geochemical insights into provenance of the middle devonian hamilton group of the central appalachian basin u s a abstract in order to better understand the relationship of clastic influx and organic matter accumulation and preservation in the middle devonian acadian foreland basin we conducted a high resolution provenance analysis of a composite section of the hamilton group the hamilton group includes the organic rich marcellus shale one of the most lucrative unconventional shale gas plays in the world and the overlying mahantango formation geochemical analysis of 121 samples from two nearby wells in monongalia county west virginia reveals subtle changes in clay provenance throughout deposition of the marcellus shale and the mahantango formation major element and trace element geochemistry suggest derivation from a felsic igneous granodioritic upper continental crustal source with additional influx from a recycled quartzose sedimentary source all weathering indices indicate increasing chemical weathering intensities throughout deposition of the marcellus shale followed by consistently moderate chemical weathering associated with accumulation of the mahantango formation sm nd isotopic analysis of ten samples throughout the hamilton group yield end values ranging from 7 06 to 11 75 and nd depleted mantle model ages dm ranging from 1 63 to 1 85 ga with ages becoming younger upsection our results suggest that the extrabasinal detritus of the hamilton group originated from a mixed sediment source with clay influx from both rocks associated with the superior craton to the north and northwest dm 2 7 ga and grenville sourced sediments derived from the adjacent acadian fold thrust belt to the east dm 1 4 1 6 ga older sm nd model ages felsic composition and evidence of sediment recycling suggest little to no contribution from the acadian volcanic arc indicating that volcanic tuff of the tioga ash beds represent isolated episodes of volcanic input into the basin model ages end values and trace element geochemistry indicate increased sediment influx from the acadian fold thrust belt throughout deposition of the hamilton group with the highest sediment influx having occurred during mahantango formation deposition increased clay influx from sediment shed off from the acadian fold thrust belt associated with marginal marine regression increased delivery of clastic detritus diluting the flux of organic matter and likely disrupting basin water stratification that aided in preservation of organic matter in the lower marcellus shale the middle devonian acadian basin serves as an example of the influence of detrital influx on organic matter accumulation and preservation in mud dominated depositional systems

surface area control of shallow cratonic to deep marine carbonate accumulation sites of calcium magnesium carbonate accumulation have been shifting from shallow shelf to deep marine settings since the late cretaceous during this time total area of flooded epicratonic shelf has decreased about 50 and areas of shelf carbonate deposition have declined from 27 106 km to present areas of 1 4 106 km while the maximum extent of platform carbonate accumulation has decreased from about 50 to 28 north latitude in terms of cratonic carbonate this decrease in tropical shelf area has resulted in a sixfold reduction in shoal water limestone accumulation from a cretaceous rate of 0 6 km m y to a present value of about 0 1 106 km m y over the same time period the global carbonate compensation depth has deepened by about 1 5 km attendant with a threefold areal increase of calcite ooze depositionn 40 to 130 106 km this expansion has resulted in an eightfold increase in pelagic limestone accumulation from a cretaceous rate of 0 05 106 km m y to a present rate of about 0 4 106 km m y transfer of accumulation from shallow cratonic to deep marine depositional settings records the central role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to phanerozoic global oceans

impact ejecta layer from the mid devonian possible connection to global mass extinctions we have found evidence for a bolide impacting earth in the mid devonian 380 million years ago including high concentrations of shocked quartz ni cr as v and co anomalies a large negative carbon isotope shift 9 per mil and microspherules and microcrysts at jebel mech irdane in the anti atlas desert near rissani morocco this impact is important because it is coincident with a major global extinction event kacak otomari event suggesting a possible cause and effect relation between the impact and the extinction the result may represent the extinction of as many as 40 of all living marine animal genera

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

a field study of the chemical weathering of ancient sedimentary organic matter weathering profiles developed on organic carbon rich black shales were studied to examine the loss and degradation of organic matter om during weathering and its role in the geochemical carbon cycle analysis of weathered shales reveals between 60 and nearly 100 total organic carbon toc loss in highly weathered samples relative to initial unweathered toc content pyrite loss coincides with or precedes organic carbon loss elemental analysis and flash pyrolysis gas chromatography py gc of kerogen concentrates indicate that there is little or no selective enrichment or depletion of norg containing sorg containing alkylaromatic branched alkyl or long chain n alkyl moieties in most profiles during weathering kerogen o c ratios consistently increase with toc and pyrite loss infrared spectroscopy ir reveals an increase in the relative abundance of c c and c o bonds relative to alkyl c h bonds in progressively weathered samples these results suggest a two component model for kerogen weathering largely non selective oxidation and hydration followed by cleavage dissolution of oxidized kerogen fragments the extent of weathering in a given outcrop is likely limited by a combination of the rate of physical erosion and exposure of the rock to oxidizing surface waters with om type composition playing a lesser role

use of hyperspectral images in the identification and mapping of expansive clay soils and the role of spatial resolution hyperspectral images were acquired along the front range urban corridor in colorado to determine the feasibility of identification and mapping of expansive clay soils with two high snr imaging spectrometers swelling soils are a major geologic hazard and cause extensive damage world wide every year the cost of postconstruction mitigation and standard engineering soil tests for creation of regional maps are immense smectite is the clay mineral group that has the greatest swelling potential and is responsible for most of the severe swelling soil damage observed in colorado data sets were acquired from 1997 to 1999 with the airborne visible infrared imaging spectrometer aviris and the hyperspectral mapper hymap using a matched filtering algorithm maps of exposed clay material were produced despite a strong vegetation cover among those exposures spectral discrimination and identification of variable clay mineralogy such as smectite smectite illite and kaolinite in decreasing order of swelling potential hazard was possible the comparison of the results from the two sensors showed that higher spatial resolution provided purer image endmembers in more heterogeneous sites but did not exhibit more endmembers and did not identify new natural outcrops that a lower spatial resolution data set would miss in a homogeneous terrain however an increase in the signal to noise ratio snr of the instrument by pixel summation made possible the identification of low reflectance exposures this work demonstrates that using recent instruments and well established methodologies imaging spectrometry can be of practical help for the detection and mapping of expansive clays

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

black shale deposition and faunal overturn in the devonian appalachian basin clastic starvation seasonal water column mixing and efficient biolimiting nutrient recycling integrated geochemical data suggest that black shale deposition in the devonian geneseo formation of western new york was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing once established however black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced n and p regeneration from sediments sustained high primary productivity by returning nutrients to the photic zone during mixing and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification this feedback was ultimately disrupted by rising siliciclastic influx which diluted organic matter and restored benthic redox stability the abrupt overturn of diverse long standing appalachian basin marine communities may have been the result of trophic resource destabilization during geneseo deposition

coal weathering and the geochemical carbon cycle abstract the weathering rate of sedimentary organic matter in the continental surficial environment is poorly constrained despite its importance to the geochemical carbon cycle during this weathering complete oxidation to carbon dioxide is normally assumed but there is little proof that this actually occurs knowledge of the rate and mechanisms of sedimentary organic matter weathering is important because it is one of the major controls on atmospheric oxygen level through geologic time we have determined the aqueous oxidation rates of pyrite free bituminous coal at 24 and 50 c by using a dual cell flow through method coal was used as an example of sedimentary organic matter because of the difficulty in obtaining pyrite free kerogen for laboratory study the aqueous oxidation rate obtained in the present study for air saturated water 270 m o2 was found to be on the order of 2 10 12 mol o2 m2 s at 25 c which is fast compared to other geologic processes such as tectonic uplift and exposure through erosion the reaction order with respect to oxygen level is 0 5 on a several thousand hour time scale for both 24 and 50 c experiments activation energies determined under 24 and 50 c conditions were 40 kj mol o2 indicating that the oxidation reaction is surface reaction controlled the oxygen consumption rate obtained in this study is two to three orders of magnitude smaller than that for pyrite oxidation in water but still rapid on a geologic time scale aqueous coal oxidation results in the formation of dissolved co2 dissolved organic carbon doc and solid oxidation products which are all quantitatively significant reaction products

phanerozoic atmospheric co2 change evaluating geochemical and paleobiological approaches abstract the theory and use of geochemical modeling of the long term carbon cycle and four paleo pco 2 proxies are reviewed and discussed in order to discern the best applications for each method geochemical models provide pco 2 predictions for the entire phanerozoic but most existing models present 5 10 m y means and so often do not resolve short term excursions error estimates based on sensitivity analyses range from 75 200 ppmv for the tertiary to as much as 3000 ppmv during the early paleozoic the 13 c of pedogenic carbonates provide the best proxy based pco 2 estimates for the pre tertiary with error estimates ranging from 500 1000 ppmv pre devonian estimates should be treated cautiously error estimates for tertiary reconstructions via this proxy are higher than other proxies and models 400 500 ppmv and should not be solely relied upon we also show the importance of measuring the 13 c of coexisting organic matter instead of inferring its value from marine carbonates the 13 c of the organic remains of phytoplankton from sediment cores provide high temporal resolution up to 10 3 10 4 year high precision 25 100 ppmv for the tertiary to 150 200 ppmv for the cretaceous pco 2 estimates that can be near continuous for most of the tertiary its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short term excursions this method however must correct for changes in growth rate and oxygen level at elevated pco 2 750 1250 ppmv this proxy loses its sensitivity and is not useful the stomatal density and stomatal index of land plants also provide high temporal resolution 2 year high precision 10 40 ppmv for the tertiary and possibly cretaceous pco 2 estimates and so also is ideal for discerning short term excursions unfortunately this proxy also loses sensitivity at some level of pco 2 above 350 ppmv which currently is largely undetermined our analysis of the recently developed 11 b technique shows that it currently is not yet well constrained most importantly it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time in addition it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur a fifth co 2 proxy based on the redox chemistry of marine cerium has several fundamental flaws and is not discussed in detail here

carbon isotope chemostratigraphy and precise dating of middle frasnian lower upper devonian alamo breccia nevada usa abstract at hancock summit west nevada western usa uppermost givetian upper middle devonian and lower and middle frasnian lower upper devonian rocks of the lower guilmette formation include in stratigraphic sequence carbonate platform facies of the conodont falsiovalis transitans and punctata zones the type alamo breccia member of the middle punctata zone and slope facies of the punctata and hassi zones the catastrophically deposited alamo breccia and related phenomena record the 382 ma alamo event produced by a km scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western north america re evaluation of conodonts from the lower guilmette formation and alamo breccia member together with regional sedimentologic and conodont biofacies comparisons now firmly locates the onset of the johnson et al 1985 transgressive regressive t r cycle iic which occurred after the start of the punctata zone within a parautochthonous megablock low in the alamo breccia whole rock carbon isotope analyses through the lower guilmette formation and alamo breccia member reveal two positive 13ccarb excursions 1 a small 3 excursion which is possibly correlative with the falsiovalis event previously identified from sections in western europe and australia occurs below the breccia in the upper falsiovalis zone to early part of the transitans zone and 2 a large multi part excursion dominated by a 6 positive shift begins above the start of the punctata zone and onset of t r cycle iic and continues above the alamo breccia ending near the punctata hassi zonal boundary this large excursion correlates with the punctata event a major positive 13c excursion previously recognized in eastern laurussia and northern gondwana consistent with previous studies at hancock summit west the punctata event is apparently not associated with any regional extinctions or ecosystem reorganizations in the study area onset of the main punctata event began after the start of both the punctata zone and t r cycle iic and preceded the alamo impact by less than 650 k y as inferred from conodont biochronologic and regional rock accumulation rate estimates although complicated by the heterolithic high energy deposits of the alamo breccia the carbon isotope record of the breccia and post breccia beds does not indicate a major impact correlative perturbation to the carbon cycle this study extends recognition of the punctata event to western laurussia further reinforcing the potential global scale of the event and its potential importance to understanding early to middle frasnian marine geochemistry and palaeoenvironments based on previous models and our observations increased tectonic activity increased nutrient flux to oceans increased marine bioproductivity widespread anoxia and increased organic carbon burial were all likely key factors in driving the punctata event excursion furthermore periodic eustatic and regional relative sea level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open marine geochemical signal and that recorded on the shallow marine lower guilmette carbonate platform

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

the multiple impacts hypothesis for mass extinction a comparison of the late devonian and the late eocene abstract application of the lag time multiple impacts hypothesis poag 1997b palaios 12 582 590 poag et al 2001 columbia univ press to the late devonian leads to the prediction that the frasnian famennian pulsed extinctions were triggered by a rapid drop in global temperature that followed an impact produced anomalous warm interval which interrupted the global cooling trend from the middle devonian greenhouse to the early carboniferous icehouse in actualistic comparison with the late eocene the lag time multiple impacts hypothesis would predict that a frasnian interval of multiple impacts should have occurred between 367 7 and 366 7 ma the fact that three impacts the alamo siljan and flynn creek do occur either within this predicted interval or close to it in time is corroborative evidence that the lag time multiple impacts hypothesis may indeed provide the causal mechanism for the frasnian famennian mass extinction based on the application of the lag time multiple impacts hypothesis to the frasnian famennian mass extinction it is here suggested that future searches for evidence of impact events in the late devonian be concentrated in strata that occur in the frasnian transitans to early hassi zonal interval and not in strata immediately below or above the frasnian famennian boundary

quantification of degree of saturation at shallow depths of earth slopes using resistivity imaging technique abstractearthen slopes encountered in engineering practice typically consist of unsaturated or partially saturated soils during rainfall water infiltrates into the slope and causes a reduction in suction slope stability is affected by the increase in the degree of saturation as a result of the reduction in normal stress and cohesive strength along potential failure surfaces therefore to determine the geohazard potential it is important to know the degree of saturation within slopes at shallow depths resistivity imaging ri is one of the most convenient techniques available for geohazard studies however only qualitative saturation profiles can be obtained using this method quantification of the degree of saturation has become important for using ri in the evaluation of the geohazard potential of slope failures the objective of this study was to quantify the degrees of saturation at shallow depths using ri electrical resistivity tests were conducted in the laboratory on high plasticity clay ch

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

rock magnetism and palaeomagnetism of the montalbano jonico section italy evidence for late diagenetic growth of greigite and implications for magnetostratigraphy summary the montalbano jonico mj section cropping out in southern italy represents a potential candidate to define the lower middle pleistocene boundary and it has been proposed as a suitable global stratotype section and point gssp of the ionian stage middle pleistocene the mj section is the only continuous benthic and planktonic 18o on land reference in the mediterranean area for the mid pleistocene transition spanning an interval between about 1240 and 645 ka combined biostratigraphy and sapropel chronology tephra stratigraphy and complete high resolution benthic and planktonic foraminiferal stable oxygen isotope records already provide a firm chronostratigraphic framework for the mj section however magnetostratigraphy was still required to precisely locate the brunhes matuyama transition and to mark the gssp for the ionian stage we carried out a palaeomagnetic study of a subsection ideale section of the mj composite section sampling 61 oriented cores from 56 stratigraphic levels spread over a ca 80 m thick stratigraphic interval that correlates to the oxygen isotopic stage 19 and should therefore include the brunhes matuyama reversal the palaeomagnetic data indicate a stable and almost single component natural remanent magnetization nrm a characteristic remanent magnetization chrm was clearly identified by stepwise demagnetization of the nrm the chrm declination values vary around 0 and the chrm inclination around the expected value 59 for a geocentric axial dipole field at the sampling locality this result indicates that the section has been remagnetized during the brunhes chron a preliminary study of 27 additional not azimuthally oriented hand samples collected at various levels from other parts of the mj composite section indicates that all the samples are of normal polarity and demonstrates that the remagnetization is widespread across the whole exposed stratigraphic sequence a series of specific rock magnetic techniques were then applied to investigate the nature of the main magnetic carrier in the study sediments and they suggest that the main magnetic mineral in the mj section is the iron sulphide greigite fe3s4 scanning electron microscope observations and elemental microanalysis reveal that greigite occurs both as individual euhedral crystals and in iron sulphides aggregates filling voids in the clay matrix therefore we infer that the remagnetization of the section is due to the late diagenetic growth of greigite under reducing conditions most likely resulting in the almost complete dissolution of the original magnetic minerals iron sulphide formation in the mj section can be linked to migration of mineralized fluids our inferred timing of the remagnetization associated with greigite growth represents the longest remanence acquisition delay documented in greigite bearing clays of the italian peninsula so far

swell and shrinkage characterizations of unsaturated expansive clays from texas abstract expansive soils have long been recognized as problematic because they cause failure to civil structures constructed above them the main problem of these soils can be attributed to poor understanding of the volume changes caused by moisture fluctuations current swell and shrinkage characterization models are limited by both the lack of standardized tests and tests that employ volume changes in uniaxial direction in the present research a comprehensive laboratory investigation was undertaken to study the volume change related swell shrinkage behaviors of five different types of expansive clayey soils sampled from various regions in texas usa extensive experimental programs consisting of basic chemical and mineralogical soil properties were first determined three dimensional free swell and shrinkage tests were performed on all soils at various compaction moisture content conditions soil water characteristic curves swccs of all test soils were determined by studying the suction potentials of these soils over a wide range of moisture contents volume change measurements of soils showed a good correlation with soil properties including plasticity and soil compaction properties swcc results also showed a clear variation in swcc profiles of soils with respect to soil plasticity overall a large database of soil properties was developed and is presented here it includes physical and mineralogical properties as well as engineering swell shrinkage and swcc test results

global events of the late paleozoic early devonian to middle permian a review abstract the late paleozoic early devonian to middle permian was an interval of profound changes in earth surface systems reflected in dynamic interplay among the biosphere hydrosphere atmosphere and geosphere major events transpired including the colonization of landmasses by vascular plants the assembly of the supercontinent pangea two first order mass extinctions the frasnian famennian and devonian carboniferous boundary events and the most severe icehouse climate mode of the phanerozoic the late paleozoic ice age lpia the goals of the present review are 1 to summarize major global developments in climate oceanography and paleobiology during the late paleozoic 2 to examine the roles of land plant evolution global tectonics and large igneous province magmatism in driving these developments and 3 to serve as an introduction to the 23 contributions to this special issue of palaeogeography palaeoclimatology palaeoecology indicating how they advance our knowledge of various scientific issues related to these developments

anoxic events in the late frasnian causes of the frasnian famennian faunal crisis carbon isotope data collected from five frasnian famennian boundary sections in central europe show two positive 13 c excursions in the late frasnian 367 ma both anomalies coincided with the deposition of the bituminous lower and upper kellwasser horizons the carbon isotope patterns indicate two phases of enhanced burial and subsequent recycling of organic carbon a maximum formation of warm saline waters on the subtropical to tropical epicontinental shelves during late frasnian transgressive episodes may have induced oceanic oxygen deficits variations in the c org burial rates may have resulted in changes in the co 2 concentrations in the oceans and in the atmosphere culminating in global climatic changes we propose that the repeated co occurrences of sea level fluctuations anoxic conditions and global climatic changes during the late frasnian would affect especially the subtropical to tropical shallow water communities and reef ecosystems which were severely affected during the frasnian famennian faunal crisis

carbon isotope stratigraphy across the silurian devonian transition in podolia ukraine evidence for a global biogeochemical perturbation the carbon and oxygen isotope composition of marine carbonates 13 c and 18 o respectively are studied in the fossiliferous stratigraphically well constrained and remarkably expanded successions of podolia sw ukraine spanning the silurian devonian transition significant isotopic shifts are directly comparable to previously published global secular trends in well preserved brachiopod calcite isotopic ratios from this region and therefore may be taken as a reliable primary record of seawater 13 c changes the sections reveal a major positive 13 c excursion with an amplitude above 6 beginning in the upper pridoli and reaching peak values as heavy as 4 2 in the lowermost lochkovian this turnover in carbon cycling is followed by a general trend toward more negative 13 c values in the upper lochkovian the podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the silurian devonian transition reflecting a complex combination of palaeogeographical biogeochemical and evolutionary processes in the late caledonian geodynamic setting with a likely undervalued role of the expanding vegetation in vast near coastal shallows and deltas

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

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

diversity patterns of the vascular plant group zosterophyllopsida in relation to devonian paleogeography abstract the zosterophyllopsida originated in the silurian and became prominent vascular components of early devonian floras worldwide an updated dataset of zosterophyllopsids at species level is analyzed to compare the taxic composition of five putative paleophytogeographic units laurussia siberia northwestern gondwana kazakhstan and northeastern gondwana i e australia china and the shan thai block the high level of endemicity shown by each unit confirms the phytogeographic differentiation and the occurrence of geographical barriers preventing massive floral exchanges between the corresponding regions for the late silurian early devonian time interval statistical analyses conducted on the three largest datasets those corresponding to laurussia siberia and northeastern gondwana indicate that the diversity dynamics of the group followed the same pattern in these regions almost all taxic diversity measures show that specific diversity was greatest in the middle and late pragian diversity dropped significantly thereafter however residual diversity reveals genuine regional patterns from this we show that the radiation of the zosterophyllopsida may have stopped earlier in northeastern gondwana and siberia than laurussia we propose that the onset of these extinctions resulted from the competitive replacement of the zosterophyllopsids by increasingly diversified lycopsids and basal euphyllophytes whose evolution could have been favored by external factors

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

the cenozoic evolution of the strontium and carbon cycles relative importance of continental erosion and mantle exchanges the past variations of the seawater 87sr86sr isotopic ratio are related to changes in the relative contribution of the mantle sr input to the ocean and the sr supply from continental weathering recently it has been postulated that the cenozoic increase in the seawater 87sr86sr isotopic ratio was associated with the uplift of the himalayan and andean mountains at that time these orogenies may have changed the sr isotopic ratio of the continental rocks undergoing weathering as a result of extensive metamorphism increased the river flux of sr through enhanced weathering in these regions and possibly caused the global climatic cooling trend of the cenozoic a model of the major geochemical cycles coupled to an energy balance climate model is used to explore the possible causes of the mesozoic cenozoic fluctuations in the seawater 87sr86sr isotopic ratio the contribution of the mantle exchanges at mid ocean ridges of the recycling of seafloor carbonates through plate margin volcanism and of the alteration of seafloor basalts to the fluctuations of the seawater 87sr86sr isotopic ratio are studied finally this model tentatively describes the impact of the himalayan orogeny on the geochemical cycles of sr and c some possible effects of the extensive metamorphism associated with the india asia collision and of the himalayan uplift are modelled the model reproduces the cenozoic increase of the seawater 87sr86sr isotopic ratio however the impact of the himalayan orogeny on the c cycle appears to be limited and insufficient to generate the global climatic cooling of the cenozoic rather in the model the cenozoic cooling is mostly due to the reduction of the co2 emission from mid ocean ridge volcanism and to changes in the chemical weathering rates in the rest of the world excluding the himalayas

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

a positive trend in seawater 87sr 86sr values over the early middle frasnian boundary late devonian recorded in well preserved conodont elements from the holy cross mountains poland abstract over the early middle frasnian boundary interval late devonian the oceans experienced major changes in global carbon cycling and nutrient dynamics with a prominent positive carbon isotope shift at the base of the palmatolepis punctata zone middle frasnian the aim of this study was to construct seawater 87 sr 86 sr ratio curves for the pa transitans pa hassi zonal interval using well preserved conodont apatite from two sections in the holy cross mountains south central poland the final curves suggest that seawater 87 sr 86 sr values were steady at 0 70794 0 70799 during the transitans zone but began to rise in the early punctata zone towards a value of 0 70814 in the late punctata zone an increase of 0 0002 the increase in seawater 87 sr 86 sr values over the early middle frasnian boundary appears to be the second of two prominent short term rises in 87 sr 86 sr values which led to an overall shift in values from a givetian plateau of between 0 70780 and 0 70785 to a late frasnian plateau of between 0 7080 and 0 7082 correlation with a regional positive 13 c anomaly and evidence for eutrophication in the marine environment suggests that the causes of these phenomena may be linked and that the positive 87 sr 86 sr shift was caused by an increase in the continental flux of sr to the oceans i e the weathering flux a possible explanation for an increase in the weathering flux is the denudation of a new acadian eovariscan orogen which would have been mostly weathered in the humid warm tropical equatorial region

magnetite dissolution diachronous greigite formation and secondary magnetizations from pyrite oxidation unravelling complex magnetizations in neogene marine sediments from new zealand detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of neogene marine sediments from the east coast of new zealand is dominated by the authigenic iron sulphide greigite the greigite is present as a mixed population of stable single domain and superparamagnetic grains which is consistent with authigenic growth from solution this growth can result from pyritization reactions soon after deposition which also leads to dissolution of most detrital magnetite however where constrained by field tests our data suggest that remanence acquisition can occur 1 myr after deposition and can vary in timing at the outcrop scale strong viscous overprints result from oxidation of the iron sulphides probably during percolation of oxic ground water this process can sometimes destroy any ancient remanent magnetization this complex magnetic behaviour particularly the presence of late forming magnetizations carried by greigite means that the remanence in new zealand cenozoic sediments and in similar sediments elsewhere cannot be assumed to be primary without confirmation by field tests the reversals test should be employed with caution in such sediments as patchy remagnetizations can lead to false polarity stratigraphies

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

iron oxyhydroxide and sulphide mineralization in hydrocarbon seep related carbonate submarine chimneys gulf of cadiz sw iberian peninsula this paper aims to investigate the iron mineralization hosted in the submarine hydrocarbon seep related carbonate chimneys from the gulf of cadiz continental slope sw iberia chimneys are made of a general fine groundmass of major fe rich dolomite ankerite and calcite small grains of quartz and phyllosilicates and some foraminifer tests and ostracod shells primary porosity is frequently generated inside foraminifer chambers and ostracod shells and is filled with closely packed microcrystals of iron oxyhydroxide and sulphide minerals forming framboids some extremely peculiar multiframboidal textures are also detected corresponding to microcrystal groups spherical framboids and euhedral crystals without framboidal texture the mineralogy observed is mainly goethite pyrite pseudomorphs traces of tiny grains of pyrite were also observed cubic octahedral and pyritohedral are the habits observed in both minerals chemical analyses of framboids and euhedral crystals display high often erratic amounts of as co ni and mo in oxyhydroxides and mo pb v and co in sulphides versus almost always negligible concentrations of cu and zn a textural geochemical and mineralogical evolution is proposed to explain the coexistence of different morphologies in the multiframboidal texture a growth and aggregation of microcrystals as typical framboidal type mineralogical associations b development of euhedral habits c coalescence and homogenization of the microcrystal into large size euhedral to anhedral crystals and d formation of euhedral crystals or polycrystalline masses with complete loss of framboidal texture along this process an increase of the concentration of fe s mn and ti linked to a decrease in mg si al as p ca and v was observed the study of the iron mineralization in the gulf of cadiz is a first and can give clues to understand the complex geobiological interactions in this and other similar extreme hydrocarbon bearing submarine ecosystems

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

modelling the phanerozoic carbon cycle and climate constraints from the 87sr 86sr isotopic ratio of seawater a numerical model describing the coupled evolution of the biogeochemical cycles of carbon sulfur calcium magnesium phosphorus and strontium has been developed to describe the long term changes of atmospheric carbon dioxide and climate during the phanerozoic the emphasis is on the effects of coupling the cycles of carbon and strontium various interpretations of the observed phanerozoic history of the seawater 87sr 86sr ratio are investigated with the model more specifically the abilities of continental weathering volcanism and surface lithology in generating that signal are tested and compared it is suggested that the observed fluctuations are mostly due to a changing weatherability over time it is shown that such a conclusion is very important for the modelling of the carbon cycle indeed it implies that the conventional belief that the evolution of atmospheric carbon dioxide and climate on a long time scale is governed by the balance between the volcanic input of co2 and the rate of silicate weathering is not true rather carbon exchanges between the mantle and the exogenic system are likely to have played a key role too further the increase of the global weathering rates with increasing surface temperature and or atmospheric co2 pressure usually postulated in long term carbon cycle and climate modelling is also inconsistent with the new model other factors appear to have modulated the weatherability of the continents through time such as mountain building and the existence of glaciers and ice sheets based on these observations a history of atmospheric carbon dioxide and climate during phanerozoic time consistent with the strontium isotopic data is reconstructed with the model and is shown to be compatible with paleoclimatic indicators such as the timing of glaciation and the estimates of cretaceous paleotemperatures

investigation of geotechnical parameters affecting electrical resistivity of compacted clays abstractthe use of resistivity imaging ri in subsurface investigation has increased in recent years ri is a non destructive method and provides a continuous image of the subsurface however only qualitative evaluation of the subsurface can be obtained from ri the correlations between ri results and geotechnical engineering properties of soils have become important for site investigation using this method the primary objective of the current study was to determine the geotechnical parameters affecting electrical resistivity of compacted clays understanding the influential factors will be helpful in determining the correlations between ri results and geotechnical properties of soil the effects of moisture content unit weight degree of saturation specific surface area percentages of pores and ion composition on soil resistivity were investigated soil samples used were classified as highly plastic clay ch according to the unified soil classification system high energy x ray fluorescence tests

characterization of eagle ford shale this paper presents the synthesized and characterized properties of eagle ford shale efs based on the collected data and laboratory testing results from the canceled superconducting super collider ssc site in texas usa index and mechanical properties of the clay shale were summarized and analyzed statistically correlations between index and mechanical properties were established the estimated shear strength of the intact shale was based on the results of drained triaxial compression and extension tests the results from uniaxial compression test on vertical and inclined cores and from the triaxial compression and extension tests show that the shale has strength anisotropy shear strength along the discontinuities was estimated based on the results from direct shear and triaxial tests on samples with preexisting joints constitutive models and bounds of drained shear strengths were evaluated and proposed for intact clay shale bedding planes and discontinuities the information on strength parameters and discontinuities can be used as the basis of future design and numerical studies

origin of red pelagic carbonates as an interplay of global climate and local basin factors insight from the lower devonian of the prague basin czech republic abstract red pelagic sediments are relatively common in the phanerozoic they are often interpreted as products of sea bottom oxidation during greenhouse climate showing a conspicuous alternation with black shales and thus carrying important palaeoceanographic information the lower devonian pragian carbonate strata of the prague basin czech republic praha formation contain a marked band of red pelagic carbonate up to 15 m thick which can be correlated for several tens of km we investigated seven sections 17 to 255 m thick of the prague basin using the methods of facies analysis outcrop gamma ray logging diffuse reflectance spectroscopy optical microscopy element geochemistry magneto mineralogy and electron microprobe analysis the aim was to find the mineral carriers of the red colour investigate the stratigraphic context of the red carbonates and evaluate the local and global prerequisites for their formation the red pigmentation represents enrichment by hematite with respect to goethite approximately 31 of the total reflectance falling in the red colour band represents a threshold for red coloration the red pigmentation is carried by submicronic hematite dispersed in argillaceous pelagic calcilutite and or inside skeletal allochems gamma ray log correlation indicates that the red carbonate band developed in stratigraphic levels with low sedimentation rates typically from 1 to 7 1 mm kyr which are comparable to the mesozoic rosso ammonitico facies the red beds and the whole praha formation pragian to early emsian are characterized by low toc values

untangling inconsistent magnetic polarity records through an integrated rock magnetic analysis a case study on neogene sections in east timor inconsistent polarity patterns in sediments are a common problem in magnetostratigraphic and paleomagnetic research multiple magnetic mineral generations result in such remanence haystacks here we test whether end member modeling of isothermal remanent magnetization acquisition curves as a basis for an integrated rock magnetic and microscopic analysis is capable of isolating original magnetic polarity patterns uppermost miocene pliocene deep marine siliciclastics and limestones in east timor originally sampled to constrain the uplift history of the young timor orogeny serve as case study an apparently straightforward polarity record was obtained that however proved impossible to reconcile with the associated biostratigraphy our analysis distinguished two magnetic end members for each section which result from various greigite suites and a detrital magnetite suite the latter yields largely viscous remanence signals and is deemed unsuited the greigite suites are late diagenetic in the cailaco river section and early diagenetic thus reliable in the viqueque type section by selecting reliable sample levels based on a quality index a revised polarity pattern of the latter section is obtained consistent with the biostratigraphy and unequivocally correlatable to the geomagnetic polarity time scale although the cailaco river section lacks a reliable magnetostratigraphy it does record a significant postremagnetization tectonic rotation our results shows that the application of well designed rock magnetic research based on the end member model and integrated with microscopy and paleomagnetic data can unravel complex and seemingly inconsistent polarity patterns we recommend this approach to assess the veracity of the polarity of strata with complex magnetic mineralogy

the influence of the late devonian kellwasser events on deep water ecosystems evidence from palaeontological and geochemical records from south china abstract the late devonian frasnian famennian f f event consisting of the lower and upper kellwasser events had a severe impact on shallow water ecosystems while the effect on deep water ecosystems is ambiguous in order to recognize the kellwasser events and to evaluate their influences on the deep water ecosystems a newly refined conodont biostratigraphic framework spanning the lower palmatolepis rhenana zone to the pa delicatula platys zone has been established in the nandong section basinal facies guangxi south china the upper devonian lower and upper kellwasser horizons lkh and ukh have been recognized in the deep water facies on the basis of lithological variations biological turnovers and positive carbon isotope 13ccarb excursions the lkh is characterized by the enrichment of marine nektonic communities e g conodonts homoctenids and entomozoacean ostracods whereas the top of the ukh is barren of fossil organisms suggesting that the upper kellwasser event had a more severe effect on nekton than the lower kellwasser event the upper kellwasser uk 13ccarb positive excursion lags behind the enrichment of cu ni mo and v indicating that the uk 13ccarb positive excursion was probably related to elevated primary productivity which induced oxygen depletion in the water column the uk 13ccarb positive excursion peak prior to the f f boundary indicates a more limited effect of riverine influx on the deep water setting which suggests that the regional palaeogeographic configuration played an important role in the carbon cycle

assessing the timing of greigite formation and the reliability of the upper olduvai polarity transition record from the crostolo river italy 1 the crostolo river section in italy yielded a detailed record of the upper olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field the paleomagnetic record is carried by the authigenic iron sulphide greigite which was interpreted to have formed shortly after deposition our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite which usually forms with greigite as a precursor this suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour

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

recognizing magnetostratigraphy in overprinted and altered marine sediments challenges and solutions from iodp site u1437 core disturbance drilling overprints post depositional acquisition of remanence authigenic growth of magnetic iron sulfides and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments we address these issues in a sequence of tuffaceous muds and volcaniclastics at international ocean discovery program site u1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling remanence measurements were filtered to remove intervals affected by fluidization plastic sediment disturbance and core biscuiting drilling overprints are concentrated in the disturbed annulus surrounding intact core material bioturbation was limited to a vertical extent of at most 15 cm changes in sediment color stiffness and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment water interface we did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums authigenic growth of greigite in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate has led to magnetic overprinting anomalous polarity artefacts extending 5 m and occurring within about 20 m below a real polarity transition appear to be due to a chemical remanence acquired by greigite produced during early diagenesis diagenetic magnetic mineral alteration resulted in the progressive loss of fine grained magnetite which enhanced susceptibility to drilling and post drilling overprints and increased the resistance of these overprints to removal by conventional demagnetization we recovered the magnetostratigraphic record from many samples with resistant overprints through low temperature demagnetization through the verwey transition this article is protected by copyright all rights reserved

marshrock formed by iron sulphide and siderite cementation in saltmarsh sediments the formation of authigenic sulphides and other iron minerals has proved interesting both in studies of palaeomagnetism1 and palaeosalinity reconstructions of pleistocene and older sediments2 3 iron sulphide and siderite cemented nodules concretions and layers are fairly common in the geological record4 6 but there is little evidence of their formation in modern environments accounts of authigenic iron minerals forming today in marine and brackish sediments generally refer only to micro concretions 10 500 m in size many of which have a framboidal texture3 7 8 i describe here macro concretions and cemented layers which are now forming within intertidal marsh sediments at warham norfolk uk parts of these marshes are only 30 yr old indicating how quickly early diagenetic cementation can occur siderite feco3 greigite fe3s4 and possibly mackinawite fes1 x are the main cementing agents the term marshrock is proposed for the cemented sediments by analogy with calcareous beachrock9 10 the formation of siderite monosulphide concretions in the warham sediments probably reflects a high reactive iron content and limited sulphate arising from rapid sedimentation and burial of organic matter during the early stages of marsh growth

geochemical consequences of intense pulse like degassing during the onset of the central atlantic magmatic province the triassic jurassic boundary tjb ismarked by one of the five largestmass extinctions of the phanerozoic and the eruption of a large igneous province the central atlanticmagmatic province camp the tjb is characterized by a carbonate production crisis by negative excursions in the carbon isotopic ratio of buried organic matter and by successive peaks in atmospheric co2 herewe use the numerical model geoclimto explore the possible connections between the camp emplacement and the observed carbon cycle perturbations different degassing scenarios linked to the camp eruption are explored we show that the emission of realistic amounts of co2 that follow a geologically constrained degassing scenario as short termpeaks less than 10 ka leads to successive decrease in carbonate production as observed in the geological record we also calculate the evolution of carbon isotopes and show that our model reproduces the amplitude of the isotopic excursion with a volcanic degassing of co2 characterized by a carbon isotopic composition of 20 such low values could be associated to carbon pools of light isotopic composition located at the transition zone cartigny p 2010 earth and planetary science letters v 296 p 329 339 and not necessarily to biogenicmethane release finally themodel predicts a succession of short term co2 rises with an amplitude in close agreement with available proxy based reconstructions

global geochemical cycles of carbon sulfur and oxygen time resolved data on the carbon isotopic composition of carbonate minerals and the sulfur isotopic composition or sulfate minerals show a strong negative correlation during the cretaceous carbonate minerals are isotopically heavy during this period while sulfate minerals are isotopically light the implication is that carbon is being transferred from the oxidized carbonate reservoir to the reservoir of isotopically light reduced organic carbon in sedimentary rocks while sulfur is being transferred from the reservoir of isotopically light sedimentary sulfide to the oxidized sulfate reservoir these apparently oppositely directed changes in the oxidation state of average sedimentary carbon and sulfur are surprising because of a well established and easy to understand correlation between the concentrations of reduced organic carbon and sulfide minerals in sedimentary rocks rocks rich in reduced carbon are also rich in reduced sulfur the isotopic and concentration data can be reconciled by a model which invokes a significant flux of hydrothermal sulfide to the deep sea at least during the cretaceous

isotopic chemostratigraphy across the early middle frasnian transition late devonian on the south polish carbonate shelf a reference for the global punctata event abstract the coupled high resolution carbon isotope data from whole rock limestone and organic matter samples of the transitans early frasnian punctata and the earliest hassi zones middle frasnian from the south polish carbonate shelf successions reveal the presence of a large multi part 13 c excursion one of the largest known carbon cycle disturbance of devonian period this early middle frasnian e mf 13 c perturbation consists of two positive excursions a minor event i and a major event iii and two negative shifts events ii and iv the major positive excursion up to 5 begins near the e mf boundary and the onset of middlesex transgressive anoxic event the latter broader scale positive 13 c perturbation correlates with the worldwide punctata event documented across eastern and western laurussia as well as northern gondwana the large magnitude of the punctata isotopic event paired with negligible biotic effects is similar to the silurian biogeochemical perturbations but contrasts markedly with the younger kellwasser events the distinctive protracted about 1 ma e mf 13 c variations may be only partly explained by escalated sea water exchange between epeiric seas and the anoxic open ocean during successive transgressive pulses very high 13 c plateau values of the punctata event correspond to a positive 87 sr 86 sr shift as well as elevated clastic input proxies and magnetic susceptibility suggesting altogether that delivery of tectonically promoted land derived nutrients to marine basins was a key factor stimulating the biogeochemical perturbation extreme increases in primary productivity and enhanced organic matter burial in restricted deeper basinal settings would promote a gradual drawdown in surface water as well as atmospheric p co 2 and consequently a climatic cooling by 5 c as indicated by the high resolution 18 o phosphate record

mountain building enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the frasnian famennian boundary c 376 ma the late devonian was a period of drastic environmental changes as exemplified by a major biotic crisis at the frasnian famennian boundary ffb and the onset in famennian times of glaciations across southern gondwana worldwide evidence for the coeval development of the major acadian eovariscan belt led us to propose a model relating the late frasnian famennian environmental perturbations to extensive continental uplift through two atmospheric co2 depleting mechanisms 1 the intensification of silicate weathering on the continental areas as attested by a major rise in the 87 sr 86 sr composition of sea water at the ffb and 2 the massive burial of organic carbon kellwasser events in partially confined basins due to the collisional induced reduction of equatorial oceanic communications between the palaeotethysian and panthalassic oceans this process is also suggested to have been controlled by an important primary productivity connected to an increased nutrient availability triggered by the enhanced continental run off

late quaternary sediments from deep sea sediment drifts on the antarctic peninsula pacific margin climatic control on provenance of minerals 1 we present results of detailed paleomagnetic investigations on deep sea cores from sediment drifts located along the pacific continental margin of the antarctic peninsula high resolution magnetic measurements on u channel samples provide detailed age models for three cores collected from drift 7 which document an age of 122 ka for the oldest sediments recovered near the drift crest at site sed 07 and a high sedimentation rate 11 cm kyr at site sed 12 located close to the alexander channel system low and high temperature magnetic measurements in conjunction with microscopic and mineralogic observations from drifts 4 5 and 7 indicate that pseudosingle domain detrital titanomagnetite partially oxidized and with limited ti substitution is the dominant magnetic mineral in the drift sediments the titanomagnetite occurs in two magnetic forms 1 a low coercivity form similar to laboratory synthesized titanomagnetite and 2 a high coercivity form bcr 60 mt these two forms vary in amount and stratigraphic distribution across the drifts we did not find evidence for diagenetic magnetic iron sulfides as has been previously suggested for these drift deposits the observed change of magnetic mineralogy in sediments deposited during heinrich events on drift 7 appears to be related to warming periods which temporarily modified the normal glacial transport pathways of glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments with a different provenance understanding this sediment provenance delivery signature at a wider spatial scale should provide information about ice sheet dynamics in west antarctica over the last 100 kyr

biogeochemical controls on black shale deposition during the frasnian famennian biotic crisis in the illinois and appalachian basins usa inferred from stable isotopes of nitrogen and carbon abstract the frasnian famennian biotic crisis is marked by two distinct intervals known as the lower and upper kellwasser events kwes that in many locations are associated with deposition of organic rich shales sedimentary nitrogen and carbon isotopes offer insight into the biogeochemical processing of nutrients production of organic matter and palaeoceanographic conditions during the kwes here we present new bulk nitrogen 15nbulk and organic carbon 13corg isotope data from the late devonian appalachian and illinois basins ab and ib with a focus on intervals encompassing the kwes black shales from the ib and ab including the kwes are 15n depleted 1 0 2 0 and have significantly lower 15nbulk than interbedded grey shales 0 5 4 0 a trend consistent with many instances of black shale deposition in the phanerozoic organic carbon isotopes exhibit the broad positive excursions 3 5 from background that are typical of the kwes globally superimposed over these positive excursions in 13corg are sharp decreases of up to 3 0 within the black shale beds to as low as 30 5 the pattern of 15nbulk and 13corg values suggests that the depth of the chemocline and the degree of water column stratification exert a primary control on both 15nbulk and 13corg during black shale deposition in the context of the frasnian famennian biotic crisis the oscillating redox state and changing temperatures would have likely placed extreme stress on organisms within the marine environment of the ab and ib and may potentially have been a contributing factor to diversity loss over this time period

paleomagnetism in lake pannon problems pitfalls and progress in using iron sulfides for magnetostratigraphy dating of upper miocene sediments of the pannonian basin hungary has proven difficult due to the endemic nature of biota scarcity of reliable radio isotopic data and generally inconsistent magnetostratigraphic results the natural remanent magnetization nrm is mostly residing in greigite fe3s4 which complicates nrm interpretation we reinvestigate the viability of these sediments for magnetostratigraphy using samples from recently drilled well cores paet 30 and paet 34 from the paks region significant intervals of the cores contain composite nrm behavior thermal demagnetization results include multipolarity m type samples consisting of a low temperature lt above 120 c a medium temperature mt and a high temperature ht component within distinct temperature ranges and all exhibiting dual polarities the lt and ht components have the same polarity and are antiparallel to the mt component rock magnetic and scanning electron microscopy results indicate that all magnetic components reside in authigenic greigite the lt and ht components represent the characteristic remanent magnetization and are of early diagenetic origin the mt component records a late diagenetic overprint alternating field demagnetization cannot resolve the individual components it yields polarities corresponding to the dominant component resulting in erratic polarity patterns interpretation of lt and ht components allows a reasonably robust magnetostratigraphic correlation to the geomagnetic polarity time scale with the base of paet 30 at 8 4 ma and its top at 6 8 ma average sedimentation rate of 30 cm kyr the base of paet 34 is correlated to 9 ma and its top to 6 8 ma average sedimentation rate of 27 cm kyr

diagenesis of magnetic minerals in a gas hydrate cold seep environment off the krishna godavari basin bay of bengal we carried out detailed magnetic measurements of the core md161 8 located in the vicinity of site nghp 01 10 where 128 m of hydrate is confirmed by drilling coring to understand the diagenesis of magnetic minerals in a gas hydrates cold seep environment the rock magnetic measurements along with sem eds and xrd analyses show a zone of reduced magnetic susceptibility zone 2 where most of the magnetic minerals are dissolved the enhanced concentration of chromium reducible sulfur crs in this zone suggests an intense pyritization process while isotopically depleted authigenic carbonates indicate sulfate reduction via anaerobic oxidation of methane aom therefore the dissolution of magnetic minerals is attributed to the hs released during aom that has resulted in the reduction in the magnitude of magnetic parameters within zone 2 a zone of enhanced susceptibility zone 2a is observed between 17 68 and 23 6 mbsf and is located beneath the present day sulfate methane transition zone smtz the frequencydependent magnetic susceptibility and low temperature magnetic measurements suggest the abundance of fine grained superparamagnetic sp sized ferrimagnetic particles the sem eds and xrd analyses show the presence of greigite which occurs in interstices between the pyrite crystals such occurrence of greigite in sediments has important implications in the interpretation of paleomagnetic records we evaluated the likely mechanism for the greigite formation in kg offshore basin and our data suggest that the formation of greigite may be related to either paleo smtz or anaerobic oxidation of pyrite it is unlikely that the formation of greigite can be explained by the downward diffusion of sulfide below the current depth of smtz however further investigations are required to ascertain the mechanism for the

the tortonian reference section at monte dei corvi italy evidence for early remanence acquisition in greigite bearing sediments summary the reliability of primary natural remanent magnetization nrm signals in greigite bearing sediments has been frequently questioned here we show that the stable nrm in the deep marine middle to late miocene sediments at monte dei corvi northern italy is mainly carried by greigite combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two greigite populations one fine grained and relatively well dispersed greigite population grain size between 60 and 200 nm is most likely of magnetotactic origin the second greigite population with larger grain sizes typically 700 nm to 1 m is most likely of authigenic bacterially mediated origin and is related to post depositional sulphidization processes greigite is the main magnetic remanence carrier in the older part of the section 12 8 to 8 7 ma whereas greigite and fine grained presumably magnetotactic magnetite are present in the younger part of the section 8 7 to 6 9 ma similar remanent magnetization directions of the magnetite and greigite components and the likelihood of a magnetotactic origin suggests that the nrm is of syn depositional age we suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine greigite this corroborates the reliability of the previously established magnetostratigraphy at monte dei corvi

investigation of a shallow slope failure on expansive clay in texas abstract highway slopes constructed with highly plastic clay soils undergo shrink swell cycles during the wet dry season which usually soften the soil and may cause failure a few years following construction there are two major wet dry cycles in texas where summer to early fall is categorized as the dry period and late fall to spring is categorized as the wet period during this study the failure of a highway slope located in fort worth texas was investigated and the effect of rainfall on the slope failure was evaluated the failure of the slope was investigated using soil test borings geophysical testing laboratory testing slope stability analysis and unsaturated flow analysis the site investigation included sample collections from different depths of borings determination of the soil index properties and various shear strength testing to determine soil strength parameters the obtained test results were further utilized in slope stability and flow analyses using the finite element method fem in plaxis the study indicated that both a fully softened condition and rainfall were responsible for the slope failure the fully softened condition provided a factor of safety of 1 46 whereas the perched water condition after rainfall provided a nearly failing factor of safety of 1 05

the global frasnian famennian kellwasser event during the uppermostp gigas conodont zone p linguiformis zone the stepwise frasnian extinction culminates in the global transgressive regressive frasnian famennian kellwasser event most pelagic organisms of the tropical marine environment are more or less affected by this mass extinction during the early upper devonian extinction rates are clearly related to the deposition of black shales and the storage of a great amount of organic carbon in the shelf sediments contemporaneous positive excursions of the 13c recorded from pelagic limestones of europe and australia prove a worldwide change in the co2 content of the oceans surface water which is in balance with the atmosphere if the climate is controlled by the co2 content of the atmosphere a cyclic operation process might have developed with the succession transgression high organic production deposition of organic carbon lowering of co2 in the atmosphere icehouse effect polar glaciation regression erosion of organic carbon and re supply of co2 to the ocean and atmosphere greenhouse effect deglaciation transgression during the change from a greenhouse to an icehouse effect an overturn of the anoxic oceans probably was one of the causes for the mass killing of low latitude marine organisms

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

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

greigite from carbonate concretions of the ediacaran doushantuo formation in south china and its environmental implications abstract greigite fe 3 s 4 is a ferrimagnetic iron sulfide that commonly forms as a precursor of pyrite in anoxic environments where the supply of reactive fe outpaces that of sulfide h 2 s and hs because of its metastability and sensitivity to redox changes during burial greigite has been rarely documented in rocks older than the cretaceous here we report well preserved greigite in carbonate concretions of the upper doushantuo formation ca 551 ma in the yangtze gorge area south china greigite in the carbonate concretions coexists with anhedral and framboidal pyrite and is distributed in clay rich carbonates with card house microtextures and dolomitic spherical structures indicative of early diagenetic formation during shallow burial preservation of greigite in carbonate concretions of the upper doushantuo formation implies that these concretions maintained a closed micro system since their formation and that they provide information about the ancient depositional and early diagenetic environments

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

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

the land plant δ13c record and plant evolution in the late palaeozoic abstract based on the evaluation of 1323 carbon isotope values for silurian to permian terrestrial organic carbon measured on plant fossils cuticules humic coals and bulk terrestrial organic matter tom we conclude that the temporal trend in 13ctom records variations in the global carbon cycle notably an increase in the fractional burial of light terrestrial organic matter in late palaeozoic sediments 13ctom values suggest that the late palaeozoic po2 peak could have been restricted to a time frame of 40 ma carbon isotope data from four taxonomic groups reveal small differences that could be a consequence of habitat conditions no significant differences in organic carbon isotopic composition in relation to variable climatic conditions are discernible the carbon isotopic composition solely reflects c3 plant metabolism

global change in the late devonian modelling the frasnian famennian short term carbon isotope excursions a model of the global biogeochemical cycles coupled to a energy balance climate model the combine model is used to identify the causes of two large n 13 c value excursions across the frasnian famennian f f boundary we test a scenario that links the sea level rise to stratification of the proto tethys ocean through the formation of warm saline deep waters in extended epicontinental seas even though this scenario can produce dysoxia below 100 m depth it fails to increase the global burial flux of organic carbon and thus seawater n 13 c values since stratification of the ocean leads to decreased productivity in surface waters several scenarios postulating a continental origin of the perturbations in the late devonian biogeochemical cycles are then tested we found that weathering of platform carbonates exposed during the early famennian sea level fall can account for a maximum positive shift in n 13 c value of 0 7x at the end of the sea level fall episode another 1 0x increase in n 13 c might originate from rapid spreading of vascular land plants near the f f boundary postulating that higher plants globally increased the weatherability of continental surface and that colonized continental area increased by 30 across the f f boundary finally the n 13 c excursion observed at the base of upper rhenana zone and the rapid increase of the carbon isotope ratios at the f f boundary require an increase of phosphorus delivery to the ocean by 40 coeval with the sea level rises once the calculated n 13 c values are in agreement with the measured data the combine model calculates a decrease in atmospheric pco2 from pre perturbation 2925 ppmv in the lower rhenana conodont zone to 1560 ppmv in the upper triangularis zone this decrease in pco2 is due to the increase in burial of organic matter during the kellwasser events and increased continental weatherability triggered by the spreading of continental vascular plants these changes occur within 4 million years the corresponding global climatic cooling reaches 4 4 c at the pole and 2 1 c at the equator f 2003 elsevier b v all rights reserved

carbon isotope stratigraphy of lochkovian to eifelian limestones from the devonian of central and southern europe lower to middle devonian carbonates of the prague syncline the carnic alps the montagne noire and the cantabrian mountains were investigated for 13ccarb and 13corg these values were measured on bulk rocks selected components and cements many carbonates exhibit primary marine values but some are altered by diagenesis a 13c curve can be presented for the latest pridolian to emsian time interval several sharp or broad positive excursions are obvious in the woschmidti postwoschmidti sulcatus kitabicus late serotinus and kockelianus conodont zones the excursion at the silurian devonian boundary is known worldwide and therefore considered global in nature some of the others are described for the first time from central and southern europe and their global nature has to be verified by further investigations in other regions most excursions relate to and or started during major regressions whereas sea level highstands correspond to minimal 13c values similar relationships between sea level changes and 13c have been observed from other early palaeozoic intervals the transgressive chotec and kacak events are marked by positive isotope excursions this type of combination is usually observed in late palaeozoic to cenozoic black shale events

carbon isotope fractionation between dissolved and gaseous carbon dioxide abstract fractionation of the stable carbon isotopes between dissolved and gaseous carbon dioxide has been measured at temperature 25 c by two methods in the first method the open system conditions and different methods of co 2 sampling were arranged in the second method the closed system conditions and co 2 gas extraction were used the results obtained by these methods are very consistent gaseous co 2 is enriched in heavy isotope 13 c by 1 03 0 02 permil in comparison to dissolved carbon dioxide

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

the diversification of paleozoic fire systems and fluctuations in atmospheric oxygen concentration by comparing silurian through end permian 250 million years myr charcoal abundance with contemporaneous macroecological changes in vegetation and climate we aim to demonstrate that long term variations in fire occurrence and fire system diversification are related to fluctuations in late paleozoic atmospheric oxygen concentration charcoal a proxy for fire occurs in the fossil record from the late silurian 420 myr to the present its presence at any interval in the fossil record is already taken to constrain atmospheric oxygen within the range of 13 to 35 the fire window herein we observe that as predicted atmospheric oxygen levels rise from 13 in the late devonian to 30 in the late permian so too fires progressively occur in an increasing diversity of ecosystems sequentially data of note include the occurrence of charcoal in the late silurian early devonian indicating the burning of a diminutive dominantly rhyniophytoid vegetation an apparent paucity of charcoal in the middle to late devonian that coincides with a predicted atmospheric oxygen low and the subsequent diversification of fire systems throughout the remainder of the late paleozoic first fires become widespread during the early mississippian they then become commonplace in mire systems in the middle mississippian in the pennsylvanian they are first recorded in upland settings and finally based on coal petrology become extremely important in many permian mire settings these trends conform well to changes in atmospheric oxygen concentration as predicted by modeling and indicate oxygen levels are a significant control on long term fire occurrence

carbon sources in arc volcanism with implications for the carbon cycle new co2 3 he data from the east sunda arc indonesia confirm earlier observations that arc volcanic gases have higher co2 3 he ratios than mor environments on average 80 of arc volcanic co2 is recycled exogene carbon addition of a few percent of carbonate bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases but can not explain the he isotope observations the co2 3he in arc volcanoes is not strongly dependent on the composition of modem trough sediments e g deep sea clays vs carbonate rich sequences and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of arcs the sum of globally deep subducted sediment and slab carbon exceeds the estimated arc co2 flux and approximately 3 5 teramole of carbon may return annually to the mantle in convergent zones the modem combined processes of mor volcanism slab alteration and subduction volcanism do not produce a substantial carbon flux into the exosphere and rate changes in ocean floor spreading are unlikely to cause major changes in atmospheric co2 as a result of changes in the volcanic co2 fluxes intense pulses of flood basalt volcanism however may alter the co2 contents of the atmosphere over the course of a millenium or so and influence global climate

modeling the carbon and sulfur isotope compositions of marine sediments climate evolution during the devonian abstract a model of global biogeochemical cycles coupled to an energy balance climatic model modified after the combine model godderis y joachimski m m 2004 global change in the late devonian modeling the frasnian famennian short term carbon isotope excursions palaeogeogr palaeoclimatol palaeoecol 202 309 329 is used to calculate the short term evolution of atmospheric p co 2 during the devonian the geochemical cycles for carbon alkalinity phosphorus sulfur and oxygen are included in this model with also 13 c and 34 s cycles high resolution records of 13 c of marine carbonates and 34 s of marine sulfates are used as forcing parameters of the geochemical cycles in an inverse modeling atmospheric p co 2 and p o 2 at the end of the silurian are calculated to have been 3000 ppmv and 0 165 bar 0 75 pal respectively a long term decrease in p co 2 is modeled for almost the entire devonian short term lowering of p co 2 to concentrations around 2000 ppmv is calculated for the silurian devonian transition and the pragian contents around 900 ppmv are modeled for the eifelian givetian givetian frasnian and frasnian famennian boundaries as a consequence of enhanced organic carbon burial during deposition of lochkovian eifelian and frasnian grey and black shales organic carbon burial is enhanced by the increase of phosphorus delivery to the ocean triggered by short term sea level falls the corresponding short term global climatic cooling at the silurian devonian boundary at the end of the pragian and the givetian frasnian as well as frasnian famennian boundaries reached 2 c at the equator the rapid colonization of continental surface by land plants during the middle and late devonian increasing chemical alteration of the continents and co 2 consumption by silicate weathering is assumed to have caused cooling of surface seawater as suggested by the 18 o values of biogenic apatites

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

oxygen isotope evolution of biogenic calcite and apatite during the middle and late devonian oxygen isotope ratios of well preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the middle and late devonian by assuming an oxygen isotopic composition of 1 v smow for devonian seawater the oxygen isotope values of eifelian and early givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 c late givetian and frasnian palaeotemperatures calculated from 18o values of conodont apatite are close to 25 c in the early frasnian and increase to 32 c in the latest frasnian and early famennian oxygen isotope ratios of late givetian and frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite 18o values and give unrealistically warm temperatures ranging from 30 to 40 c diagenetic recrystallization of shell calcite different habitats of conodonts and brachiopods as well as non equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the 18o depletion of brachiopod calcite moreover the 18o depletion of brachiopod calcite with respect to equilibrium 18o values calculated from conodont apatite is too large to be explained by a change in seawater ph that might have influenced the oxygen isotopic composition of brachiopod calcite the realistic palaeotemperatures derived from 18o apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of palaeozoic oceans more faithfully than brachiopod calcite and do not support the hypothesis that the 18o 16o ratio of devonian seawater was significantly different from that of the modern ocean

a high resolution record of the matuyama brunhes transition from the mediterranean region the valle di manche section calabria southern italy abstract high resolution palaeomagnetic and rock magnetic investigations on the valle di manche section crotone basin calabria southern italy provide a detailed record of the matuyama brunhes m b reversal that to our best knowledge is the only available record of the last geomagnetic reversal for the mediterranean on land marine stratigraphy the m b transition can be pinpointed precisely as it develops within a 3 cm thick interval located just above a prominent tephra layer the pitagora ash where the sedimentation rates are about 27 cm kyr demagnetization analyses indicate a stable palaeomagnetic behaviour throughout the section for both normal and reversed polarity directions with demagnetization vectors aligned toward the origin of zijderveld diagrams after the removal of a small viscous low coercivity remanence component in the lower part of the studied interval some samples acquired a spurious gyromagnetic remanent magnetization grm during af demagnetization in high fields rock magnetic analyses confirm that magnetite is the main magnetic carrier for all measured specimens which also have an abundant paramagnetic fraction only the lower part of the record well below the m b boundary is characterized by a downward increasing presence of iron sulphides greigite according to our chronology which is based on a robust cross validated age model the final reverse to normal directional change of the m b transition occurred at ca 786 9 5 ka error includes uncertainty in orbital tuning and was very rapid of the order of 100 years or less

greigite detected as dominating remanence carrier in late pleistocene sediments lisan formation from lake kinneret sea of galilee israel summary a rock magnetic investigation of three sedimentary cores of lisan formation of late pleistocene age from lake kinneret sea of galilee northern israel demonstrates that the magnetization of these sediments is controlled by various degrees of a secondary chemical remanent magnetization crm carried by greigite fe3s4 this crm is superimposed on a primary detrital remanent magnetization drm that resides in ti magnetite this finding is independently confirmed by x ray diffraction xrd measurements performed on magnetic extracts of the sediments the domain state of the greigite is largely single domain behaviour sd thus dominating the magnetization therefore the magnetic record retrieve from of these sediments is not reflecting geomagnetic variations but rather chemical rock magnetic properties resulting from diagenetic processes the results of our study suggest that paleomagnetic record of greigite bearing sediments should be interpreted with caution because of the following reasons 1 geomagnetic secular variations can be biased due to large coercivity overlap between magnetite and greigite 2 alternating field af demagnetization can produce erroneous directions due to vector distortion by acquisition of a gyro remnant magnetization grm 3 estimation of relative paleointensity can be hampered by large scale variations in natural remnant magnetization nrm intensity caused by the acquisition of a secondary crm of unknown age and unknown extent superimposed on the drm the precipitation of greigite requires reducing conditions at the sediment water interface and or interstitial water associated with the presence of sulphur and iron oxides reducing conditions are typical of stratified lakes and other stratified water bodies such as the dead sea paleo lake lisan temporarily the eastern mediterranean sea the caspian sea the black sea and many other marine basins according to our findings paleomagnetic records from such environments carried dominantly by greigite therefore should be treated with care

a global biogeochemical perturbation across the silurian devonian boundary ocean continent biosphere feedbacks abstract the large scale global biogeochemical perturbation across the silurian devonian s d boundary recorded in the major positive excursion of the 13c time curve klonk event of an amplitude of 2 5 to 3 0 max 4 0 in europe and up to 5 0 in north america reflects a unique combination of palaeogeographic biogeochemical and evolutionary processes in the late caledonian geodynamic setting the steady sulfur isotopic ratios show an overall stability of the s d oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean continent biosphere feedback coastal zones of the latest silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation which explosively expanded to inhabit vast near coastal shallows and deltas large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for co2 drawdown which have resulted in reversed greenhouse effect and a global climatic cooling tendency this feedback was blocked when the sea level gradually dropped and led to shrinking of the silurian epicontinental seas and the growing climatic deterioration during the s d transition limited primary production furthermore continued processes of regressive abrasion and erosion limited the storage of organic carbon as well as the efficiency of the carbonate factory during the following early devonian greenhouse interval marine regression and active latest caledonian tectonism promoted progressive weathering of the sedimentary organic matter the oxidation of c rich deposits caused the subsequent growth of co2 levels in the early devonian culminating in the warming of global climate

modeling atmospheric o2 over phanerozoic time a carbon and sulfur isotope mass balance model has been constructed for calculating the variation of atmospheric o2 over phanerozoic time in order to obtain realistic o2 levels rapid sediment recycling and o2 dependent isotope fractionation have been employed by the modelling the dependence of isotope fractionation on o2 is based for carbon on the results of laboratory photosynthesis experiments and for sulfur on the observed relation between oxidation reduction recycling and s isotope fractionation during early diagenetic pyrite formation the range of fractionations used in the modeling agree with measurements of phanerozoic sediments by others results derived from extensive sensitivity analysis suggest that there was a positive excursion of o2 to levels as high as 35 during the permo carboniferous high o2 at this time agrees with independent modeling based on the abundances of organic matter and pyrite in sediments and with the occurrence of giant insects during this period the cause of the excursion is believed to be the rise of vascular land plants and the consequent increased production of o2 by the burial in sediments of lignin rich organic matter that was resistant to biological decomposition

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

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

a numerical model of the evolution of ocean sulfate and sedimentary sulfur during the last 800 million years abstract a reservoir model describing the time evolution of the sedimentary cycle of sulfur over the past 800 my has been developed as a first approximation the ocean sulfate concentration is assumed to be time independent with this assumption the model is integrated backward in time and a new initialization procedure is derived in order to calculate the present state of the system which must be compatible with both observational data and model equations the effects of a variation of the present state of the cycle on its past evolution are investigated it is found that when the present gypsum reservoir content is too low or when the weathering rate constants are too high no acceptable solution can be obtained for the evolution of the cycle since one reservoir is forced to depletion the sensitivity of the model to the mean isotopic composition of the sedimentary system and to the fractionation factor during pyrite formation is also studied moreover a model with time dependent ocean sulfate concentration was developed the existence of an acceptable solution appears to be linked to the steady state hypothesis for ocean sulfate since a model with no acceptable steady state solution may be integrated until t 800 my without any problem of reservoir depletion when the time dependent equations are used a tentative evolution of the ocean sulfate concentration is calculated it is shown that this concentration is negatively correlated to the 34s of seawater sulfate the carbon cycle is modelled in order to compare the calculated 13c of carbonate deposits to the observational data

oxygen and carbon stable isotope records of the lochkovian pragian boundary interval from the prague basin lower devonian czech republic abstract climate changes close to the lochkovian pragian event are still widely discussed carbonate 18o and 13c and conodont apatite 18o from medial to distal carbonate ramp sediments were analysed to provide further stable isotope data from the stratotype area in the prague basin the uppermost silurian to lower emsian 18o trends are put into an updated conodont biostratigraphy framework carbonate 18o and 13c are discussed in the context of facies dependent diagenesis the 13c pattern measured from different sections enables the correlation of intervals with development of different facies inside the prague basin positive 13c shifts are often coupled with trends of decreasing computed gamma ray cgr values interpreted as regressions whereas negative 13c shifts commonly coincide with opposite trends in cgr the lochkovian pragian boundary interval coincided with an increase in carbonate 13c and an increase in the 18o of conodont apatite and carbonate which we interpret as consequence of climate cooling generally the lochkovian was warmer than the pragian and earliest emsian