed in different climatic and weathering environments were grouped according to their amount of al substitution and crystallinity as determined by xrd line broadening al substitution in goethite formed in hydromorphic moderately acid soils and calcareous environments was generally low 0 15 mole goethite formed in non hydromorphic highly weathered environments was generally high in al substitution 15 32 mole and increased in the following order saprolites and strongly acid freely drained soils this study showed that the degree of al substitution and crystallinity of goethites may reflect the environment in which they have formed and serve as an indicator of soilforming processes

three dimensional micromagnetic calculations for naturally shaped magnetite octahedra and magnetosomes abstract a three dimensional micromagnetic algorithm for modelling naturally shaped particles is presented it uses a fft accelerated calculation of the demagnetizing energy exchange energy calculation directly takes into account the micromagnetic boundary condition a rectangular grid enclosing the particle is chosen to economically fit the shape after verifying that the new program reliably reproduces previous calculations the program is used to determine room temperature magnetization structures and single domain to vortex transition sizes these calculations are performed for octahedral magnetite and magnetosomes of different elongations the least energy magnetization structures of octahedral particles are similar to the flower and vortex states found for cubic particles the critical size d 0 above which an inhomogeneous magnetization structure has lower energy than the homogeneous flower state as well as the grain size dependence of m rs m s closely resemble cubic particles however meta stable flower states persist up to much larger grain size in octahedral magnetite this explains previous observations of large trm arm peaks in hydrothermal magnetite in realistic magnetosome geometries the sd psd transition is shifted towards larger grain sizes as compared to previous estimates based on elongated rectangular grains for substantially elongated grains this is a result of the alignment of the particles long edge with the crystallographic easy axis for more equidimensional grains the rounded ends of magnetosomes reduce spin deflection at the top and bottom edges thereby the formation of nucleation centers is effectively inhibited even without additional stabilization by magnetostatic interaction within a magnetosome chain

comment on the inference of mantle viscosity from an inversion of the fennoscandian relaxation spectrum by j x mitrovica and w r peltier mitrovica peltier 1993 hereafter mp have recently analysed the fennoscandian uplift data using inversion techniques they concluded that although observations support the presence of a weak asthenosphere under fennoscandia the decay spectrum for the best fit model 1 3 x lo1 pa s asthenosphere overlying a 10zlpas mantle found by previous workers fjeldskaar cathles 1991a b cathles 1975 1980 hereafter fc lies outside the decay spectrum observed in fennoscandia and is thus excluded by the observed data we believe this conclusion results from the authors inadvertantly using a different asthenosphere thickness in their calculations than the one that they report the purpose of this communication is to argue this case and to show that when proper asthenosphere thicknesses are used the models of mp and fc are both compatible with the observed uplift data and that each in fact defines one edge of the observed uplift spectrum the essential difference in the two end member models is the elected tradeoff between lithosphere thickness and asthenosphere viscosity mp choose a stiff lithosphere and a minor asthenosphere whereas fc choose the opposite we believe lithosphere rigidity data not discussed in mp argue for a weak lithosphere in fennoscandia and suggest that the fc model is to be preferred the relaxation spectrum for the best fit model of fc as presented in m p s fig 11 is shown by the dashed curve labelled lvz in fig 1 it clearly lies below and outside the observed fennoscandian relaxation data shaded area the decay spectrum for the lvz model is according to mp based on loading of a 70 km lithosphere on a 90 km asthenosphere of viscosity 2 x lo pa s overlying a 10 p a s mantle the maximum relaxation time for the lvz model exceeds 6000 years however the maximum relaxation time for fc s bestfit asthenosphere model that the lvz curve is supposed to

introduction to the special section on physical and biogeochemical processes responsible for the magnetization of sediments sediment palcomagnetism has made important contributions to magnetostratigraphy tectonic reconstructions and studies of geomagnetic field behavior interpretations of the sedimentary palcomagnetic record depend critically on when and how the natural remanent magnetization nrm is acquired and what postdepositional modifications have occurred this special section of the journal of geophysical research is a collection of 16 papers that address recent advances in our understanding of the physical chemical and biological processes which affect the acquisition and alteration of the nrm in sediments the issue is an outgrowth of a special symposium on magnetization processes in sediments that was convened at the 1987 spring agu meeting in baltimore maryland

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

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

diagenetic dissolution of biogenic magnetite in surface sediments of the benguela upwelling system intense primary biologic productivity in the surface waters of the benguela upwelling system provides a high supply of organic matter to the sea floor at the continental slope off namibia and sustains extreme concentrations of magnetite producing bacteria in the top sediment layers biogenic magnetite is thus by far the dominant carrier of the magnetic signal in these deposits also because of a very minor input of terrigenous ferrimagnetic minerals reducing conditions in the sediment column cause a selective dissolution of the bacterial magnetite fraction just a few centimeters below the main mineralization horizon this diagenetic process is documented in detail by high resolution rock magnetic analyses and transmission electron microscopy concentration dependent and grain size sensitive magnetic parameters such as susceptibility laboratory imparted remanences and hysteresis data reveal a significant drop in ferrimagnetic mineral content within the upper 10 cm of the sediments accompanied by a gradual downward coarsening of the ferrimagnetic mineral assemblage from primarily magnetic single domain particles in the top centimeters to multi domain grains in deeper strata electron microscope observations enable both an unequivocal identification of bacterial magnetite on the basis of shape and grain size and to trace dissolution effects on the biogenic magnetic mineral component to depth

early diagenesis of magnetic minerals in marine transitional environments geochemical signatures of hydrodynamic forcing abstract the low field magnetic susceptibility of surficial sediments from the ria de pontevedra nw spain shows a characteristic hydrodynamically driven textural control where diamagnetic sandy bioclastic carbonates occur toward the coastline leaving the finer muddy sediments to be deposited in the central and deeper areas of the ria three gravity cores were collected from the inner core 1 middle core 2 and outer core 3 zones of the clayey rich central axis of the ria from which the magnetic properties geochemical characteristics and hydrodynamic regime of the uppermost 90 cm of the sediment record were reconstructed profiles have a distinctive pattern that is controlled by depth isothermal remanent magnetization irm acquisition curves facilitated the identification of three well defined zones with depth depending on their degree of magnetic saturation and thus of their dominant magnetic behaviour magnetite like goethite like and greigite like this zonation is interpreted as the result of mineralogical changes during early diagenesis the relative depths of each zone are mostly dependent on distance to the open sea with the magnetite like zones being thicker in the outermost part of the ria toward the inner part of the ria the three zones become shallower and thinner in the innermost areas only greigite like coercivities are observed good correlation between magnetic and geochemical parameters such as vs 2fe s or h cr vs c s demonstrates that magnetic property variations with depth are diagenetically controlled and that these parameters can be used as magnetic proxies for early diagenesis similar significant positive correlations were found between of the clay fraction and modelled wave heights and between other magnetic and hydrodynamic parameters providing magnetic evidence of cryptic wave climate forcing of the diagenetic pathway the combined analysis of the magnetic geochemical and hydrodynamic data ultimately indicates that the sediment distribution and subsequent diagenetic pathway in this type of transitional environment mostly depend on the local distribution of wave energy and water depth rather than on estuarine like circulation processes

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

structure and morphology of magnetite anaerobically produced by a marine magnetotactic bacterium and a dissimilatory iron reducing bacterium intracellular crystals of magnetite synthesized by cells of the magnetotactic vibroid organism mv 1 and extracellular crystals of magnetite produced by the non magnetotactic dissimilatory iron reducing bacterium strain gs 15 were examined using high resolution transmission electron microscopy electron diffraction and57fe mo ssbauer spectroscopy the magnetotactic bacterium contained a single chain of approximately 10 crystals aligned along the long axis of the cell the crystals were essentially pure stoichiometric magnetite when viewed along the crystal long axis the particles had a hexagonal cross section whereas side on they appeared as rectangules or truncated rectangles of average dimension 53 35 nm these findings are explained in terms of a three dimensional morphology comprising a hexagonal prism of 110 faces which are capped and truncated by 111 end faces electron diffraction and lattice imaging studies indicated that the particles were structurally well defined single crystals in contrast magnetite particles produced by the strain gs 15 were irregular in shape and had smaller mean dimensions 14 nm single crystals were imaged but these were not of high structural perfection these results highlight the influence of intracellular control on the crystallochemical specificity of bacterial magnetites the characterization of these crystals is important in aiding the identification of biogenic magnetic materials in paleomagnetism and in studies of sediment magnetization

biogenic and lithogenic magnetic minerals in atlantic and pacific deep sea sediments and their paleomagnetic significance the carrier of the natural magnetization of deep sea sediments was characterized by mineralogical electron microscopic and rock magnetic investigations magnetic single domain sd and pseudo single domain psd particles which are most important for the stable remanent magnetization were separated from the magnetic coarse fraction and concentrated as magnetic fine fraction the magnetic coarse fraction consists of lithogenic magnetite and titanomagnetite which often contains exsolution lamellae of ilmenite both minerals are partially maghematized and occur isolated in the sediment or embedded in rock particles in regionally different concentrations the magnetic fine fraction consists of lithogenic magnetite and titanomagnetite and biogenic magnetite magnetofossils fossil bacterial magnetosomes the latter generally maghematized

volcanic ash particles as carriers of remanent magnetization in deep sea sediments from the kerguelen plateau abstract carbonate sediments from the kerguelen plateau odp leg 120 of eocene to pliocene age were investigated with rock magnetic petrographic and geochemical methods to determine the carriers of remanent magnetization magnetic methods showed that the major magnetic minerals were titanomagnetites slightly larger than single domain particles submicrometre to micrometre size grains of titanomagnetite were identified as inclusions in volcanic glass particles or as crystals in lithic clasts volcanic fallout ash particles formed the major fraction of the magnetic extract from each sediment sample three groups of volcanic ashes were identified trachytic ashes basaltic ashes with sideromelane and tachylite shards and palagonitic ashes these three groups could be equally well defined based on their magnetic hysteresis properties and alternating field demagnetization curves the highest coercivities of all samples were found for the tachylite due to the submicrometre size titanomagnetite inclusions in the matrix trachytic ashes had intermediate magnetic properties between the single domain type tachylites and the palagonitic altered basaltic ashes with low coercivities samples which contained mixtures of these different volcanic ashes could be distinguished from the three types of ashes based on their magnetic characteristics there was neither evidence of biogenic magnetofossils in the transmission electron micrographs nor did we find magnetic particles derived from continental antarctica the presence of dispersed volcanic fallout ashes between visible ash layers suggests continuous explosive volcanic activity on the kerguelen plateau in the south indian ocean since the early eocene the continuous fallout of volcanic ash from explosive volcanism on the kerguelen archipelago is the source of the magnetic particles and thus responsible for the magnetostratigraphy of the nannofossil oozes drilled during leg 120

magnetic characterization of holocene sedimentation in the south atlantic surface sediment samples representative for the tropical and subtropical south atlantic 15 n to 40 s were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant holocene climatic and oceanographic controls individual parameters reveal distinct yet frequently overlapping regional sedimentation characteristics a probabilistic fuzzy c means cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine particle magnetite grain size and coercivity spectra the resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes authigenic biogenic magnetite accumulation in high productivity areas transport by ocean current systems and effects of bottom water velocity on depositional regimes distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the south atlantic reflect prominent contrasts of both oceanic and continental influences

