microfossils in cherts from the middle riphean mesoproterozoic avzyan formation southern ural mountains russian federation a diverse assemblage of well preserved microorganisms has been detected in black cherts from the approximately 1200 ma old avzyan formation suite of the southern ural mountains russian federation the lower kataskin member contains a diverse abundant microbiota dominated by mat forming filamentous cyanobacteria several types of colonial unicells and morphologically distinctive stalked cyanobacteria the upper revet member contains a less diverse biota dominated by unicellular cyanobacteria palaeoecological evidence indicates that the microbial community of the kataskin member inhabited a shallow water presumably marine carbonate environment revet microorganisms possibly lived in restricted peritidal environments the biostratigraphic significance of the avzyan microbiota is limited many of the taxa are long ranging they were already abundant in palaeoproterozoic successions and continue into the neoproterozoic nevertheless in many respects the kataskin assemblage is comparable to those reported from the middle late riphean deposits of northern america australia and eurasia the following taxa are here described chroococcaceae eogloeocapsa avzyanica sergeev gloeodiniopsis lamellosa schopf emend knoll et golubic entophysalidaceae eoentophysalis belcherensis hofmann dermocarpaceae polybessurus bipartitus fairchild ex green et al nostocaceae eosphaeronostoc kataskinicum sergeev nostocaceae or oscillatoriaceae siphonophycus robustum schopf emend knoll et golubic emend knoll et al siphonophycus sp incertae sedis eosynechococcus amadeus knoll et golubic

a simple ice ocean model for the greenland sea abstract a steady coupled model of the upper ocean circulation and ice cover in a meridional channel is presented and applied to the greenland sea the main ocean gyre is driven by wind stress however a nonzero boundary condition on meridional velocity is applied at the western side to produce a southward flowing coastal current representative of the east greenland current egc to match the egc to the interior sverdrup solution a munk type western boundary layer is introduced the upper ocean temperature is governed by an advection diffusion equation which is forced at the boundaries and by heat exchange with the atmosphere the ice model is purely thermodynamic and the ice edge position is determined iteratively since the streamfunction and temperature fields depend on the ice cover and in turn the ice cover is determined by both the ocean and air temperature the range of behavior of the temperature field and ice edge position is studied as a function of the ice ocean drag the wind stress tra

comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones for white magnetite bearing mesozoic pelagic limestones from italy which carry a primary magnetization the values of mrs ms and hcr hc generally lie in the pseudo single domain psd field of the day et al 1977 plot the logarithmic plot of mrs ms against hcr hc gives a straight line r 0 814 with slope and intercept close to the empirical mixing line of parry 1982 for single domain sd and multidomain md magnetite for one of the white pelagic limestone formations maiolica formation samples with hysteresis ratios closer to the md field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay we therefore associate the increased md magnetite with increased detrital influx for pinkish and reddish varieties of the italian pelagic limestones the presence of hematite is manifest by high saturation fields a wide range of hcr hc and wasp waisted hysteresis loops attributed to the mixing of magnetite and high coercivity authigenic hematite the hysteresis ratios for a collection of paleozoic and mesozoic remagnetized magnetite bearing limestones from britain nevada alaska and the appalachians lie mainly outside the psd field and appear to follow a power law trend following jackson et al 1993 the high values of hcr hc and the characteristically wasp waisted hysteresis loops can be interpreted in terms of a fine grained subspherical high coercivity sd magnetite mixed with a high proportion of superparamagnetic magnetite the slope and intercept of the power law relationship for mrs ms and hcr hc in the remagnetized limestones are distinct from those observed for the italian limestones and may provide a means of fingerprinting magnetite of primary as opposed to diagenetic origin

isotopic evidence for carbonate cementation and recrystallization and for tectonic expulsion of fluids into the western canada sedimentary basin carbon oxygen and strontium isotope data for calcites and dolomites from the devonian obed platform in alberta canada demonstrate that 1 both limestones and dolostones of the obed platform underwent significant deep burial cementation and recrystallization 2 calcites experienced more extensive geochemical alteration than did dolomites under deep burial conditions and 3 the fluids that facilitated deep burial carbonate diagenesis probably were partially derived from the rocky mountain fold and thrust belt the more extensive degree of recrystallization of calcite is shown especially by its higher 87 sr 86 sr ratios a lesser degree of 13 c depletion in dolomites indicates that dolomite recrystallization partially coincided with hydrocarbon oxidation evidence supporting interpretation 3 above includes fractures and vugs bearing late diagenetic calcite cements that have extremely high 87 sr 86 sr ratios including the highest ratios reported thus far for any diagenetic carbonates from western canada 0 7252 in carbonates values this high are found only in tectonic veins in proterozoic clastic rocks in the rocky mountains and in the obed platform about 100 km into the foreland basin the late diagenetic calcite cements also have highly depleted 13 c values minimum 27 1 relative to pdb peedee belemnite indicating incorporation of oxidized carbon from thermochemical sulfate reduction the process of carbonate cementation and recrystallization in strata of the obed platform probably occurred during deep burial maximum 5 7 km and was effected by a hot 100 c mixture of connate brines and hydrothermal or metamorphic fluids that were expelled from the rocky mountain fold and thrust belt during the laramide orogeny the data also suggest that 1 the common practice of using limestones to establish marine or original geochemical baselines for stable and radiogenic isotope interpretations must be conducted with caution and 2 replacement burial dolostones are quite resistant to burial recrystallization finally the geochemical trace of tectonically expelled fluids may be limited to about 100 km into the foreland basin implying that the volumes and or fluxes of fluids produced by tectonic expulsion are rather low

epibiotic relationships in mesoproterozoic fossil record gaoyuzhuang formation china epibiotic coccoid microfossils of probable cyanobacterial affinity were discovered in cherty stratiform stromatolites of the 1400 1500 ma gaoyuzhuang formation hebei province northern china sessile coccoids with polarized growth settled on cyanobacterial sheaths and developed dense populations these microfossils represent the earliest known occurrence of epibiosis in the fossil record with characteristics similar to those observed in modern aquatic environments the reported fossil epibionts are also the first known occurrence of proterozoic cyanobacteria of chamaesiphonalean affinity the presence of specialized epibiotic communities in gaoyuzhuang stromatolites demonstrates that epibiosis was well established and diversified by mesoproterozoic time reflecting early adaptation in response to competitive pressures for microbial attachment sites in ancient densely populated shallow water benthos or possibly the onset of specialized mutualistic relationships among cooccurring microorganisms

late paleozoic remagnetization in limestones of the craven basin northern england and the rock magnetic fingerprint of remagnetized sedimentary carbonates samples of lower carboniferous chatburn and pendleside limestones collected from 27 sites in the craven basin northern england carry a syn hercynian remagnetization residing in magnetite the paleomagnetic directions are all of reversed polarity consistent with a kiaman superchron remagnetization the partially 62 tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization the resulting pole appears to be late carboniferous in age and there is no indication of any magnetization component that predates the remagnetization event the magnetization appears to be of the same type as the synfolding remagnetizations documented in several appalachian carbonate units jackson 1990 reported that remagnetized appalachian carbonates have distinctive hysteresis properties and we have obtained similar hysteresis loops from remagnetized paleozoic carbonates from the craven basin england the great basin nevada alaska and new york state the hysteresis loops are all wasp waisted and yield highly anomalous hysteresis ratios compared to data from synthetic or typical magnetite bearing samples in contrast unremagnetized limestones from italy show hysteresis behavior which is more typical for magnetite bearing rocks channell and mccabe this issue these results suggest that remagnetization in magnetite bearing carbonates can be readily detected on the basis of hysteresis properties prior to a full scale paleomagnetic investigation

preservation and detection of microstructural and taxonomic correlations in the carbon isotopic compositions of individual precambrian microfossils abstract here we present techniques for and new data from in situ carbon isotope 13c analysis of precambrian permineralized microscopic fossils with a reproducibility of 1 2 using secondary ion mass spectrometry sims individual microfossils selected for their excellent preservation were analyzed in petrographic thin sections of stromatolitic cherts from the proterozoic gunflint 1880 ma bitter springs 830 ma min yar 740 ma and chichkan 775 ma formations the range of 13c values 34 6 to 22 1 vpdb among the 46 individuals analyzed falls within that expected for photoautotrophic carbon fixation by ribulose bisphosphate carboxylase rubisco consistent with morphology based taxonomic assignments for these specimens microfossils classified as cyanobacteria from the gunflint bitter springs and min yar formations for which published carbonate carbon isotope data can be used to estimate the 13c of the original dissolved inorganic carbon substrate exhibit a consistent 19 total fractionation 13c of dissolved inorganic carbon 13c of biomass similar to that observed in living cyanobacteria over a wide range of 13ccarb values 2 9 to 3 4 in stromatolitic chert of the min yar formation morphologically diverse microfossils preserved in a 1 mm2 part of a microbial mat exhibit systematic isotopic differences among and within taxa that correlate with their morphologically inferred biological affinities and suggest that isotopic signatures of their original biosynthetic processes e g lipid and peptidoglycan synthesis are preserved isotopic offsets consistent with the different rubisco based fractionations typical of cyanobacteria and photosynthetic eukaryotes are documented by the differing 13c values of a colonial cyanobacterium 22 6 0 5 and a phytoplanktonic protistan acritarch 28 9 1 0 situated

