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

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

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

model dependent 10be sedimentation rates for the taiwan strait and their tectonic significance in a new method to estimate the sedimentation rate along continental margins we made use of a model in which the 10 be in the sediment is considered as a mixture of terrigenous and pelagic components this method does not depend directly on radioactive decay it has many advantages over conventional techniques and is especially suitable for areas undergoing rapid deposition sedimentation rates on the continental shelf around taiwan obtained by this method compare favorably with those from 14 c dating our estimated rates for modern basins in the taiwan strait are comparable to the rates in the paleobasins now on land in taiwan this similarity suggests that the arc continent collision on the east side of taiwan has been driving a migration of its western shoreline toward mainland china

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

environmental magnetic implications of greigite fe3s4 formation in a 3 m y lake sediment record from butte valley northern california authigenic greigite fe 3 s 4 has been identified in several horizons of lake beds in a 102 m core from butte valley northern california using mineral magnetic methods and x ray diffraction analysis the presence of greigite has several implications for the paleoenvironmental record from butte valley first its occurrence in 2 5 3 0 ma strata confirms that greigite can persist in the geological record for long periods of time second the detrital mineral magnetic record may be partially obscured by the presence of authigenic greigite and care must be taken in interpreting magnetic variations in the greigite bearing zones as paleoclimate proxies third differences in the timing of remanence acquisition for authigenic and detrital phases may compromise studies of high frequency geomagnetic field variations fourth greigite may also be significant as a paleoenvironmental indicator of lake and sediment chemistry the magnetic detection of greigite may therefore provide important information about paleolimnological conditions

pyrite formation in an anoxic estuarine basin in a shallow fjord like estuary the upper and lower basins of the pettaquamscutt river rhode island each contain water columns stratified into oxic and anoxic plus sulfidic layers both water columns contain iron monosulfides and pyrite from just below the oxic anoxic interface down to the sediment water interface concentrations of iron monosulfides increase with increasing depth in the water columns but concentrations of pyrite are relatively constant with depth suspended pyrite is present only as framboids whereas the underlying sediments contain dominantly framboids but also euhedral grains and infilled framboids based on pyrite textures s isotopic compositions of water column and sediment sulfides and settling rate calculations about 70 of the pyrite burial flux to the pettaquamscutt sediments is accounted for by framboids forming at the oxic anoxic interface and settling to the sediment water interface the remaining pyrite burial flux is secondary pyrite nucleation and growth within the sediments represented by euhedral grains and infilled framboids the s isotopic composition of pyrite in the pettaquamscutt sediments and presumably of pyrite in the sediments of other modern anoxic basins and analogous ancient sediments therefore contains a component originating near the oxic anoxic interface and a component added below the sediment water interface pyrite framboids form in the water column where there is high supersaturation with respect to pyrite and slight undersaturation with respect to the iron monosulfides we propose that framboid nucleation and growth occurs in the water column adjacent to and below the oxic anoxic interface where supplies of ferrous species sulfide species and suitable electron acceptors are available infilled and overgrown framboids from in the sediments as a result of surface nucleation and continued pyrite growth on framboids that originate above the sediment water interface

changing environmental conditions in recent past reading through the study of geochemical characteristics magnetic parameters and sedimentation rate of mudflats central west coast of india abstract geochemical sediment components major and trace metals magnetic related parameters concentration mineralogy and grain size and radionuclide 210 pb analyses were carried out on sediment cores collected from mudflat regions of central west coast of india viz kolamb creek malvan mandovi estuary panaji and tadri creek gokarn in order to study the variations in sediment characteristics with time the changes in spatial and temporal depth distributions of sediment composition were observed to be mainly controlled by the catchment area geology geomorphology and anthropogenic activities the sedimentation rate in the study area varied from 0 13 to 2 cm yr the rate was extremely low at the lower half of the cores 0 13 0 31 cm yr before 1980 as compared to the near surface 1 21 2 cm yr after 1980 with two clear phases of sedimentation rates the higher sedimentation rate in the upper portions of the cores was found to correspond to increased deposition of finer sediment components metals and magnetic minerals which in turn were found to be mainly controlled by geology and or human activities the results of the present study reveal variations in sediment properties with time and support the use of geochemical in addition to magnetic susceptibility parameters on mudflats to understand changing environmental conditions of recent past

