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

depositional history of quaternary sediments on the continental shelf off the southeastern coast of korea korea strait abstract the depositional history of quaternary sediments on the continental shelf off the southeastern coast of korea korea strait was investigated by means of analysing high resolution seismic profiles and sediment samples quaternary sediments up to 50 m thick overlie the irregular surface of the acoustic basement in the inner shelf the sedimentary column is comprised of two main sequences separated by a mid reflector which is interpreted as the pre holocene surface the holocene sequence reaching up to 30 m in thickness with a depocenter near the present river mouth is characterized by deposition in a prodelta environment while the upper pleistocene sequence represents deposition in deltaic or shallow water environments during the late glacial period most of the shelf was subaerially eroded and deposition took place only in the trough region the present location of marine built terraces indicates that the lowest position of sea level was about 130 m below its present level the subsequent postglacial transgression reworked pleistocene sediments to form a thin veneer of transgressive facies sea level rose rapidly in the early stages of the postglacial transgression and approached the present level at approximately 7000 5000 yrs b p recent sedimentation the holocene sequence probably began at that time

inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan abstract to establish a magnetobiostratigraphy for a 620 m thick middle to late pleistocene mudstone sequence in the lower gutingkeng formation of the tsailiao chi tlc section in southwestern taiwan we conducted paleomagnetic and mineral magnetic measurements together with sediment granulometry and calcareous nannofossil identification paleomagnetic samples from 65 sites revealed two types of thermal demagnetization 25 400 c behavior 1 single component stable characteristic remanence in magnetite and pyrrhotite dominated samples type s and 2 abrupt changes in polarity when samples with significant greigite concentrations were heated above 320 340 c type c the characteristic polarities derived from type s samples and from magnetite dominated type c samples obtained above 340 c are consistent with those determined from nannofossil biostratigraphy this implies that the nrm carried by magnetite and pyrrhotite is reliable the essentially antiparallel remanence components in type c samples below 340 c are attributed to greigite the almost antiparallel direction could have resulted from delayed formation of greigite but in this case the different direction of this component must have resulted from variable remanence lock in times alternatively the opposite polarities may result from self reversal which warrants further investigation pyrrhotite and greigite may have both formed authigenically but there is no clear explanation for the observed differences in direction

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

variation of magnetic properties in sediments from lake towuti indonesia and its paleoclimatic significance abstract we investigated the mineral magnetic behavior of sediments from lake towuti located in the eastern sulawesi ophiolite belt indonesia rock magnetic analysis supplemented by x ray diffraction and scanning electron microscopy analysis were performed on sediment core tow10 9b from the north basin of lake towuti to give insights on the environmental and sedimentary processes controlling the magnetic properties of the sediment and its paleoclimatic significance the results show that the core has three distinct zones of varying magnetic properties careful examination demonstrates that these zones correspond to varying levels of iron oxide dissolution and magnetite precipitation that are climatically and environmentally dependent the magnetically strongest zone is characterized by weak iron oxide dissolution and intense magnetite precipitation likely driven by changes in the stratification and or water level of the lake during dry conditions in marine isotope stage 2 mis2 period whereas the two magnetically weaker zones are characterized by signs of dissolution and correspond to relatively wet conditions respectively during marine isotope stage 3 mis3 and the holocene although our data show that major changes in concentration dependent parameters such as magnetic susceptibility and saturation isothermal remanent magnetization sirm in lake towuti sediment correlate with changes in regional rainfall many of the concentration changes are more strongly affected by in situ chemical processes than by changes in erosion and terrestrial sediment supply these findings urge caution in the interpretation of magnetic mineral concentration profiles as indicators of clastic sediment inputs