concurrence of folding and remagnetization events in the monterrey salient ne mexico abstract carbonate formations from many locations reveal magnetizations that are secondary attaching a numerical age to these remagnetizations would provide information on orogenic processes to contribute to the effort of assigning an age to remagnetization events lower cretaceous formations from the monterrey salient in northeast mexico were sampled individual fold tests reveal eight site pair synfolding remagnetizations set a of eocene age the remanence is a chemical remanent magnetization crm carried by magnetite the magnetization of set b is possibly due to an early carbonate alteration that produced a remanence significantly older than folding a complexity of set b is that the site pairs reveal k versus unfolding percentage diagrams with maximum unfolding peaks at 100 to 125 an explanation of multiple deformation phases with an earlier horizontal folding phase around a non plunging horizontal axis and a later vertical axis rotation is suggested for the past 100 peaks of the b sites the latter deformation rotation about a vertical axes is revealed by a declination strike test in set a sites and site means of the full collection that deviate counterclockwise from the reference declination by combining synfolding results with published 40ar 39ar illite ages of folding fitz diaz et al 2016 remagnetization ages in set a sites of 48 52 ma are obtained the magnetization in set b sites is likely older though not well constrained interestingly prior results from the central sierra madre oriental to the south show two remagnetization events late cretaceous and early eocene in age in a succession from w to e nemkin et al 2015 the age of folding and remagnetization acquisition in set a sites along the monterrey salient are concurrent with the frontal remagnetized folds in the central sierra madre oriental thus the timing of major remagnetization in the monterrey salient occurred in the eocene

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

proterozoic stromatolitic microbiotas of the 1400 1500 ma old gaoyuzhuang formation near jixian northern china abstract two communities of diverse well preserved fossil prokaryotic microorganisms have been discovered in petrographic thin sections of carbonaceous black chert from the ca 1400 1500 ma old gaoyuzhuang formation at the stratotype section of the sinian suberathem near jixian northern china one of these communities is preserved in bedded essentially flat laminated stromatolitic chert the other occurs in silicified conical stromatolites of the forms conophyton cylindricum and c garganicum the gaoyuzhuang cherts comprise one of the few microfossiliferous deposits now known from the precambrian of china the permineralized microbiotas they contain are among the first such stromatolitic assemblages to be discovered in the sinian stratotype section

remagnetization and orogenic fluids testing the hypothesis in the central appalachians summary palaeomagnetic and geochemical studies on devonian sedimentary rocks in the valley and ridge province in west virginia indicate that the hypothesized relationship between orogenic fluids and remagnetization is not straightforward the helderberg group an aquitard and the oriskany formation a palaeoaquifer both contain similar synfolding late palaeozoic chemical remanent magnetizations crms residing in magnetite the results from the oriskany formation are consistent with remagnetization by orogenic fluids because the unit contains geochemical evidence for the migration of such fluids in contrast there is no geochemical evidence for the widespread migration of orogenic fluids into the helderberg group and thus the crm cannot be related to such fluids another remagnetization mechanism which remains elusive must be responsible for the crm in the helderberg group

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

extending the applicability of the raman carbonaceous material geothermometer using data from contact metamorphic rocks the degree of graphitization of carbonaceous material cm has been widely used as an indicator of metamorphic grade previous work has demonstrated that peak metamorphic temperature t o f regional metamorphic rocks can be estimated by an area ratio r2 of peaks recognized in raman spectra of cm the applicability of this method to low pressure 50 that adequately reflects the range of sample heterogeneity using this procedure with 532 nm laser and adapting our new calibration the errors of the raman cm geothermometer for contact metamorphic rocks decrease to 30 c

a box model study of the greenland sea norwegian sea and arctic ocean we develop a simple dynamical system model of the arctic ocean and marginal seas by applying the martinson killworth and gordon box model of a high latitude two layer ocean to four regions connected together the greenland sea the norwegian sea the arctic ocean and the greenland gyre the latter is a small convective region embedded in the northwest corner of the norwegian sea the model for each region consists of a thermodynamic ice layer that covers two layers of saline water which can under specific conditions become statically unstable and hence create a state of active overturning the system is forced by monthly mean atmospheric temperatures in the four regions by continental runoffs and by inflows from adjacent oceans the model predicts the ice thickness and the temperature and salinity of the water in the upper layer of the four regions also determined are the water temperature and salinity of the lower layer in the arctic ocean box the convective state of any given region i e whether it is in an active overturning mode or not is also determined as a continuous function of time the different output variables of the model which are the response to climatological forcing conditions compare favourably with observed data in the control run the arctic ocean region is characterized by continuous ice cover the greenland sea and greenland gyre have ice cover only during winter and the norwegian sea region never forms an ice cover another feature of the control run is the winter time occurrence of convective overturning in the upper 200 m in the greenland gyre region the model is also used for different anomaly experiments a positive air temperature anomaly which represents a global warming of the earth a negative salt anomaly in the norwegian sea which simulates the great salinity anomaly of the 1960s and 1970s and an increase in the ice flux through fram strait which parameterizes anomalous ice production in the arctic

synthesis and model of formation water flow alberta basin canada based on a large amount of publicly available data several studies have previously examined the flow of formation waters in different parts of the alberta basin offering various interpretations as to the causes of the observed pressure regime and flow pattern however there has been no synthesis of these diverse studies on a basin wide basis accordingly these studies are critically reviewed in this paper and synthesized in a new basin scale model of the flow of formation waters in the alberta basin the proposed regional scale model has significant implications for understanding hydrocarbon migration pathways ore genesis the geothermal regime and deep waste disposal in the alberta basin several flow systems each one driven by a different mechanism are identified together with the main processes leading to the nonhydrostatic pressures observed in the basin two megahydrostratigraphic successions and associated flow systems are recognized the first succession corresponds to the pre cretaceous passive margin stage of basin development and consists of thick carbonate dominated aquifer systems separated by shaly aquitards and evaporitic aquicludes a northeastward basin scale flow system is driven by basin topography with recharge in montana and discharge in northeastern alberta southwest to northeast regional scale flow adjacent to the fold and thrust belt is probably the result of past tectonic processes the salinity of formation water in pre cretaceous aquifers is high and generally increases both northward and with depth this variability is the result of increased water rock reactions mineral solubility with increased temperatu e and of incomplete flushing by meteoric water as a result of salinity variations flow retarding buoyancy effects can be important the second megahydrostratigraphic succession corresponds to the post jurassic foreland stage of basin evolution and consists of thick shaly aquitard systems and relatively thin sandstone aquifers in the southwestern part of the basin flow in isolated aquifers is driven southwestward by erosional rebound in the thick intervening shales downdip toward the fold and thrust belt the salinity of formation waters in the post jurassic aquifers is low mixing of waters and interference between the major flow systems takes place along the pre cretaceous unconformity where successively older pre cretaceous aquifers subcrop from west to east flow in shallow lo al systems at the top of the sedimentary succession is driven by local topography

synfolding remagnetization and deformation results from palaeozoic sedimentary rocks in west virginia summary palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent palaeozoic units along two transects meldey and rig of the patterson creek anticline in the appalachian orogen of west virginia to better understand the relationship between deformation and remagnetization while no primary directions were observed we isolated identical well defined secondary late palaeozoic magnetizations in carbonates from the late silurian tonoloway formation and the early devonian helderberg group standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the helderberg group and pre folding to early synfolding in the tonoloway formation a fold test on all data combined indicates a synfolding magnetization best grouping at 60 65 per cent unfolding but the resulting distributions are not circular an alternative interpretation based on optimal differential unfolding at the site mean level yields one group with a nearly pre folding or early synfolding magnetization 70 per cent unfolding that is both circular and concordant with north america s apparent polar wander path the second group has a variable synfolding magnetization that is elliptical at the optimum unfolding level and falls off the path differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result the sites falling into the pre folding group are biased towards the tonoloway formation the medley transect the northwest limb away from small folds and lower dip angle relative to the sites in the synfolding group the two groups defined by the fold test results also have a relationship with both total strain and strain partitioning the rocks containing the early synfolding remanence exhibit less total strain and in particular less pressure solution strain relative to grain boundary sliding and calcite twinning it is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions the variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization in any event the relationship between deformation and magnetization is not simple apparently identical magnetizations from adjacent sites may have very different histories and or responses to deformation while the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

the cordilleran ribbon continent of north america the north american cordilleran orogen is the result of a two stage process a triassic jurassic accretion within panthalassa forming saybia a composite ribbon continent and b late cretaceous collision of saybia with north america this model requires that a large portion of the continental foreland of the orogen is exotic the exotic continental component of saybia cassiar platform is distinguished from the autochthon on the basis of its a triassic eurasian fauna b involvement in a major late triassic early jurassic orogenic event and c young in part grenvillian basement and mantle a mid cretaceous magmatic arc records west dipping subduction beneath the east margin of saybia the related accretionary prism consists of imbricated shale chert and deep water limestones the medial basin and overlies an isotopically juvenile mantle domain carbonatite complexes delineate the cryptic suture separating saybia and the autochthon paleomagnetic and paleobotanical data place saybia 2000 km to the south relative to the autochthon at 80 ma late cretaceous thrust belt development records transpression between the north moving ribbon continent and the autochthon pinning against the okhotsk chukotka arc in siberia buckled saybia giving rise to the alaskan promontory

