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

magnetic polarity stratigraphy of a plio pleistocene marine sequence of north island new zealand abstract thick sequences of relatively undisturbed plio pleistocene sediments in the wanganui basin north island new zealand consist of well exposed silts clayey silts sandstones rare limestones and several tephras oriented specimens were collected from a section more than 2500 m thick and palaeomagnetic measurements were made using a c demagnetisations in fields up to 35 mt with the aid of tephrochronology the age of the upper sequence is now well established and falls within the matuyama epoch the lower two thirds of the section except for the basal 500 m is predominantly normally magnetised and is interpreted as a very extended sequence of the gauss epoch the lowest 500 m then represents the gilbert epoch the plio pleistocene boundary as defined at vrica italy falls within the upper part of the section studied in the upper nukumaruan stage for the first time a reliable correlation is made with the international boundary using as intermediaries the palaeomagnetic and tephrostratigraphy of deep sea cores from the southwest pacific as a result of the high deposition rate of the order of 1 2 m ky and the apparent lack of unconformities the temporal resolution is high short lived magnetic events are detected especially in the lower matuyama and upper gauss epochs these generally correlate well with events reported from other extended sections

magnetic properties of hydrothermally synthesized greigite fe3s4 i rock magnetic parameters at room temperature summary greigite of variable purity has been synthesized hydrothermally at 140 c by injecting a mohr s salt solution into a sodium sulphide solution containing dissolved zerovalent sulphur ageing for 30 960 min and rapidly quenching oxygen was rigorously excluded during the synthesis scanning electron microscope sem observations yield a grainsize range up to 400nm the variable purity was deduced from a varying specific saturation magnetization 3 29 am2 kg i the mjm or ors os ratio varies between 0 3 and 0 4 which is indicative of fine grained greigite concurring with sem observations hysteresis measurements at low temperatures indicate the presence of grains that are superparamagnetic at room temperature coercivity parameters b 13 43 mt bo cr 37 73 mt are the highest obtained to date for synthetic greigite and compare well with those measured for natural greigite ranges for other rock magnetic parameters are discussed greigite hysteresis parameters overlap those of magnetite to a large degree a diagnostic interparametric ratio for greigite is its high o xin ratio the range of the present greigite is 40 75 kam

reconstructing past seawater mg ca and sr ca from mid ocean ridge flank calcium carbonate veins proxies for past seawater chemistry such as mg ca and sr ca ratios provide a record of the dynamic exchanges of elements between the solid earth the atmosphere and the hydrosphere and the evolving influence of life we estimated past oceanic mg ca and sr ca ratios from suites of 1 6 to 170 million year old calcium carbonate veins that had precipitated from seawater derived fluids in ocean ridge flank basalts our data indicate that before the neogene oceanic mg ca and sr ca ratios were lower than in the modern ocean decreased ocean spreading since the cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early tertiary may explain the recent rise in these ratios

sources of aeromagnetic anomalies over cement oil field oklahoma simpson oil field alaska and the wyoming idaho utah thrust belt geochemical and rock magnetic studies undertaken to determine the cause of magnetic anomalies over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the wyoming idaho utah thrust belt have revealed different magnetic sources developed under different sedimentologic geochemical and structural settings at cement ferrimagnetic pyrrhotite fe7s8 typically intergrown with more abundant nonmagnetic pyrite fes2 formed as a result of hydrocarbon seepage sulfur isotopic data indicate that sulfur in the fe s minerals was probably derived from two different sources 1 isotopically heavy thermochemical h2s in petroleum and 2 isotopically light h2s generated by sulfate reducing bacteria that derived metabolic energy from leaking hydrocarbons or organic compounds derived from hydrocarbons although pyrrhotite may make a minor contribution to the reported total magnetic field anomalies at cement the measured anomalies are probably mostly caused by man mad

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

pyritization processes and greigite formation in the advancing sulfidization front in the upper pleistocene sediments of the black sea pyritization in late pleistocene sediments of the black sea is driven by sulfide formed during anaerobic methane oxidation a sulfidization front is formed by the opposing gradients of sulfide and dissolved iron the sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content two processes of diffusion limited pyrite formation were identified the first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of fesn n 1 10 1 29 including greigite elemental sulfur and polysulfides formed from h2s by a reductive dissolution of fe iii containing minerals serve as intermediates to convert iron sulfides into pyrite in the second process a direct pyrite precipitation occurs through prolonged exposure of iron containing minerals to dissolved sulfide methane driven sulfate reduction at depth causes a progressive formation of pyrite with a 34s of up to 15 0 the s isotopic composition of fes2 evolves due to contributions of different sulfur pools formed at different times steady state model calculations for the advancement of the sulfidization front showed that the process started at the pleistocene holocene transition between 6360 and 11 600 yr bp our study highlights the importance of anaerobic methane oxidation in generating and maintaining s enriched layers in marine sediments and has paleoenvironmental implications

