stromatolitic framework of carbonate mud mounds abstract mud mounds are an important phanerozoic reef type generally formed in deeper water on carbonate shelf to basin slopes they commonly contain radiaxial calcite spar filled depositional cavities stromatactis but lack abundant metazoan framebuilders mud mounds are often flanked by steeply dipping bioclastic grainstones and sometimes attained widths and depositional reliefs of over a hundred meters they occasionally occur as basal cores on which developed shallow water metazoan constructed reefs stromatactoid mud mounds are composed of a framework of submarine cemented crudely reticulate masses or a succession of laminar crusts surrounded by cement filled cavities these cavities developed when unconsolidated sediment around the framework was removed by winnowing mud mound frameworks of all ages show strong geometric and petrographic similarities to shallow water cryptalgal structures especially thrombolites unlaminated stromatolites which occur in many phanerozoic shallow water reefs both thrombolites and mud mound frameworks are largely composed of lime mudstone cemented on the sea floor and contain fenestral fabric and diagnostic cryptalgal microstructure their depositional surfaces were irregular not bored or burrowed and rarely encrusted stromatolitic lamina ion although commonly not well developed in the subtidal zone anyway occasionally occurs in mud mounds it is proposed that mud mounds accumulated from organic probably blue green algal binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension patchy distribution of algal mats resulted in areas of both bound and unbound sediment sporadic floodlike episodes of rapid sedimentation however temporarily smothered the living algal mat and also left layers of unbound sediment periods of winnowing washed out unconsolidated unbound sediment and promoted synsedimentary cementation of resulting stromatactoid cavities and algal bound framework

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

origin of manganese carbonates in jurassic red shale central japan manganese carbonate deposits in japanese jurassic sedimentary rocks were studied petrogeochemically the deposits are characteristically composed of spheroidal micronodules up to 1 mm in diameter and always contain well preserved radiolarian shells chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition mn86 7 92 2ca2 2 2 9mg2 6 6 7fe2 6 5 6 co3 carbon and oxygen isotope values which range from 7 99 to 4 78 and 4 05 to 0 28 relative to pdb respectively suggest that the manganese carbonate was precipitated in a suboxic zone the micronodules closely resemble agglutinated benthic foraminifera in shape we suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

grain size distribution of pedogenic magnetic particles in chinese loess paleosols 1 variations in magnetic parameters of the long chinese loess paleosol sequences reflect paleoclimatic fluctuations however there remain ambiguities whether the magnetic proxies are controlled by the concentration or by the distribution of magnetic grain sizes we systematically investigated the frequency dependence of low field magnetic susceptibility fd and the decay rate dr dependence of anhysteretic remanence magnetization arm susceptibility arm dr for a set of representative loess paleosol samples from the yuanbao section western chinese loess plateau after removing the effect of aeolian coarse grained magnetites both fd and arm dr are nearly independent of the degree of pedogenesis indicating a presence of fine grained magnetic particles ranging from superparamagnetic to single domain with an almost constant grain size distribution therefore the magnetic enhancement of magnetic susceptibility or arm is caused dominantly by changes in the concentration of these fine grained pedogenic grains

fluid flow compartmentalization in the sicilian fold and thrust belt implications for the regional aqueous fluid flow and oil migration history abstract the fluid flow history in the frontal part of the sicilian fold and thrust belt ftb has been reconstructed using an integrated structural petrographic geochemical and microthermometric approach the study focused on comparing fluid flow during progressive deformation along major thrust horizons and in pelagic sediments occurring in the associated thrust sheets foot and hanging wall a fluid flow model is constructed for the frontal part of the sicilian ftb syn deformational quartz and calcite have been precipitated along decollement horizons in the iudica scalpello study area the microthermometric analysis of fluid inclusions in the quartz and calcite indicated migration of low saline high temperature aqueous fluids 1 5 h the fluid flow history in the thrust sheets can be subdivided into two stages calcite of types 1 and 2 has identical light orange cathodoluminescence as the surrounding mudstone furthermore its isotope signature 2 13 c 18 o m h between 80 and 140 c type 3 calcite has less depleted 18 o values compared to calcite of types 1 and 2 and the 18 o from calcite in faults is even positive during the final stages of fluid flow with precipitation of calcite type 3 the fluid flow model invokes infiltration of overpressured fluids that migrated along the decollement zone these fluids only infiltrate the thrust sheet in the hanging wall of the thrust leading to a compartmentalized fluid flow pattern an identical fluid flow pattern with migration of low and hot saline fluids associated with hydrocarbons was observed in other parts of the sicilian fold and thrust belt with identical structural deformation and fluid flow patterns the study confirms the suitability of the frontal part of the sicilian fold and thrust belt as an outcrop analog for deeply submerged accretionary prisms the study thus offers unique insights in expected fluid flow patterns in thrust sheets of deeply submerged accretionary complexes

extension of berner s new geochemical classification of sedimentary environments to ancient sediments using stable isotopes of carbon and oxygen in siderite nodules and lithology of host shales one can recognize geochemical environments proposed by berner in devonian carboniferous sediments of the appalachian basin his oxic post oxic and sulfidic diagenetic environments are well represented sediments of the methanic environment may be present but the evidence is equivocal the sulfidic sediments can be further subdivided into well laminated and bioturbated corresponding to the presence of h 2 s in the overlying water in the first case but confined to the sediment pore waters in the second journal abstract

submarine cementation in a high energy platform reef one tree reef southern great barrier reef three major framework facies algal pavement coral heads and branching corals show varying degrees of cementation the widest distribution and greatest variety of submarine cements occur within those facies or subfacies that have extensive encrustation by coralline algae these are found beneath the windward margin and within the patch reef drilled both mg calcite and aragonite cements exhibit diverse fabrics and textures mg calcite in the dominant cement and is present as both an interskeletal and intraskeletal infill whereas aragonite cement is exclusively intraskeletal interskeletal cements are only developed within those facies capable of trapping significant quantities of fine grained internal sediment lithified crusts with either a laminated or columnar morphology are a prominent cavity infill within many of the reef framework facies organic influence during formation peloids which are the major component of the lithified crusts as well as many other cavity infillings are a particular form of mg calcite micrite and have nucleated within the interstitial waters of the reef microcrystalline rim cements around the peloids have often cemented them into a coherent mass that resembles detrital lime mudstones modified journal abstract

fine grained carbonates in nearly closed reef environments mataiva and takapoto atolls central pacific ocean abstract the nature origin and mode of accumulation of fine grained carbonate sediments have been investigated in the nearly enclosed lagoons of two french polynesian atolls mataiva and takapoto the fine sized fraction 20 m is substantially higher in amount 70 of the total fine sediments on average than the mud portion and may increase volumetrically 90 99 in some localities aragonite is the predominant mineral present 90 of the total calcium carbonate the fines appear to be mainly skeletally derived the silt fraction includes coral and molluscan detritus sponge chips juvenile benthic foraminiferans and other biogenic debris the mud is chiefly composed of aggregated nanograins ascidian spicules skeletal nanograins coral generated mainly and coccoliths a minor part of the mud is certainly of non skeletal origin some portion of freely deposited fines fill cavities or are attached upon sand grain surfaces they are associated with scattered inorganic precipitates and contribute to the formation of micrite envelopes and aggradation of host grains such a process is highly significant in the total carbonate budget of the reef the fine carbonate sediments result directly from the biological disintegration of nearby buildups mainly through the boring activity of sponges and lithophagid molluscs we suggest that the authigenic mud sized crystallites are controlled in their precipitation both by carbonate supersaturation and anoxic conditions seasonally affecting the interstitial sediment waters enclosed atoll lagoons are relatively self governing environments of deposition which are influenced by long term quiescent water situations rather than by rare catastrophic events

magnetic enhancement is linked to and precedes hematite formation in aerobic soil 1 soil formation usually increases magnetic susceptibility most often by increasing the concentrations of magnetite and maghemite which are two ferrimagnetic iron oxides here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite which is later transformed into hematite the iron oxide that gives red color to soil we show that the maghemite hematite ratio is influenced by the particular environment and the degree of soil development so it constitutes an effective tool for paleoenvironmental and planetary studies

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

sponge constructed stromatactis mud mounds silurian of gaspe quebec abstract stromatactis calcilutite mounds from upper silurian rocks of gaspe peninsula quebec canada are important constituents of a reef complex in the southern part of the peninsula they reach thicknesses up to 115 m and had paleoslopes of about 40 stromatactis constitutes up to 30 percent of the rock volume the microfacies of the calcilutite consists of agglomerates of pelletlike bodies forming a pelletoidal network surrounded by uniform mudstone sponge spicules are ubiquitous by analogy with textural features of a well documented sponge malumispongium hartnageli clark 1924 which occurs slightly above the mounds it is suggested that the pelletoidal network of the mound calcilutite represents a former sponge network that underwent very early dingenetic change and cemen ation stromatactis would have originated as early marine cementation of growth cavities in the sponge network and of cavities created by decay of local uncemented sponge tissues

magnetic properties of surface sediments as proxies of recent anthropogenic pollution in the anllons riverbed nw spain a study of surface sediments from the anllons riverbed northwestern spain was conducted to assess the use of their magnetic properties as proxies of recent trace element contamination i e as zn pb cr ni and cu thermomagnetic curves hysteresis parameters specific magnetic susceptibility frequency dependent magnetic susceptibility fd magnetic remanence and saturation isothermal remanent magnetization ratios i e s and sirm reveal the presence of mostly fine grained magnetite and maghemite in these samples the tomlinson pollution load index pli as well as the magnetic parameters fd sirm and s ratios vary more or less in agreement to each other when they are sequentially arranged throughout a 30 km long transect that goes from the town of carballo to the river mouth in ponteceso from that profile it is possible to identify an anomaly of that lies between two small peaks of the pli and seems to define a contaminated area probably linked to a diffuse low impact source of pollution there is also a conspicuous peak of the pli that is likely associated to a nearby gold mine the variations of the fd s along the transect are the ones that best reflect the changes in the pli values regardless of the level of contamination this result suggests that adsorption is the main mechanism of trace elements removal by fe oxides in these samples direct signal analyses used to decompose the isothermal remanent magnetization curves reveal the presence of four magnetic fractions with varying relative concentrations all over the profile namely single domain magnetite close to the superparamagnetic threshold sd sp and or maghemite log b 1 2 ca 1 2 single domain magnetite log b 1 2 ca 2 hematite log b 1 2 ca 2 5 and goethite log b 1 2 ca 3 5 integrating all the available information it was sketched out a likely sequence of events that included magnetic minerals neoformation in parent soils their subsequent chemical transformations and the contamination episodes that affected the region

new insights into partial oxidation model of magnetites and thermal alteration of magnetic mineralogy of the chinese loess in air summary new rock magnetic data including low and high temperature thermomagnetic runs and hysteresis loops as well as x ray diffraction measurements are presented from the chinese loess palaeosols at jiuzhoutai on the northwestern margin of the chinese loess plateau to provide sound constraints on the partial oxidation model of magnetites and variations in magnetic mineralogy grain size and oxidation states of loess samples during thermal treatments from room temperature up to 700 c the results show that the partially oxidized magnetites in samples are first reduced probably as a result of the burning of organic matter below 300 c and then reoxidized above that however even after heating to 700 c the highly diminished in size magnetite core still remains stoichiometric as revealed by the 120 k verwey transition we conclude that the diffusion of fe2 from the magnetite core to the surrounding maghemite rim is confined to the oxidation front magnetite core maghemite rim boundary and has less effect on the stoichiometry of the magnetite core the new results also reveal that the absolute number of superparamagnetic grains magnetite maghemite remains relatively stable during heating furthermore the left downward shift in the data points of day plots mainly between 25 and 150 c is caused by a decrease in the oxidation gradient in contrast the right upward mainly between 500 and 650 c shift of the day plots corresponds to a much higher degree of oxidation resulting in the increase of a new oxidation gradient between the magnetite core or the residual maghemite mantle and the newly developed haematite outer rim which is transformed from maghemite by heating these new results place strong constraints on the fidelity of palaeomagnetic results from the chinese loess constructed by thermal demagnetization and provide a more accurate interpretation of the day plot and low temperature magnetic behaviours for these partially oxidized magnetites furthermore this study provides robust lines of evidence for the core rim coupling model between different magnetic phases magnetite maghemite and haematite

testing lake level reconstructions based on rock magnetic proxies for the sediment record of laguna chaltel patagonia argentina this study was carried out on sediment cores collected with a gravity corer from laguna chaltel an almost circular crater lake located in patagonia argentina 49 9 s 71 w the main magnetic carrier was ti magnetite in the pseudo single domain range a model using magnetic grain size and concentration previously applied to laguna potrok aike to infer lake level changes was used for laguna chaltel the main requirement to apply the model is uniform magnetic mineralogy which is the case for laguna chaltel after magnetic data were compared with previously studied lake levels it was found that the magnetic proxies that best follow hydrologic changes are arm sirm anhysteretic remanent magnetisation saturation of isothermal remanent magnetisation and arm the concentration proxy arm measured with a 100 mt alternating field and 0 05 mt direct field was also used as wind indicator high wind strength was found at around 3650 cal yr bp and low wind strength for the last century arm sirm and arm were used to infer the strength of fluvial runoff into the lake for a core collected close to the shore and near a tributary the results show that the magnetic model is promising for inferring lake level variations particularly in patagonian lakes