biogenicity of earth s earliest fossils a resolution of the controversy abstract the abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the 3465 ma apex chert of northwestern australia widely regarded as among the oldest records of life have been investigated intensively first reported in 1987 and formally described in 1992 and 1993 the biogenicity of the apex fossils was questioned in 2002 and in three subsequent reports however as is shown here by use of analytical techniques unavailable twenty years ago the apex filaments are now established to be bona fide fossil microbes composed of three dimensionally cylindrical organic kerogenous walled cells backed by a large body of supporting evidence of similar age other microfossils stromatolites and carbon isotopic data it seems clear that microbial life was present and flourishing on the early earth 3500 ma ago

paleomagnetic constraints on the tectonic history of the foreland belt southern canadian cordillera preliminary results in the southern canadian cordillera the paleomagnetic memory of paleozoic carbonate strata in the front ranges and inner foothills of the foreland thrust and fold belt retains no record of their known deposition at low latitudes instead each folded structure exhibits a similar but asynchronous sequence of events including an eastwardly progressing predeformational chemical remagnetization during the cordilleran orogeny the remagnetization of a western front ranges structure occurs during a period of normal polarity before 130 ma the paleomagnetic pole requires that the subsequent deformation of the western front ranges is jurassic or younger the remagnetization of a front ranges structure in the lewis thrust sheet occurs during a period of normal polarity after 130 ma but before deformation which from other evidence occurred around 75 ma the predeformational remagnetization of an inner foothills structure occurs during a reversed magnetic period that we interpret to be after 75 ma an early cretaceous sill in the lewis thrust sheet was remagnetized during a reverse polarity chron prior to the end of lewis thrust deformation when about 70 of the present dip of the sill was acquired remagnetization consistently predates deformation whereas it occurs later at more easterly localities there are also similarities in character and style of the remagnetizations among localities when coupled with the eastward progression of the deformation our observations suggest that an important and pervasive but hitherto unrecognized and unappreciated orogenic chemical process affected paleozoic carbonate strata in the van of the deforming cordilleran tectonic wedge

direct dating of carbonate remagnetization by 40ar 39ar analysis of the smectite illite transformation abstract the occurrence of remagnetizations in carbonate rocks is a longstanding issue the result of a cryptic process that affects rocks in both fold thrust belts and undisturbed continental interiors this paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the cantabrian asturian belt northern spain detailed x ray diffraction patterns indicate that mixed layer clays from these rocks are mostly illitic with an increasing proportion of authigenic illite polytype 1m d in finer grain size fractions the 40 ar 39 ar age data from the different grain size fractions coupled with quantitative polytype modeling indicates an authigenic age that is coeval with the established late paleozoic remagnetization age of these rocks we propose that the transformation of fe rich smectite to fe poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks the irreversible kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization and may explain such vexing phenomena as synfolding magnetizations this technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences

a new paleomagnetic pole for the neoproterozoic uinta mountain supergroup central rocky mountain states usa abstract hematite cemented strata of the neoproterozoic age 800 750 ma uinta mountain group umg provide a new well defined mid neoproterozoic paleomagnetic pole for laurentia thermal demagnetization results and rock magnetic experiments reveal the presence of two discrete magnetizations in umg rocks both magnetizations are carried by hematite the dominant characteristic remanent magnetization chrm is an east directed or antipode shallow positive to negative inclination magnetization that is typically unblocked over a narrow range of high laboratory temperatures between 660 and 680 c the second magnetization exclusively of normal polarity is north to northeast directed and has a moderate to steep positive inclination and probably represents a recent or modern day viscous remanent magnetization the response of umg samples to thermal and chemical demagnetization comparison with previously obtained data from umg rocks in the area results from rock magnetic experiments and the inferred low latitude of laurentia during umg deposition in the neoproterozoic are all consistent with the interpretation that the west or east directed shallow magnetization is a near primary remanence the presence of dual polarities of the chrm with the west directed remanence inferred to be of normal polarity and the east directed of reverse polarity suggests that magnetization acquisition in the umg rocks took place over a sufficient amount of time to average secular variation of the geomagnetic field the mean paleomagnetic pole derived from the inferred primary magnetizations from umg strata 0 8 n 161 3 e 95 4 6 n 9 sampling localities consisting of 79 sites is fully consistent with recently reported results from rocks of mid neoproterozoic age for southwest laurentia which define a tight counterclockwise loop located in the south pacific in the apparent polar wander path for laurentia during the neoproterozoic

zebra dolomitization as a result of focused fluid flow in the rocky mountains fold and thrust belt canada discordant zebra dolomite bodies occur locally in the middle cambrian cathedral and eldon formations of the main ranges of the canadian rocky mountains fold and thrust belt they are characterized by alternating dark grey a and white b bands forming an abba diagenetic cyclicity these bands developed parallel to both bedding and cleavage dark grey a bands consist of fine 300 m non planar crystalline impure dolomite the white b bands are composed of coarse up to several millimetres milky white pure saddle dolomites b1 which are often covered by pore lining zoned dolomite b2 the b phases often possess a saddle shaped morphology in contrast to the replacement origin of the a dolomite the zoned b2 dolomite rims are interpreted as a cement formed in open cavities the b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non carbonate components all the zebra dolomites studied are nearly stoichiometric and are characterized by enriched na and depleted sr concentrations fe and mn concentrations in these dolomites differ depending on the sample locality fluid inclusion data indicate that the dolomites formed from relatively hot th 130 200 c saline 20 23 wt cacl2 eq fluids a diagenetic high temperature origin is also supported by depleted 18o values 20 to 14 vpdb a contribution of 87sr enriched fluids is reflected in the 87sr 86sr values 0 7091 0 7123 zebra dolomite development is explained by focused fluid flow which exploited areas of structural weaknesses e g basin platform rim areas faults etc expulsion of hot basinal brines in a tectonically active regime generated overpressures which explains the development of secondary porosity during zebra dolomitization as well as the intra zebra fracturing at decimetre to micrometre scale

synfolding magnetization detection testing and geological applications summary a correct method for determining the direction of a synfolding remanence is described and a new test which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre and post folding components is suggested it is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object

mineralized microbes from giggenbach submarine volcano 1 the giggenbach submarine volcano which forms part of the kermadec active arc front is located 780 km nne of the north island of new zealand samples collected from chimneys associated with seafloor hydrothermal vents on this volcano at a depth of 160 180 m contain silicified microbes and microbes entombed in reticular fe rich precipitates the mineralized biota includes filamentous rod shaped and rare coccoid microbes in the absence of organic carbon for rdna analysis or preserved cells the taxonomic affinity of these microbes in terms of extant taxa remains questionable because of their architectural simplicity and the paucity of taxonomically significant features the three dimensional preservation of the microbes indicates rapid mineralization with a steady supply of supersaturated fluids to the nucleation sites present on the surfaces of the microbes the mineralization styles evident in the microbes from the giggenbach submarine volcano are similar to those associated with mineralized microbes found in terrestrial hot spring deposits in new zealand iceland yellowstone and kenya these similarities exist even though the microbes are probably different and the fluids become supersaturated with respect to opal a by different mechanisms for ancient rocks it means that interpretations of the depositional settings cannot be based solely on the silicified microbes or their style of silicification

sea ice anomalies observed in the greenland and labrador seas during 1901 1984 and their relation to an interdecadal arctic climate cycle two independent ice data sets from the greenland and labrador seas have been analyzed for the purpose of characterizing interannual and decadal time scale sea ice extent anomalies during this century sea ice concentration data for the 1953 1984 period revealed the presence of a large positive anomaly in the greenland sea during the 1960s which coincided with the great salinity anomaly an upper ocean low salinity water mass that was observed to travel cyclonically around the northern north atlantic during 1968 1982 this ice anomaly as well as several smaller ones propagated into the labrador sea and then across to the labrador and east newfoundland coast over a period of 3 to 5 years a complex empirical orthogonal function analysis of the same data also confirmed this propagation phenomenon an inverse relation between sea ice and salinity anomalies in the greenland labrador sea region was also generally found an analysis of spring and summer ice limit data obtained from danish meteorological institute charts for the period 1901 1956 indicated the presence of heavy ice conditions i e positive ice anomalies in the greenland sea during 1902 1920 and in the late 1940s and generally negative ice anomalies during the 1920s and 1930s only limited evidence of the propagation of greenland sea ice anomalies into the labrador sea was observed however probably because the data were from the ice melt seasons on the other hand several large ice anomalies in the greenland sea occurred 2 3 years after large runoffs in the early 1930s and the late 1940s from northern canada into the western arctic ocean similarly a large runoff into the arctic during 1964 1966 preceded the large greenland sea ice anomaly of the 1960s these facts together with recent evidence of climatic jumps in the northern hemisphere tropospheric circulation suggest the existence of an interdecadal self sustained climate cycle in the arctic in the greenland sea this cycle is characterized by a state of large sea ice extent overlying an upper layer of cool relatively fresh water that does not convectively overturn which alternates every 10 15 years with a state of small sea ice extent and relatively warm saline surface water that frequently overturns