complex polarity pattern at the former plio pleistocene global stratotype section at vrica italy remagnetization by magnetic iron sulphides the vrica section in calabria southern italy was the global stratotype for the pliocene pleistocene boundary until this boundary was redefined in 2009 several paleomagnetic investigations have been carried out at vrica to determine the age of the formerly defined pliocene pleistocene boundary which was a key calibration point for the astronomical polarity timescale apts each study has documented a complex polarity pattern at and above the top of the olduvai subchron and in relation to the existence of the so called vrica subchron when constructing the apts two alternative interpretations for the vrica section were proposed neither of which could be conclusively supported authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record availability of a fresh 50 m sediment core enabled us to test this possibility our magnetostratigraphic record is similar to that of previous studies but it is also complex above the olduvai subchron we confirm abundant occurrences of authigenic greigite and pyrrhotite along with detrital titanomagnetite authigenic monoclinic pyrrhotite indicates growth significantly later than deposition and greigite can grow at any time during diagenesis depending on the availability of dissolved iron and sulphide the spatially variable magnetic polarity pattern at vrica is therefore interpreted to have resulted from post depositional magnetic iron sulphide formation at variable times tectonism along the calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization the complex magnetostratigraphic record at vrica was taken into account when the apts was developed and alternative interpretations result in a maximum age difference of 50 kyr for the upper olduvai reversal our results therefore do not undermine the apts rather they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments

identification and origins of iron sulfides in czech loess multiple rock magnetic and non magnetic techniques were employed to identify iron sulfides in late pleistocene czech loess deposits the results indicate that iron sulfides in the loess at znojmo section are mainly pyrrhotite and pyrite with stoichiometry ranging from fe10s11 to fes2 although early pedogenic origin of fine sulfide fraction can not be ruled out scanning electron microscopy sem of individual grains and microscopic observations of magnetic extracts show that iron sulfide grains are probably of detrital origin potential sources of these sulfides may have been eluvium loams above hypothetical ore deposits in the vicinity of the section or glacifluvial sediments from nearby glacial margins framboidal pyrite forms now completely replaced by iron oxides suggest that some particles of biogenic sulfides may have also been transported into loess from presumably fluvial sediments

magnetic tracing of sediment dynamics of mudbanks off southwest coast of india in this study we utilized environmental magnetic in combination with sedimentological and hydrodynamic data to investigate the formative processes of mudbanks along southwest coast of india we document the linkages between enrichment of silt sized magnetic particles and formation processes of mudbanks along alappuzha coast a trend of increasing magnetite concentration and coarsening in magnetic grain size is observed at mudbank stations m2 and m3 while the mud deficient station m1 showed an opposite trend a strong relationship between magnetic and physical grain size for all samples implies that the magnetic particle size and clastic grain size are largely adjunct analysis of rock magnetic and grain size data of surficial and suspended sediments from non mudbank m1 and two mudbank stations m2 m3 reflect the differential sediment partitioning and transport regimes which controlled the formation of mudbanks along alappuzha coast two plausible mechanisms responsible for the formation of mudbanks are identified grain size selective entrainment is the dominant process during pre monsoon weaker hydrodynamics waves and bottom currents favors accumulation of silt sized fine and coarse magnetic and non magnetic fractions resulting in the formation of magnetically low enriched sediment bed of mixed grain sizes at the onset of monsoon wave induced energetic bottom currents enhance the suspension of entire sediment bedload at stations m2 m3 to form fluid mud concurrently mineral density based selective fractionation allows the settling of coarse silt sized magnetic particles while the fine magnetic silt size particles accumulate forming thick fluid mud as a suspension load resulting in the formation of mudbanks an observed increase in magnetic susceptibility and coarsening in magnetic grain size of surficial sediments at mudbank stations m2 m3 during monsoon period supports the interpretation our findings are summarized in a conceptual model which can be very well applied to investigate sediment dynamics associated with mudbank formation in coastal and shelf sedimentary systems