greigite and the magnetic properties of sediments x ray diffraction spectra of freeze dried sediments from a lake in the english lake district positively identified the presence of greigite a magnetic iron sulfide fe3s4 mineral the sediments also show considerable loss of magnetic intensity when wet samples are allowed to oxidize several hypotheses have been proposed elsewhere to explain the intensity loss but an assessment of the evidence suggests that oxidation of greigite is responsible in these sediments data from several lakes and from laboratory experiments are used to deduce the environmental requirements for greigite formation these conditions of high labile c concentrations good sources of labile fe and s and an oscillating oxycline depth in the sediments are likely to be found only in mono or dimictic eutrophic lakes estuaries and salt marshes and are unlikely to be found in meromictic lakes unstratified lakes oligotrophic lakes or deep sea sediments

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

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

preliminary results of a study of four successive sedimentary geomagnetic reversal records from the mediterranean upper thvera lower and upper sidufjall and lower nunivak abstract the results of a study of four successive early pliocene geomagnetic reversal records are presented the polarity transitions have been recorded in the trubi formation of calabria s italy which is composed of alternating carbonate poor and carbonate rich marly layers with moderately high 5 cm ka 1 accumulation rates the multicomponent nrm contains a well defined primary component in which two dominant magnetic phases can be recognized a magnetite and a low temperature lt i e removed at moderate temperatures phase most probably the lt phase is pyrrhotite the combined presence of pyrrhotite and magnetite in the sediment restricts the validity of a xarm x test for paleointensity studies the magnetite magnetizations reveal a smoother record relative to the records of the lt phase magnetizations between the two phases no significant delay in the moment of remanence acquisition is observed the durations of the major directional changes for the reversals are estimated to be in the range 0 3 3 0 ka the vgp path of the upper thvera record follows a great circle 90 w of the site meridian indicating a clearly non zonal transitional field whereas the vgp paths of the other reversal records are close to the site meridian indicating a mainly zonal transitional field the lower sidufjall and lower nunivak records are nearly identical and have a zonal harmonic content similar to records reported for the matuyama brunhes polarity transition

authigenic or detrital origin of pyrrhotite in sediments resolving a paleomagnetic conundrum monoclinic pyrrhotite fe7s8 is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments in contrast geochemical literature indicates that pyrrhotite formation is extremely slow below 180 c which makes it a highly unlikely carrier of early diagenetic remanences in sediments this view is confirmed by the occurrence of late diagenetic fe7s8 in ancient sediments and the general lack of fe7s8 in modern sediments horng et al c s horng m torii k s shea s j kao inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan earth planet sci lett 164 1998 467 481 documented the presence of fe7s8 that carries a magnetic signal indistinguishable from that of detrital magnetite in pleistocene marine sediments from taiwan we tested the possibility that the fe7s8 could have a detrital origin by conducting a source to sink study and found fe7s8 in metamorphic rocks of the taiwan central range and in material eroded from these rocks in the transportation pathway and in the depositional sink this confirms that the fe7s8 has a detrital origin rapid transportation from source to sink e g by typhoon associated flood events probably assists preservation of the fe7s8 which might otherwise oxidize during transportation the widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of fe7s8 in marginal sedimentary basins detrital fe7s8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance

rock magnetism and palaeomagnetism of greigite bearing mudstones in the italian peninsula magnetic measurements were carried out on a collection of samples from different geological settings throughout the italian peninsula the samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments thermal static and tumbling alternating field af demagnetization greigite fe3s4 was positively identified in most samples in agreement with previous studies of greigite the sediments display thermal decomposition of the magnetic carriers at temperatures above ca 230 c resulting in decreased magnetic susceptibility values and maximum unblocking temperatures tub mostly in the range 320 350 c high saturation isothermal remanent magnetization sirm to low field magnetic susceptibility k ratios sirm k 10 ka m hysteresis ratios that are typical for single domain sd grains mrs ms 0 5 and b0 cr b0 c 1 5 where mrs is the saturation remanence ms is the saturation magnetization b0 c is the coercive force and b0 cr is the coercivity of remanence moderate coercivity with b0 cr values that range between 52 and 81 mt and a tendency to acquire a significant rotational remanent magnetization rrm we also found that greigite bearing sediments are particularly sensitive to field impressed anisotropy we propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite which relies on its sd behaviour in field impressed anisotropy experiments in greigite bearing sediments the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field 0 9 t in our experiments at right angles to this direction the susceptibility increase is directly correlated to the concentration of greigite in the sediment the comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization in close steps up to 350 380 c is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite whereas af demagnetization treatments display significant acquisition of unwanted magnetic remanences gyroremanent remanent magnetization grm and rrm at fields above ca 40 mt

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

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

carbonate deposition pyramid lake subbasin nevada 4 comparison of the stable isotope values of carbonate deposits tufas and the lahontan lake level record abstract in this paper the fundamental importance of changes in hydrologic balance and hydrologic state on the 18o and 13c values of water and dissolved inorganic carbon dic in lakes of the lahontan basin is illustrated abrupt changes in 18o and 13c values of carbonate deposits tufas from the pyramid lake subbasin nevada coincide with abrupt changes in lake level and hydrologic state minima in lake level at 26 000 15 500 and 12 000 yr b p are associated with relatively heavy 18o and 13c values maxima in the lake level record at 14 000 and 10 500 yr b p are associated with relatively light 18o and 13c values we believe that the correlation between maxima and minima in the lake level and 18o records reflect the fundamental effect of lake level dynamics on the 18o value of lake water evaporation increases the 18o value of lake water whereas streamflow discharge and on lake precipitation decrease the 18o value variation in the 18o value of lake water therefore indicates change in the hydrologic balance increases in 18o accompany decreases in lake volume and decreases in 18o accompany increases in lake volume covariance of 13c and 18o indicates that change in 13c values of dic also accompany change in lake volume we offer the hypothesis first put forward by j a mckenzie that change in the productivity photosynthesis respiration balance is responsible for much of the observed variation in 13c most great basin lakes including lake lahontan experienced changes in hydrologic state during the late wisconsin when a lake becomes hydrologically open the residence time of water decreases the greater the rate of spill the greater the volume of evaporated 18o enriched water removed from the spilling lake and the more negative the 18o value of water remaining in the spilling lake the concentration of dic as well as the concentrations of photosynthesis limiting nutrients e g phosphorus nitrogen silica molybdenum decrease as spill increases increasing rates of spill therefore lead to overall decreases in photosynthetic rates relative to respiration rates and as a consequence the 13c values of dic become more negative

geodesy by radio interferometry determinations of baseline vector earth rotation and solid earth tide parameters with the mark i very long baseline radio interferometry system we analyzed 37 very long baseline radio interferometry experiments performed between 1972 and 1978 and derived estimates of baseline vectors between six sites five in the continental united states and one in europe we found no evidence of significant changes in baseline length for example with a statistical level of confidence of approximately 85 upper bounds on such changes within the united states ranged from a low of 10 mm yr for the 850 km baseline between westford massachusetts and green bank west virginia to a high of 90 mm yr for the nearly 4000 km baseline between westford and goldstone california we also obtained estimates for universal time and for the x component of the position of the earth s pole for the last 15 experiments the only ones employing wideband receivers the root mean square differences between our values and the corresponding ones published by the bureau international de l heure are 0 0012 s and 0 018 arc sec respectively the average value we obtained for the radial love number h for the solid earth is 0 62 0 02 estimated standard error

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

a record of reversed polarity carried by the iron sulphide greigite in british early pleistocene sediments abstract palaeomagnetic measurements were made on samples extracted from a short sequence of early pleistocene estuarine clays now exposed in a sea cliff near sheringham on the north norfolk coast uk on the basis of earlier palynological work these clays had been ascribed a pastonian late tiglian age the clays show marked changes in colour from reddish brown at the top of the unit to blue grey in the middle and grey brown at the base the palaeomagnetic data vary in close association with these colour changes the top and basal brown clays show scattered normal directions of low intensity while the middle blue clays show strongly clustered reversed directions of much higher intensities some samples taken from the boundary between the middle blue and upper red clays show upon demagnetisation a normal overprint on a stable reversed polarity using high gradient magnetic extraction magnetic concentrates have been obtained from the strongly magnetic middle blue clays the presence of iron sulphide minerals in these concentrates was identified using energy dispersive x ray analysis during scanning electron microscopy more specifically x ray diffraction identifies greigite as the only detectable ferrimagnetic mineral in the magnetic concentrates rock magnetic measurements show clear qualitative differences in the magnetic mineralogies of the three clay subunits but absolute identification of the magnetic mineralogy of the weakly magnetic upper and basal brown clays has not yet been possible we interpret the sequence as a primary reversed polarity record this record is carried by the iron sulphide greigite as a chemical remanence acquired during syn depositional reduction of iron via the decomposition of organic material in these anoxic tidal clays subsequently the upper and basal subunits of the clay have been oxidised by permeation of groundwater from the adjacent coarse grained sediments most of the griegite in the oxidised margins of the clay has been altered as a result to a new less efficient magnetic recording material which thus carries a later scattered low intensity normal overprint

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

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

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 an indicator of paleosalinity changes in the middle miocene paratethys sea of central europe the miocene epicontinental paratethys sea of central eurasia has experienced multiple restriction and reconnection events to the open ocean magnetostratigraphy is an important dating tool to better understand the temporal and spatial paleoenvironmental variations associated with these changes magnetostratigraphy in the paratethys domain however is complicated by the presence of greigite fe3s4 here we report rock magnetic and x ray fluorescence data of the tisa section romania which was previously magnetostratigraphically dated at the middle miocene base at 12 8 ma and top at 12 2 ma this section comprises the badenian sarmatian extinction event bsee which is marked by a major salinity change from marine to brackish environments related to the opening of the connection between the central and the eastern paratethys basins in the marine badenian sediments below the bsee the pyritization process is shown to be complete because of abundant sulfate supply in the brackish sarmatian deposits four intervals with early diagenetic greigite are observed and linked to insufficient sulfate in the water column these four greigite intervals appear to correspond to maxima in the 100 kyr eccentricity cycle we propose that increased fresh water from the eastern paratethys basin during eccentricity maxima restricted the sulfate availability in the tisa area leading to a reduced hs production and enhanced greigite preservation the early diagenetic formation of greigite enables a quasi syn depositional recording of the paleomagnetic field which allows reliable paleomagnetic dating in this section our results further suggest greigite as a potential indicator for salinity changes during marine brackish transitions

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

pyrite and organic matter in phanerozoic normal marine shales data have been obtained from our own chemical analyses and from the literature for the concentrations of organic carbon c and pyrite sulfur s in over 600 samples of cambrian to recent normal marine shales normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and or indicators of bioturbation all samples were selected to minimize 1 1 loss of c and or s due to weathering at the outcrop by emphasizing the use of drill core material 2 2 analytical errors and non diagenetic effects by avoiding sediments low in c or s content such as sandstones 3 3 fe limitation of pyrite formation by avoiding limestones cherts and euxinic shales 4 4 metamorphic loss of c relative to s by avoiding rocks obviously subjected to even low grade metamorphism our results indicate that 1 there is generally a good positive linear correlation between organic c and pyrite s for normal marine shales of all ages and 2 the mean cs ratio for normal marine shales has varied over time devonian to tertiary shales exhibit mean cs weight ratios 1 8 0 5 somewhat lower than quaternary sediments meancs 2 8 this we believe is due to preferential c loss relative to s during biogenic methanogenesis plus diagenetic heating distinctly lower mean c s values for the cambrian and ordovician 0 5 0 1 cannot be explained solely in terms of diagenetic c loss and instead must represent low original ratios the low original ratios we suggest were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time because the major source of such material vascular land plants had not yet evolved this along with a possibly lower degree of bioturbation contributed to enhanced pyrite formation and preservation and thus lower early paleozoic cs ratios