laboratory simulation of exchange through fram strait laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy driven and wind driven rotating fluids in lock exchange experiments water with one density flows into a second basin after a sliding gate is removed water of a second density flows back into the first basin the size and location of the currents for various values of density difference rotation rate and assorted sidewall geometries was recorded volume flux of the fluid was also measured and compared with a theory for lock exchange flow of a rotating fluid in a separate group of experiments with a passive upper layer easterly winds like those in the arctic ocean drive the upper level water into the arctic ocean and therefore oppose the buoyant exchange westerly winds would drive the water out of the arctic ocean this indicates that the exchange between the arctic ocean and the greenland norwegian sea is likely to be driven by buoyancy rather than by driven by wind crude estimates of the volumetric and fresh water exchange rate from the lock exchange formulas are compared with observed ocean fluxes and approximate agreement is found

genesis of large siliceous stromatolites at frying pan lake waimangu geothermal field north island new zealand lilypad stromatolites up to 3 m long and 1ae5 m wide were found to be actively growing in the shallow marginal waters of frying pan lake and its outflow channel these stromatolites composed of phormidium 90 fischerella and a variety of other microbes develop through a series of distinct growth stages dark green microbial mats cover the floor of the outflow channel and give rise to columns of various sizes and shapes in the shallower marginal waters once the columns reach the water level the mats spread laterally to form a lilypad stromatolite the lilypads are characterized by a raised dark green rim 4 5 mm high that encircles a flat interior covered with a distinctive orange red mat the microbes forming the columns and lilypad plate are being actively silicified the stromatolites are formed of i flat lying phormidium filaments p laminae ii upright filaments of phormidium that are commonly associated with fischerella u laminae and iii mucus diatoms and pyrite framboids m laminae p laminae dominate most of the columns with tripartite cycles of p u to m laminae being found mostly in the upper parts of the stromatolites the transition from the p to u laminae is marked by a change in the growth pattern of the phormidium and branching of fischerella which was probably triggered by a change in environmental conditions in the frying pan lake outflow channel this change may be related to fluctuations in water level and flow rates that are caused by periods of heavy rain seasonal changes long term variations in rainfall and or the unique 40 day hydrological cycle that exists between frying pan lake and inferno crater which is a nearby hydrothermal crater lake

formation of coarsely crystalline hydrothermal dolomite reservoirs in the presqu ile barrier western canada sedimentary basin late stage coarsely crystalline replacement dolomite and associated saddle dolomite cement form a widespread diagenetic facies in the middle devonian presqu ile barrier that extends southwestward from pine point to the subsurface of the foothills in northeastern british columbia these dolomites create hydrocarbon reservoirs in otherwise tight limestones in the subsurface of the northwest territories and northeastern british columbia and host mississippi valley type deposits at pine point the coarsely crystalline replacement dolomite and saddle dolomite cement are interpreted to have formed during burial because they replace blocky sparry calcite cements occur continuously across the sub watt mountain unconformity postdate stylolites and earlier replacement dolomites and overlap sulfide mineralization the dgr18o values of both replacement and saddle dolomites increase eastward updip along the presqu ile barrier from 16 pmil pdb in the deeper subsurface of northeastern british columbia to 7 pmil pdb at pine point whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92 c the 87sr 86sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the pres u ile barrier from about 0 7106 in the subsurface of northeastern british columbia to 0 7081 at pine point these geochemical trends suggest a possible basin scale migration of hot radiogenic dolomitizing fluids updip eastward along the presqu ile barrier such large scale fluid movements probably were related to western canada sedimentary basin tectonic compression and sedimentary loading which occurred at least twice during early burial between the late devonian and early carboniferous and during deep burial between the late jurassic and early tertiary

microfossils from the neoarchean campbell group griqualand west sequence of the transvaal supergroup and their paleoenvironmental and evolutionary implications the oldest filament and colonial coccoid containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the neoarchean approximately 2600 ma old campbell group ghaap plateau dolomite lime acres member obtained at lime acres northern cape province south africa the assemblage is biologically diverse including entophysalidacean eoentophysalis sp probable chroococcacean unnamed colonial coccoids and oscillatoriacean cyanobacteria eomycetopsis cf filiformis and siphonophycus transvaalensis as well as filamentous fossil bacteria archaeotrichion sp filamentous possible microfossils unnamed hematitic filaments also occur the campbell group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the transvaal intracratonic sea although only moderately to poorly preserved they provide new evidence regarding the paleoenvironmental setting of the campbell group sediments extend the known time range of entophysalidacean cyanobacteria by more than 400 million years substantiate the antiquity and role in stromatolite formation of archean oscillatoriacean cyanobacteria and document the exceedingly slow hypobradytelic evolutionary rate characteristic of this early evolving prokaryotic lineage

remagnetizations and thrusting in the idaho wyoming overthrust belt a study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the idaho wyoming overthrust belt samples were taken from the jurassic twin creek unit of the most easterly thrust the prospect thrust for paleomagnetic study detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole latitude longitude 83 n 286 e located on the tertiary segment of the north american apparent polar wander path apwp the age of the paleopole roughly coincides with the age of thrusting on the prospect thrust as determined by synorogenic deposits the magnetization was found to reside in magnetite spherules previous data from the absaroka and darby thrust sheets that lie to the west of the prospect thrust were reevaluated and they also indicate a synfolding age for the remagnetization the corresponding paleopole 64 n 162 e lies on the late mesozoic segment of the north american apwp for these more interior thrust sheets the apparent age of the remagnetization is older than that for the exterior prospect thrust sheet but again roughly corresponds with the inferred age of thrusting for the sampled thrusts we conclude that remagnetization in the idaho wyoming overthrust belt is temporally related to deformation and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

taphonomy of very ancient microfossils from the 3400ma strelley pool formation and 1900ma gunflint formation new insights using a focused ion beam abstract focused ion beam fib milling permits the accurate extraction of ultrathin c 100 nm cross sectional lamellae from microfossils found in geological thin sections subsequent tem analysis of these lamellae can provide unique insights into the ultrastructure chemistry and taphonomy of precambrian microfossils at the micrometer to nanometer scale combining serial fib milling with sem imaging extends this capability to three dimensional 3d tomographic reconstruction and visualization of precambrian microfossils revealing information not available in light microscopy here we apply these techniques to two iconic silicified microfossil assemblages from the 3400 ma strelley pool formation of western australia and the 1900 ma gunflint formation of canada all the examined microfossils have carbonaceous walls surrounded by pure silica impregnation of microfossil walls by nano grains of silica is common together with variable degrees of wall displacement and replacement by silica all microfossils are rigidly preserved in 3d and show little or no folding or compression however there are also notable differences in taphonomic preservation our examples of the spheroidal gunflint microfossil huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano silica grains that resemble those found on bacterial surfaces in modern silicifying hot spring environments the nucleation of these silica nano spheres on the microfossil walls has induced an artificial saw tooth like ridged wall texture that may subsequently hinder species level identification the strelley pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica plus extensive recrystallization of spheroidal silica nano grains to angular micro quartz our examples of the sheath like filamentous gunflint microfossil siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains a model is presented to explain these observations criteria for distinguishing highly probable microfossils from non cellular carbonaceous microstructures e g botryoids and grain coatings using fib based imaging are put forward for the first time here using examples drawn from the strelley pool formation and comparisons with younger gunflint material the combined in situ techniques of fib tem and fib sem nano tomography potentially provide a wealth of new nano scale information regarding the biogenicity antiquity and taphonomy of precambrian microfossils however the destructive nature of both techniques makes their application to unique palaeontological specimens problematical

carbonaceous biosignatures of diverse chemotrophic microbial communities from chert nodules of the ediacaran doushantuo formation the ediacaran doushantuo formation dst is renowned for exceptionally preserved precambrian fossils including metazoans some of these fossils particularly microfossils such as multicellular algae and acanthomorphic acritarchs are preserved in dst chert nodules to better understand the geomicrobiological processes that contributed to the authigenic formation of dst chert nodules and facilitated exceptional fossil preservation we analyzed organic matter in these chert nodules and the surrounding matrix calcareous mudstone using multiple in situ techniques confocal laser raman spectroscopy micro fourier transform infrared spectroscopy ftir and secondary ion mass spectroscopy sims we found strong ultrastructural chemical and isotopic heterogeneities in the organic matter as indicated by the raman spectral parameter i 1350 1600 ranging from 0 49 to 0 88 the infrared spectral index r3 2 from 0 12 to 0 90 and an estimated 13corg sims range of 44 v pdb these micron scale heterogeneities imply that the organic matter preserved in the dst chert nodules is derived from different carbonaceous sources in a diverse microbial ecosystem including eukaryotic and or prokaryotic photoautotrophs as well as chemotrophs involved in the fermentation and probably anaerobic oxidation of organic remains thus the microbial ecosystems in ediacaran ocean waters and sediments were more complex than previously thought and these microbial processes controlled dynamic micro environments in dst sediments where chert nodules were formed and fossils were mineralized the results also show that variations in the relative abundances activities and interactions of co existing microorganisms in dst sediments may have modulated 13corg shifts causing local decoupling between 13corg and 13ccarb as measured in bulk samples

possible origins of permian remagnetizations in devonian and carboniferous limestones from the moroccan anti atlas tafilalet and meseta abstract permian overprints have been recognized for several years in different regions of the hercynian chain this process seems to affect not only large internal parts but also external zones of the chain devonian limestones sampled in the tafilalet and maider massifs near erfoud and msissi reveal a strong permian component carried mainly by magnetite scanning electron microscope sem and x ray investigations were performed in order to unravel the origin of this magnetite permian remagnetizations are also present in visean limestones from the meseta isothermal remanent magnetization studies and sem analysis of magnetic separates show that this magnetization is carried mainly by pyrrhotite which could be a result of oxidation of pre existing pyrite