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

atmospheric carbon dioxide through the eocene oligocene climate transition geological and geochemical evidence1 2 3 indicates that the antarctic ice sheet formed during the eocene oligocene transition4 33 5 34 0 million years ago modelling studies5 6 suggest that such ice sheet formation might have been triggered when atmospheric carbon dioxide levels fell below a critical threshold of 750 p p m v but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear here we use the boron isotope ph proxy7 8 on exceptionally well preserved carbonate microfossils from a recently discovered geological section in tanzania9 10 to estimate before during and after the climate transition our data suggest that a reduction in occurred before the main phase of ice growth followed by a sharp recovery to pre transition values and then a more gradual decline during maximum ice sheet growth was between 450 and 1 500 p p m v with a central estimate of 760 p p m v the ice cap survived the period of recovery although possibly with some reduction in its volume implying as models predict11 a nonlinear response to climate forcing during melting overall our results confirm the central role of declining in the development of the antarctic ice sheet in broad agreement with carbon cycle modelling12 and help to constrain mechanisms and feedbacks associated with the earth s biggest climate switch of the past 65 myr

a new zealand late miocene magnetostratigraphy glacioeustatic and biostratigraphic correlations magnetostratigraphy is described for a sequence of predominantly hemipelagic marine mudstone 2200 m thick representing the late miocene and earliest pliocene from the mangapoike river section eastern north island new zealand together with the early pliocene magnetostratigraphy previously described from mangapoike it now provides a 3000 m thick magnetostratigraphic reference section for new zealand ranging in age from 10 5 ma to 3 5 ma thirteen magnetozones are recognised in the 2200 m described here and all but the highest are confidently correlated with the polarity timescale from the base of chron 11 to chron 7 correlation of the highest normal magnetozone to the timescale is ambiguous and two possible interpretations are presented both of which imply a previously unrecognised latest miocene unconformity the inferred unconformity is supported by biostratigraphy it is shown that the integration of biostratigraphy lithostratigraphy and magnetostratigraphy can discern diastems of the order of 104 years magnetostratigraphy and 13c data also indicates a diastem between 6 30 ma to 5 41 ma at the miocene pliocene angular unconformity at mangapoike these two unconformities and a distinct lithological change at the base of the late miocene are synchronous with deep sea benthic 18o records and unconformities expected from postulated eustatic sea level changes a coherent correlation model for new zealand on shore magnetostratigraphic biostratigraphic and isotopic data conflicts with the two magnetostratigraphic interpretations proposed for dsdp site 594 150 km east of new zealand both of which are probably incorrect

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

towards an astrochronological framework for the eastern paratethys mio pliocene sedimentary sequences of the focsani basin romania abstract a reliable geological timescale for the neogene sedimentary deposits of the romanian carpathian foredeep did not exist so far due to the lack of accurate isotopic datings and straightforward magnetostratigraphic correlations here we present a new chronology for the upper miocene to pliocene deposits of the westernmost part of the eastern paratethys based on high resolution magnetostratigraphic data from the focsani basin the thick and continuous river sections of putna 2 35 km and rimnicu s rat 7 32 km yield excellent magnetostratigraphic results and allow an unambiguous correlation to the polarity timescale for the interval between 7 and 2 5 ma the new age control permits the calculation of accumulation rates for each polarity zone which shows a significant increase at approximately 5 8 6 my the upper miocene to pliocene sedimentary rocks of the focsani basin display cyclic alternations of sandy units and finer intervals the calculated values for the average duration of the observed sedimentary cycles are very close to the average duration of precession 21 7 ky which indicates that the sedimentary cycles from the eastern carpathian foredeep deposits are astronomically forced we show that the usage of the unambiguous time control offered by magnetic polarity stratigraphy would give a more realistic picture of the processes that have taken place in this tectonically active region

vegetation sea level and climate changes during the messinian salinity crisis the messinian salinity crisis late miocene is one of the most fascinating paleoceanographic events in the recent geological history of the mediterranean sea defining a time when it partly or nearly completely dried out however the relative roles of tectonic processes and sea level changes as triggers for restriction and isolation of the mediterranean sea from the open ocean are still under debate in this study we present a detailed pollen dinoflagellate cyst dinocyst and magnetic susceptibility analysis of a sequence of late neogene between ca 7 3 and 5 2 ma marine sediments from the montemayor 1 core lower guadalquivir basin southwestern spain which provides a continuous record of paleoenvironmental variations in the atlantic side of the betic corridors during the late miocene our results show that significant paired vegetation and sea level changes occurred during the messinian likely triggered by orbital scale climate change important cooling events and corresponding glacio eustatic sea level drops are observed in this study at ca 5 95 and 5 75 ma coinciding with the timing and duration of oxygen isotopic events tg32 and tg22 20 recorded in marine sediments worldwide it is generally accepted that the onset of the messinian salinity crisis began at ca 5 96 0 02 ma therefore this study suggests that the restriction of the mediterranean could have been triggered at least in part by a strong glacio eustatic sea level drop linked to a climate cooling event occurring at the time of initiation of the messinian salinity crisis

