palaeomagnetism and magnetic anisotropy of late neoproterozoic strata south australia implications for the palaeolatitude of late cryogenian glaciation cap carbonate and the ediacaran system abstract the ediacaran global stratotype section and point gssp is defined near the base of the nuccaleena formation the cap carbonate overlying the terminal cryogenian glaciogenic elatina formation at a basin wide sequence boundary in the adelaide geosyncline south australia previous palaeomagnetic data for 205 specimens from the elatina formation yielded a structurally corrected mean declination d 208 3 and inclination i 12 9 95 4 2 palaeolatitude 6 5 2 2 and pole position at latitude p 47 3 s longitude p 359 3 e dp 2 1 and dm 4 2 new palaeomagnetic data for 141 specimens from the nuccaleena formation yield a structurally corrected mean direction of d 208 3 i 34 9 95 3 4 19 2 2 3 2 2 and pole position at p 32 3 s p 350 8 e dp 2 2 and dm 3 9 there is a substantial difference in inclination between the elatina and nuccaleena formations despite their juxtaposition positive fold tests for both formations seem to rule out remagnetisation of one or both formations fold tests on tepee like structures in the nuccaleena formation accord with the magnetisation having been acquired at or near the time the structures formed during early diagenesis magnetisation therefore variably lags deposition causing the three identified magnetostratigraphic horizons to appear highly bedding transgressive whereas local correlations may seem satisfactory regional correlations are not nevertheless the number and duration of polarity chrons at various localities appear valid implying that the nuccaleena formation accumulated over an interval of 10 5 years the question of compaction related inclination shallowing was addressed by examining the anisotropy of remanence using a superconducting 12 t magnet the effective flattening factors f determined were nuccaleena dolostones n 4 f 0 98 0 02 elatina tidal rhythmites n 6 f 0 97 0 01 elatina arenites n 9 f 0 92 0 02 the mean elatina inclination is corrected to 14 0 palaeolatitude of 7 1 and the mean nuccaleena inclination to 35 4 palaeolatitude of 19 6 inclination shallowing also was investigated using the elongation inclination method the distributions of directions for the elatina were somewhat skewed with respect to bedding possibly invalidating use of this method nevertheless for the elatina the method yields an inclination of 19 palaeolatitude of 10 for the nuccaleena the corrected inclination is 41 9 palaeolatitude of 24 these corrections do not materially alter the near equatorial palaeolatitude for the elatina formation which is supported by data for other neoproterozoic strata in southern australia however the substantial increase in palaeolatitude between the late cryogenian and the early ediacaran is maintained the sequence boundary below the nuccaleena formation is a disconformity to low angle unconformity that in places cuts deeply into the 1500 m thick late cryogenian glaciogenic succession and may represent a significant time gap marked by rapid polar wander such a hiatus may be an important factor in the juxtaposition of glacial deposits and cap carbonates

ediacaran δ13c chemostratigraphy of south china abstract we report new 13 c chemostratigraphic data of the ediacaran system in south china using litho and biostratigraphic markers as independent calibrators we combine new and previously published 13 c data to construct a composite 13 c profile for the ediacaran system in south china the composite curve shows in chronostratigraphic order 1 a negative 13 c excursion en1 in the doushantuo cap carbonate that overlies the ghaub age nantuo diamictite 2 a pronounced positive 13 c excursion ep1 in the lower doushantuo formation where doushantuo type acanthomorphic acritarchs red algae and animal embryos first appear 3 a mild negative 13 c excursion en2 in the middle doushantuo formation 4 a positive 13 c excursion ep2 in the upper doushantuo formation where doushantuo type acanthomorphic acritarchs micrometazoans and stem group florideophyte red algae become abundant and diverse 5 a pronounced negative 13 c excursion en3 near the uppermost doushantuo formation 6 a strongly positive 13 c excursion up to 6 3 pdb ep3 in the lower dengying formation 7 a stable 13 c plateau ca 2 5 pdb ei in the middle and upper dengying formation where abundant vendotaenid algae macroscopic ediacaran fossils biomineralized animal tubes and mostly horizontal animal traces occur and 8 a 10 negative 13 c excursion en4 immediately below the ediacaran cambrian boundary the general pattern of this composite 13 c curve is robust despite a considerable amount of variation among the 13 c profiles of different sections this composite 13 c curve provides a first order chemostratigraphic framework for the subdivision and correlation of the ediacaran system in south china