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

carbon isotopic composition of individual precambrian microfossils ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 ma bitter springs formation australia and the approximately 2100 ma gunflint formation canada the delta 13c pdb values from individual microfossils of the bitter springs formation ranged from 21 3 1 7 to 31 9 1 2 and the delta 13c pdb values from microfossils of the gunflint formation ranged from 32 4 0 7 to 45 4 1 2 with the exception of two highly 13c depleted gunflint microfossils the results generally yield values consistent with carbon fixation via either the calvin cycle or the acetyl coa pathway however the isotopic results are not consistent with the degree of fractionation expected from either the 3 hydroxypropionate cycle or the reductive tricarboxylic acid cycle suggesting that the microfossils studied did not use either of these pathways for carbon fixation the morphologies of the microfossils suggest an affinity to the cyanobacteria and our carbon isotopic data are consistent with this assignment

paleomagnetism of paleozoic strata brabant and ardennes massifs belgium implications of prefolding and postfolding late carboniferous secondary magnetizations for european apparent polar wander paleozoic carbonate rocks of the brabant massif namur basin and dinant synclinorium belgium are characterized by two secondary late carboniferous magnetizations of uniform reverse polarity a postfolding low unblocking temperature 300 375 c component lt and a prefolding high unblocking temperature 400 550 c component ht in the namur basin lt magnetizations postdate late westphalian deformation 305 ma in the dinant synclinorium ht magnetizations predate early westphalian deformation 316 ma lt magnetizations for the combined brabant massif and namur basin sites give a south paleopole at 42 3 s 339 6 e dp 1 9 dm 3 8 consistent with the european cratonic reference the lt magnetizations in the dinant synclinorium give a paleopole at 36 2 s 321 0 e dp 2 4 dm 4 7 the ht magnetization gives paleopoles at 34 1 s 333 4 e dp 2 6 dm 5 1 brabant and namur and at 25 9 s 321 5 e dp 3 5 dm 6 8 dinant synclinorium declinations indicate 15 clockwise rotation of the dinant synclinorium with respect to the brabant massif after acquisition of the lt magnetization devonian red beds yield near univectorial apparent synfolding magnetizations of distributed unblocking temperature that we interpret as artifacts of the remanence acquisition or deformation processes the ht paleopole for the brabant massif is indistinguishable from other early late carboniferous european paleopoles and is similar to poles interpreted to be derived from remagnetizations in the british isles age constraints on the time of remanence acquisition of secondary magetizations allow a refined calibration of the european apparent polar wander path inclinations of the lt and ht components indicate about 6 8 700 km northward motion during the late carboniferous

constraining clastic input controls on magnetic susceptibility and trace element anomalies during the late devonian punctata event in the western canada sedimentary basin terra nova 24 301 309 2012 abstract factor analyses were applied to trace element and magnetic susceptibility ms records of the late devonian palaeoceanographic punctata event perturbation in the western canada sedimentary basin to constrain the extent to which geochemical anomalies were driven by 1 changes in clastic input factor 1 55 1 of total variance vs 2 anoxia in the depositional environment factor 2 35 2 of total variance ms associates only with factor 1 r 0 971 and is significantly correlated 99 9 level with acid insoluble limestone residues and clastic proxies al si k ti zr r 0 542 0 639 based on xrd analyses of a small subset of these residues n 7 a multivariate linear regression model accounts for 97 7 of total ms variance as a function of variable admixing of illite pyrite quartz and feldspar in turn variably diluted by stratigraphic changes in the total carbonate content

excellent source rocks discovered in the cryogenian interglacial deposits in south china geology geochemistry and hydrocarbon potential abstract the cryogenian 720 635 ma is characterized by the well known sturtian and marinoan glaciations which plays a key role in understanding remarkable climate and sea level changes and oxygenation events that occurred during the neoproterozoic era however less attention has been paid to the interglacial deposits recently we paid great attentions to the excellent hydrocarbon source rocks toc 2 in the cryogenian interglacial datangpo formation 663 4 ma 654 3 8 ma in south china which mainly consists of black shales with thin manganese interbeds making the interglacial interval a new focus to elucidate their characteristics and depositional environments petrological and geochemical studies were conducted on fresh rock samples collected from six sections gucheng sanlian xiaoxi gaodongyuan xiaochayuan yanglizhang in manganese mining areas at southeastern south china sedimentary reconstructions suggest a transition from restricted to open ocean environment during the deposition of the datangpo formation and this formation records variations from warm and humid to dry and cold climate black shales were deposited in its lowest member during the warm period and showed greatest thicknesses more than 20 m at depocenters microscopic observations of the black shale samples exhibited abundant benthic algae with great diversities indicating a gradual recovery of the ecosystem biomarker analyses further reveal that these algae combined with bacterial microorganisms prosperous in low salinity reducing environments were the dominant organic contributors to the source rocks which were inferred by low pristane pr to phytane ph ratio pr ph low gammacerane high tricyclic terpene and high c27 sterane contents trace element records combined with isotopic data suggest that the deposition of source rocks in datangpo formation were related to a burst in surface water productivity due to warm climate and better preservations resulted from anoxic waters within a stratified water column these findings indicate the cryogenian source rocks may provide a new potential field for future explorations of shale gas and deep oil and gas

microbial silicification in iodine pool waimangu geothermal area north island new zealand implications for recognition and identification of ancient silicified microbes silicified microbes provide evidence for some of the earliest life forms on earth they are extremely important to understanding the early development of life and the conditions that allowed its development such discussions commonly rely on comparisons with extant taxa and therefore depend upon the preservation style of the microbes and in particular the preservation of the taxonomically important features silicified microbes are deceptive they commonly appear to be well preserved even though their taxonomically critical features have been destroyed by silicification an understanding of the early taphonomic processes that influence microbial silicification can be obtained by studying extant microbes that are being silicified in modern hot spring pools iodine pool located in the waimangu geothermal area on the north island of new zealand is ideal for this purpose the spring water has a temperature of 69 100 c a ph of 8 3 9 0 and 440 457 ppm sio 2 glass slides left in the shallow marginal waters of this pool for 90 h became covered with thin layers of opaline silica discrete opal a spheres unsilicified microbes pseudofilaments partly silicified microbes and silicified microbes the rapidly silicified microbes appear well preserved with their general morphology diameter length and presence or absence of septa being readily apparent most of the silicified microbes however lack the key features that would allow accurate comparisons with extant taxa only two of the silicified microbes can be tentatively allied with the unsilicified forms despite being found side by side on the same glass slide these problems in identifying modern rapidly silicified microbes suggest that identifications of ancient silicified microbes can be problematic and must be treated with caution

nanosims insights to biogenicity and syngeneity of archaean carbonaceous structures nanosims is a relatively new technology that is being applied to ancient carbonaceous structures to gain insight into their biogenicity and syngeneity nanosims studies of well preserved organic microfossils from the neoproterozoic similar to 0 8ga bitter springs formation have established elemental distributions in undisputedly biogenic structures results demonstrate that sub micron scale maps of metabolically important elements carbon c nitrogen measured as cn ion and sulfur s can be correlated with kerogenous structures identified by optical microscopy spatial distributions of c cn and s in individual microfossils are nearly identical and variations in concentrations of these elements parallel one another in elemental maps c cn and s appear as globules aligned to form remnant walls or sheaths of fossiliferous structures the aligned character and parallel variation of c and cn are the strongest indicators of biogenicity

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

confocal laser scanning microscopy and raman imagery of ancient microscopic fossils abstract among all problems confronting the study of ancient permineralized petrified microscopic fossils two stand out the need for 1 accurate documentation of their three dimensional morphology and 2 direct analysis of their chemical composition and that of their surrounding mineral matrices to address these problems we demonstrate the use of two techniques that we have recently introduced to precambrian paleobiology confocal laser scanning microscopy and raman imagery these techniques both of which are non intrusive and non destructive can provide data by which to characterize in situ and at micron scale resolution the cellular and organismal morphology of thin section embedded organic walled fossils in addition raman imagery provides direct analyses of the molecular structural composition of the kerogenous components of such fossils and of their surrounding matrices and a means to assess quantitatively the geochemical maturity of the preserved organics use of these techniques for studies of ancient microscopic fossils can provide information in three dimensions at high spatial resolution about their morphology and cellular anatomy taphonomy and fidelity of preservation composition and mode of preservation and their biogenicity and syngenetic origin with the rocks in which they occur

interdecadal variability in a hybrid coupled ocean atmosphere sea ice model interdecadal climate variability in an idealized coupled ocean atmosphere sea ice model is studied the ocean component is a fully three dimensional primitive equation model and the atmospheric component is a two dimensional 2d energy balance model of budyko sellers north type while sea ice is represented by a 2d thermodynamic model in a wide range of parameters the model climatology resembles certain aspects of observed climate two types of interdecadal variability are found the first one is characterized by northward propagating upper ocean temperature anomalies in the northwestern part of the ocean basin and a westward propagating wavelike temperature pattern at depth the other type has larger scale temperature anomalies that propagate westward in both the upper and deep ocean along the sea ice edge both types of oscillations have been found previously in similar models that do not include sea ice therefore the oscillation mechanism does not depend on sea ice feedbacks nor is it modified very much by the inclusion of sea ice for some parameter values the interdecadal oscillations are self sustained while for others they are damped stochastic forcing experiments show that in the latter case significant interdecadal signals can still be identified in the time series of oceanic heat transport the periods of these signals however do not closely match those identified in a stability analysis of the deterministic model when linearized about its steady state the authors show that linearization around the actual climatology of the stochastically forced integrations provides a better match for some of the modes that were poorly explained when linearizing about the deterministic model s steady state the main difference between the two basic states is in the distribution of climatological convective depth which is affected strongly by intermittent atmospheric forcing

microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation mesoproterozoic gaoyuzhuang formation china abstract silicified stratiform stromatolites of the mesoproterozoic gaoyuzhuang formation 1 4 1 5 ga china contain well preserved microfossils the cherts also harbor varied synsedimentary precipitates and void filling cements replaced by early diagenetic silica minerals these precipitates disclose microenvironments characterized by supersaturated solutions in protected shallow depressions within an intertidal setting the precipitates provided surfaces for selective microbial settlement and rigid sediment matrix for microbial growth together with early silicification the frequent precipitation events contributed to preservation of microfossils by reducing sediment compaction and shearing fossiliferous cherts display fine flat and wavy lamination characterized by an alternation of highly silicified thin organic rich layers with thick sediment rich layers organic rich layers are dominated either by coccoid or by filamentous microfossils whereas sediment rich layers contain abundant synsedimentary precipitates within which the microfossils are preserved in their growth position four dominant microfossils coccostratus dispergens n gen et sp eoentophysalis belcherensis eoschizothrix composita and siphonophycus inornatum occur contiguously through several tens of laminae and are identified as main frame building biological components of gaoyuzhuang stromatolites community composition microbial density distribution orientation and developmental patterns of the frame building microfossils are closely correlated with the changing depositional events of gaoyuzhuang cherts contrasting conditions of sedimentary kinetics with those of sedimentary stasis each assemblage of frame building microfossils responded to sedimentation with different mechanisms to escape burial high sedimentation rates correlate with scattered colonies of coccoids and with loose webs of predominantly upright filaments low sedimentation rates correlate with dense laterally connected colonies of coccoids and with a change in filament orientation from vertical to horizontal in multi trichomous microfossil eoschizothrix composita low sedimentation rates are also accompanied with an increase in number of trichomes per filament the observed morphological variability of the frame building microfossils is explained by microbial development reproduction and behavior by interactions between sedimentological and biological controls

new study of microbial mats from the mesoproterozoic jixian group north china evidence for photosynthesis and oxygen release abstract microbial mats were well established and widespread during the proterozoic eon playing a pivotal role in atmospheric oxygen content in this study microbial mats were systematically investigated from the 1600 ma to 1400 ma jixian group north china three types of microbial mats were recognized including microbial mat dominated by filaments mmf microbial mat dominated by coccoids mmc microbial mat dominated by filaments and coccoids mmfc these microbial mats were mainly built by siphonophycus kestron siphonophycus solidum myxococcoides sp and coniunctiophycus majorinum due to in situ preservation five sedimentary structures related to photosynthesis and oxygen release were discovered in these microbial mats the alternating light dark laminae and bottleneck structure may be caused by immediate mineralization and phototropism of filaments in a hypersaline environment bubble structure fenestrae structure and hourglass structure probably were related to oxygen release and their emergence fill the gap between oxygenic photosynthesis of microbial mat and atmospheric oxygen content in proterozoic eon

remagnetization of the folded belden formation northwest colorado paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in belden formation pennsylvanian limestones on an asymmetric fold in colorado the natural remanent magnetizations contain three components a modern viscous magnetization removed at low temperatures a reversed tertiary magnetization declination d 186 7 inclination i 54 6 of thermoviscous origin removed at intermediate temperatures and an apparently synfolding late cretaceous early tertiary characteristic magnetization d 338 7 e i 62 5 precision parameter k 54 9 radius of 95 probability cone 95 5 9 65 unfolding removed by 580 c hysteresis properties suggest that the characteristic magnetization resides in single domain magnetite low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization the 87sr 86sr values 0 7088 0 7100 show the limestones to be radiogenic with 87sr enriched halos around calcite cemented tectonic fractures there is no control on the distribution and intensity of the magnetization by the veins or structural position e g highly mineralized versus less mineralized rock lithology rather than fluid alteration controls the magnetization fecal pellets contain pyrite with alteration rims identified as magnetite by scanning transmission electron microscope x ray diffraction patterns selective demagnetization has established that the fecal pellets carry the characteristic magnetization it is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding a synfolding magnetization caused by the syndeformational mineralizing fluids however can be ruled out structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result the replacement of pyrite by magnetite is interpreted as prefolding in origin perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons

the age and timing of folding in the central appalachians from paleomagnetic results comparison of the paleomagnetic pole positions of late paleozoic secondary magnetizations with a time averaged reference apparent polar wander path for north america shows that sedimentary rocks within the central appalachians were remagnetized in the later half of the permian between ca 255 and 275 ma ages of remagnetization do not vary for sites distributed along or across strike of the central appalachian fold and thrust belt or across the appalachian plateau in new york nor do they vary with stratigraphic position of the host rocks however the pattern of folding relative to remagnetization across the valley and ridge and great valley in pennsylvania maryland and west virginia shows a distinct temporal variation individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt synfolding magnetizations in the central part of the belt and prefolding magnetizations near the foreland because of the similar age of the late paleozoic magnetization we conclude that rocks across the central appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland we interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization folds in the central region grew during remagnetization and folds near the foreland grew after remagnetization rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized synfolding folds in such folds rocks were apparently remagnetized during an increment of fold growth in which there was partial and possibly negligible fold limb rotation collectively these results may constrain the mechanism for basinal fluid mobilization during deformation current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism if realistic our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets as has been suggested for other fold and thrust belts

in situ silicification of an icelandic hot spring microbial mat implications for microfossil formation transmission electron microscopy and energy dispersive x ray analysis revealed that filamentous phototrophic bacteria resembling chlorojlexus aurantiacus underwent rapid silicification in an icelandic hot spring microbial mat the mineralization associated with the cells occurred both extracellularly within and on the external sheaths of the bacteria and intracellularly within the cytoplasm the exceptional preservation of the bacterial sheaths is due to the presence of distinct mineral nucleation sites this results in the production of silica casts of the bacteria which bear a striking resemblance to microbial remains in ancient microfossil assemblages rcsumc les analyses par microscopie clectronique i transmission et par 1 6nergie dispersive des rayons x ont rcvcl6 que les bactkries filamenteuses phototrophes ressemblant b chlorojlexus aurantiacus ont subi une silicification rapide au sein d un tapis microbien de source therrnale en islande la mineralisation associee aux cellules se presente sous forme extracellulaire b l interieur de et sur la membrane externe des bacteries et sous forrne intracellulaire i l interieur du cytoplasme la preservation exceptionnelle de la membrane des bacteries est due b la presence de sites de nucleation minerale disperses le resultat est la production des moulages siliceux des bactkries affichant une resemblance remarquable avec les vestiges microbiens d anciens assemblages microfossiles traduit par la raaction fossil evidence of the earth s earliest biosphere shows that complex communities of microorganisms were abundant and well distributed throughout the precambrian era among the richest beds of microbial fossils are those found in the gunflint iron formation north of lake superior in canada barghoorn and tyler 1965 the early archaean apex chert of australia schopf and packer 1987 schopf 1993 and the riphean section of the anabar uplift in russia sergeev 1993 all ranging in age from 0 8 to 3 5 ga the distribution and types of microorganisms preserved as cellularly intact microfossils are remarkably similar to those in modern microbial mats in which organic rich layers formed by cyanobacteria and or other types of filamentous bacteria alternate with layers that are comprised of detrital inorganic sediment and authigenic mineral precipitates morse and mackenzie 1990 these benthic microbial communities are considered to be analogous and possibly homologous with those that were preserved billions of years ago as stromato

evidence of early cretaceous remagnetization in the crimean peninsula a palaeomagnetic study from mesozoic rocks in the crimean and western pontides conjugate margins of the western black sea

mineral templated growth of natural graphite films abstract organic material in sediments is progressively altered during diagenesis and metamorphism leading to the formation of kerogen and ultimately crystalline graphite bulk carbonaceous material in metamorphic terrains typically has attained an overall degree of structural order that is in line with peak metamorphic temperature on a micron to nano scale however carbonaceous material can display strong structural variation the main factor that drives this variation is the chemical and molecular heterogeneity of the precursor biologic material specific conditions during metamorphism however can also play a role in shaping the microstructure of carbonaceous material here we describe the structural variation of carbonaceous material in rocks of the 2 0 ga zaonega formation karelia russia raman spectroscopy indicates that bulk carbonaceous matter in these rocks has experienced peak temperatures between 350 and 400 c consistent with greenschist facies metamorphism on a nano scale however a strong structural heterogeneity is observed transmission electron microscopy tem reveals the occurrence of thin films of highly ordered graphitic carbon at mineral surfaces these graphite films consisting of 20 100 individual layers completely envelop quartz crystals and occur on specific crystal surfaces of chlorite it is proposed that minerals can act as templates for the parallel ordering of carbon crystallites causing enhanced graphitization within narrow zones at mineral surfaces alternatively oriented organic precursor molecules could have been adsorbed onto charged mineral surfaces leading to thin graphitic films during later metamorphic heating episodes overall the presented observations demonstrate that mineral surfaces can initiate and accelerate localized graphitization of sedimentary organic material during metamorphism and therefore cause distinct nano scale variation in structural order

paleomagnetism and age of three canadian rocky mountain diatremes paleomagnetic results and age estimates derived from them arc presented for three diatremes using as a basis of comparison the combined apparent polar wander apw path for north america and europe of van der voo the cross diatreme of the front ranges of the canadian rocky mountains has yielded a radiometric age of 241 ma earliest triassic and is hosted by the flat lying pennsylvanian tunnel mountain formation it has normal polarity magnetization and yields a paleopole correctly placed according to its radiometric age on the apw path the blackpool diatreme for which no radiometric age is available which is located in the main ranges of the rocky mountains is known to be post late ordovician because it is hosted by rocks of that age it also has magnetization of normal polarity and yields a paleopole that when calculated with respect to present horizontal is coincident with the latest cretaceous to paleocene paleopole for north america the paleopole when calculated with respect to bedding