diagenesis in anoxic sediments from the california continental borderland and its influence on iron sulfur and magnetite behavior solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical magnetic and x ray diffraction techniques in three california continental borderland basins the observed profiles have been fit by simple reaction diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur more than 95 of the solid phase reduced sulfur is pyrite and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent depth integrated sulfate reduction rates for modern sediments in san pedro santa catalina and san nicolas basins are 11 4 6 3 and 6 3 8 8 mol cm 2yr 1 respectively measurements of solid phase iron species indicate that surficial sediments are enriched in easily reducible ferric oxyhydroxides the enrichment is maintained by a combination of oxidation of fe2 diffusing upward from underlying anoxic sediments as well as input of fresh sediment enriched in ferric oxyhydroxides the three primary sources for iron converted to pyrite and the sequence in which they are utilized are ferric oxyhydroxides magnetite and other crystalline oxides and exchangeable iron in phyllosilicates the majority 50 80 of the iron converted to pyrite is from the silicates and budgetary calculations indicate the amount of iron released from san pedro basin silicates agrees within 35 with the amount of magnesium removed from pore water to solid phases fe2 is enriched in near surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation at depth pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates sulfide produced at depth diffuses upward until it reaches sediments with available iron causing a step like increase in solid phase sulfur concentration over 90 of the magnetite present in surficial sediments is dissolved at depth due to reaction with h2s a model is developed to predict the depth at which magnetite dissolution should occur based on sulfate reduction rates and the flux of ferric oxyhydroxides the results of this model predict the onset of dissolution at depths of 5 40 cm in different basins and agree well with the observed depths of magnetite dissolution

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

paleomagnetic research on black sea quaternary sediments abstract the paper reports the first paleomagnetic study of a black sea quaternary sediment core made by a bulgarian research group the experimental results show an important difference in magnetic behaviour between the samples from the holocene part and those from the pleistocene part of the core suggestions are made for the character of the remanent magnetization and the main carriers of it the presence of greigite fe 3 s 4 and pyrite fes 2 is discussed a rough correlation is established between the variation of inclination for the holocene part of the sediment and for the bulgarian archeomagnetic curve on the basis of the comparison of inclination data obtained from the pleistocene part of the core and previously studied black sea core related to the absolute time scale it is suggested that the lower boundary of the pleistocene part is 16000 years old bp

trace elements and their isotopes in bones and teeth diet environments diagenesis and dating of archeological and paleontological samples fossil biogenic apatites display trace element te compositions that can record environmental and biological signals give insights into past water compositions or be used for dating paleontological and archeological bones and teeth processes of te incorporation into apatites of skeletal phosphatic tissues are described ranging from those active in living organisms to those active during diagenesis abiotic incorporation mechanisms have been modeled theoretically and experimentally and include crystallographic controls on te partitioning coefficients inorganic surface adsorption and adsorption mediated through chelation and diffusion adsorption processes each leading to specific fractionation patterns predictions from models and experiments have been tested against te concentration and isotopic composition data on remains of contemporary wild or raised animals and on fossils of various ages and environments in living organisms tes incorporated in apatite are separated in two categories non essential elements whose concentration is compared to that of an essential element with similar behavior e g sr ca to reconstruct trophic chains and essential elements whose isotopic ratios are used to trace metabolic activity diet etc in fossils elements are incorporated during diagenesis such as rare earth elements rees and trace diagenetic processes ranging from very early quantitative adsorption likely mediated by chelators protracted diagenesis through inorganic adsorption and fractionation through diffusion adsorption to low grade metamorphism associated with intense redistribution under crystal chemical control these different steps are also identified as steps of radionuclide incorporation only samples where the earliest steps can be deciphered are useful for determining stratigraphic and archeological ages other samples will date late diagenetic events that have obliterated the paleoenvironmental signals but may be of geological significance e g tectonic events

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

a petromagnetic study of miocene rocks bearing micro organic material and the magnetic mineral greigite sokolov and cheb basins czechoslovakia abstract the middle miocene sediments of the cypris series from the sokolov and cheb brown coal basins western bohemia were subjected to petromagnetic investigation during the laboratory thermal treatment at a temperature of 350 400 c the samples rich in micro organic material changed into carbonified samples which exhibited similar physical and technological properties to coal samples collected from the underlying coal seam the experimental results may have a bearing on several problems concerning the origin of fossil fuels and highlight specific rock properties which may have potential for identifying the micro organic content a strongly magnetic mineral greigite fe 3 s 4 was identified in magnetic zones above the coal seam the authigenic mineral greigite occurs in strata of greater thickness and it was found that a reducing environment generated by the presence of organic matter was required for the formation of the greigite metastable properties and the low coercivity spectra of this mineral are adverse for palaeomagnetic investigations