the age of the nantuo formation and nantuo glaciation in south china a u pb sensitive high resolution ion microprobe shrimp age of 654 5 3 8 ma from an ash bed immediately below the nantuo formation in south china provides the lowest age constraint for the nantuo glaciation which has been correlated with the global marinoan glaciation a u pb shrimp age of 636 3 4 9 ma from a fallout tuff within the basal layer of the nantuo formation along with the existing age of 635 2 0 6 ma from its overlying doushantuo cap carbonate suggests that the traditionally defined nantuo formation may have been deposited in a short time period at the end of the nantuo glaciation in combination with available ages globally the data support a relatively short duration and rapid termination of marinoan glaciations

new paleomagnetic results from the ediacaran doushantuo formation in south china and their paleogeographic implications abstract a new paleomagnetic pole is obtained from the top of member 3 of the doushantuo formation in the jiulongwan section of the yangtze gorges area in south china a total of 191 samples from 23 sites were collected and subjected to stepwise alternating field af and thermal demagnetization after the removal of a soft component sc of viscous magnetic remanence acquired in present geomagnetic field a high temperature component hc likely carried by magnetite was isolated the hc includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great circle analysis both the vectors only and the combined vectors and arc constraints of the hc passed a reversal test this is the first ediacaran paleomagnetic remanence from the south china block scb that passes a reversal test and it is interpreted as a primary remanence the hc vectors were thus used to calculate the paleomagnetic pole and in combination with arc constraints were used for polarity interpretation the vectors give a mean direction of d 75 2 i 41 0 95 2 5 n 147 after bedding correction and a corresponding paleomagnetic pole at 23 9 n 187 0 e dm dp 3 0 1 8 this pole differs significantly from previously published results of the doushantuo formation which may have been subjected to paleozoic remagnetization our results provide a 23 5 1 8 n paleolatitude for the yangtze gorges area of the scb and suggest several alternative paleogeographic options for the ediacaran cambrian amalgamation of east gondwana depending on chronostratigraphic correlations

eco environmental implications of elemental and carbon isotope distributions in ornithogenic sediments from the ross sea region antarctica abstract seabirds have substantial influence on geochemical circulation of elements serving as a link for substance exchange between their foraging area and colonies in this study we investigated the elemental and carbon isotopic composition of five penguin affected sediment profiles excavated from ross island and beaufort island in the ross sea region antarctica among the three main constituents of the sediments including weathered bedrock guano and algae guano was the main source of organic matter and nutrients causing selective enrichment of several elements in each of the sediment profiles in the 22 measured elements as cd cu p s se and zn were identified as penguin bio elements in the ross sea region through statistical analysis and comparison with local end member environmental media such as weathered bedrock fresh guano and fresh algae carbon isotopic composition in the ornithogenic sediments showed a mixing feature of guano and algae using a two member isotope mixing equation we were able to reconstruct the historical change of guano input and algal bio mass compared with research in other parts of antarctic arctic and south china sea we found apparent overlap of avian bio elements including as cd cu p se and zn information on the composition and behavior of bio elements in seabird guano on a global scale and the role that bio vectors play in the geochemical circulation between land and sea will facilitate future research on avian ecology and paleoclimatic reconstruction

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

rapid early cambrian rotation of gondwana based on the history of mesozoic cenozoic plate motions as well as simple dynamical considerations a speed limit for tectonic plates has been suggested at 20 cm yr previous paleomagnetic data from the early cambrian of gondwana are confl icting but generally imply rapid motions approaching that limit herein we describe results from a continuous paleomagnetic sampling of lower to middle cambrian strata from the amadeus basin central australia we fi characteristic remanence directions that show an 60 declination shift through the section assuming a tectonically assembled gondwana supercontinent by early cambrian time this large vertical axis rotation of its australian sector corresponds to an equally large translation across paleolatitudes for its brazilian and west african sectors analysis of all high quality paleomagnetic data from gondwana both confi rms and constrains the 60 rotation to have occurred toward the end of early cambrian time at rates exceeding 16 12 8 cm yr these observations suggest that either nonuniformitarian plate tectonics or an episode of rapid true polar wander occurred during the cambrian explosion of animal life

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

paleozoic terranes of eastern australia and the drift history of gondwana abstract critical assessment of paleozoic paleomagnetic results from australia shows that paleopoles from locations on the main craton and in the various terranes of the tasman fold belt of eastern australia follow the same path since 400 ma for the lachlan and thomson superterranes but not until 250 ma or younger for the new england superterrane most of the paleopoles from the tasman fold belt are derived from the lolworth ravenswood terrane of the thomson superterrane and the molong monaro terrane of the lachlan superterrane consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic australia by the late early devonian the lolworth ravenswood terrane is interpreted to have undergone a 90 clockwise rotation between 425 and 380 ma although the tamworth terrane of the western new england superterrane is thought to have amalgamated with the lachlan superterrane by the late carboniferous geological syntheses suggest that movements between these regions may have persisted until the middle triassic this view is supported by the available paleomagnetic data with these constraints an apparent polar wander path for gondwana during the paleozoic has been constructed after review of the gondwana paleomagnetic data the drift history of gondwana with respect to laurentia and baltica during the paleozoic is shown in a series of paleogeographic maps