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

orogenic remagnetizations in the front ranges and inner foothills of the southern canadian cordillera chemical harbinger and thermal handmaiden of cordilleran deformation abstract paleomagnetic studies of carbonate rocks in mountain belts have been helpful to the understanding of orogenic mational origin with only one exception the a component has normal polarity in the front ranges and reverse processes the timing of deformation and the nature of fluid flow in actively deforming foreland basins most studies on shallow water carbonates indicate that their remanence is not primary but rather they record remagnetization events contemporaneous with orogeny by considering the remanent directions fold test results and rock magnetic properties and tying these results to complementary structural geochronological and geochemical studies we are able to interpret deformation history hydrological environment and orogenic diagenesis we study the relationship between remagnetization and deformation of paleozoic carbonates of the front ranges and inner foothills of the southern canadian rockies the magnetic properties are remarkably constant along 500 km strike length as sampled at 124 sites through 4 transects primary paleozoic remanent directions which would have shallow inclination are never observed rather the paleomagnetic signal is dominated by geographically persistent remagnetizations characterized by steep inclination as well as a soft present field overprint we observed two distinctive secondary magnetizations named the a and b components carried by fine grained magnetite pervasive diagenesis induced the a component a total chemical remanent remagnetization poles for the a component are better concentrated after bedding correction indicating a pre or early syndefor polarity in the inner foothills pole positions polarity and geological and thermal constraints indicate that the a component was acquired diachronously in advance of the eastward migrating cordilleran tectonic wedge subsequently an intermediate temperature partial thermo remanent remagnetization the b component was superimposed on large regions of the front ranges and inner foothills b component directions are brought into optimal concentration by differential untilting of 0 to 50 indicating that the component was acquired after the rocks were incorporated into the orogenic wedge but before the end of contractional deformation the b component is strongest within a couple of kilometers of the frontal thrust of the front ranges the relative magnitude of the b to a components and the maximum unblocking temperature of the b component decrease away from the frontal thrust over about 30 km both to the west and to the east the b component thermal overprint was attained by 250 c heating in response to tectonic or possibly sedimentary loading it was preserved by a rapid cooling accompanying a differential uplift and erosion of up to 8 km in the vicinity of the frontal thrust late in or post dating its local tilting history the likely cause was uplift of the exposed structural panel by contraction of younger underlying thrust structures

geochemistry of syntectonic crustal fluid regimes along the lithoprobe southern canadian cordillera transect in conjunction with the lithoprobe southern canadian cordillera program an extensive examination of geochemical indicators of origins movement chemical evolution and economic significance of paleocrustal fluids was conducted the study area covers approximately 360 000 km2from the canadian rockies to vancouver island research incorporated petrological mineralogical fluid inclusion 18o d 13c and rb sr studies of samples of quartz carbonate veins and other rock types the results of the study document a variety of pre syn and postorogenic crustal fluid events in the rockies a major pre laramide hydrothermal event was identified which was comprised of a west to east migration of warm saline brines this was followed by a major circulation of meteoric water in the rockies during laramide uplift in the southern omineca extensional zone convecting surface fluids penetrated to the brittle ductile transition at 350 450 c and locally into the underlying more ductile rocks a principal c

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

sims and nanosims analyses of well preserved microfossils imply oxygen producing photosynthesis in the mesoproterozoic anoxic ocean abstract well preserved microfossils in the stromatolitic cherts from the gaoyuzhuang formation 1500 ma which have one of the best state of preservation in the proterozoic era may play key roles in systematizing information about the evolution of early life in the proterozoic ocean here a combination of light microscopy lm scanning electron microscopy sem nano scale secondary ion mass spectrometry nanosims secondary ion mass spectrometry sims and geochemical techniques was employed to characterize the morphology elemental distributions and carbon isotope values of individual gaoyuzhuang microfossils and their growing environment light microscopy analyses show that abundant filamentous and coccoid microfossils classified as oscillatoriacean and chroococcacean cyanobacteria respectively are exceptionally well preserved in chert nanosims analyses show that metabolically important elements such as 12 c 13 c 12 c 14 n 32 s and 34 s are concentrated in these microfossils and that the variations in the concentrations of these elements are similar to each other establishing the elemental patterns in undoubtedly biogenic microstructures the carbon isotope 13 c values of individual microfossils range from 31 7 0 9 to 23 1 1 0 weighted mean 28 5 0 1 consistent not only with a calvin cycle based photosynthesis but typically for chroococcacean cyanobacteria as well our results for the first time provide the element distributions and cell specific carbon isotope values on convincing mesoproterozoic cyanobacterial fossils supporting sustained oxygen producing photosynthesis in the proterozoic ocean the geochemical data indicate these fossil microorganisms may grow in an anoxic seawater potentially supporting the delayed oxygenation on the earth in this regard this integrated approach may be a step towards a more comprehensive picture of the evolution of early life on the earth

syntectonic remagnetization in the southern methow block resolving large displacements in the southern canadian cordillera 1 the upper cretaceous ventura member of the goat wall unit in the southern methow block of southern british columbia and northern washington state holds a syntectonic magnetization eight new sites from manning park in british columbia give a mean direction of d 27 5 i 60 1 k 304 7 95 3 2 after optimal partial tilt correction of five groups of bedded sites from farther south in the basin reported by bazard et al 1990 four have a syntectonic remanence with a direction similar to what we observe the exception is one group which has optimal concentration of remanence directions on 100 untilting and an abherent direction which must be rejected combining the accepted sites the optimal differential syntilting direction is d 11 8 i 61 5 k 39 3 95 3 4 n 47 giving a mean pole of 79 8 n 359 2 e k 19 5 and a95 4 8 the age of the remagnetization is constrained to be between 88 and 80 ma compared to cratonic north america this result indicates that the southern methow block was displaced from the south by 1800 500 km meaning it lay south of the sierra nevada subduction zone but well north of other paleomagnetically constrained cretaceous rock units from the insular superterrane including correlative strata of the mount tatlow area in the northern methow block among several possibilities to reconcile this discrepancy the most plausible has the whole methow block translated coherently but with the southern methow block strata remagnetized during transit

sulfur oxidizing bacteria prior to the great oxidation event from the 2 52 ga gamohaan formation of south africa the first 2 b y of earth s history was an important time for life when microbes evolved and diversified into essentially all of the metabolic forms that now exist because of feedbacks between biology and the surface environment understanding earth s biological history can help us understand the evolution of earth itself the morphological and geochemical evidence for this ancient biological history is sparse but is increasing here we report evidence for 2 52 ga exceptionally large organic smooth walled coccoidal microfossils preserved in a deep water black chert in the gamohaan formation of the kaapvaal craton of south africa these fossils occur mainly as compressed solitary coccoids that range in size from 20 to 265 m but occasionally occur in short chains of cells morphologically these fossils are similar to proterozoic and phanerozoic acritarchs and to certain archean fossils interpreted as possible cyanobacteria however their exceptionally large size simple cell wall microstructure and paleoecological setting as well as multiple sulfur isotope systematics of pyrite within the unit suggest that the gamohaan formation fossils were sulfur oxidizing bacteria similar to those of the modern genus thiomargarita organisms that live in anoxic and sulfidic deep water settings these are the oldest reported fossil sulfur bacteria and reveal a diversity of life and ecosystems previously only interpreted from geochemical proxies just prior to the great oxidation event a time of major atmospheric evolution

cellular taphonomy of well preserved gaoyuzhuang microfossils a window into the preservation of ancient cyanobacteria abstract the 1500 ma gaoyuzhuang microfossils a representative mesoproterozoic cyanobacteria assemblage are crucial for understanding and searching for early precambrian life on earth the cellular taphonomy of fossils in this assemblage is poorly known however here we combined in situ microscopic and microanalytical techniques to study the detailed taphonomy of these microfossils light microscopy lm scanning electron microscopy sem and transmission electron microscopy tem observations show that gaoyuzhuang microfossils are mainly preserved in black chert layers and that silica particles can be found within fossilized cell walls and sheaths raman spectra show the characteristic first order bands at 1350 and 1605 cm 1 of carbonaceous material comprising microfossils indicating that they have experienced peak temperatures of 215 to 308 c raman maps show the spatial distribution of the carbonaceous matter as well as that of the silica matrix consistent with nano scale secondary ion mass spectrometry nanosims analyses variations in 13c versus 12c and 34s versus 32s within individual microfossils are likely the result of original differences in the isotopic compositions of cellular components confocal laser scanning microscopy clsm analyses provide fluorescence based 2 d and 3 d images of the cellular components of such microfossils at high spatial resolution the micro scale chemical isotopic and structural heterogeneities together with the subcellular morphological features support the preferential preservation of cyanobacterial cell walls and sheaths over cell contents in precambrian chert we proposed a potential taphonomic model perhaps responsible for the high fidelity of preservation for such microfossils permineralized in precambrian rocks the integrated approach has potential for deciphering the enigmatic nature of mesoproterozoic microorganisms preserved in stromatolitic cherts with significant implications for microbial preservation in the geological record