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

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

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

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

reversal records in marine marls and delayed acquisition of remanent magnetization records of geomagnetic reversals frozen into the magnetic components of sediments provide a means to study the time dependent behaviour of the geomagnetic field during polarity transitions although sedimentary records have the advantage of being readily available and continuous the process by which they acquire a remanent magnetization is still not fully understood1 magnetites which lose their magnetization at relatively high temperatures 580 c are generally considered to carry the primary remanence of the sediment that is to record the palaeomagnetic direction here we describe two reversed to normal transitional records from marine marls in which this high temperature component shows a delayed remanence acquisition relative to a lower temperature component in these samples therefore the high temperature component does not reflect geo magnetic changes during the reversal at least for marine marls detailed palaeomagnetic rock magnetic and geochemical studies are apparently necessary to judge the validity of reversal records

the evolution of pco2 ice volume and climate during the middle miocene abstract the middle miocene climatic optimum 17 15 ma mco is a period of global warmth and relatively high co2 and is thought to be associated with a significant retreat of the antarctic ice sheet ais we present here a new planktic foraminiferal 11b record from 16 6 to 11 8 ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to co2 these new data demonstrate that the evolution of global climate during the middle miocene as reflected by changes in the cyrosphere was well correlated to variations in the concentration of atmospheric co2 what is more within our sampling resolution 1 sample per 300 kyr there is no evidence of hysteresis in the response of ice volume to co2 forcing during the middle miocene contrary to what is understood about the antarctic ice sheet from ice sheet modelling studies in agreement with previous data we show that absolute levels of co2 during the mco were relatively modest 350 400 ppm and levels either side of the mco are similar or lower than the pre industrial 200 260 ppm these new data imply the presence of either a very dynamic ais at relatively low co2 during the middle miocene or the advance and retreat of significant northern hemisphere ice recent drilling on the antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the ais did occur during the middle miocene but the complete loss of the ais was unlikely consequently it seems that ice volume and climate variations during the middle miocene probably involved a more dynamic ais than the modern but also some component of land based ice in the northern hemisphere

conical folding in the core of an orocline a geometric analysis from the cantabrian arc variscan belt of nw iberia abstract the cantabrian arc of the variscan belt has been recently defined as a true orocline constraining kinematics and deformation timing this curved sector of the orogenic belt is characterized by two different fold sets 1 one runs parallel to the outcrops of the main thrusts and describes a horseshoe shape concave towards the east and 2 another is radial to the arc a detailed geometric study of the fold interference patterns in the cantabrian arc revealed the conical nature of the folds belonging to the radial set these conical folds developed with different geometrical characteristics semi apical angles and axis attitudes depending on the initial orientation and geometry of the folded surfaces they are interpreted to result from a vertical axis rotation during oroclinal buckling of the variscan belt in nw iberia this study of conical folds in the cantabrian arc highlights that conical folds in curved orogenic arcs are a powerful tool for establishing the sequence of tectonic events because interference patterns due to vertical axis secondary differential rotations provide unique geometrical characteristics observed in the cantabrian arc that can be extrapolated to other oroclines

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

framboidal and idiomorphic pyrite in the upper maastrichtian sedimentary rocks at gabal oweina nile valley egypt formation processes oxidation products and genetic implications to the origin of framboidal pyrite abstract the upper maastrichtian organic rich sediments studied at gabal oweina egypt are moderately enriched in syngenetic and diagenetic pyrite pyrite occurs mostly as layers or bands group of lamina lenses diagenetic intercalated pockets burrow fills and disseminated individual pyrite framboids and crystals within the host sediments the pyritic thin bands and lamina consist mostly of unconsolidated to compact oriented pyrite oriented along the bedding planes in gypsiferous clayey matrix and less common as poorly oriented pyrite crystallites in several cases pyrite crystals of the latter type depict zoning fracturing and micro concretions pyritic burrow fills are composed mainly of pyrite phosphatic ooids microfossils glauconitic grains poorly graphitized carbon and native sulfur pyrite replaces minerals other than gypsum sulfur or carbon it also replaces microfossils thus turning some of the phosphatic ooids and microfossils to pyritized pseudomorphs none of the studied phosphate ooids or framboids contains any mackinawite pyrrhotite or greigite based on the microscopic and sem observations of the micro textures of disseminated pyrite found at gabal oweina section four morphological forms of primary pyrite could be identified 1 grouped multiple framboids 2 individual framboids 3 pyrite idiomorphic crystal overgrowths on framboids and 4 single and aggregates of euhedral pyrite crystals the multiple framboid formation may have emerged from three successive processes nucleation and growth of individual aggregates of the microcrystals to form combined micro framboids the growth of framboids and followed by grouping of the several pyrite framboids direct pyrite nucleation shell formation crystallization and aggregation processes might complete a single framboid the disseminated single and aggregated euhedral pyrite crystals bear evidence indicating that their formation was via nucleation and growth of pyrite crystallites and their aggregation to individual framboids infilling in the interstices by additional pyritic material compaction and homogenization of all these materials furthermore we encounter for the first time in nature idiomorphic pyrite crystals that integrated numerous framboids using them as their nucleation and growth sites without erasing or modifying their pristine morphology elemental sulfur contains minor concentration of sb ni cd and cu strongly suggesting their presence as submicron sulfide crystallite inclusions sem and microprobe investigations revealed that goethite is present as a weathering product in all morphological types of pyrite however only an iron sulfate presumably melanterite was encountered as oxidation product of the multi framboids and the euhedral aggregate crystals the upper maastrichtian sediments not only contain a menagerie of pyrite morphologies but probably a complete record of the formation process and the geochemical conditions of the growth of framboids single pyrite crystals pyrite burrows pyritized mn fe oxide framboids and finally their weathered products the various pyrite forms strongly suggest a multistage process that led to their formation without any evidence for mackinawite pyrrhotite or greigite precursors there is also no evidence in the oweina sediments for post pyrite formation of mackinawite pyrrhotite or greigite the presence of elemental sulfur containing minor concentrations of sb ni cd and cu with pyrite framboids indicates that the pore solutions were geochemically supersaturated in sulfur thus inhibiting the crystallization of any iron sulfide other than pyrite this cast considerable doubt on the assumed mackinawite or greigite precursors as prerequisite for formation of pyrite framboids

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

magnetic properties of sediments in a polish lake evidence of a relation between the rock magnetic record and environmental changes in late pleistocene and holocene sediments summary a study of mineral magnetic parameters was carried out on a late pleistocene and holocene sedimentary sequence of nearly 18 m from lake bledowo central poland sediments of lake bledowo have already been analysed for bulk sediment mineralogy and biogenic materials the mineral magnetic stratigraphy confirms the major changes in palaeo environmental conditions that have been deduced from other methods the most important mineral magnetic change results from the authigenetic formation of ferrimagnetic greigite fe3s4 during the beginning of lacustrine conditions 12 000 yr bp our data also indicate a detrital origin of overlying ferrimagnetic iron oxides it is suggested that they originate from brown soils developed on the boulder clay constituting the west side of the lake shore variations of ferrimagnetic iron oxide size are related to the early diagenetic processes in the sediment larger particles are present in periods with early diagenesis of organic matter in anoxic conditions this indicates the dissolution of fine magnetic particles by iron oxide reducing bacteria and results in homogeneous magnetic grain sizes despite their origin from soils characterized by a multimodal grain size distribution

magnetostratigraphy of pliocene sediments from the stirone river po valley summary the miocene pliocene and pliocene pleistocene boundaries bracket a pliocene section approximately 800m thick along the stirone river near parma northern italy from the base to the top the sequence consists mainly of blue clays zanclean in age whitish laminates silty clays and sandstones of piacenzian age a debris flow which is the sedimentary expression of a hiatus separates the zanclean from the piacenzian sediments thermomagnetic analysis thermal evolution of magnetic susceptibility acquisition and removal of anhysteretic and isothermal remanent magnetization arm and irm by alternating field and thermal methods x ray diffraction and scanning electron microscopy sem have been used in conjunction to try and unravel the complex magnetic mineralogy of the stirone samples two major carriers of the magnetic remanence appear to be pseudo single domain psd magnetite and high coercivity pyrrhotite in varying amounts and grain sizes five different magnetic indicators yield roughly consistent logs with magnetite being important in the lower part of the section and pyrrhotite becoming dominant upsection six zones with more or less uniform properties have been outlined except in the lowermost zone where magnetite is also an important carrier pyrrhotite in the 150 350 c range is taken to be the main carrier of the characteristic and we believe primary magnetization the overall mean direction for about 200 samples is d 0 i 52 95 3 corresponding to a lack of local rotation about a vertical axis but implying some 10 of inclination shallowing the magnetostratigraphy encompasses ten successive polarity zones correlation of magnetic and biostratigraphic results to the geomagnetic polarity time scale and to the biozonation of the mediterranean area leads to the proposal that the thvera subchron and part of the above reversed polarity zone are missing as revealed by the absence of the sphaeroidinellopsis seminulina s l zone and by the occurence of globorotalia margaritae from the very base of the section the sidufjall and nunivak subchrons are recognized and the first occurrence of globorotalia puncticulata occurs just above nunivak subchron the cochiti subchron is observed next a remarkable hiatus involves the upper part of the gilbert chron up to the lower part of the kaena subchron based on the record of the last occurrence of g margaritae and g puncticulata and the first occurence of g aemiliana and g bononiensis another hiatus involving the reunion and olduvai subchrons is suggested to occur in the upper part of the pliocene sequence based on the absence of the g inflate zone and on the first occurrence of artica islandica which marks the pliocene pleistocene boundary towards the top of the section the average sedimentation rate is estimated to be 450 500 m ma 1 for the early pliocene and 200 300 m ma 1 for the late pliocene despite highly non uniform sedimentation both reflecting the episodic nature of coupled tectonic and sedimentary processes on

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

the magnetic structure and palaeomagnetic recording fidelity of sub micron greigite fe3s4 abstract we present the results of a finite element micromagnetic model of 30 nm to 300 nm greigite fe3s4 grains with a variety of equant morphologies this grain size range covers the magnetic single domain sd to pseudo single domain psd transition and possibly also the psd to multi domain md transition the sd psd threshold d 0 is determined to be 50 nm d 0 56 nm depending on grain shape the nudged elastic band method was used to determine the room temperature energy barriers between stable states and thus the blocking volumes it is found that in the absence of interparticle magnetostatic interactions the magnetisation of equant sd greigite is not stable on a geological scale and only psd grains 70 nm can be expected to carry a stable magnetisation over billion year timescales i e all non interacting sd particles are essentially superparamagnetic we further identify a mechanism for the psd to multi domain md transition which is of a continuous nature from psd nucleation up to 300 nm when structures typical of md behaviour like closure domains begin to form

the occurrence of greigite in sediments from loch lomond greigite has been identified in the sediments of loch lomond using x ray diffraction greigite is the dominant magnetic mineral in the oldest sediments recovered while magnetite predominates in the younger sediments which had previously been used for palaeomagnetic secular variation studies a layer of sediment in between these two magnetic mineral regimes has very low magnetic concentrations probably as a consequence of magnetite dissolution associated with sulphide rich pore waters produced at the time of a marine incursion the greigite largely oxidises once the sediment is exposed to air but if freeze dried the greigite becomes surprisingly stable following freeze drying the greigite can be heated to 280 c in air before it alters and loses its strong ferrimagnetic properties

palaeomagnetic location of the jaramillo subchron and the matuyama brunhes transition in the castlecliffian stratotype section wanganui basin new zealand abstract magnetostratigraphy is described for the lower part of the castlecliff section in wanganui basin new zealand based on analysis of multiple samples from 37 horizons the castlecliff section is the stratotype of the middle pleistocene castlecliffian stage and comprises eleven unconformity bound sequences which accumulated at inner shelf depths during interglacials a range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition in the stable single domain or pseudo single domain size range however at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral chiefly within the superparamagnetic single domain size range resulting in hard secondary components of magnetization nevertheless at most sites the primary component of remanence was clearly identified by stepwise demagnetization four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints the section sampled spans the interval from below the jaramillo subchron to above the matuyama brunhes boundary the matuyama brunhes transition extends over 10 5 m of section within the upper westmere siltstone virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path these represent the first published data on the occurrence of the matuyama brunhes boundary on land in new zealand

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

magnetic characterization and mossbauer spectroscopy of magnetic concentrates from greek lake sediments abstract bulk magnetic properties of three greek lake sediments indicated that the main magnetic constituent was predominantly pseudo single domain magnetite this was confirmed by successfully extracting and directly identifying the magnetite the majority of the magnetite in the magnetic concentrates is present as grains of about 2 3 m in size the overall size ranging from with occasional grains up to 20 m the grains are roughly equidimensional and angular in form indicating that they are of primarily detrital origin although an authigenic origin for the very fine grains must be a possibility mossbauer spectra of the magnetic concentrates suggest that either the magnetite is slightly non stoichiometric to a similar degree in the three lakes or contains impurity ions haematite was also found in the concentrates but its abundance is only 10 15 that of magnetite and its is not detectable in any of the magnetic properties of the sediments