magnetic properties of muddy sediments on the northeastern continental shelves of china implication for provenance and transportation mud deposits are widespread in estuaries and on continental shelves and contain important information regarding interactions between terrestrial and coastal processes magnetic properties of muddy sediments from the northeastern continental shelves of china including shelves close to korean coasts were analyzed using bulk and size fractionated samples to elucidate muddy sediment sources and transport types concentration and granulometry of magnetic minerals were determined and all samples were found to be dominated by magnetite muddy sediments from the zhemin coast off the zhejiang and fujian coasts have the highest magnetic susceptibility and saturation isothermal remanent magnetization sirm values while those from the central and southeastern south yellow sea and the western korea strait have lower values reflecting lower magnetic mineral concentrations single domain ferrimagnetic grains dominate the magnetic particles in the central south yellow sea while superparamagnetic sp grains contribute significantly to the magnetic grains in the bohai sea sp and coarse multi domain md grains dominate magnetic grain assemblages in the north yellow sea and along the zhemin coast rock sediment particle size is not a major factor affecting the magnetic properties of the mud deposits in the study area rather terrigenous inputs and hydrodynamic transport processes primarily control the magnetic properties and sediments from the changjiang and huanghe rivers make the most significant contribution this observation applies to the whole study area although terrigeneous magnetic mineral concentrations decrease with increasing distance from the river mouths due to both sorting and mixing with other sediments with weak magnetic signatures bacterial magnetosomes also play a role in the central south yellow sea

magnetic properties of light and dark sediment layers from the japan sea diagenetic and paleoclimatic implications abstract rock magnetic paleoclimatic diagenetic relationships of sediments spanning the last 0 78 ma have been investigated using samples collected from light and dark layers recovered at odp sites 794 yamato basin and 795 japan basin rock magnetic parameters k k fd arm sirm s ratio are shown to reflect diagenetic processes and climate related variations in the concentration mineralogy and grain size of the magnetic minerals contained within the sediments the magnetic mineralogy is dominated by ferrimagnetic magnetite type minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and or greigite magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size magnetite dissolution related to sulfate reduction due to bacterial degradation of organic matter is the process responsible for the magnetic characteristics observed in the dark layers testifying to the reducing conditions in the basin variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages major downcore changes in the magnetic properties observed at site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm tsushima current into the japan sea at about 0 35 0 40 ma ago

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

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

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

mineral magnetic study of lacustrine sediments from lake pumoyum co southern tibet over the last 19 ka and paleoenvironmental significance abstract high resolution proxy records from tibetan plateau are essential to understand the past climatic and environmental changes in this work we conducted systematic environmental magnetic studies on lacustrine sediments from lake pumoyum co southern tibet spanning from the last deglaciation 19 ka to present we correlated the magnetic proxies in the lake core with environmental changes the magnetic proxies from the 3 units were studied both unit 3 19 13 2 ka and unit 2 13 2 8 4 ka contain evidences for an anoxic or sub anoxic environment fine grained greigite dominates the bulk magnetic properties of unit 3 while both magnetite and greigite coexist in unit 2 overall this reveals a transition from an anoxic environment to an oxic environment in contrast unit 1 8 4 ka present spans the holocene when the lake environment was oxic there is an increasing trend in the concentration of magnetic minerals which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post glacial melt water flow although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units our results suggest that the magnetic proxies of the local lake environment on southern tibet are sensitive to environmental changes

partitioning of transgressive deposits in the southeastern yellow sea a sequence stratigraphic interpretation abstract the last glacio eustatic sea level fluctuation in the southeastern part of the yellow sea formed a number of unconformity bounded sedimentary sequences these sequences comprise 1 regressive to transgressive incision fills unit c 2 transgressive sand ridges and swales unit b and 3 transgressive to highstand muds lower mud unit a2 upper mud unit a1 the sequence boundary between unit c and the lower unit comprises erosional surface which can be traced for the entire study area the even and sharp boundary between the incision fills unit c and the sand ridges unit b represents a transgressive surface of erosion the depositional boundary between the lower mud unit a2 and the upper mud unit a1 suggests a change in oceanographic conditions the boundary is confined landward and correlated offshore with the top of units b and c that is exposed on the seafloor these transgressive deposits show partitioning controlled by currents tides and sediment supply as well as physiography of the seafloor