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

mineral magnetic record of environment in late pleistocene and holocene sediments lake manas xinjiang china a study of magnetic parameters was carried out on a late pleistocene and holocene sedimentary sequence 5 m from lake manas northern xinjiang china that has already been ana lysed for bulk sediment mineralogy stables isotopes from carbon ates and biogenic material magnetic minerals confirm major changes in palaeoenvironmental and palaeohydrological conditions that were deduced from other methods they also provide complementary information on changes in the lake system during the early and middle holocene the most important mineral mag netic change results from the authigenic formation of ferrimagnetic greigite fe3s 4 during the lacustrine episodes dated from 8 7 kyr b p to 5 5 kyr b p 380 335 cm and from 4 5 kyr b p to 3 5 kyr b p 320 285 cm respectively our data also indicate an abrupt break in sedimentary conditions which coincides with a short term retmxtowards dry climatic conditions around 3 5 kyr b p which was only suggested from other proxy data

sedimentation rate dilution preservation and total organic carbon some results of a modelling study abstract a modelling exercise was undertaken to evaluate the positive preservational and negative diluent effects of sedimentation rate sr on the total organic carbon toc content of marine sediments and to use multiple regression analysis of modern data to predict toc data presented show the predicted marine toc using the carbon burial efficiency be versus sr relationship of betts and holland betts j n holland h d 1991 the oxygen content of ocean bottom waters the burial efficiency of organic carbon and the regulation of atmospheric oxygen palaeogeography palaeoclimatology palaeoecology global and planetary change section 97 5 18 calculated for a range of sr water depths primary productivities and four fixed be values the results suggest that the preservation effect of sr only enhances toc at 5 cm ka in oxic regimes after which dilution occurs if carbon supply is held constant above 35 cm ka there is no appreciable difference in toc regardless of whether the sea floor is anoxic or not below 10 cm ka there is a strong divergence in oxic and anoxic toc values observed positive correlations of sr and toc at 5 cm ka are largely an artefact of including sites from shallower depths and or higher productivities multiple regression analysis mra of modern marine sediment data suggests that once re scaled to allow for their non linear effect dissolved oxygen values may be a significant additional variable in determining the toc predicting an approximately 2 5 4 0 times higher toc in anoxic facies compared with fully oxic ones 4 ml l dissolved oxygen all other things being equal the mra also indicates a generally negative partial correlation between carbon delivery flux and be in modern sediments producing a significant moderating effect on the predicted toc