a new devonian palaeomagnetic pole for the russian platform and baltica and related apparent polar wander summary of published palaeomagnetic data from the russian platform and britain for mid palaeozoic times places an ocean between the two of the order of 2000 km latitudinal width this contradicts geological evidence which favours contiguity between the two continental units by devonian times we propose that this discrepancy can be attributed to the incorrect dating of palaeomagnetic remanence we strove to remedy this situation by conducting a new palaeomagnetic investigation of silurian and devonian sediments in west ukraine ussr latitude 48 7 n longitude 26 e that investigation uncovered a pervasive upper palaeozoic remagnetization corresponding to a palaeomagnetic pole at lat 45 8 s long 348 2 e dpldm 1 6 3 1 n 25 sites reversed polarity a second primary or near primary devonian remanence was identified with pole at lat 3 7 n long 325 5 e dpldm 5 2 8 7 n 25 samples mixed polarity the age determination for this devonian result is based on its dual polarity evidence for a detrital remanent mode and early host mineral phase haematite the new devonian result for the russian platform is conformable with the devonian parts of the british apparent polar wander paths and therefore eliminates the aforementioned discrepancy between palaeomagnetic and geological evidence for the timing of assembly of the old red continent

eocene rotation of the coast plutonic complex and intermontane belt paleomagnetism of eocene plutons along the klondike highway paleomagnetic results are reported for the 59 ma skagway 54 ma fraser 53 ma summit lake and 48 ma clifton felsic plutons of the eastern coast plutonic complex cpc that outcrop along the south klondike highway in alaska and british columbia thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization chrm directions for all 29 sites of normal reversed and mixed polarity the chrm resides in single or pseudosingle domain magnetite and or pyrrhotite that is shown to be primary by contact tests with the 47 ma vertical white pass mafic dikes paleopoles from six 56 to 50 ma mean 52 2 ma intermontane belt yukon tanana terrane imb ytt units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 ma mean 52 4 ma eastern cpc paleopoles both paleopole populations show nonsignificant poleward northward translation relative to north america imb ytt 3 7 5 3 n cpc 4 3 6 4 s overall 1 2 4 9 s ind

record of deformation by secondary magnetic remanences and magnetic anisotropy in the nar phu valley central himalaya secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility ams were studied in low grade metamorphic carbonates of the tethyan himalaya in nar phu valley central nepal and used for interpretation of tectonic deformations the characteristic remanence chrm is likely of thermomagnetic origin related to post peak metamorphic cooling occurring after the eohimalayan phase 35 32 ma the chrm postdates small scale folding main himalayan folding f1 and f2 as shown by a negative fold test of site mean directions at 99 confidence level and has been probably acquired between 32 and 25 ma late orogenic long wavelength folding associated with the chako antiform ca is recorded by the spatial dispersion of chrm directions and the distribution of the main axes of the ams tensor the mean tilting of the chrm direction since remanence acquisition c20 30j approximately coincides with the tilting of the ca 31j at the study area indicating that the pyrrhotite remanence predates the ca ca formed at 18 ma according to preliminary u pb dating however comparison of tilt angles of remanence directions and ams tensor axes suggests that remanence acquisition was not completed before the onset of the ca formation this could imply a younger age early miocene or even younger of the chrm using the distribution of remanence directions along a small circle as well as the distribution of ams tensor axes a clockwise mean rotation of 16j is obtained for a remanence age of c30 ma an early miocene remanence age would not change this result substantially compilation of rotations in the tethyan himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the hidden valley in the w to the shiar valley in the e c150 km distance incompatible with an oroclinal bending model d 2003 elsevier b v all rights reserved

origin of orogenic remagnetizations in mississippian carbonates sawtooth range montana 1 paleomagnetic results are presented from mississippian madison group carbonates in the sawtooth range northwestern montana samples were collected from sites along two east west trending transects perpendicular to the thrust faults in the sun river canyon and in the north and south forks of the teton river and from three folds the madison group contains a widespread characteristic remanent magnetization chrm that resides in magnetite with southerly declinations and moderately steep up inclinations tilt test results suggest that the chrm is pretilting in the thrust sheets and teton anticline but syntilting in the clary coulee and swift dam folds the chrms all have the same characteristics and were probably caused by the same remagnetization event yet the tilt test results are different one explanation involves the difference in fold types between the teton anticline fault bend fold geometry and the clary coulee and swift dam folds fault propagation fold geometries the deformation that produced the two geometries could have caused variations in strain stress which may have altered an original pretilting into a syntilting chrm a mean paleopole for the three pretilting tilt test results 67 2 n 177 9 e a95 13 1 suggests remanence acquisition in the late jurassic early tertiary the chrm is interpreted as a chemical remanent magnetization crm geochemical studies indicate that the mississippian carbonates were altered by evolved fluids with radiogenic 87sr 86sr values and petrographic analysis indicates that hydrocarbons migrated through the carbonates the crm is interpreted to be related to alteration by one of these fluids

chemical remagnetization and burial diagenesis testing the hypothesis in the pennsylvanian belden formation colorado lower pennsylvanian belden formation carbonate rocks from colorado were subjected to paleomagnetic rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization crm carried by authigenic magnetite and burial diagenesis thermal demagnetization results indicate the presence of two components of natural remanent magnetization nrm after removal of a low unblocking temperature nrm 250 c remanence that is interpreted to be a modern viscous magnetization an intermediate unblocking temperature 250 400 c remanence component with normal and reversed polarity tertiary directions is interpreted to be a thermoviscous remanent magnetization many limestones also contain a high unblocking temperature 400 570 c remanence component which is interpreted to be a crm fold tests from different parts of the basin indicate that the crm was acquired either before or during laramide folding this crm is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the crm is carried by single domain pseudo single domain magnetite although elevated 87sr 86sr values indicate passage of radiogenic fluids through the limestones the results of contact vein tests do not support the hypothesis that these fluids were responsible for the crm the time of crm acquisition which varies from late paleozoic to cretaceous coincides with the modeled time of organic matter maturation in different parts of the basin this suggests that diagenetic reactions that were triggered by low to moderate burial temperatures may have caused the magnetite authigenesis and probably gave rise to the crm

structural characterization by raman hyperspectral mapping of organic carbon in the 3 46 billion year old apex chert western australia abstract the 3 46 billion years old apex chert pilbara craton western australia is well known for hosting the oldest highly disputed microfossils on earth this rock has a complex history of thermal alteration that includes circulation of hydrothermal fluids lower greenschist facies regional metamorphism and post metamorphic weathering by meteoric fluids carbonaceous material occurs in the sedimentary stratiform part of the chert as well as the underlying intruding hydrothermal black chert veins in order to identify a least altered remnant of early life it is necessary to develop a method that enables rapid evaluation of cm structural order on a small spatial scale here we present the detailed characterization of cm in the apex chert by raman hyperspectral mapping it is shown that this approach gives better estimates of average raman band ratios than individual point analyses and it is demonstrated that significant differences in structure exist between cm in the stratiform part of the apex chert and cm in an underlying black chert vein the large raman map based datasets also reveal that significant mixing took place between these two end members cm s indicating that the apex chert has been thoroughly altered by hydrothermal fluid circulation at the brecciated intersection between the stratiform chert and the intrusive hydrothermal chert vein very poorly ordered cm was found that is not in line with lower greenschist facies regional metamorphism it is speculated here that this cm represents an organic fraction that was introduced or thoroughly altered by late stage meteoric fluids alternatively the ubiquitous presence of hematite in this sample caused a perturbation in the raman spectra of the cm leading to an artifact in the calculated raman based band ratios overall it can be concluded that the best preserved cm occurs in the stratiform parts of the apex chert while earlier discussions on organic microfossils in this rock often focused on parts of the chert that either represented the hydrothermal veins or the brecciated intersection between the veins and the stratiform part

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

a kinematic model for the rinconada fault system in central california based on structural analysis of en echelon folds and paleomagnetism deformation associated with the rinconada fault one of the major strands of the san andreas fault system in central california is accommodated by both discrete fault offset w18 km and distributed off fault deformation en echelon folds adjacent to the rinconada fault were studied in detail at two locations near williams hill and lake san antonio to characterize the magnitude and style of distributed deformation the obliquity between fold hinges and the local strike of the fault was 27 and 14 at these two sites respectively systematic outcrop scale fault displacement measurements along roadcuts indicate that the maximum horizontal elongation occurs parallel to local fold hinges and ranges from 4 to 9 we used the orientation and stretch of fold hinges to construct a transpressional kinematic model for distributed deformation this modeling suggests a 20e50 angle of oblique convergence 5 km of fault parallel wrench deformation and 2e4 km of fault perpendicular shortening between 3 and 16 of clockwise rotation is also predicted by our model this rotation is independently confirmed by a 14 7 vertical

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference