a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily van hoof a a m van os b j h and langereis c g 1993 the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily phys earth planet inter 77 297 313 the detailed paleomagnetic records of the upper and lower nunivak polarity transitions have been determined from pliocene marine marls in southern sicily magnetites are the most important carrier of the remanence in both records the transitions are recorded in two components a low temperature and a high temperature component the two components do not represent the geomagnetic field because the changes in these components take place at lithological boundaries in addition the directional changes do not completely match directional changes of the same transitions recorded in calabria some 250 km away the character of the virtual geomagnetic poles paths is probably caused by smoothing of the stable directions before and after the transitions the directional changes as well as the smoothing mechanism can be explained by the diagenetic magnetite formation model van hoof et al 1992 in which shortly after burial the remanence carried by newly formed secondary magnetites is superposed on the initial remanence carried by primary magnetite

the geochemistry of iron in recent tidal flat sediments of the wash area england a mineralogical mossbauer and magnetic study abstract undisturbed core samples of recent sediments from the wash tidal flats east anglia england obtained using a delft corer were studied with special reference to the diagenesis and geochemical behaviour of iron the mossbauer effect in 57fe was used to monitor the distribution of fe between different phases as a function of depth together with the magnetic mineralogy and palaeomagnetic properties the cores consist of successively downwards 0 36 m brown clay 1 5 m finely laminated silts and fine sands and 7 14 m homogeneous fine sands the dominant minerals are quartz feldspar calcite and clay minerals and chemical analysis for al si mg mn ca fe na k showed variations closely linked to lithological changes illite is the most abundant clay mineral mean 48 followed by mixed layer illite montmorillonite and montmorillonite kaolinite and chlorite chlorite is the major iron bearing clay mineral and represents 4 to 10 of the computer fit analysis of the mossbauer spectra of best quality showed contributions from fe2 and fe3 in clay minerals essentially chlorite low spin fe2 in pyrite and magnetically ordered iron in greigite fe3s4 systematic variations as a function of sample depth indicate a relative increase in the amount of fe in pyrite at the expense of the clay minerals magnetite and titanium bearing magnetite are the carriers of natural magnetic remanence in these sediments the direction and intensity of natural remanence in the samples compare well with the known secular variation of the earth s magnetic field derived from the historic archaeomagnetic record and this enables the samples to be dated and sedimentation rates to be determined 1 5 mm yr 1 for the upper 2 m and 7 7 mm yr 1 for the lower 7 m

reductive diagenesis magnetite dissolution greigite growth and paleomagnetic smoothing in marine sediments a new view abstract in many anoxic sedimentary environments the onset of sulfate reduction and pyritization of detrital iron bearing minerals leads to a precipitous decline in magnetic mineral concentration during early diagenesis the usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution detailed rock magnetic and electron microscope analyses of rapidly deposited 7 cm kyr latest pleistocene holocene sediments from the continental margins of oman 22 22 4 n 60 08 0 e and northern california 38 24 8 n 123 58 2 w demonstrate that pyritization during early diagenesis also leads to the progressive down core growth of the ferrimagnetic iron sulfide greigite greigite growth begins with nucleation of large concentrations of superparamagnetic sp nanoparticles at the inferred position of the sulfate methane transition which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of sp particles looping of hysteresis parameters on a day plot records the dissolution of single domain sd titano magnetite and the formation of sp greigite which then slowly and progressively grows through its sd blocking volume and acquires a stable paleomagnetic signal this looping trend is also evident in data from several published records oregon margin korea strait japan sea niger fan argentine margin and the ontong java plateau indicating that these processes may be widespread in reducing environments our observations have profound implications for paleomagnetic records from sulfate reducing environments the paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10 s of kyr and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10 s to 100 s of kyr we therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis

the early diagenesis of iron in pelagic sediments a multidisciplinary approach abstract biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial atlantic are investigated by means of pore water chemistry chemical leaching experiments and total elemental determinations color reflectance spectroscopy rock magnetic measurements and tem observations of the magnetic fraction results indicate that in the presence of nitrate ascorbate a weak reducing agent leachable iron decreased with depth from the sediment surface within this upper sediment region iron assimilation by bacteria is indicated as magnetosomes were found by tem observations throughout the whole core extractions with a strong reducing agent dithionite representing iron bound to iron oxides correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone the abrupt decrease of red color reflection intensity relative to the amount of iron oxides below the iron redox boundary is the result of a decrease in the specific surface area of iron oxyhydroxide oxides magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary arising from an increase in biogenic magnetite formed by magnetotactic assimilatory bacteria dissolution of very fine grained magnetite and the gradual decrease of coarser grained terrigenous titano magnetite with depth the early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content

resistivity and self potential tomography applied to groundwater remediation and contaminant plumes sandbox and field experiments summary geophysical methods can be used to remotely characterize contaminated sites and monitor in situ enhanced remediation processes we have conducted one sandbox experiment and one contaminated field investigation to show the robustness of electrical resistivity tomography and self potential sp tomography for these applications in the sandbox experiment we injected permanganate in a trichloroethylene tce contaminated environment under a constant hydraulic gradient inverted resistivity tomograms are able to track the evolution of the permanganate plume in agreement with visual observations made on the side of the tank self potential measurements were also performed at the surface of the sandbox using non polarizing ag agcl electrodes these data were inverted to obtain the source density distribution with and without the resistivity information a compact horizontal dipole source located at the front of the plume was obtained from the inversion of these self potential data this current dipole may be related to the redox reaction occurring between tce and permanganate and the strong concentration gradient at the front of the plume we demonstrate that time lapse self potential signals can be used to track the kinetics of an advecting oxidizer plume with acceptable accuracy and if needed in real time but are unable to completely resolve the shape of the plume in the field investigation a 3d resistivity tomography is used to characterize an organic contaminant plume resistive domain and an overlying zone of solid waste materials conductive domain after removing the influence of the streaming potential the identified source current density had a magnitude of 0 5 a m 2 the strong source current density may be attributed to charge movement between the neighboring zones that encourage abiotic and microbially enhanced reduction and oxidation reactions in both cases the self potential source current density is located in the area of strong resistivity gradient

magnetic mineralogy of sandstones from the lunde formation late triassic northern north sea uk origin of the palaeomagnetic signal the minerals responsible for the magnetic properties of the late triassic lunde formation have been examined using a combination of magnetic mineral extraction microscopy and rock magnetic measurements the magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis by a process of differential preservation and modification of the original detrital minerals the magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material which contains most of the magnetic minerals paramagnetic minerals both of depositional origin and produced during diagenesis are largely responsible for the magnetic susceptibility a suite of residual ferrimagnetic oxides chromite and mn substituted magnetite is responsible for the remanence properties in grey sandstones because most of the originally deposited magnetic fe ti oxides have been removed by reducing conditions during diagenesis however a small number of reddened horizons preserve a higher proportion of original detrital fe ti oxides ferrimagnetic ilmenite ferrimagnetic chromite and magnetite although authigenic hematite has replaced most of the original mineralogy in the reddened horizons magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal particularly in horizons which have a low content of discrete detrital ferrimagnetic grains either through diagenetic dissolution or paucity of the silt and clay fraction within the grey sandstones the triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates with a contribution from detrital ferrimagnetic chromite a model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed

deciphering the impact of diagenesis overprint on negative δ13c excursions using rock magnetism case study of ediacaran carbonates yangjiaping section south china rock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the neoproterozoic in the yangtze platform south china a detailed characterization of the magnetic mineralogy which includes low temperature and high field magnetometry and classical magnetic measurement arm irm susceptibility was therefore performed along upper doushantuo and lower dengying formations outcropping in the yangjiaping section the derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents they show that the magnetic content is significantly reduced in samples presenting negative 13ccalcite values we interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction combined with c and o isotopic data paleomagnetic techniques thus show that the upper doushantuo lower dengying negative excursion of the yangjiaping section is largely due to diagenesis although the preservation of a genuine 13c excursion of lower magnitude from 7 down to 0 instead of down to 9 as usually considered cannot be ruled out a corrected 13ccarbonate chemostratigraphic curve is therefore proposed the unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the yangtze platform and thus to its correlation to negative excursions in other cratons i e shuram excursion more generally this study illustrates the potential of magnetic mineralogy characterization a low cost time efficient and non destructive technique as screening tool for diagenetic overprints of 13c and 18o

the magnetization of clay rich rocks in sedimentary basins low temperature experimental formation of magnetic carriers in natural samples abstract rocks are often magnetically altered during burial thus providing a useful method to study their thermo chemical history in order to improve the understanding of burial diagenetic magnetization we experimentally synthesized magnetite in natural samples lower jurassic argillites from the paris basin were heated to 150 c under a confined atmosphere of argon when a magnetic field was applied during the heating a chemical remanent magnetization was acquired rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite using anhysteretic remanent magnetization as a measure of the quantity of magnetite an increase of up to 150 after heating was observed ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3 calcite and new magnetite is sometimes completely absent in samples containing less than 0 5 calcite hysteresis measurements suggest that the new magnetite is in the pseudo single domain state scanning electron microscope studies indicate that the new magnetic carriers are associated with pre existing pyrite the magnetic transformations during the experiments involved a fluid which reacted with iron bearing and sulfur bearing minerals in ways depending on local redox conditions this study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins without any external supply from long range fluid flow

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

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

mineral magnetic properties of holocene sediments in the subaqueous yangtze delta and the implications for human activity and early diagenesis abstract this study examines the temporal and spatial distribution of magnetic properties in holocene sediment cores taken from the subaqueous yangtze delta china to evaluate depositional environmental changes including the sediment with an anthropogenic provenance and changes in the bottom water chemistry our approach is to compare the magnetic properties in holocene cores with those from a suite of surficial sediment samples taken at various locations from the yangtze delta and the adjacent continental shelf the results indicate that the magnetic properties in sediment cores change generally with sedimentary facies mainly due to their in phase changes with sediment grain size and redox conditions but that they have also been significantly altered by effects of human activity and early diagenesis magnetic parameters that exhibit soil erosion associations show remarkable increases over the past 800 years reflecting an increase in the terrestrial supply of fine grained magnetic minerals induced by the intensification of human activity in the yangtze river catchment early diagenesis was strong in core hzk8 located at the depocenter of the subaqueous yangtze delta including dissolution of fine grained ferrimagnetic minerals and the precipitation of authigenic greigite and pyrite as evidenced by both room temperature and thermo magnetic analyses the dissolution of ferrimagnetic minerals coincides with changes in sedimentation rate at different sites and time periods suggesting that this factor is important for controlling the rate of early diagenesis authigenic iron sulfides suggest the function of sulfate reducing bacteria which implies a hypoxic environment at this site from 6000 cal yr bp

magnetostratigraphy of a long quaternary sediment core in the south yellow sea abstract continental shelves serve as a bridge between the continent and ocean and sediments in this region are sensitive to land sea interaction sea level variation and local subsidence in this study we present a comprehensive magnetic study of the longest sediment core csdp 1 300 1 m recovered from the south yellow sea the major magnetic minerals in the studied sediments are magnetite hematite and greigite greigite records a chemical remanent magnetization which can be removed effectively by thermal demagnetization the magnetostratigraphy defined in this study contains the matuyama brunhes boundary m b 781 ka at 73 68 m which is consistent with results from adjacent cores the base of the quaternary 2 6 ma in the yellow sea is recovered for the first time at a depth of 227 16 m the basal age of the core is estimated to be 3 50 ma it indicates that the first transgression of the yellow sea occurred no later than 1 7 ma succeeding large amplitude regressions occurred in some cold periods such as during mis 20 mis 18 and mis 10 our results provide the first chronology that brackets the entire quaternary and we reconstruct the sedimentary evolution of the yellow sea with robust age constraints which provides an important framework for further paleoenvironmental and tectonic studies

diagenetic magnetic enhancement of sapropels from the eastern mediterranean sea diagenetic dissolution of magnetic minerals has been widely observed in organic rich sediments from many environments organic rich sediments from the eastern mediterranean sea sapropels recovered during leg 160 of the ocean drilling program reveal a surprising catalogue of magnetic properties sapropels from all sites studied across the eastern mediterranean sea are strongly magnetic and the magnetization is directly proportional to the organic carbon content the magnetization of the sapropels is dominated by a low coercivity probably single domain magnetic mineral with an inverse magnetic fabric that exhibits a clear decay in magnetic properties when exposed to air during heating the magnetic particles irreversibly break down between 360 and 400 c the contrast between the magnetic properties of sapropels and surrounding sediments is marked with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments the contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non steady state diagenesis sulphate reducing conditions dominated during sapropel deposition while overlying sediments were deposited under oxic conditions the mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate reducing conditions that existed during times of sapropel formation attempts to identify this mineral have been unsuccessful but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase the influence of diagenesis on the magnetic properties of cyclically deposited eastern mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments

a 580 kyr paleomagnetic record from the sub antarctic south atlantic ocean drilling program site 1089 1 we report geomagnetic directional paleosecular variation relative paleointensity proxies and oxygen isotope data from the upper 88 m composite depth mcd at south atlantic ocean drilling program odp site 1089 40 56 2 s 9 53 64 e 4620 m water depth the age model is provided by high resolution oxygen isotope stratigraphy augmented by radiocarbon dates from the upper 8 mcd of nearby piston core rc11 83 mean sedimentation rates at site 1089 are in the range of 15 to 20 cm kyr two intervals during the brunhes chron at 29 6 mcd 190 ka and at 48 mcd 335 ka have component magnetization directions with positive reverse polarity inclination however the excursional directions are heavily overprinted by the postexcursional field magnetite is the dominant carrier of magnetic remanence and occurs in the pseudosingle domain psd grain size an additional higher coercivity magnetic carrier characterized by low unblocking temperatures 350 c is assumed to be authigenic pyrrhotite a decrease in magnetization intensity down core is mirrored by a reduction in pore water sulfate indicating diagenetic reduction of magnetite despite down core changes in magnetic mineralogy normalized intensity records from site 1089 are comparable with high resolution paleointensity records from the north atlantic e g odp sites 983 and 984 sediment properties and sedimentation patterns within the cape site 1089 and iceland sites 983 and 984 basins are distinctly different at both millennial and orbital timescales and therefore preclude lithologic variability from being the source of this correlation variations in normalized intensity from site 1089 therefore appear to reflect changes in global scale geomagnetic field intensity

magnetofossils in the sediment of lake baikal siberia abstract a multidisciplinary approach involving rock magnetics transmission electron microscopy and x ray diffraction was used to identify a biogenic magnetite component in the lake baikal siberia sedimentary magnetic record the distinctive biogenic component to the magnetic record occurs as chains of single domain elongate hexagonal and tear drop cone shaped magnetite particles these magnetofossils are inferred to be magnetosomes produced by magnetotactic bacteria living in the surficial sediment throughout lake baikal postdepositional reduction diagenesis results in the loss of the fine grained magnetofossils at depth in addition this study shows that the fine grained magnetofossils are removed by the process of storage diagenesis during long periods 21 months of core storage which results in a change to a coarser grained slightly higher coercivity bulk magnetic mineral assemblage although the lake baikal sedimentary magnetic record has several distinct and complex components by determining their origin this study has shown that the sediments are well suited for environmental magnetic study

sulfur and iron diagenesis in temperate unsteady sediments of the east china sea inner shelf and a comparison with tropical mobile mud belts mmbs redox cycling of iron fe and sulfur s exerts profound influences on fates of numerous elements in coastal marine sediments in this study s and fe cycling and its geochemical expressions in the east china sea ecs inner shelf a representative of temperate mobile mud belts mmbs were characterized and compared with tropical counterparts the amazon shelf and the gulf of papua fe and s speciation consistently points to the dominance of authigenic nonsulfidized fe ii phases i e poorly crystalline clays pccs and carbonates and the prevalence of fe redox cycling in the suboxic or weakly sulfidic regimes of the ecs mmbs high contents of authigenic magnetite may be a common diagenetic expression in all mmbs compared to the tropical mmbs three main differences of diagenetic expressions in the ecs mmbs are i light 34spyrite in the ecs mmb vs characteristically heavy 34spyrite in the amazon shelf mmbs ii lower total reactive fe fetr total diagenetic fe ii and ratio of fetr to total fe in the ecs mmbs iii fe ii carbonates and pccs are equally important sinks for nonsulfidized fe ii in the ecs mmbs whereas pccs are the predominant sink in the tropical counterparts these differences are ascribable to factors including low degradability of organic matter small diffusion scales less intense chemical weathering in the drainage basin and or weaker reverse weathering in the ecs mmbs despite the differences above fe and s diagenetic expressions that characterize the prevalence of fe redox cycling in the unsteady suboxic regimes are shared by the ecs and tropical mmbs

magnetic measurements used to identify paleoxidation fronts in deep sea sediments from the madeira abyssal plain we describe the results of rock magnetic measurements of a sediment core from the madeira abyssal plain which contains multiple turbidites previous studies of trace element e g u concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts we show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements low concentrations of magnetic minerals occur in turbidites with a high organic content reflecting the destruction of magnetic material via the process of reductive diagenesis in addition fossil redox fronts are readily identified by rock magnetic properties

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

source of the climate signal recorded by magnetic susceptibility variations in indian ocean sediments the origins of magnetic susceptibility variations in deep sea sediments from the indian ocean ocean drilling program odp leg 117 have been examined to identify the specific connections between climate shifts and sediment magnetic properties in a previous study the magnetic susceptibility variations in odp hole 722b were identified as an outstanding proxy paleoceanographic record despite evidence of postdepositional loss of magnetic minerals through reductive diagenesis this paradox is addressed by assessing the contributions of ferrimagnetic detrital iron oxides bacterial magnetosomes and paramagnetic detrital fe silicate minerals to the magnetic susceptibility signal in addition to detailed magnetic analyses mineralogical morphological and grain size data have been obtained from representative magnetic extracts for hole 722b we find that magnetic responses to climate change result from 1 fluctuations in the volume magnetic susceptibility which is primarily controlled by carbonate dilution 2 a ferrimagnetic signal which is restricted to the upper 7 meters below sea floor mbsf and which largely reflects source area aridity and 3 a paramagnetic susceptibility record below 7 mbsf which is coincident in frequency with variations in lithogenic grain size

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

chronologies of sedimentary processes in sediments of the foam site long island sound connecticut a detailed study of the sedimentation and diagenetic history of diver taken sediment cores from the foam site in long island sound was made by determining the distribution with depth of the following properties sup 234 th sub xs sup 7 be sup 210 pb sub xs sup 239 240 pu selected trace elements sup 14 c in both shells and organic fraction pore water sulfate and the diagenetic reaction products solid phase organic carbon and sulfur the sup 234 th sub xs and sup 7 be data indicate that the upper 4 cm are well mixed on the time scale of about a year the distributions of sup 210 pb sub xs sup 239 240 pu and anthropogenic trace metal excesses in the 4 15 cm section are controlled by the mixing of the surface 0 4 cm sediment into this layer the net sediment accumulation rate in the 0 15 cm layer is about 0 03 cm yr the average sediment accumulation rate in the approx 18 100 cm layer as determined from sup 14 c ages of shell layers is 0 094 0 021 cm yr perhaps a factor of approx 2 lower than that estimated from diagenetic modelling of solid phase sulfur and organic carbon depth profiles the sediment structure indicates that the sediment accumulation in this depth zone has beenmore episodic with the possibility of erosion as well as deposition having occurred their study shows that in high energy and dynamic sedimentary environments such as the foam site the results of steady state diagenetic modelling should be accepted only over time and depth scales permitted by the physical evidence less

assessment of the controls on 234u 238u activity ratios recorded in detrital lacustrine sediments abstract understanding landscape evolution on geological time scales has become increasingly important in the light of recent climate warming and intensified land use uranium isotopes 234u 238u activity ratios can be used to reconstruct catchment wide erosion from fine grained detrital matter as the lighter nuclide 234u is lost from grains loss of 234u is mainly attributed to recoil of 234th during decay of 238u however mobilisation of 234u is also controlled by preferential oxidation and leaching of 234u from detrital grains further considerations are required concerning the sediment mineralogy and uranium bound to authigenic or endogenic matter here we extend the discussion about the controls on previously published 234u 238u activity ratios and uranium concentrations of detrital matter along a 5 43 m long late glacial to holocene sedimentary record from lake ohrid north macedonia albania lake ohrid is chosen to study how erosion responds to anthropogenic disturbances and climate variability which are common through the late pleistocene and holocene in the mediterranean region we compared uranium concentration and isotope data to redox sensitive mineralogical and biogeochemical proxies mineralogical and biogeochemical data show no control on uranium concentration and 234u 238u activity ratios rock magnetic redox proxies indicate uranium mobilisation from detrital matter in more oxic environments no correlation between 234u 238u activity ratios and rock magnetic data is observed which implies that the redox environment has no control on the isotope signal monte carlo simulations reveal that post depositional preferential mobilisation of 234u has only negligible impact on the calculated sediment residence times our model implies that pre depositional leaching can results in shorter sediment residence times but low 234u 238u activity ratios during cold and dry intervals imply that recoil is likely the main processes explaining loss of 234u from the detrital grain this is explained by the time scales studied herein in 10 000 years during which preferential leaching is seen to be not significant

the magnetic properties of quaternary and tertiary sediments in the southern north sea cores from boreholes penetrating late quaternary glacial interglacial and postglacial sediments and the underlying late cenozoic delta complex of the southern north sea have been examined for their magnetic properties a magnetic polarity stratigraphy has been established as an aid to biostratigraphic dating of the sediments the kaena gauss and gauss matuyama transitions and the base and top of the olduvai subchron have been identified the strength and stability of laboratory induced isothermal remanent magnetisation display clear magneto petrological variations which match lithostratigraphic changes in the cores principal component analysis has picked out a basin wide and palaeoenvironmental consistency in the magnetic data large multi domain magnetite grains predominate in the post deltaic and fluvio deltaic sediments whereas smaller greigite or titanomagnetite grains are concentrated in the intertidal and marine deltaic facies since heavy mineral analysis indicates that most of the deltaic detritus derived from common source areas the differences in magnetic mineralogy have probably been caused by the sediment transport processes operating within the delta complex

magnetic hysteresis properties of greigite fe3s4 and a new occurrence in holocene sediments from swedish lappland a new occurrence of ferrimagnetic greigite fe3s4 is reported in freshwater holocene sediments from gaskkamus gorsajavri in swedish lappland the presence of greigite was inferred from the mineral magnetic analysis of sediment cores the greigite bearing sediments possess a high saturation isothermal remanent magnetisation to susceptibility sirm ratio a characteristic shared by other natural sediments known to contain greigite the hysteresis properties measured using a vibrating sample magnetometer of natural greigite extracted from lake sediments show much higher saturation coercivity b0 c and mrsms ratio values approximately 50 mt and 0 5 respectively than synthetic samples of greigite approximately 12 mt and 0 2 the results suggest that synthetic greigite samples contain crystals ranging from small non remanence carrying superparamagnetic grains to large multi domain grains in contrast the natural samples probably consist predominantly of pseudo single domain and single domain crystals capable of carrying a chemical remanent magnetisation however the unstable nature of greigite under laboratory conditions creates difficulty in characterising its physical attributes

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

a model for particle selective transport of tracers in sediments with conveyor belt deposit feeders conveyor belt deposit feeding organisms prevalent in both marine and freshwater systems have a profound effect on sediment properties and transport processes these organisms ingest sediments over a range of depths while depositing gut contents from tails protruding above the sediment surface this action results in particle selective transfer of buried materials to the sediment surface and imposes an accelerated rate of sediment and pore water burial within the feeding zone most previous efforts to combine sediment diagenesis with the effects of biogenic reworking characterize mixing as exclusively diffusive and ignore such major advective effects here a model is developed based on fundamental diagenetic equations for transport and reaction for the distribution of tracers in accumulating sediments subject to compaction and diffusive as well as advective redistribution by benthic organisms conveyor belt cb feeding is characterized as a first order process with a depth dependent rate constant which is either localized gaussian or distributed integrated gaussian biogenic diffusivity of bulk sediments is allowed these alternative depth dependences as well the model assumes simple linear adsorption of the tracer between solid and solution phases and uses a time dependent flux at x 0 which is a combination of that originating externally and the depth integrated contributions from feeding particle selectivity is introduced by applying mass conservation separately to transport of the tracer and bulk sediments properties of the model are illustrated for tracers in nondispersive systems strongly bound to sediment solids cb recycling gives rise to transient reflections on passage of a tracer pulse through the zone of bioturbation reflections readily disappear in the presence of various integrative processes the system time resolution is defined in terms of the downward propagation of dual tracer pulses and shown to be systematically degraded by intensified feeding and selective transport a simple expression is developed for estimating the magnitude of particle selective changes in steady state surface concentrations in terms of the particle selectivity factor and the efficiency of sediment reworking unrecognized particle selectivity can lead to false inferences about changes in the rate of delivery of tracers to sediments selective cb recycling can cause a tracer such as 210pb to be retained largely within the bioturbation zone so that relatively little activity reaches the region of sediment useful for geochronological interpretation increased selectivity accentuates the sensitivity of the profile to details of the reworking process the model applied to previous studies of cb deposit feeders in tracer labeled freshwater microcosms shows the need to include particle diffusion as well as advection in the treatment of cb transport

magnetite dissolution in siliceous sediments 1 magnetite dissolution and consequent loss of magnetization is widely observed in reducing sedimentary environments where the decrease in eh ph values with depth is driven by bacterially mediated degradation of organic carbon we have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and or silicic volcanic ash these depletions in magnetization are greater than can be accounted for by dilution with magnetite poor sediments and suggest that postdepositional destruction of magnetite has occurred biosiliceous sediments usually also contain elevated concentrations of organic carbon which makes it difficult to separate organic carbon related magnetite dissolution from other possible mechanisms for magnetite dissolution however the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox state of the environment this suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations and appropriate eh ph conditions and predict that magnetite will break down to produce iron bearing smectite a survey of magnetic susceptibility and pore water geochemical data from widely distributed ocean drilling program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities this observed link also holds for environments with low biogenic silica productivity and low organic carbon content but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments

effects of diagenesis on magnetic mineralogy in a jurassic claystone limestone succession from the paris basin we performed detailed rock magnetic and geochemical analyses on a previously published early jurassic magnetostratigraphic section the results improve our understanding of acquisition and preservation processes of magnetization in a series of alternating claystones and limestones the main carrier of magnetization is magnetite anhysteretic remanent magnetization arm varies by a factor of 40 and the arm variations are linked to magnetite grain size comparison of magnetic and geochemical data shows that when carbonate content is high 30 and 13 c0 magnetite is characterized by small grain sizes whereas when carbonate content is low 20 magnetite is coarse grained it appears that the oxidation of organic matter by sulfate reduction controls both 13 c and magnetite grain size h 2 s produced during sulfate reduction causes partial dissolution of magnetite grains with the finest magnetite grains those that best record the magnetic signal being dissolved first despite this partial dissolution both the direction and polarity of the original remanent magnetization are preserved

sources deposition rates and decomposition of organic carbon in recent sediment of sachem head harbor long island sound the vertical distributions of excess 210pb and fall out 239 240pu imply a uniform sedimentation rate of 1 4 1 6 cm year 1 from 0 to 105 110 cm this sediment accumulation rate is compatible with sulfate reduction rate data from this location below 70 cm only aged refractory carbon is present cr 1 8 c with an age of approximately 2400 years this phase is present in a number of locations across long island sound planktonic carbon cp is present above the 60 67 cm horizon a value of 1 0 for ap 14c activity at 32 37 cm was taken ap 1 285 was used for contemporary plankton this was obtained by correcting the measured ap of a plankton tow sample for admixed refractory carbon these values were then used to calculate cr cp and cf fossil carbon at 32 37 cm and 6 12 cm the only values compatible with the known sulfate reduction rate data are cr equal to pre 60 67 cm levels 1 6 1 8 c cf being 0 3 c at both depths and cp decreasing with depth from 0 3 to 0 4 c at 6 12 cm to close to zero at 32 37 cm

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

detrital and early diagenetic processes in late pleistocene and holocene sediments from the sw galicia bank inferred from high resolution enviromagnetic and geochemical records abstract u channel based high resolution magnetic and geochemical measurements supported by scanning electron microscopy x ray diffraction and fluorescence and grain size analyses of discrete samples were carried out on six sediment cores recovered from the sw flank of the galicia bank at the prestige s wreck area five magnetochemical facies reflecting the different sedimentary environments i e hemipelagic contouritic turbiditic sediment provenances and early diagenetic redoxomorphic processes i e mn peaks are defined on the basis of a cluster analysis the records showed climatically forced detrital signatures of regional importance associated with continental slope sedimentary processes the chronostratigraphy and correlation of the cores is based on 20 14 c ams dating recognition of heinrich like events h1 to h3 and well documented holocene climatic features from a geochemical and enviromagnetic perspective the late pleistocene to holocene evolution of the area shows two different sedimentation contexts mis 2 is dominated by down slope depositional processes the highest sedimentation rates 9 1 cm ky were achieved during this interval the major turbidite activity is recorded between 31 000 yr bp and 13 500 yr bp successive arrivals of ird h1 to h3 are also identified during this period in contrast holocene sedimentation was dominated by a mixture of pelagic hemipelagic and along slope depositional processes and presented considerable lower sedimentation rates 3 cm ky bottom currents intensification at 7000 yr bp and 4500 yr bp coinciding with late holocene warm events are also recognised during this stage

a sedimentary record of the deposition of heavy metals and magnetic oxides in the loch dee basin galloway scotland since c ad 1500 geochemical analyses of a 210pb dated piston core from loch dee have highlighted trends of increased pb and cu deposition within the catchment as early as ad 1500 probably as a consequence of local mining and smelting activity more marked atmospheric pollutant deposition appears to have occurred in association with european industrialization after c 1840 causing a conspicuous enhancement of pb zn cu and magnetically detectable fe oxide concentrations in the uppermost 8 cm of sediment the co variability of these contaminants through the surficial sediment suggests that they emanate from a common industrial process and the presence of magnetic oxides in cenospheric particulates implicates thermal power generation plant as a principal pollutant source this interpretation is further supported by the temporal comparability of trace metal magnetic oxide influxes at loch dee and documented records regarding european fossil fuel combustion levels over the past 150 years the reduced heavy metal concentrat

carbon sulfur iron relationships in sedimentary rocks from southwestern taiwan influence of geochemical environment on greigite and pyrrhotite formation the importance of the magnetic iron sulfide minerals greigite fe3s4 and pyrrhotite fe7s8 is often underappreciated in geochemical studies because they are metastable with respect to pyrite fes2 based on magnetic properties and x ray diffraction analysis previous studies have reported widespread occurrences of these magnetic minerals along with magnetite fe3o4 in two thick plio pleistocene marine sedimentary sequences from southwestern taiwan different stratigraphic zones were classified according to the dominant magnetic mineral assemblages greigite pyrrhotite and magnetite dominated zones greigite and pyrrhotite are intimately associated with fine grained sediments whereas magnetite is more abundant in coarse grained sediments we measured total organic carbon toc total sulfur ts total iron fet 1n hcl extractable iron fea and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals the studied sediments have low ts fea weight ratios 0 03 0 2 far below that of pyrite 1 15 which indicates that an excess of reactive iron was available for pyritization observed low ts 0 05 0 27 is attributed to the low organic carbon contents toc 0 25 0 55 which resulted from dilution by rapid terrigenous sedimentation the fine grained sediments also have the highest fet and fea values we suggest that under conditions of low organic carbon provision the high iron activity in the fine grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded which in turn favored preservation of the intermediate magnetic iron sulfides the relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous fes into greigite or into pyrrhotite compared to pyrrhotite dominated sediments greigite dominated sediments are finer grained and have higher fea but lower ts we suggest that diagenetic environments with higher supply of reactive iron lower supply of labile organic matter and consequently lower sulfide concentration result in relatively high eh conditions which favor formation of greigite relative to pyrrhotite

how environmental magnetism can enhance the interpretational value of grain size analysis a time slice study on sediment export to the nw african margin in heinrich stadial 1 and mid holocene abstract sediment dynamics in limnic fluvial and marine environments can be assessed by granulometric and rock magnetic methodologies while classical grain size analysis by sieving or settling mainly bears information on composition and transport the magnetic mineral assemblages reflect to a larger extent the petrology and weathering conditions in the sediment source areas here we combine both methods to investigate late quaternary marine sediments from five cores along a transect across the continental slope off senegal this region near the modern summer intertropical convergence zone is particularly sensitive to climate change and receives sediments from several aeolian fluvial and marine sources from each of the investigated five geob sediment cores 494 2956 m water depth two time slices were processed which represent contrasting climatic conditions the arid heinrich stadial 1 15 kyr bp and the humid mid holocene 6 kyr bp each sediment sample was split into 16 grain size fractions ranging from 1 6 to 500 m concentration and grain size indicative magnetic parameters susceptibility sirm hirm arm and arm irm were determined at room temperature for each of these fractions the joint consideration of whole sediment and magnetic mineral grain size distributions allows to address several important issues i distinction of two aeolian sediment fractions one carried by the north easterly trade winds 40 63 m and the other by the overlying easterly harmattan wind 10 20 m as well as a fluvial fraction assigned to the senegal river

magnetotactic bacteria and single domain magnetite in hemipelagic sediments remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time scale study geomagnetic reversals and secular variation and measure the rates of seafloor spreading although these sediments may contain different magnetic minerals magnetite is the most commonly observed and magnetically stable phase and its size shape and post depositional fate affect the magnetic remanence of the sediments biogenic magnetites are single domain with a high natural magnetic remanence nrm and have been suggested as a significant source of magnetic remanence in marine sediments we have studied surface sediments from the santa barbara basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra fine grained single domain magnetite using a novel extraction technique transmission electron microscopy and squid magnetometry we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments

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

rock magnetic and geochemical record in a sediment core from the eastern arabian sea diagenetic and environmental implications during the late quaternary abstract rock magnetic concentration grain size and mineralogy parameters together with organic carbon calcium carbonate redox sensitive elements 18 o of globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off mangalore southwestern margin of india the top 290 cm sediments of the core correspond to the last 18 kabp the 18 o and magnetic records exhibit major events at 16 kabp 14 5 kabp 11 5 kabp and 9 8 8 6 kabp related to start and intensity of the summer monsoon and climate change and are synchronous with that of the western arabian sea and north atlantic the sediments with high magnetic susceptibility correlate with high sedimentation rates the sediments are dominated by fine grained magnetite but intervals of 1 2 3 8 kabp and 10 13 5 kabp were subjected to diagenetic changes resulting in the dissolution of fine grained magnetites and enrichment of redox sensitive trace elements cu ni zn v mo and u the sediments between 290 cm and 445 cm correspond to 18 27 kabp and are characterized by distinct decrease in magnetic concentration grain size and mineralogy parameters high organic carbon low concentrations of redox sensitive trace elements and abundant pyritized tubules the reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub oxic conditions revealed by low concentrations of trace elements in the sediments the seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum lgm intermittent bioturbation however allowed oxidants to penetrate into the sediments remobilized redox sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment

biogenic magnetite from surface holocene carbonate sediments great bahama bank magnetic separates and rock magnetic tests from holocene carbonate surface sediments of the bahamas indicate that fine grained magnetite maghemite is common at or near the sediment water interface and forms a major portion of the magnetic fraction magnetite crystals range from 400 to 1000 a in diameter as observed under the transmission electron microscope tem and often occur as elongate chains or multi grain clusters crystals are very similar in dimension and grain habit to those of known biogenic origin mainly bacteria all measured grains are within the single domain stability field for magnetite based on crystal dimensions coercivity distributions are fairly broad 5 100 mt or greater likely the result of partial oxidation of the magnetic minerals no large multidomain grains were observed in the tem study individual grains occur in several shapes in the microscope image mainly hexagonal and cuboidal prismatic widespread occurrence of single domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization a contribution of magnetite from authigenic precipitation during burial has not been documented in these near surface environments

early diagenetic greigite as a recorder of the palaeomagnetic signal in miocene pliocene sedimentary rocks of the carpathian foredeep romania summary during the miocene pliocene the carpathian region represented the westernmost part of the so called eastern paratethys a palaeobioprovince that covered central and eastern europe as well as parts of southwest asia previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides in chron c3r here we demonstrate using detailed rock magnetic investigations and scanning electron microscope sem analyses that the major magnetic iron sulphide mineral is greigite fe3s4 thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 c and sem observations indicate octahedral grain morphologies and fe s ratios that are indicative of greigite hysteresis loops have rectangular shapes which are typical of single domain behaviour with coercivity and coercivity of remanence at values of bc 35 45 mt and bcr 52 67 mt respectively first order reversal curve diagrams have contours that close around single domain peaks with bc values of 45 90 mt isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component dispersion parameter dp 0 15 log units indicating a narrow grain size distribution a positive fold test inclination shallowing and two positive reversal tests are additional arguments for the syn depositional formation of this greigite we thus argue that most of the greigite was formed under early diagenetic conditions that is within 1000 yr of deposition of the sediment in this setting and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation the appearance of greigite during chron c3r between 6 0 and 5 5 ma in the carpathian foredeep is most likely related to regional tectonic and or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions

river avulsions and sedimentary evolution of the luanhe fan delta system north china since the late pleistocene abstract the luanhe fan delta lfd is one of the most important coastal deltas in north china numerous investigations on the late quaternary evolution of this fan delta system have been implemented during the 1980s however few detailed sedimentologic studies have been undertaken based on comprehensive analysis of sedimentary facies grain size microfossils radiocarbon dates ams14c and optically stimulated luminescence osl ages from three 30 m long cores along with recent reports from a series of cores and seismic profiles from the coastal areas we reconstructed the detailed late quaternary sedimentary evolution of the northern bohai coast which evolved from a coastal barrier lagoon system during mis5 to an alluvial plain with river channels and floodplains between mis4 and the early holocene a variety of transgressive coastal depositional environments freshwater marshes beach ridges sandy barriers shallow seas developed during the early holocene these environments were followed by delta progradation the delta system then gradually turned into a coastal barrier lagoon system under the combined impact of prevailing wave driven currents and local subsidence two dramatic 90 degree diversions of the lower luanhe river course likely influenced the sedimentary evolution of the lfd in its inner portion the first avulsion took place in qianxi probably at the end of the late pleistocene 15 000 cal a bp it led to the abandonment of the luanhe alluvial fan in the western part of the northern bohai coastal plain nbcp and to the formation of a new alluvial fan in the eastern nbcp this alluvial fan evolved into a fan delta system 7000 3500 cal a bp during the holocene transgression the second avulsion occurred at zhuacun during the late holocene probably at 3600 100 cal a bp following this latter avulsion the luanhe river shifted toward the east and formed a new fan delta system 3700 cal a bp present with its apex in luanzhou abrupt tectonic uplift in this highly seismic region likely controlled the two avulsions and the dramatic shift of the alluvial fan complexes climate and sea level changes may also have partly influenced the development of the lfd

nutrient and iron cycling in a modern analogue for the redoxcline of a proterozoic ocean shelf abstract proterozoic oceans are characterized by a thin veneer of oxygenated surface waters underlain by anoxic or euxinic deeper waters and it has been hypothesized that metabolically flexible microbes such as cyanobacteria would have thrived under these conditions however relatively little is known about the relationship between water chemistry microbial mats and the biogeochemical cycling of macronutrients c n p and s and fe in these redox transition zones we investigated the burial of these elements in a modern analogue for the proterozoic to constrain these relationships better the low oxygen and high sulfate water chemistry of middle island sinkhole mis a sinkhole within lake huron allows for the proliferation of a microbial mat community with diverse metabolisms within the photic zone at the sediment water interface similar to what has been hypothesized following the great oxidation event 2 35 ga sediments from mis were compared to those of a fully oxygenated lake huron control site in order to determine how water chemistry and microbial mat presence impact biogeochemical cycling mis sediments have a greater accumulation of macronutrients than lake huron with stable c isotopes and c n ratios demonstrating that the c buried within mis and lake huron sediments comes from the same phytoplankton source given that both sites bury c from the same source the increased organic matter burial in mis is not dependent on microbial mat presence and instead must be controlled by differences in preservation sequential extraction of iron indicates that mis is ferruginous but not euxinic and that lake huron is oxic both results are consistent with measured water chemistry at the same time the presence of ferruginous iron speciation in sediments under a partly oxygenated water column suggests that the sediment is recording either rapid transformations across the redoxcline rather than the whole water column some in situ re speciation of iron or both this has important implications for our interpretations of fe speciation results in the geologic record taken together nutrient and fe burial in the analogue mis site demonstrate that proterozoic biogeochemical cycling in surficial sediments may be predominately driven by a combination of water column and porewater redox chemistry and not microbial activity

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

a 37 000 year environmental magnetic record of aeolian dust deposition from burial lake arctic alaska abstract environmental magnetism and radiocarbon dating of burial lake sediments constrain the timing and magnitude of regional aeolian deposition for the noatak region of western arctic alaska for the last 37 000 years burial lake 68 43 n 159 17 w 21 5 m water depth is optimally located to monitor regional dust deposition because it is perched above local drainage and isolated from glacial processes cores collected in the summer of 2010 were studied through the application of magnetizations and progressive alternating field af demagnetization of u channel samples with additional data provided by computed tomography ct derived density hysteresis measurements isothermal remanent magnetization irm acquisition experiments organic carbon content biogenic silica physical grain size radiocarbon dating of wood seeds and plant macrofossils point source magnetic susceptibility and x ray fluorescence xrf with similar magnetic properties to regional alaskan loess deposits low coercivity highly magnetic material deposited during the late pleistocene contrasts with a high coercivity weakly magnetic component found throughout the record consistent with locally derived detritus the relative proportion of low coercivity to high coercivity magnetic material defined by the s ratios is used to reconstruct the regional input of dust to the basin over time a four fold decrease in the low coercivity component through the deglacial transition is interpreted to reflect diminished dust input to the region comparisons with potential sources of dust show that the timing of deposition in burial lake is largely consistent with general aridity lack of vegetative cover and increased windiness rather than glacial advances or retreats the influence from subaerial exposure of continental shelves cannot be ruled out as a significant far field source of dust to interior alaska during the last glacial maximum lgm but is unlikely to have been the sole source or to have contributed to increased dust in both the early and late holocene

variations of calcium carbonate organic carbon and iron sulfides in anoxic sediment from the orca basin gulf of mexico abstract sediment from the anoxic orca basin is highly laminated and enriched in iron sulfides and organic carbon unlike sediment from the surrounding northwest gulf of mexico continental slope black laminae are enriched in calcium carbonate 19 vs 15 organic carbon 1 8 vs 0 7 and iron sulfides 0 7 vs 0 4 compared to light gray laminae however more of the fes is converted to fes2 in the light gray laminae dark gray laminae of intermediate composition are also common the laminae probably reflect annual and longer term variations in organic matter supply to the basin and variations in mixing at the brine seawater interface

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

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

organic carbon sulphur and iron in recent semi euxinic sediments of kau bay indonesia kau bay island of halmahera eastern indonesia is a 470 m deep basin separated from the pacific ocean by a sill that is at present only 40 m below sea level during weichselian time the sea level dropped below the depth of the sill and freshwater sediments were deposited these sediments are rich in organic matter corg 2 6 wt with terrigenous isotope characteristics 13corg 28 to 30 and are also rich in sulphur s 4 wt this sulphur enrichment is interpreted to be a late diagenetic feature related to diffusion of dissolved sulphate and sulphide from the marine sediments into the freshwater sediments the sulphur in these sediments has a 34s value of 15 the overlying holocene sediments consist of marine muds deposited in alternating low oxygen and h2s bearing bottom waters the marine sediments are also rich in organic matter corg 2 6 wt but of mixed terrigenous and marine origin 13c 25 to 19 moreover the sulphur content of these holocene sediments is lower s 1 2 wt and isotopically lighter 34s 20 in addition to pyrite large amounts of acid volatile sulphur avs compounds accumulate in kau bay the reactive iron content of the sediment limits the amount of iron that is sulphidized while the supply of oxidants controls the limited conversion of avs to pyrite total and reactive iron contents are strongly correlated with the amount of terrigenous organic matter as a consequence of their common provenance and depositional histories the marine sediments in kau bay show no simple relation between organic carbon and sulphur

a multiproxy study distinguishes environmental change from diagenetic alteration in the recent sedimentary record of the inner cadiz bay sw spain in this study we reconstruct the recent environmental evolution of the inner cadiz bay using sedimentary records reaching back as far as ad 1700 we report lithological descriptions of the sedimen

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern mediterranean sea summary magnetic properties irm arm xin s ratio at 0 3 t room temperature rt hysteresis and thermomagnetic curves and geochemical data fe s mn al ti organic c were studied in two eastern mediterranean boxcores abc26 and bc19 at a resolution of 3 5 mm the boxcores contain sapropel s1 9 6 kyr bp at a few decimetres below seafloor the magnetic fraction consists predominantly of single domain sd to pseudo singledomain psd magnetite in the entire cores the original input of magnetic grains comes from two sources aeolian dust both cores and volcanic ash from the minoan eruption of santorini core bc19 only non steady state diagenesis has changed the magnetic mineralogy considerably in these alternating organic rich organic poor sediments during deposition of sapropel s1 reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities coarser magnetic grain sizes and partial maghemitization in thermomagnetic curves two types of pyrite can be identified one oxidizes below 450 uc and the other above 450 uc the higher oxidation temperature is predominantly found below the sapropel this may be related to the microtexture of pyrite which is euhedral below sapropels and mainly framboidal within sapropels since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half c 5 cm of the sapropel the oxidized part of the sapropel is enriched in diagenetically formed fe oxides with relatively high coercivity and arm the maximum coercivity is found in a distinct layer between the present day mn and fe redox boundaries at the top of the unoxidized sapropel the freshly precipitated fe oxides in this centimetre thick layer contain a mixture of superparamagnetic sp grains and high coercivity sd magnetite higher in the oxidized zone the freshly precipitated fe oxides have aged into generally slightly lower coercivity sd grains with relatively high arm in addition to the diagenetic formation of fe oxides at the top of the sapropel formation of a ferrimagnetic fe monosulphide may have occurred within the sapropel during later stages of diagenesis which may have enhanced the arm signal in the organic rich interval in particular

sea level change and remagnetization of continental shelf sediments off new jersey odp leg 174a magnetite and greigite diagenesis summary palaeomagnetic and rock magnetic studies were performed on a seaward transect of continental shelf sediments from sites 1071 and 1072 of ocean drilling programme leg 174a on the new jersey margin the sediments recorded a polarity change from reversed to normal which was tentatively interpreted as the matuyama brunhes boundary the polarity boundary is closely correlated with the sequence boundary pp3 s which is considered to have formed as an erosional unconformity during regression and subsequent transgression related to global eustacy rock magnetic analyses indicate that the sediment contains comparable amounts of magnetite and greigite fe3s4 with possible trace amounts of haematite a marked change in rock magnetic properties is recorded at site 1072 with an increase in greigite concentration at the polarity boundary 62 4 metres below seafloor where the grain size does not change at site 1071 greigite is dominant from 2 m below pp3 s to 0 8 m above it these intervals are characterized by higher peak coercivities of remanence and subdued remanence in both the magnetite and greigite components palaeomagnetic analysis indicates that the intervals just below pp3 s have dual component magnetizations of reversed polarity and are considered to have been deposited during the matuyama chron c1r 1r and were possibly remagnetized during the brunhes chron through the formation of the pp3 s surface detrital magnetite and early diagenetic greigite might have carried the reversed polarity magnetization during the formation of pp3 s in the brunhes chron greigite might have formed at the oxidation front with ongoing downward formation due to oxidation of pyrite by percolating fresh water which might be a cause of the remagnetization our study indicates that careful rock magnetic investigation is necessary for magnetostratigraphic studies of continental shelf deposits in order to recognize remagnetizations induced by sea level changes

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

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

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

rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the california continental borderland the rock magnetic properties of marine sediments from the california continental borderland san pedro santa catalina and san nicolas basins have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals previous studies have noted that the natural remanent magnetization in these sediments primarily carried by detrital magnetite decays to 10 or less of its surface value soon after deposition this decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process the shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest grained magnetic material the intensity decreases which are linearly proportional to magnetite concentration decreases indicate that dissolution occurs with rate constants ranging from 0 3 to 1 6 kyr 1 the rate constants sulfide concentrations and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite

quantifying magnetite magnetofossil contributions to sedimentary magnetizations under suitable conditions magnetofossils the inorganic remains of magnetotactic bacteria can contribute to the natural remanent magnetization nrm of sediments in recent years magnetofossils have been shown to be preserved commonly in marine sediments which makes it essential to quantify their importance in palaeomagnetic recording in this study we examine a deep sea sediment core from offshore of northwestern western australia the magnetic mineral assemblage is dominated by continental detritus and magnetite magnetofossils by separating magnetofossil and detrital components based on their different demagnetization characteristics it is possible to quantify their respective contributions to the sedimentary nrm throughout the brunhes chron in the studied core the contribution of magnetofossils to the nrm is controlled by large scale climate changes with their relative importance increasing during glacial periods when detrital inputs were low our results demonstrate that magnetite magnetofossils can dominate sedimentary nrms in settings where they are preserved in significant abundances

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

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

multiple mechanisms of remagnetization involving sedimentary greigite fe3s4 sedimentary greigite fe3s4 is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine grained marine and terrestrial sediments we have conducted detailed scanning electron microscope observations on polished sections from such sediments coupled with elemental microanalysis to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases this detailed observational work in conjunction with recently published work has enabled identification of several mechanisms for remagnetization involving greigite 1 neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite 2 greigite growth within cleavages of detrital sheet silicate grains 3 neoformation of greigite on the surfaces of authigenic clays smectite illite 4 greigite nucleation on the surfaces of siderite 5 greigite growth on the surface of gypsum that resulted from earlier oxidation of nodular pyrite many other mechanisms for greigite neoformation are conceivable the key variables are appropriate redox conditions for generation of sulphide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron bearing minerals documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization which will complicate or compromise studies of environmental magnetism and geomagnetic field behaviour

a paleomagnetic and geochemical record of the upper cochiti reversal and two subsequent precessional cycles from southern sicily italy abstract a detailed paleomagnetic and geochemical study of the upper cochiti n r reversal recorded in marine marls on southern sicily shows two consecutive and very rapid transitions r n and n r that coincide with distinct lithological boundaries a fe migration model is presented in which magnetite is formed under different diagenetic conditions during sulphate reduction originally formed primary magnetite is preserved this primary magnetite has recorded the pre transitional normal polarity after burial migration of ferrous iron occurs and subsequent oxidation produces secondary magnetite which records the post transitional reversed polarity and produces the two apparent transitions in addition low temperature lt and high temperature ht components record the reversal in a slightly different way the ht component being acquired with a delay it is probable that both components reside in magnetite but have a different origin and grain size distribution giving different blocking temperature spectra the actual upper cochiti reversal occurs at a level with an estimated astronomically calibrated age of 4 165 ma

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr

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

magnetic mineral dissolution in pleistocene fluvio lacustrine sediments nihewan basin north china abstract the nihewan fluvio lacustrine sequence north china has recorded late pliocene pleistocene climatic and environmental changes and contains valuable information on early human evolution in high latitude east asia we carried out a combined mineral magnetic and geochemical investigation on a sequence from the xiantai section eastern nihewan basin results suggest that large amplitude magnetic property variations between so called high magnetic and low magnetic units mainly result from preservation dissolution cycles of detrital magnetic minerals in alternating oxic and anoxic depositional environments in our preservation dissolution model the high and low magnetic units represent glacial and interglacial deposits respectively this contributes to a better understanding of the link between magnetic properties and climate in the xiantai fluvio lacustrine sequence based upon this relationship early humans may have occupied the xiantai and xiaochangliang sites during an interglacial period during both interglacial and glacial periods at the donggutuo site and during a glacial period at the maliang site our work provides a paleoenvironmental context for early human adaptation and occupation in mainland east asia during the early pleistocene

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

the geochemistry of river particulates from the continental usa major elements river particulates have been collected from twenty three rivers from throughout the continental usa the rivers drain mostly large basins basin areas range from 2 1 x 10 3 to 2970 x 10 3 km2 composed of mixed lithologies and spanning a wide range of climatic conditions as evidenced by a large variation in air and ground water temperatures precipitation and runoff suspended particulates have been analyzed for organic and inorganic carbon as well as the major elements al fe mn k si ca mg and na also determined were suspended sediment loads at the time of sampling particle surface areas and grain size distributions five year average river water chemistry and suspended sediment concentrations obtained from usgs water supply reports are also included as supporting information particle chemistry systematically varies with rates of runoff with high runoff rivers transporting the most heavily altered particulates and low runoff rivers carrying the least altered degrees of alteration are indicated by the extent to which the refractory nonmobile elements al and fe are concentrated into the particulates and the extent to which the most easily weathered elements na ca and mg have been leached overall the susceptibility of elements towards leaching is consistent with numerous previous descriptions of element mobility on weathering a simple predictive model has been developed to explore and explain the observed trends between particle composition and river runoff thus the elemental composition of riverine particulates may be given by the expression pcoi propi tm dcoi ss where pcoi is the concentration of element i as the oxide in a riverine particulate propi is the proportion of i as the oxide in unweathered rock dcoi is the dissolved concentration of i as the equivalent oxide tm is the total rock derived mass dissolved in river water and ss is suspended sediment concentration to utilize this equation a model was first developed to predict the concentration of river solutes with the input of temperature precipitation and a limited number of weathering parameters combining this solute model with the above predictive equation for particle composition it was discovered that the observed trends between particle composition and runoff require the special circumstance of decreasing ss with increasing runoff hence the composition of river particulates depends both on the climate parameters of runoff and temperature as they control dissolved river chemistry and the nonclimate parameters including elevation relief tectonics and basin area that control the ss load of rivers

dissolution of titanomagnetite and sulphidization in sediments from lake kinneret israel summary magnetic extracts obtained from sediment cores recovered from lake kinneret israel were subjected to comprehensive scanning electron microscope and in line energy dispersive x ray spectroscopy analyses a variety of ferrimagnetic iron oxides and iron sulphide particles were identified along with apparently non magnetic iron sulphides the majority of the iron oxides are titanomagnetites that have undergone pervasive dissolution and in many cases were only preserved as skeletal 3 d lattices of ilmenite lamellae often the spaces between the submicron ilmenite lamellae are empty also sponge like dissolution features are observed in some residual titanomagnetite grains another group of titanomagnetite grains is characterized by superficial shrinkage cracks which indicate low temperature maghemitization in some cases these cracks appear to be where dissolution started most titanomagnetites contain impurities of al mg si and mn on the order of 1 per cent chromium magnetites with minor ti mg and al content on the order of a few percent but without mn were also found chromium magnetites mostly occur as octahedra in a well preserved state which indicates that they are less prone to dissolution magnetite spherules of 40 60 m in size which were mostly hollow were only found in material collected from the outer zone of the core section that was in contact with the steel core barrel these spherules were not found in material collected from the centre of the core they are therefore interpreted as contaminants that were produced during fabrication of the steel core barrel iron sulphides are present as pyrite either as larger aggregates of cubic crystals each of up to 20 m in size or as 5 10 m framboidal aggregates consisting mostly about 0 3 m idiomorphic octahedra the cubic pyrite crystals have heterogeneous compositions pure pyrite should be non magnetic but even aggregates of up to 150 m in size were magnetically separated they probably therefore contain a magnetic core of greigite or overgrown titanomagnetite the latter possibility is supported by observations of residual titanomagnetites with individual pyrite crystals residing directly on their surfaces or filling space between ilmenite lamellae irregular aggregates of flaky submicron iron sulphides were also found which are interpreted to be greigite the greigite has a low degree of visible crystallization pyrite and or greigite grains were found inside the aggregates or in agglomerates with organic remnants together visual observations and elemental analyses provide clear evidence of pervasive dissolution of primary titanomagnetite and concurrent sulphidization of sediments from lake kinneret

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

rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal bearing sediments offshore shimokita peninsula japan iodp site c0020 sediments recovered at integrated ocean drilling program iodp site c0020 in a forearc basin offshore shimokita peninsula japan include numerous coal beds 0 3 7 m thick that are associated with a transition from a terrestrial to marine depositional environment within the primary coal bearing unit 2 km depth below seafloor there are sharp increases in magnetic susceptibility in close proximity to the coal beds superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence we investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record using isothermal remanent magnetization irm thermal demagnetization anhysteretic remanent magnetization arm iron speciation and iron isotopes magnetic mineral assemblages in all samples are dominated by very low coercivity minerals with unblocking temperatures between 350 580 c that are interpreted to be magnetite samples with lower unblocking temperatures 300 400 c higher arm higher frequency dependence and isotopically heavy 56fe across a range of lithologies in the coal bearing unit between 1925 1995 mbsf indicate the presence of fine grained authigenic magnetite we suggest that iron reducing bacteria facilitated the production of fine grained magnetite within the coal bearing unit during burial and interaction with pore waters the coal peat acted as a source of electron donors during burial mediated by humic acids to supply iron reducing bacteria in the surrounding siliciclastic sediments these results indicate that coal bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time within the subsequent deep biosphere

plio pleistocene magnetic polarity stratigraphies and diagenetic magnetite dissolution at odp leg 177 sites 1089 1091 1093 and 1094 abstract magnetic polarity stratigraphies from odp leg 177 high resolution sites indicate brunhes sedimentation rates in the 12 25 cm kyr range with a trend of decreasing sedimentation rates with increasing age magnetite is the principal remanence carrying mineral downcore alteration of magnetite and authigenic growth of iron sulfides introduces a high coercivity diagenetic remanence carrier pyrrhotite the change in pore water sulfate with depth in the sediment tends to be in step with the decrease in magnetization intensity indicating the link between sulfate reduction and magnetite dissolution shipboard pass through magnetometer data are generally very noisy due to a combination of weak magnetization intensities drilling related core deformation and the influence of authigenic iron sulfides post cruise progressive demagnetization of discrete samples aids the magnetostratigraphic interpretation as these measurements are less influenced by low magnetization intensities and drilling related deformation the magnetostratigraphic interpretations provide much needed calibration for biostratigraphic events in the high latitude southern oceans apart from the odp hole 745b kerguelen plateau published plio pleistocene magnetostratigraphies from odp sites in the southern ocean are poorly constrained for this reason we compare interpolated ages of 11 radiolarian events and one diatom event that occur at hole 745b and leg 177 sites