spatial and temporal variations in early diagenetic organic matter oxidation pathways in lower jurassic mudstones of eastern england abstract the lower liassic succession of the east midlands platform eastern england comprises shallow marine mudstones and oolitic ironstones in these sediments temporal and spatial variations in the relative importance of early diagenetic anaerobic organic matter oxidation pathways are recorded the ironstones are stratigraphically condensed and were deposited upon the market weighton swell mws a topographic high in the basin in shallow water and high energy conditions at times of low relative sea level at times of higher sea level stratigraphically condensed mudstones were deposited upon the mws away from the mws stratigraphically expanded mudstones were deposited throughout the lower liassic the ironstones contain berthierine ooids and pore filling cement and siderite pore filling cement with rare pyrite replacing berthierine and siderite mudstones deposited away from the mws contain pyrite as the sole early diagenetic iron mineral contrastingly mudstones deposited upon the mws contain both early diagenetic pyrite and mn rich siderite the presence of berthierine and siderite in the ironstones indicates that sulphide was absent from early diagenetic pore waters and that fe iii reduction dominated anaerobic organic matter oxidation in contrast in mudstones deposited away from the mws the presence of pyrite indicates that sulphate reduction was a predominant pathway for anaerobic organic matter oxidation in mudstones deposited upon the mws the presence of both pyrite and mn rich siderite suggests that both sulphate reduction and fe iii reduction were important during early diagenesis it is proposed that this spatial and temporal variation in relative importance of early diagenetic anaerobic organic matter oxidation reactions was a result of variation in physical sedimentary parameters sediment accumulation rate and energy of environment at times of low net sediment accumulation rates in shallow water high energy environments suboxic diagenesis fe iii and mn iv reduction dominated giving rise to the characteristic berthierine siderite ironstone assemblage at times of higher sedimentation accumulation rates in deeper lower energy environments sulphate reduction dominated leading to pyrite formation at intermediate conditions fe iii mn iv and sulphate reduction were all important leading to precipitation of both pyrite and mn rich siderite

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

depositional controls on lower carboniferous microbial buildups eastern midland valley of scotland in the eastern midland valley of scotland non skeletal carbonate buildup complexes are present in the charlestown main limestone brigantian a thin marine carbonate unit within a lower carboniferous basin fill otherwise dominated by fluvio deltaic sediments and volcanics the buildups are restricted geographically to west lothian and central fife and correspond to an area of reduced subsidence the burntisland high buildup geometry and local facies mosaics reflect the relative water depth and position on the burntisland high that because of its slight topographical relief and more distal position with respect to the location of fluvio deltaic systems became an area of increased carbonate productivity and hence a locus for buildup development away from the burntisland high in the fife midlothian low more monotonous argillaceous limestone facies of low energy and somewhat deeper water aspect accumulated buildup facies contain a diverse marine fauna and are characterized by peloidal clotted and homogeneous micrites interpreted as predominantly microbial in origin two types of buildup complex occur at charlestown vertically stacked tabular banks up to 2 m thick and 100 m wide form the western margin of an asymmetric buildup accumulation east of this margin a complex mosaic of isolated low relief buildups and inter buildup sediments was deposited skeletal sand shoals present within these inter buildup sediments indicate that high energy conditions prevailed during buildup growth in contrast at roscobie inter buildup sediments are of low energy aspect and buildup geometry is domical rather than tabular initiated in slightly deeper and calmer water on the flank of the burntisland high the roscobie buildups up to 15 m thick and 46 m wide accreted vertically rather than laterally as at charlestown and coalesced to form an elongate ridge shaped buildup complex

analysis of the degree of contamination and evolution in the last 100 years of the composition of the bed sediments of the anllons basin in this study five cores of the anllons river bed sediments were analyzed in order to evaluate the downcore and downstream variations in their chemical composition the first step was the evaluation of the metal distribution in the bulk 2 mm and fine fractions 63 m the analysis revealed that most of the metals followed the same trend in both fractions although the fine fraction presented usually higher concentrations however the concentration of both fractions tended to equalize with increasing contamination no general increase was observed in the metal concentrations toward the surface which could be attributed to recent anthropogenic contributions instead the distributions were homogeneous or peaked at various depths downcore the most important historical feature was observed at the mouth of the river at 96 cm depth corresponding to the end of the eighteenth century upcore increased metal concentrations in parallel with increased fine fraction occurred from this depth which were attributed to a bridge construction and consequent changes in sediment dynamics as the metal concentrations can be influenced by variations in texture or other sediment characteristics the second step was to evaluate the efficiency of several normalized indexes in the assessment of the degree of contamination by calculating the enrichment factor ef the geoaccumulation index igeo and the pollution load index pli the efs obtained were 10 thus revealing little anthropogenic inputs to the basin the igeo produced higher values when compared with the efs instead of absolute ef or igeo absolute values the use of cumulative probability plots allowed identifying more accurately potential outliers indicating contamination only one population was identified for zn and pb with a reduced number of outliers at the highest concentrations for pb as shown a more complex plot with the outliers identified at c4 and c5 finally the pli allowed determining the absence of a significant contamination in the bed sediments the bioavailable and non geogenic fractions contribute up to a 90 of the total concentrations in the case of as and cu and showed similar dissimilar profiles in comparison with total metals also the quality guidelines were surpassed so the high solubility of as zn pb and ni in the sediments revealed the need to monitor the bed sediment quality of the anllons river

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

an integrated study of the grain size dependent magnetic mineralogy of the chinese loess paleosol and its environmental significance 1 to investigate the grain size dependent properties of the magnetic minerals in chinese loess paleosol samples touxiangdao xining qinghai province china magnetic extracts were divided into two size fractions by gravitational settling on the basis of hysteresis measurements thermal demagnetization of low temperature saturation isothermal remanent magnetization and nonmagnetic studies sem and xrd we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples the results show that the oxidation degree nonstoichiometry of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered in ascending order of the oxidation degree of our samples we find 1 loess coarse particles lc are multidomain md magnetite with slight oxidation 2 paleosol coarse pc particles are also md magnetite but with a higher oxidation degree compared to lc 3 loess fine lf particles are pseudo single domain psd magnetite with a high oxidation degree and 4 paleosol fine pf particles are psd maghemite single domain sd and superparamagnetic sp maghemite mainly stay in the residues further thermomagnetic analysis of pf psd maghemite revealed that this natural maghemite has a curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50 after a 700 c heating cooling cycle these new results identify the sources of multicomponent nrm in chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components

the habit and origin of siderite spherules in the eocene coal bearing prestfjall formation faroe islands abstract siderite spherules are described and chemically analysed from two localities holi i helli section and rokhagi coal mine within the coal bearing prestfjall formation the formation marks a significant hiatus in the eruption of lava flows of the paleogene faroe islands basalt group and shortly follows the paleocene eocene thermal maximum the spherules are chemically pure although feco 3 80 6 99 2 mol shows a marked substitution for mnco 3 0 1 13 9 mol the low concentrations of mgco 3 0 0 0 4 mol and the lack of associated sulphide minerals e g pyrite confirm the freshwater lake environment affinity of the formation the siderite spherules from the holi i helli section contain significantly higher concentrations of mnco 3 3 may also account for the abundance of siderite spherules formed by the split crystal process in the holi i helli section whereas prefect spherules consisting of individual crystals growing concentrically around a cryptocrystalline centre are dominant from the rokhagi coal mine

a geochemical constraint on the formation process of a manganese carbonate nodule in the siliceous mudstone of the jurassic accretionary complex in the mino belt japan abstract manganese mn carbonate nodules which differ from seafloor mn nodules mainly composed of mno 2 are occasionally embedded in the form of a lens shape in the jurassic accretionary complexes such as the mino belt in japan the interpretation of the formation process of mn carbonate is still controversial particularly concerning whether the mn carbonate was formed primarily or secondarily in this study a fresh mn carbonate nodule incorporated into the red siliceous mudstone was collected for geochemical and sedimentological analysis the optical observation of thin sections indicates that the mn carbonate nodules are composed of abraded grains of rhodochrosite spherule with radiolarians and are sedimentary embedded in siliceous mudstone microfossil radiolarians from the mn carbonate nodules and the host red siliceous mudstone are dated as the bajocian but the radiolarians in the nodules are somewhat older than those in the host red siliceous mudstone geochemical analysis using the x ray absorption near edge structure on ce indicates the dominance of trivalent ce at present despite the observation of a positive ce anomaly in the paas normalized ree pattern of mn carbonate the ree adsorption experiment on synthesized mnco 3 does not show any distinctive positive ce anomaly and a thermodynamic calculation suggests the possible coexistence of rhodochrosite and spontaneous oxidation of ce a leaching experiment that can selectively decompose the carbonate phase demonstrated no ce anomaly in the carbonate phase of the mn carbonate and a poor contribution to the bulk ree concentration the carbon isotope data of mn carbonate nodule implied the dominance of inorganic marine carbonate origin with small contribution of organic decomposition the most plausible account of all of the observational and geochemical results is that the rhodochrosite grains were primarily formed on the depositional site and subsequently transferred to a different site where siliceous mudstone was deposited

the lower cretaceous mesogean benthic ecosystems palaeoecologic aspects and palaeobiogeographic implications abstract douville s mesogee was based on an organismic assemblage and therefore defined mainly as a palaeobiogeographic entity the palaeoecologic analysis of douville s lower cretaceous reference taxa as well as that of a lot of other taxonomic groups and the discussion on their palaeobiogeographic meaning according to the concepts of modern marine biogeography especially the bathymetric thermic relationships result in the definition of three main organismic assemblages whose mesogean significance is discussed the key reference taxa and associated biota indicate that the definition of the lower cretaceous mesogee implies characteristic palaeoenvironmental conditions this definition substantial for the whole cretaceous means that mesogee is not simply a palaeobiogeographic entity but also an ecosystem corresponding to the area of distribution of warm shallow water carbonate platforms

hauterivian shallow marine calcareous biogenic mounds s e spain abstract hauterivian biogenic deposits from the prebetic northern margin near caudete albacete province southeastern spain are represented by low domed bodies less than 10 m thick surrounded by bioclastic sediments and capped by siliciclastics they consist of a coral stromatoporoid and microbial framework with cavities filled by mud rapidly lithified intermound bioclastics derived from the mound organic community reflect high energy conditions and shallow water settings mound growth and architecture were controlled by nutrient fluxes and internal light gradients comparisons with closely related structures from the palaeozoic and the recent show some similarities but also significant differences the example documented here also departs from other cretaceous occurrences

components facies and ramps redefining upper oligocene shallow water carbonates using coralline red algae and larger foraminifera venetian area northeast italy abstract chattian mixed carbonate siliciclastic deposits from two palaeogeographic units of southern alps the lessini shelf and venetian foreland basin of northeast italy are studied with respect to facies distributions and controlling ecological factors on dominant biogenic components these ramp depositional systems are located in the western southern alps on the lessini shelf and in the eastern southern alps in the venetian foreland basin the successions show an extensive geographic distribution of at least 150 km 40 km at the southern margin of the southern alps palaeoenvironments are inferred following microfacies characteristics and floral and faunal indicators especially those of coralline red algae and larger foraminifera an inner to middle homoclinal ramp morphology is reconstructed based on facies distributions and taxonomic and coralline algal growth form proxy data the distribution along depositional strike shows variation from thinner carbonate dominated sequence in the west monti berici and monti lessini to a more variable mixed carbonate siliciclastic dominated environment to the east vittorio veneto alpago facies character and occurrence are controlled by local hydrodynamic conditions and varying water turbidity of the studied ramp depositional system the general facies succession shows a transgressive sequence from the proximal inner to proximal middle ramp a shoal belt of trough cross bedded sandstones occurs in a proximal inner ramp position in all of the studied areas in a gentle dipping homoclinal ramp most benthic organisms inhabited the relatively quite environments off these shoals in a distal inner ramp setting basinward environments proximal middle ramp constitute areas of maximal carbonate production and consist primarily of larger foraminiferal facies and rhodolith pavements larger foraminifera represented by two larger foraminiferal assemblages are most diversified in the shallower settings near the fair weather wave base distal inner proximal middle ramp whereas coralline algae predominate in deeper areas proximal middle ramp constructing extensive rhodolith pavements rhodolith pavements are distinguished with respect to rhodolith shape size growth forms and coralline algal taxonomic composition larger foraminifera are interpreted with respect to shell structures shell architectures and comparable recent counterparts the analysed benthic communities thrived in a mesotrophic regime within trophic gradients constrained by fluvial influence coralline algal dominance and a moderately diverse larger foraminiferal assemblage suggest a tropical warm temperate transition the considerable stability imposed on sediments by the rhodolith pavement must also have significantly contributed to the maintenance of a middle ramp profile bulge this study will contribute to a better understanding of oligocene carbonates which have received less attention than their eocene or miocene counterparts

hysteresis and thermomagnetic properties of particle sized fractions from loess and palaeosol samples spanning 22 myr of accumulation on the chinese loess plateau summary hysteresis loop measurements and thermomagnetic experiments were carried out to characterize magnetic mineralogy and magnetic grain size in a suite of particle size fractioned samples from the chinese loess plateau representing the main changes in magnetic properties over the last 22 myr we find that 1 the ferrimagnets in the detrital fractions 8 m younger than 14 ma show a magnetite type curie temperature whereas those older than 15 ma have a curie temperature slightly over 600 c owing to low temperature oxidation 2 the dominant ferrimagnets in the clay fractions 2 m are most likely to be maghemite 3 both the hysteresis plots and thermomagnetic experiments reinforce previous indications from low temperature experiments that paramagnetic minerals make a major contribution to the clay fractions in all the samples considered 4 the degree of oxidation indicated by the thermomagnetic curves is highest in the clays and next highest in the oldest coarse fractions 5 day plots provide limited and potentially misleading information on the differences between fine grained pedogenic and coarse grained detrital ferrimagnetic assemblages even when allowance is made for the effects of hematite on the quotients used this study reinforces the view that measurements on bulk samples alone obscure critical distinctions between pedogenic and detrital components

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

a model for the origin and petrogenesis of the red stromatactis limestone of paleozoic carbonate mounds abstract from study of the devonian recifs rouges of belgium and the silurian mounds of the quebec appalachians we propose that deposition of the red stromatactis limestone facies of cambrian devonian carbonate mounds was controlled by sponges and that the red color and stromatactis result from early diagenesis within a few meters below the substrate water interface in the deep marine environment the common presence of sponge body fossils as well as various stages of sponge preservation ranging from easily delineated to indiscernible bodies in the belgian recifs rouges support the conclusion that a large part of the red finely crystalline limestone originated from early cementation of sponge communities or spicule rich organic mats derived from degradation of sponge communities the role of bacteria and other microbes although widely advocated as the main primary builders of carbonate mounds is difficult to assess microbial communities may have contributed to mound accretion as primary builders and or mud producers but their presence cannot be documented we suggest that they may have acted as agents for concurrent sponge putrefaction and early cementation biodiagene is during the sulfate reduction phase under alkaline ph and anoxic conditions we interpret stromatactis as a spar body that resulted from early marine cementation of a cavity network created by excavation of uncemented material in partly indurated decaying sponges and spicule rich organic mats derived from degradation of sponge communities through circulation of interstitial water in the uppermost few meters of the sediment this cavity network was connected to sea floor and flushed by oxic waters that changed the diagenetic environment from anoxic to oxic converting amorphous iron sulfide to hematite and giving the facies its red color recognition of the dominant role of sponge communities in the construction of red stromatactis limestone mounds of the cambrian devonian time interval has important implications for the geological record this community was the main deep water mound building community during this time but is difficult to recognize because of its variable but often poor preservation

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

magnetic stratigraphy of chinese loess as a record of natural fires magnetic susceptibility records of paleosols and loess show high correlation with oxygen isotope stratigraphy of ocean sediments kukla 1987 providing a global paleoclimatic record different models have been put forth to explain the nature and cause of susceptibility variations but consensus has not yet been achieved our low temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher verwey transition 115k than that for the magnetite 100 k in unaltered loess the same shift in the verwey transition can be achieved by heating and cooling loess samples this is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past natural fire intensity is sensitive to the amount of annual precipitation so that increased fire induced susceptibilities should reflect an increase in the humidity of regional climate

paleomagnetism of alkaline complexes and remagnetization in the kapuskasing structural zone ontario canada the shenango and nemegosenda alkaline complexes of the southern kapuskasing structural zone ksz an uplifted crustal block in northern ontario preserve primary thermoremanent magnetizations with mean directions d 112 i 46 k 13 95 13 n 9 sites and d 304 i 51 k 31 95 10 n 7 sites for shenango and d 124 i 57 k 13 9520 n 4 sites and d 312 i 54 k 17 95 13 n 7 sites for nemegosenda their average paleomagnetic pole is 45 n 189 e dp 6 dm 8 falling on the keweenawan track of the north american apparent polar wander path apwp between 1098 1107 ma reverse polarity r and 1087 1098 ma normal polarity n poles of lake superior lavas and dikes unlike the keweenawan paleopoles which have a reversal asymmetry of 30 the shenango and nemegosenda r and n poles are almost 180 reversed which was found also by lewchuk and symons 1990b this symmetry of r and n poles casts doubt on the previously accepted predominance of asymmetric reversals of the earth s field in keweenawan time the archean shawmere anorthosite is overprinted by a b magnetization with mean direction d 97 i 68 k 12 95 10 n 18 sites the shawmere b paleopole at 38 n 226 e dp 13 dm l6 falls between the carbonatite poles and 1850 ma cratonic poles and the b magnetization seems to be a composite of middle and late proterozoic overprints sites in the western part of the shawmere body close to the carbonatite line of faults linking the various alkaline complexes are severely altered with an abundance of secondary magnetite and hematite virtual geomagnetic poles for these sites fall close to the keweenawan track and have an average pole of 189 53 n dp 15 dm 20 suggesting that the b nrm at these sites is an 1100 ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion more easterly sites are less altered their vgps resemble 1850 ma cratonic poles and average to 235 e 30 n dp 15 dm 17 these b natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle proterozoic hydrothermal activity associated with intrusion of early alkaline bodies fault movement on the ivanhoe lake fault zone or late stage uplift of the ksz

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

geochemical normalization of magnetic susceptibility for investigation of floodplain sediments magnetic susceptibility ms is commonly used as a proxy for industrial pollution in natural sediments or as a proxy for the percentage of detrital components in peat or carbonates the ms may also reflect the geology of the sediment source and post depositional processes in sediments such as soil development the aim of our research was to test the usefulness of fe normalized mass specific ms and ti normalized fe in a study of floodplain sediments we sampled 27 floodplain sediment cores from several geologically different catchments throughout the czech republic analysed their fe and ti concentrations using x ray fluorescence spectroscopy and determined their to decipher sediment grain size dependence and possible magnetic enrichment background functions for were constructed using similar approach as that used for geochemical background functions of the risk element concentrations with fe concentrations as an independent variable it provides a mechanism to calculate of sediments as it would be pristine i e without post depositional changes and pollution sediments derived from mafic source rocks had fe larger by two orders of magnitude than sediments derived from felsic rocks sediments derived from mafic source rocks also exhibit lower mean fe ti ratio in pristine sediment strata than the average upper continental crust the magnetic carriers inherited from mafic rocks are stepwise destroyed by pedogenesis in the floodplains and thus slowly approach of sediments derived from felsic rocks gleying processes may change fe fe ti ratio allows identifying a past action of those post depositional processes

temperature dependence of magnetic susceptibility in an argon environment implications for pedogenesis of chinese loess palaeosols summary temperature dependence of magnetic susceptibility t has been widely used to determine changes in mineralogy of natural samples during heat treatment we carried out integrated rock magnetic experiments to interpret the t curves of the chinese loess palaeosols in argon we used both raw materials and heated samples in addition we also investigated the magnetic properties of magnetic extracts and residues to quantify contributions from each fraction to the bulk magnetic properties for the heating curves the susceptibility loss 30 per cent between 300 400 c is caused by the inversion from pedogenic fine grained maghemite to haematite suggesting that the susceptibility loss can be used as a new concentration index of the pedogenic fine grained superparamagnetic sp particles in the chinese loess palaeosols unlike the warming curves the cooling curves are dominated by newly formed fine grained magnetites with a dominant size of 35 nm the onset for the new production of these fine grained magnetic particles occurs at 400 c it is interesting that the room temperature magnetic susceptibility ph of the samples heated after a 700 c run is independent of the degree of pedogenesis and saturates at approximately 33 35 10 7 m3 kg 1 indicating that the susceptibility enhancement is controlled only by the reduction of fe bearing aeolian minerals during heating it appears that the 700 c thermal treatment in argon could be in some sense an analogue to the pedogenic processes thus we predict that 33 35 10 7 m3 kg 1 is the maximum susceptibility that pedogenesis can generate for the last interglacial palaeosol unit s1 in practice ph would be useful to quantify the aeolian inputs to the chinese loess plateau

the geochemistry of manganese carbonate in panama basin sediments abstract the mn distribution in panama basin area sediments and interstitial waters is discussed striking surficial mn enrichments produced by a well known diagenetic recycling process characterize the sediments of the region thermodynamic solubility calculations indicate that in at least one core interstitial waters approach saturation with respect to mnco3 a mixed carbonate phase of composition mn48 ca47 mg5 co3 was recovered from an ash band in the same core the association of this material with the coarse volcaniclastic debris is thought to result from facile manganous carbonate precipitation in sediment horizons of coarser mean grain size since sulphate reduction in the upper two metres of panama basin sediments is fairly minor little increase in alkalinity is observed and it is postulated that significant production of carbonate alkalinity is not a prerequisite for manganous carbonate generation in hemipelagic sediments a more important factor appears to be the availability of mn oxides for solution during early diagenesis stable c isotopic analyses indicate that little c of organic origin is used in the precipitation reaction in either panama basin or loch fyne scotland sediments

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

rudist coral frameworks associated with submarine volcanism in the maastrichtian of the pachino area sicily maastrichtian strata from the pachino area se sicily provide a model of association between rudist coral frameworks and submarine volcanic activity two successive carbonate units are distinguished a coral rudist bioherms and biostromes developed in a high energy environment b hippuritid build ups and banks overlain by rudist coral clusters which grew under harsher ecological conditions in a weaker current regime at the top of the maastrichtian sequence the growth of rudist coral frameworks ceased as ecological conditions shifted toward restricted environments the inferred succession reflects a gradual decrease in current strengths and water depths related to the maastrichtian regressive phase the frameworks display consistent evolutionary sequences reflecting progressive changes in their biota structure size and shape occurrences of rudist coral frameworks are clearly linked with submarine volcanoes which provided opportunities for their development they sometimes grew in the vicinity of active vent centres a feature reported for the first time in the upper cretaceous

behavioural properties of trace metals in soils abstract the speciation and spatial distribution of trace metals in soils are closely related to their behaviour in multiphase and dynamic soil systems the surface reactions at soil solution root interfaces are the key to the determination of the amounts of mobile and plant available metals the behaviour of trace metals in the soil environment depends to some extent on their origin lithogenic metals are associated mainly with primary minerals and may be phytoavailable under specific conditions in the soil plant system anthropogenic metals show great diversity but in most cases they are likely to be very mobile and easily available to plants forms of the metals of both origins may be transformed due to pedogenic processes and they become pedogenic metals greatly controlled by soil properties in order to assess the phytoavailability and to accept proper guidelines for permissible trace metal loading their behaviour in the specific soil environments has to be defined

stromatolites associated with coralgal communities in holocene high energy reefs thick laminar to columnar stromatolitic growths encrust typical high energy coralgal reef communities in a french polynesian outer reef edge the reef communities accumulated during vertical growth from 5870 100 yr b p to the present at depths never exceeding 5 m below mean sea level oxygen and carbon isotope records and high calcification rates of stromatolite encrusted corals strongly suggest that for about the past 6000 yr the local environmental conditions were optimal for reef development accordingly stromatolites can grow in healthy holocene reefs because similar stromatolites have been recorded in other cenozoic reefs we suggest that microbial structures may have played a more prominent role in cenozoic reef growth than is currently recognized