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

characteristics and origin of lacustrine source rocks in the lower cretaceous erlian basin northern china abstract four lacustrine rift sub basins in the erlian basin one of the most petroliferous basins in northern china have been analyzed to characterize source rock potential to reveal environmental and ecological changes and to construct depositional models for lacustrine source rocks in different tectonic settings the first member of the tengger formation in these sub basins is characterized by a wide range of total organic carbon contents hydrogen indices and kerogen types these sub basins have dissimilar boundary fault activities and sedimentation rates the differences in border fault activity and sedimentation rate are consistent with changes in productivity redox conditions and clastic influx input the carbon isotopic composition of carbonates in mudstone samples suggests that the productivity of aer and wuli sub basins is higher than that of naoer and saihan sub basins differences in tectonic conditions also result in different redox conditions of depositional environments total reduced sulfur and pr ph ratios show an oxidizing environment in aer and wuli sub basins while a reducing environment was present in naoer and saihan sub basins with more active boundary faults and higher sedimentation rates the terrigenous organic matter inputs to aer and wuli sub basins are higher than naoer and saihan sub basins this can be confirmed by relative steranes abundances different tectonic conditions existed in different sub basins which lead to variations in ecological communities in the lake systems these environmental and ecological changes enable us to construct models for the deposition of lacustrine source rocks under different tectonic backgrounds during source rock deposition in aer and wuli sub basins the high influx of clastic sediments carrying high terrigenous organic matter and dissolved inorganic carbon and nitrate into the lake might have sustained high productivity and oxidizing conditions due to high influx of clastic sediments and unstable water column stratification the depositional environment of aer and wuli sub basins are oxidizing in a high productivity lake there is still significant organic matter accumulation in oxidized environment after oxidative degradation the high productivity is the dominant factor controlling the formation of organic rich sediments and the deposition of source rocks in aer and wuli sub basins in naoer and saihan sub basins the depositional environment during source rock formation is reducing due to low influx of clastic sediments with low productivity in low productivity lakes such as naoer and saihan sub basins which are low productivity lakes redox condition substitutes for productivity and becomes the predominant factor controlling the formation of lacustrine source rocks it is evident that the synergetic evolution of depositional environments and organisms in a lake system accounts for the formation of source rocks with different characteristics the proposed models may have important implications for source rock prediction in other lacustrine rift basins

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

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

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

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

rates of reaction between silicate iron and dissolved sulfide in peru margin sediments porewaters in site 680 peru margin sediments contain dissolved sulfide over a depth of approximately 70 m which at a sedimentation rate of 0 005 cm yr 1 gives a sediment exposure time to dissolved sulfide of about 1 4 106 yr reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly reactive silicate iron fraction defined as the difference between iron extracted by dithionite fed at room temperature and that extracted by boiling concentrated hcl feh normalised to the total iron content fet straight line plots are obtained for 1n feh fed fet against time of burial from which a first order rate constant of 0 29 10 6 yr 1 equivalent to a half life of 2 4 106 yr can be derived for the sulfidation of this silicate iron comparable half lives are also found for the same poorly reactive iron fraction in the nearby site 681 and 684 sediments this silicate fe fraction comprises 0 8 1 0 fe only 30 60 of which reacts even with 1 5 3 million years exposure to dissolved sulfide diagenetic models based on porewater concentrations of sulfate and sulfide and solid phase iron contents at site 680 are consistent in indicating that this poorly reactive iron fraction is only sulfidized on a million year time scale silicate iron not extracted by hcl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments

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

impact of climate change on the magnetic mineral assemblage in marine sediments from izu rear arc nw pacific ocean over the last 1 myr abstract a rock magnetic study was conducted on upper pleistocene marine sediments from international ocean discovery program expedition 350 site u1437 in order to highlight the paleoenvironmental changes in the nw pacific ocean influenced by the east asian monsoon regime remanent magnetization analyses hysteresis properties first order reversal curves and low temperature magnetic measurements were carried out along with electron microscope observations the results indicate that coarse grained titano magnetite is the dominant magnetic phase in the magnetic mineral assemblage time series analysis supports that this assemblage is modulated by global climate changes over the last 1 myr during the interglacial stages magnetic minerals are more abundant and are dominated by coarse grained titano magnetite of both terrigenous likely from mainland china and volcanic izu arc front japan origin during the glacial stages the magnetic mineral content is lower probably reflecting partial dissolution of ti magnetite and the magnetic assemblage is composed of terrigenous coarse grained titano magnetite and of higher coercivity presumably finer eolian particles likely hematite as a result of the enhancement of the winter monsoon in continental asia the magnetic mineral assemblage reflects a superimposition of volcanic and global climate signals bulk organic geochemical analyses for total and organic carbon as well as nitrogen and sulfur contents confirm a climatic signature in the composition of the sediments with more oxygenated water masses being present during glacial periods additional x ray fluorescence measurements on bulk samples indicate various origins of the sediment particles with both proximal and distal sources

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