magnetic characterisation of present day deep sea sediments and sources in the north atlantic the north atlantic plays a key role in global climate through formation of north atlantic deep water which drives the thermohaline circulation to understand past and future climate change it is essential to understand the processes occurring within the present day north atlantic but ocean wide studies of present day sediment distribution and sources are relatively sparse here we use magnetic measurements to characterise the surface sediments of the north atlantic and identify the major climatic and oceanographic controls on their magnetic signatures the magnetic data and subsequent cluster analysis identify distinct spatial patterns of sediment sources and transport pathways much of the sedimentary magnetic signal appears to be controlled by detrital inputs especially of windblown dust and ice rafted debris ird with a range of different sources these sediment transport pathways can be validated by making direct comparison of the sediment magnetic properties with source rocks and soils and with iceberg trajectory and observation data the spatial distributions of the ird dominated sediments substantiate those mapped previously using lithological tracers provide additional spatial information on sediment sources and pathways and suggest that deep water currents are less significant than proposed in controlling present day sediment mineralogy and distribution the data form a present day basis for comparison with glacial stage ird and dust distributions and modes of ocean circulation

changes in the strength of the iceland scotland overflow water in the last 200 000 years evidence from magnetic anisotropy analysis of core su90 33 abstract results from a high resolution study of magnetic anisotropy combined with mineral magnetic analysis clay analysis and oxygen isotopes stratigraphy are reported for core su90 33 located 60 34 4n 22 05 1w at 2400 m water depth along the iceland scotland overflow water isow a branch of the north atlantic deep water nadw this core covers the last 6 climatic stages the main magnetic mineral is low ti content magnetite with very slight changes in the grain size the susceptibility record corrected for the carbonate content indicates down core variations in the amount of magnetite which largely co vary with the changes in the proportion of smectite in the clay fraction both parameters are climatically controlled with lower values during glacial than during interglacial periods the measurements of the anisotropy of magnetic susceptibility document down core changes in the degree of anisotropy with significantly higher values during interglacial periods than during glacial times these changes appear to be related to differences in the degree of alignment of the magnetic particles and not to changes in grain shape providing evidence that they arise from changes in strength of the bottom water circulation therefore the strength of the contour current associated to the transport of the isow appears to have been significantly larger during climatic stages 5 3 and 1 than during stages 6 4 and 2

determination of lattice parameter oxidation state and composition of individual titanomagnetite titanomaghemite grains by transmission electron microscopy a transmission electron microscopy tem method higher order laue zone holz diffraction has been applied to measurement of accurate lattice parameters of individual titanomagnetite and titanomaghemite grains lattice parameters and cation composition data are obtained with tem analytical electron microscopy aem for individual grains smaller than 0 1 m in diameter a suite of ocean drilling program deep sea drilling project ocean floor basalts containing titanomagnetite titanomaghemite with a wide range of oxidation degree z were employed to establish the relationship between lattice parameter a z and ti content x for naturally occurring samples standardization was accomplished with measurements on grains sufficiently large to allow the determination of absolute oxygen contents by electron microprobe analyses which directly yielded z values the probe data confirm that the maghemitization process in ocean floor basalts is one involving loss of iron and indicate a nearly linear relationship between lattice parameters and oxidation degrees for a given x value except for the smallest and largest values of z the standardized relation permits determination of z for individual submicrometer sized grains using only measured a values and cation compositions as determined by holz diffraction and aem

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

detection of live magnetotactic bacteria in south atlantic deep sea sediments abstract since they were first descrbed in 1975 1 magnetotactic bacteria have been observed in a wide variety of environments including soils 2 rivers freshwater lakes and coastal marine sediments 3 4 and in one case at a water depth of about 600 m in a hemipelagic sediment 5 here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern south atlantic different morphologies were identified cocci spirilla vibrionic and rod shaped forms at water depths to about 3000 m on the african continental margin between the equator and 30 s and on the walvis ridge in a pelagic environment about 1400 km off the coast no actively swimming magnetotactic bacteria could be detected in deeper waters magnetotactic bacteria were observed only in the upper 10 cm of the sediments with the highest concentrations always found in a well defined narrow subsurface layer the depth of this layer is dependent on the input and turnover rates of organic matter in the sediment measurements of oxygen and nitrate concentrations 6 7 provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment the data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone in contrast to former assumptions that they may inhabit microaerobic layers 8 11

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

volume loss from bering glacier alaska 1972 2003 comment on muskett and others 2009 alaskan glaciers experienced rapid and accelerating wastage over the last 4 5 decades arendt and others 2002 and accounted for 0 12 0 02 mm yr or 7 5 of the total sea level rise between 1962 2006 berthier and others 2010 ice loss in alaska is dominated by a few large glaciers located in the vicinity of the gulf of alaska e g columbia malaspina and bering glaciers among them the bering glacier system bgs is often regarded as the largest glacier system in north america with an area of nearly 4400 km beedle and others 2008 and over 5000 km if the accumulation area of tana glacier is included molnia 2007

the northwest african slope apron a modern analogue for deep water systems with complex seafloor topography the northwest african slope apron is an interesting modern analogue for deep water systems with complex seafloor topography a sediment process map of the northwest african continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin fine grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin and is considered to result from background sedimentary processes alongslope bottom currents smooth and mould the seafloor sediments and produce bedforms such as erosional furrows sediment waves and contourite drifts downslope gravity flows debris avalanches debris flows and turbidity currents are infrequent but important events on the margin and are the dominant processes shaping the morphology of the slope and rise the overall distribution of sedimentary facies and morphological elements on the northwest african margin is characteristic of a fine grained clastic slope apron however the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models in particular the distribution and thickness of turbidite sands are controlled by the location of the break of slope which is itself controlled by the pre existing submarine topography

observation of shrinkage cracks in ocean floor titanomagnetites abstract a characteristic of low temperature ocean floor titanomagnetite oxidation is the formation of irregular curved cracks which are caused by shrinkage of the spinel lattice the spatial distribution of the shrinkage cracks within individual titanomagnetite grains magnetically extracted from different ocean floor basalts was studied by scanning electron microscopy the cracks do not develop in grains smaller than approximately 5 microns in diameter under ocean floor conditions the critical titanomagnetite oxidation state z for the onset of cracking is between z 0 23 and z 0 37 shortly before the onset of cracking internal stress reaches a maximum which if reflected in the magnetic coercivity of the material

modern and last glacial maximum eolian sedimentation patterns in the atlantic ocean interpreted from sediment iron oxide content eolian dust derived from the desert regions of north africa is blown far into the tropical atlantic ocean by persistent easterly and northeasterly winds in this paper we demonstrate that the iron oxides hematite and goethite are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical atlantic iron oxides are identified by diffuse reflectance spectrophotometry a technique capable of identifying these minerals in concentrations as low as 0 01 by weight we analyze samples from both the modern and last glacial maximum lgm synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the atlantic ocean to determine the relative contribution of the iron oxides we factor analyzed the modern and lgm levels as a single data set the iron oxide factor explains about 25 of the variance in the combined core top and lgm data set mapped factor scores for the lgm and modern ocean indicate high iron oxide values are present in just two regions one off eastern north america and the other off northwest africa in the region off eastern north america iron oxide occurs primarily during the lgm as the previously noted brick red lutite a unique sediment type derived from the erosion of permo carboniferous red beds in atlantic canada a larger lobe shaped area of iron oxide rich sediment is present off northwest africa in both the modern and lgm levels the modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships satellites and analysis of air samples during the lgm iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

variations in terrigenous input into the deep equatorial atlantic during the past 24 000 years estimates of terrigenous fluxes at three different water depths at two sites in the equatorial atlantic by normalization against excess 230 th flux indicate that the flux of terrigenous material to the seafloor was significantly higher during the last glacial period than it is today fluxes started to decrease during deglaciation and reached minimal values in the middle of the holocene from 15 000 to 5 000 years ago there was a substantial increase in flux with increasing water depth below 2 800 meters this increase may reflect resuspension and lateral transport of slope and rise sediment possibly because of intensification of deepwater circulation during that period

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

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

provenance of present day eolian dust collected off nw africa 1 atmospheric dust samples collected along a transect off the west african coast have been investigated for their physical grain size distribution mineralogical and chemical major elements composition on the basis of these data the samples were grouped into sets of samples that most likely originated from the same source area in addition shipboard collected atmospheric meteorological data modeled 4 day back trajectories for each sampling day and location and total ozone mapping spectrometer aerosol index data for the time period of dust collection february march 1998 were combined and used to reconstruct the sources of the groups of dust samples on the basis of these data we were able to determine the provenance of the various dust samples it appears that the bulk of the wind blown sediments that are deposited in the proximal equatorial atlantic ocean are transported in the lower level 900 hpa ne trade wind layer which is a very dominant feature north of the intertropical convergence zone itcz however south of the surface expression of the itcz down to 5 s where surface winds are southwesterly we still collected sediments that originated from the north and east carried there by the ne trade wind layer as well as by easterly winds from higher altitudes the fact that the size of the wind blown dust depends not only on the wind strength of the transporting agent but also on the distance to the source hampers a direct comparison of the dust s size distributions and measured wind strengths however a comparison between eolian dust and terrigenous sediments collected in three submarine sediment traps off the west coast of nw africa shows that knowledge of the composition of eolian dust is a prerequisite for the interpretation of paleorecords obtained from sediment cores in the equatorial atlantic

low temperature partial magnetic self reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths with various low temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the argentine continental slope near the rio de la plata estuary a so far unreported style of partial magnetic self reversal has been detected in these sediments the sulphate methane transition smt zone is situated at depths between 4 and 8 m where reductive diagenesis severely alters the magnetic mineral assemblage throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions in the smt zone titano magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium rich titanohematite originating from high temperature deuteric oxidation within the volcanic host rocks these abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy warming of field cooled and zero field cooled low temperature saturation remanence displays magnetic phase transitions of titanium rich titanohematite below 50 k and the verwey transition of magnetite a prominent irreversible decline characterizes zero field cooling of room temperature saturation remanence it typically sets out at 210 k and is most clearly developed in the lower part of the smt zone where low temperature hysteresis measurements identified 210 k as the blocking temperature range of a titanohematite phase with a curie temperature of around 240 k the mechanism responsible for the marked loss of remanence is therefore sought in partial magnetic self reversal by magnetostatic interaction of titano magnetite and titanohematite when titanohematite becomes ferrimagnetic upon cooling its spontaneous magnetic moments order antiparallel to the titano magnetite remanence causing an drastic initial decrease of global magnetization the loss of remanence during subsequent further cooling appears to result from two combined effects 1 magnetic interaction between the two phases by which the titano magnetite domain structure is substantially modified and 2 low temperature demagnetization of titano magnetite due to decreasing magnetocrystalline anisotropy the depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics low temperature remanence cycling is therefore proposed as a diagnostic method to magnetically characterize such alteration palaeo environments 2007 the authors journal compilation 2007 ras

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals

progressive dissolution of titanomagnetites at odp site 653 tyrrhenian sea abstract magnetic properties microscopy and ferrous ferric iron ratio document increasing down core alteration of titanomagnetite to iron sulfide at odp site 653 leg 107 tyrrhenian sea changes in magnetic properties are interpreted to indicate a grain size increase in titanomagnetite to 40 m below seafloor bsf and a grain size decrease from this level to about 150 m bsf as dissolution affects first the fine grained and then the coarser grained magnetite the availability of dissolved sulfate from underlying messinian evaporites may account for the sulfide supply at depth and hence for the prolonged dissolution process dissolution at leg 107 sites adjacent to site 653 is less pronounced possibly by the availability of organic matter that can be metabolised by sulfate reducing bacteria