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

fuzzy c means cluster analysis of early diagenetic effects on natural remanent magnetisation acquisition in a 1 1 myr piston core from the central mediterranean abstract the influence of early diagenesis on the natural remanent magnetisation nrm in sediments from the calabrian ridge central mediterranean is analysed with the help of fuzzy c means fcm cluster analysis and non linear mapping nlm the sediments are variably coloured white beige purplish greenish and grey layers occur with occasionally intercalated sapropels the nrm acquired depends on both depositional conditions and diagenetic processes to describe these fcm was performed with in arm caco3 ba mn and s as variables an eight cluster model was derived with the clusters belonging to two categories one expressing mainly diagenetic processes i e dissolution and precipitation and the other expressing mainly depositional conditions the impact of diagenesis on nrm acquisition is profound and not restricted to the close vicinity of the anoxic sapropelitic layers as a consequence the influence of diagenetic processes on the nrm should be thoroughly assessed when selecting samples e g for the determination of the relative palaeointensity of the geomagnetic field application of multivariate classification techniques appears to be useful because it links rock magnetic parameters to the geochemical environment in the present piston core three short reversed geomagnetic events in the brunhes chron are preserved and indeed occur in clusters expressing no or minor diagenesis the recording of the blake event however has been prevented by later precipitation of magnetite in the corresponding interval

high resolution analysis of early diagenetic effects on magnetic minerals in post middle holocene continental shelf sediments from the korea strait 1 two studied sediment cores from the korea strait contain mud sequences 14 m and 32 62 m in thickness that were deposited during the last 6 000 years the sediments have uniform lithology and geochemical properties however marked down core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties an expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep sea sediments the studied sediments are divided into four descending intervals based on magnetic property variations interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite and the lowest solid phase sulfur contents interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations which suggest active reductive dissolution of detrital magnetic minerals interval 3 is marked by a progressive loss of hematite with depth and coincides with the minimum magnetite concentration interval 4 has an increasing down core enhancement of authigenic greigite which apparently formed at depths of 3 30 m below the sediment water interface due to arrested pyritization reactions these results indicate delays of thousands of years for acquisition of magnetizations carried by greigite which suggests that studies of geomagnetic field behavior from greigite bearing continental shelf sediments should be conducted with care also virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn depositional environmental processes

a new proxy for bottom water ventilation in the eastern mediterranean based on diagenetically controlled magnetic properties of sapropel bearing sediments magnetic properties of eastern mediterranean sediments recovered during leg 160 of the ocean drilling program odp provide insight into non steady state diagenetic reactions associated with accumulation and degradation of organic matter in sapropels according to their magnetic properties sapropels can be classified as one of three types that correspond to increasingly anoxic conditions at the time of sapropel formation a combination of magnetic and geochemical data suggests a causal relationship that enables determination of the relative role of bottom water ventilation versus productivity in the resulting diagenetic stage reached for the three types of sapropels it appears that increased productivity is a prerequisite for sapropel formation but once organic matter is available in sufficient amounts variable efficiencies in bottom water ventilation are more important for modulating the diagenetic context in which different types of sapropels formed magnetic properties are more sensitive to variations in bottom water ventilation than to productivity and can be used to establish relative variations in bottom water ventilation both at and after periods of sapropel formation magnetic results and the distribution and type of sapropels at odp site 966 eratosthenes seamount between 2 3 and 4 0 ma suggest that bottom water ventilation was modulated by the orbital eccentricity component with ventilation being restricted during 400 kyr eccentricity maxima and enhanced during eccentricity minima enhanced ventilation during eccentricity minima as indicated by magnetic data is consistent with the occurrence of red intervals at site 966 and at other eastern mediterranean sites drilled during odp leg 160 and also with astronomically modulated variations in caco3 content found in mediterranean land sections this suggests that variations in bottom water ventilation modulated by 400 kyr eccentricity cycles operated at a basin wide scale

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

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

early diagenesis in north atlantic abyssal plain sediments characterized by rock magnetic and geochemical indices processes of early diagenesis are investigated in a sequence of late quaternary turbiditic sediments from the madeira abyssal plain map west of the nw african continental margin in three sediment cores from this region solid phase geochemical indices of redox sensitive ion mobilization are compared with rock magnetic parameters as proxies for diagenesis characterization the mud dominated distal turbidites are emplaced mainly as a result of glacio eustatic changes in sea level and derive both from different sites on the nw african margin and from the flanks of the canary islands thus the turbidites are of variable composition especially in terms of carbonate detrital magnetic mineral and organic carbon content within organic rich 0 5 corg turbidite horizons remineralization of organic detritus proceeds in two stages the initial stage involves suboxic reductive diagenesis following depletion of pore water o2 while the later stage involves oxidative diagenesis associated with the slow descent of an oxidation front through the sediment due to diffusion of o2 from the overlying seawater rock magnetic parameters of these sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within the organic rich turbidites during suboxic diagenesis normalized quotients of magnetic parameters and remanence coercivity profiles of selected samples also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are progressively dissolved in all three cores studied however there is clear evidence for the presence of ultrafine ferrimagnetic iron oxide grains at depth within the suboxic zones of organic rich turbidites beneath both active and fossil oxidation fronts these grains are probably associated with populations of live magnetotactic bacteria which inhabit such organic rich horizons and which form a link in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results comply with previous observations made in studies of suboxic diagenesis in hemipelagic sediments and demonstrate the value of rapid and non destructive rock magnetic measurements for illuminating redoxomorphic processes of diagenesis involving transformation of iron phases in deep sea sediments

a stable chemical remanence in holocene sediments the ferrimagnetic iron sulphide greigite has been extracted from freshwater sediments from loch lomond and llyn geirionydd and identified by x ray diffraction and thermomagnetic analyses the authigenic greigite is probably the dominant carrier of a stable natural remanent magnetization in the freshwater sediments underlying marine sediments in loch lomond and in the lateglacial interstadial sediments from llyn geirionydd mineral magnetic analysis shows that greigite tends to have a higher sirm x ratio approximately 70 kam 1 than many natural magnetite assemblages this magnetic characteristic coupled with greigite s ability to oxidize easily and to change to a less magnetic mineral upon exposure to air may be used to indicate its presence in downcore magnetic profiles alternating field demagnetization cannot be used to distinguish chemical from detrital remanences in loch lomond or llyn geirionydd on account of the similar magnetic stabilities of greigite and magnetite the time lag between the acquisition of the primary detrital remanence and the secondary chemical remanence remains unknown and creates difficulties in understanding the record of paleomagnetic secular variation contained in these lake sediments

transgressive sand ridges on the mid shelf of the southern sea of korea korea strait formation and development in high energy environments abstract the mid shelf of the southern sea of korea korea strait is covered by a series of shore parallel sand ridges up to 63 km long 3 9 km wide and 22 m thick the ridges are a moribund type but show well organized dipping strata indicating appreciable progradation the submerged paleo channel system the paleo seomjin river associated with the sand ridge field suggests that the ridges primarily originated from river mouth sandy shoals which subsequently developed into shore parallel linear sand bodies in high energy environments the combined effect of coastal tidal and longshore and offshore ocean currents was possibly responsible for shore parallel sediment transport and growth of the ridges 14c dates core lithology and sea level data suggest that the sand ridges mainly formed and developed during the middle period of the postglacial transgression they are postulated to have developed sequentially from offshore to onshore in response to sea level rise during that time very slow sea level rise and high sediment supply through the paleo channel system probably provided favorable conditions for forming the sand ridges the ridges became inactive during the late transgression period as a result of a substantial reduction of wave and current strength and remained on the mid shelf

magnetic discrimination of pyrrhotite and greigite bearing sediment samples by using bulk samples rock magnetic measurements were performed to discriminate between pyrrhotite and greigite bearing shallow marine sediments that are now uplifted above sea level in southwestern taiwan thermal demagnetization of a composite isothermal remanent magnetization irm was found to be effective in differentiating between the two types of sediments to check the thermal instability and estimate the true unblocking temperature tb spectra of sediments containing these minerals saturation irms sirms were imparted at each temperature step during demagnetization while pyrrhotite bearing samples showed unambiguous tb temperature spectra greigite bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization such thermal instability of greigite is a practical and important clue for its identification zero field warming of irm from 5 to 300 k sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation

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

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

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

palaeomagnetism and rock magnetism in the upper pliocene valle ricca rome italy section summary a revision of the magnetostratigraphy in the valle ricca monterotondo rome pliopleistocene section was carried out this section consisting mainly of blue grey marly clays was previously investigated for palaeomagnetic effects by arias et al 1980 1990 who detected a normal polarity interval close to a volcanic ash layer which was correlated to the reunion subchron this interpretation conflicts however with new biostratigraphic data and palaeomagnetic observations in order to investigate the possible origin of these discrepancies we performed a detailed sampling for palaeomagnetic and rock magnetic analyses in the controversial interval around the volcanic ash layer the present results show strong variations in the rock magnetic parameters and suggest significant changes in the magnetic mineralogy two levels with a high intensity low temperature normal component of magnetization were distinguished at these two levels the rock magnetic parameters are typical of greigite and a widespread occurrence of framboids of micron sized iron sulphide grains was found by mineralogical analyses a high temperature reversed component of magnetization carried by magnetite was isolated for the whole interval investigated we demonstrate that the previously recognized normal polarity interval is the effect of changes in the magnetic mineralogy and it does not represent a true reversal of the earth s magnetic field the origin of the low temperature normal component of magnetization is discussed in light of the present knowledge about greigite formation and remanence acquisition

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

origin of the sedimentary magnetic record at ocean drilling program sites on the owen ridge western arabian sea the 3 2 m y whole core magnetic susceptibility record obtained during ocean drilling program leg 117 from the owen ridge in the western arabian sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information the salient features of this record are that 1 it correlates strongly with variations in the concentration and flux of eolian dust 2 it is driven strongly at earth orbital periodicities and 3 there is a significant change in the spectral character at 2 4 m y which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas in view of its potential paleoclimatic significance we have examined the origin of the rock magnetic signal from the owen ridge in more detail we find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis we ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process the first occurs at about 1 5 m depth and is characterized by a loss of fine grained ferrimagnetic material the second stepshift in magnetic properties occurs at a depth of 6 8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity magnetite and high coercivity hematite and or goethite magnetic mineral components scanning electron microscope and energy dispersive xray analyses show that the bulk of the coarse grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the fe ti ratio of these grains across the lower diagenetic front this suggests that titanomagnetite grains with higher ti contents may be more resistant to the process of reductive diagenesis and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front

post deformational growth of late diagenetic greigite in lacustrine sediments from southern italy 1 paleomagnetic rock magnetic and micro textural analyses from a middle pleistocene lacustrine sequence in the southern apennines italy indicate the presence of greigite and magnetite as the main magnetic minerals at different stratigraphic levels in all cases a normal polarity characteristic remanent magnetization chrm was observed in agreement with an 40ar 39ar age of 0 712 0 018 ma from volcaniclastic sanidine after correction for bedding tilt chrm directions carried by greigite do not coincide with the expected geocentric axial dipole field direction at the site latitude whereas the magnetite chrms directions do the data indicate that the greigite magnetization was acquired after tilting and after lock in of the magnetite chrm the estimated delay for the remanence carried by greigite with respect to deposition is 300 ka scanning electron microscope analyses indicate that alteration of detrital volcanic minerals has occurred and that authigenic greigite is generally present in agglomerates and around volcanic grains this observation is consistent with a late diagenetic origin of greigite due to anoxic conditions and availability of dissolved sulfide associated with decomposition of organic matter in the paleolake documentation of a late diagenetic magnetization confirms that care should be taken when using greigite bearing sediments for magnetostratigraphic and tectonic studies

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

rock magnetic properties of eemian maar lake sediments from massif central france a climatic signature the hypothesis of a climate variability within the last interglacial eemian suggested by oxygen isotope data of a greenland ice core grip was later supported by magnetic susceptibility organic matter and pollen signatures in the eemian lake sediments of the lac du bouchet france in the present paper we analyse in detail the rock magnetic properties of these sediments in order to determine the origin of the magnetic susceptibility variations furthermore in order to verify the empirical relation between magnetic susceptibility and climate we studied two eemian sequences from neighbouring lakes lac st front and ribains maar lake rock magnetic investigations mainly consisted of analyses of anhysteretic and isothermal remanent magnetizations along with hysteresis measurements carried out on pilot samples lac du bouchet susceptibility variations are at least partly affected by postdepositional magnetite dissolution lac st front sediments show a complex behaviour resulting in partly contradicting interpretations of rock magnetic parameters possible explanations may be the presence of a very fine grained magnetite or the presence of maghemite ribains maar lake sediments show a homogeneous magnetic mineralogy without major variations in most of the rock magnetic parameters susceptibility records of the three lakes were compared under the simplified assumption that magnetic susceptibility mainly reflects variations of the detrital titano magnetite input only a poor correlation between the three eemian susceptibility sequences is observed suggesting that local effects including dissolution and dilution effects may have altered the original detrital climatic signal the purely detrital origin of the lac du bouchet eemian susceptibility variations must thus be questioned but it nevertheless appears to be the only lake whose susceptibility variations can tentatively be correlated with the grip eemian oxygen isotope record

high resolution seismic study as a tool for sequence stratigraphic evidence of high frequency sea level changes latest pleistocene holocene example from the korea strait abstract sequence stratigraphic analysis of high resolution seismic reflection profiles and sediment data reveals that the latest pleistocene holocene deposits in the korea strait shelf off the southeastern korean peninsula form a high frequency sequence consisting of a set of lowstand transgressive and highstand systems tracts that corresponds to a fifth order 20 kyr sea level cycle eight depositional systems each with different seismic facies constitute the systems tracts the lowstand systems tract consisting of sandy mud forms a deltaic wedge that pinches out updip near or at the relict shelf edge the transgressive systems tract consisting mainly of sands includes six depositional systems 1 inner shelf transgressive layer 2 transgressive estuarine deltaic complex 3 transgressive sand ridge 4 mid shelf transgressive layer 5 incised channel fill and 6 beach shoreface complex although the transgressive systems tract is widely distributed it is much thinner than the lowstand and highstand systems tracts the highstand systems tract is composed of a prodelta shelf complex consisting almost exclusively of holocene muds the distribution and geometry of the systems tracts in the latest pleistocene holocene sequence in the korea strait shelf is different from that of vail s model in that 1 the lowstand systems tract consists only of the lowstand deltaic wedge which forms an elongated sediment body along the paleoshelf edge and 2 the highstand systems tract is completely confined to the inner shelf and forms a nearshore belt parallel to the coastline high frequency 20 kyr high amplitude magnitude of about 130 m sea level change together with strong currents flowing northeastward along the southeastern coast of the korean peninsula and varying sediment supply have resulted in this rather unusual stratigraphic architecture

on the applicability of oil shale to palaeomagnetic investigations abstract samples of oil shales of autunian age from the krkonose riesengebirge piedmont basin northern bohemia which represent rocks with fossil micro organic matter in the initial stage of carbonification were subjected to palaeomagnetic investigations the palaeomagnetic data obtained from these rocks are fully in accordance with those derived previously on rocks of the red bed facies in the same area preliminary results obtained on lower carboniferous coal bearing shales collected from the upper silesian basin ostrava karvina region indicate also that these rocks characterized by a higher degree of carbonification are suitable for palaeomagnetic investigations these results with those of laboratory carbonification indicate that such rocks as bituminous rocks black shales oil shales coal bearing shales or generally rocks containing micro organic matter at different stages of natural carbonification are worthy of being studied palaeomagnetically

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

depositional patterns of sand ridges in tide dominated shallow water environments yellow sea coast and south sea of korea abstract the continental shelf of the yellow sea and the korean south sea is a tide dominated shelf a number of sand ridges occur in this area which form bathymetric highs surveyed were one large sand ridge jangan satoe along the western yellow sea coast and a group of mid shelf sand ridges in the south sea to delineate their depositional pattern and probable origin in tide dominated shallow water environments analyses of sediment samples side scan sonographs and seismic profiles reveal that these ridges show regionally different characteristics in morphology bedforms and sediment the sand ridge in the yellow sea shows a composite form of various bedforms including sand waves and megaripples this ridge is interpreted to be in an accretional active stage maintained by strong tidal currents in contrast the sand ridges on the mid shelf of the south sea show rounded single forms without bedforms on them they are interpreted to represent moribund type sand ridges in the shelf environment which were formed near the shoreline during the holocene sea level transgression these ridges developed off the paleo seomjin river during a period of a stillstand or slow sea level rise approximately between 10 000 and 7000 yrs b p

contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite abstract in several recent published studies paleomagnetic results from greigite bearing sediments reveal characteristic remanences that are anti parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit these observations have important implications for the reliability of paleomagnetic data from greigite bearing sediments we have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the lower gutingkeng formation southwestern taiwan scanning electron microscope observations indicate that the gutingkeng greigite has three modes of occurrence including nodular framboidal and matrix greigite microtextural observations including transection of bedding by iron sulfide nodules with no deviation of sediment textures the presence of partially dissolved edges around detrital and early diagenetic phases and neoformation of greigite and fe rich clays around detrital phyllosilicates indicate that all three types of greigite have a diagenetic origin that post dates early diagenetic pyrite in addition paleomagnetic data yield contradictory polarities even for greigite bearing sister samples from the same stratigraphic horizon the data are collectively interpreted to indicate that neoformation of the gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite the timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon direct petrographic observation of authigenic magnetic iron sulfide phases as carried out in this study can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite bearing sediments

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

rock magnetism of lower middle pleistocene marine sediments wanganui basin new zealand rock magnetic studies were carried out in order to identify the remanence carrying minerals and the domain state of magnetic grains in lower middle pleistocene marine sediments from the wanganui basin southwestern north island new zealand isothermal remanent magnetization irm acquisition and stepwise thermal demagnetization of a three axis composite irm suggest that titanomagnetite or titanomaghaemite is the dominant remanence carrier in these sediments other rock magnetic data suggest that these grains occur in a range of sizes in the superparamagnetic to pseudo single domain range which helps account for the wide range of observed palaeomagnetic behaviour isolated occurrences of ferrimagnetic iron sulphides cannot be ruled out

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

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

a multi parameter rock magnetic record of the last glacial interglacial paleoclimate from south central illinois usa pittsburg basin a small kettle lake in south central illinois was formed after the retreat of the illinoian ice sheet 130 ka b p and the sedimentary record includes the last interglacial glacial cycle curie temperature measurements xrd analyses and thermal demagnetization of a low temperature j indicate that the magnetic fraction is characterized by rs oxidized magnetite and magnetic iron sulfides the grain size of the magnetic fraction was estimated from armrj ratios rs frequency dependent susceptibility and hysteresis parameters these measurements show that glacial sediments contain coarse grained presumably detrital material while interglacial samples have an additional fine grained magnetic component which cannot be explained by a single catchment or climatic process early interglacial samples show magnetic properties characteristic of authigenic biogenically produced magnetite while late interglacial samples are probably influenced by pedogenesis around the basin q 1997 elsevier science b v

diagenesis and remanence acquisition in the lower pliocene trubi marls at punta di maiata southern sicily palaeomagnetic and rock magnetic observations three new profiles lcm pmd and cmd through the lower cochiti polarity zone enable comparison of the effects of weathering and diagenesis on the magnetic properties of the early pliocene trubi marls these marls have been well dated by means of astronomical calibration and are of particular interest in terms of polarity zonation and regional tectonics the freshest exposures lcm profile carry a remanence associated with pyrrhotite which is particularly enhanced in some grey layers this is the first time that occurence of pyrrhotite as the mam remanence carrier in the trubi marls is documented sd magnetite was previously recognised as the fundamental ferromagnetic phase the mean chrm direction from the lcm record does not show the expected 30 35 clockwise rotation found in other studies and does not record the normal cochiti interval either i e is reverse throughout the sampled interval suggesting a relatively late origin for the nrm standard rock magnetic techniques are used to investigate the nature of the magnetization in the three laterally equivalent profiles at punta di maiata the actual data set seems to be better explained by a complex diagenetic history involving a multistage redox history that invokes a late anoxia event the previously postulated iron migration model van hoof et al 1993 was put forward for the suboxic situation and does not take into account the presence of magnetic sulfides which are clearly demonstrated in the zone under study the extended diagenetic model inferred here proposes a pathway that accounts for most of the new observations the salient feature is the postulated existence of a diagenetic phase that has altered the original nrm in some areas dissolution of magnetite resulting in a later stage acquisition of chemical remanence that does not date to the timing of deposition of the sediments the extension and the detailed mechanisms by which such a process has taken part deserve further research nonetheless the present investigation accounts for previously unrecognised features in the trubi marls from sicily and outlines how intricate the nature of the mechanisms contributing to the blocking of remanence in sediments can be

critical differences in sediment delivery and partitioning between marine and lacustrine basins a comparison of marine and lacustrine aggradational to progradational clinothem pairs

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 late diagenetic syn folding magnetization carried by pyrrhotite implications for paleomagnetic studies from magnetic iron sulphide bearing sediments paleomagnetic rock magnetic and sedimentary micro textural data from an early miocene mudstone sequence exposed in okhta river sakhalin russia indicate the presence of pyrrhotite and magnetite at different stratigraphic levels sites that contain only magnetite have a reversed polarity characteristic remanent magnetization chrm with a low coercivity overprint which coincides with the present day geomagnetic field direction pyrrhotite bearing sites have stable normal polarity chrms that are significantly different from the present day field direction after correction for bedding tilt the chrm data fail a reversals test however the normal polarity pyrrhotite chrm directions become antipodal to the tilt corrected magnetite chrm directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40 partial unfolding these data suggest that the pyrrhotite magnetization was acquired during folding and after lock in of the magnetite remanences electron microscope observations of polished sections indicate that fluid associated halos surround iron sulphide nodules pyrrhotite is present in randomly oriented laths in and around the nodules and the nodules do not appear to have been deformed by sediment compaction this observation is consistent with a late diagenetic origin of pyrrhotite documentation of a late diagenetic magnetization in pyrrhotite bearing sediments here and in recent studies of greigite bearing sediments suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn depositional magnetization

evolution of late quaternary deposits on the inner shelf of the south sea of korea abstract high resolution 3 5 khz seismic profiles and sediment samples were analysed to delineate the shallow seismic stratigraphy of late quaternary deposits in the central part of the korean south sea the stratigraphy above the acoustic basement is represented by three distinct sedimentary sequences separated by erosional boundaries the upper sequence a has an acoustically semi transparent signature usually without distinct internal reflectors and overlies the flat lying mid reflector r the sediments in this sequence consist of bioturbated terrigenous mud derived mainly from the seomijin river they have developed during the last 5000 yrs and have an accumulate rate of 10 40 cm 100 yrs reflector r represents the postglacial transgressive layer formed during the last stage of the holocene sea level rise approximately between 7000 yrs b p and 5000 yrs b p the middle sedimentary sequence b below r shows parallel to subparallel internal reflectors and prograding clinoforms that have formed largely during the last glacial period the top of this sequence displays various erosional channels and transparent fills produced by subaerial and fluvial erosion of the pre existing shelf the submerged channels indicate the existence of a paleochannel system presumambly the extension of the paleo seomijin river during the sea level lowstand the lower sequence c has an irregular erosional upper surface and an acoustically transparent subbottom it is interpreted as the erosional remnants of earlier pleistocene sediments formed in shallow water environments

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

late quaternary lowstand wedges on the shelf margin and trough region of the korea strait abstract high resolution sparker profiles collected from the korea strait reveal a distinct pattern of stacked progradational sedimentary wedges on the shelf margin and trough region these sedimentary wedges comprise a succession of three distinct units a lower b middle c upper unit bounded by erosional unconformities and their internal structures show well stratified complex sigmoid oblique types the isopach map shows that the wedges thicken seaward and their landward limit is in about 100 120 m water depth according to the internal structure distribution pattern and radiocarbon dates these sedimentary wedges are interpreted to have been formed during the glacioeustatic sea level lowstands multiple falls of sea level during the late quaternary coupled with high sediment discharge from the paleo nakdong river system resulted in the formation of thick lowstand wedges three intervals of wedge building are recorded the youngest one unit c is interpreted to mark the last glacial period while the middle one unit b represents the lowstand period before the last interstadial