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

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

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

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

oroclines of the variscan orogen of iberia paleocurrent analysis and paleogeographic implications abstract coupled structural and paleomagnetic analyses have shown that the northern iberian bend of the variscan orogen referred to as the cantabrian orocline developed by vertical axis rotation of an originally linear orogen however palinspastic restoration of the orocline has proven difficult owing to 1 an unusually great orogenic width of over 700 km and 2 exposure of shallow water strata of the gondwanan margin in the northern and southern portions of the orogen we present paleocurrent data from lower ordovician shallow marine clastic sedimentary rocks across the variscan of northern and central iberia collected to constrain palinspastic restoration of the orogen paleocurrent data were collected from over 50 sites and include cross bed foresets ripple crests and casts as well as rare ball and pillow structures syn sedimentary slump folds and incised channels paleocurrent directions fan around the cantabrian orocline are consistently oriented at a high angle to structural strike and yield a consistent offshore direction outward from the oroclinal core similarly changes in structural strike and paleocurrent direction across central iberia imply the presence of a second more southerly orocline the central iberian orocline that is continuous with but convex in the opposite direction of the cantabrian orocline together the cantabrian and central iberian oroclines define an s shaped pair of continental scale buckle folds palinspastic restoration of the oroclines yields a linear continental margin 1500 km long characterized by consistent offshore paleoflow to the west defining a westerly oceanic domain presumably the rheic ocean and an easterly landward direction presumably gondwana recognition of the southern orocline explains the unusual width of the orogen the geometry of aeromagnetic anomalies attributable to variscan rocks and is consistent with available structural data paleomagnetic declination data and the distribution of correlative paleozoic and older rock sequences including shallow water strata of the gondwanan margin the s geometry of the oroclines is similar to the z geometry of the alaskan oroclines of the north america cordillera suggesting that as in the cordillera the iberian oroclines may record a significant margin parallel translation event

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

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

mid to late holocene climate response from the triloknath palaeolake lahaul himalaya based on multiproxy data abstract a 2 13 m thick palaeolake sequence deposited between the right lateral moraine and the valley wall of triloknath glacier has been studied at high resolution to understand the past climatic changes in lahaul himalaya the osl and ams 14c chronology suggests that the lake building activity was initiated post cooling phase since 6484 cal ybp the combination of sedimentological geochemical and environmental magnetic parameters helped to infer five first order climatic zones the influx of magnetic minerals along with a notable increase in antiferromagnetic mineralogy appearance of hematite and higher chemical index of alteration cia values during 5925 cal ybp to 5676 cal ybp suggests warm and wet conditions owing to enhanced indian summer monsoon ism precipitation corresponding to the mid holocene warming further reduction in the magnetic mineral concentration with few peaks of antiferromagnetic mineralogy and higher cia values suggests warmer episodes within an overall cold climate and the finer ferrimagnetic grain size suggests intermittent low energy and ponding conditions during 3904 cal ybp to 2365 cal ybp this was followed by fluctuating cold arid conditions in response to reduced ism precipitation during 2365 cal ybp and 1758 cal ybp further between 1758 cal ybp to the present episodic warming pedogenic enhancement and increased monsoon precipitation have been recorded a major warm episode medieval warm period as depicted from enhanced antiferromagnetic mineralogy and higher cia values has been recorded between 742 cal yrs bp and 593 cal ybp the sudden drop in the magnetic mineral concentration as well as the cia value around 300 cal ybp 1650 ce is likely because of colder conditions prevailing during the little ice age lia the study illustrates high sensitivity of the lake sediment sequence to witness the climatic episodes experienced in the glaciated regime of the triloknath glacier

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

orocline timing through joint analysis insights from the ibero armorican arc abstract the timing and kinematics of oroclinal bending in the core of the ibero armorican arc iaa has recently been constrained using paleomagnetic data from the cantabrian zone in northern iberia this study analyzes the joint patterns present in rock units deposited pre syn and post oroclinal bending systematic changes in the orientations of tensional joint sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the iaa time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units joint azimuth variability in the pre orocline rocks neoproterozoic to pre upper carboniferous is comparable to the present arc curvature of the orocline about 180 the joints in the syn orocline rocks upper pennsylvanian or stephanian 304 to 299 ma show a lower azimuthal variability that is comparable to about 50 70 of the total curvature seen in pre orocline rocks finally post orocline rocks permian contain joints that have uniform azimuths for each set across the entirety of the present day arc together these spatially and temporally distinct joint sets suggest that rotations in the cantabrian zone took place in the upper pennsylvanian during a ca 10 ma time period which agrees well with previous paleomagnetic arguments the data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear or nearly linear orogenic belt and finally these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development

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