incompatible ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis abstract paleomagnetic results obtained from rocks of ediacaran age in several localities in laurentia and baltica persistently display co existence of two magnetization components one shallowly and the other steeply inclined both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age the conventional interpretation of these results translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near polar latitudes in a geographic reference frame such large scale and fast 45 cm yr migrations of a continent have been rejected as dynamically implausible neither plate tectonics nor true polar wander are thought to be able to attain the required velocities a highly irregular behavior of the geomagnetic field during the ediacaran possibly an alternation of the geomagnetic dipole axis between a co axial and an equatorial alignment remains the only viable explanation for the paleomagnetic data such a behavior entails specific outer core conditions which in turn impose strong constraints on the possible models for the thermal evolution of the earth s interior

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

dynamic ice wallow relief of northern alaska s nearshore abstract contour maps with 0 5 m depth interval were prepared for a small area seaward of reindeer island a barrier island in the beaufort sea alaska by repeated surveys with very accurate navigation and very close trackline spacing the maps reveal numerous closed depressions and mounds about 50 to 100 m in diameter and 2 to 3 m in relief presumably related to grounded ice floes common in the area year round some of the features were obliterated over the course of three seasons while new ones formed although the depressions resemble kettles they are formed by very different mechanisms we believe that these bedforms represent erosion and deposition caused by a intensified flow around stationary ice floes serving as obstacles and b pulsating currents generated by vertical oscillation or rocking motions of grounded floes in a seaway because sediment transport occurs around the ice not where it directly touches the sea floor the depressions are much larger than the base of the acting floes ice wallow bedforms although not found everywhere are characteristic of arctic nearshore regions with non cohesive sediments and most likely occur in other ice stressed coastal environments in differing degrees the bedforms studied here are highly active and must be considered in planning nearshore construction activities

a statistical dynamical scheme for reconstructing ocean forcing in the atlantic part i weather regimes as predictors for ocean surface variables the links between the observed variability of the surface ocean variables estimated from reanalysis and the overlying atmosphere decomposed in classes of large scale atmospheric circulation via clustering are investigated over the atlantic from 1958 to 2002 daily 500 hpa geopotential height and 1 000 hpa wind anomaly maps are classified following a weather typing approach to describe the north atlantic and tropical atlantic atmospheric dynamics respectively the algorithm yields patterns that correspond in the extratropics to the well known north atlantic europe weather regimes nae wr accounting for the barotropic dynamics and in the tropics to wind classes t wc representing the alteration of the trades 10 m wind and 2 m temperature t2 anomaly composites derived from regime wind class occurrence are indicative of strong relationships between daily large scale atmospheric circulation and ocean surface over the entire atlantic basin high temporal correlation values are obtained basin wide at low frequency between the observed fields and their reconstruction by multiple linear regressions with the frequencies of occurrence of both nae wr and t wc used as sole predictors additional multiple linear regressions also emphasize the importance of accounting for the strength of the daily anomalous atmospheric circulation estimated by the combined distances to all regimes centroids in order to reproduce the daily to interannual variability of the atlantic ocean we show that for most of the north atlantic basin the occurrence of nae wr generally sets the sign of the ocean surface anomaly for a given day and that the inter regime distances are valuable predictors for the magnitude of that anomaly finally we provide evidence that a large fraction of the low frequency trends in the atlantic observed at the surface over the last 50 years can be traced back except for t2 to changes in occurrence of tropical and extratropical weather classes all together our findings are encouraging for the prospects of basin scale ocean dynamical downscaling using a weather typing approach to reconstruct forcing fields for high resolution ocean models part ii from coarse resolution climate models

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

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

paleomagnetic results from the silurian of the yangtze paraplatform gpdyke n d huang k xu g zhang w y and kent d v 1987 paleomagnetic results from the silurian of the yangtze paraplatform in d v kent and m km editors laurasian paleoma etism and tectonics teetonophysics 139 123 132 samples have been collected from sites from three localities located in sichuan and in yunnan provinces the samples were taken from limestones and red siltstones covering most of the silurian of the south china block the results from the limestone sites were disappointing and all the sites that yielded successful results were from red siltstones characteristic high temperature components were successfully isolated from fifteen of these sites the sites from xiushan county sichuan 28 48o n 108 99 0 e successfully pass the fold test at the 99 level indicating that the magnetization was acquired before folding in the mesozoic the direction obtained from this locality is directed toward the east with shallow positive inclinations d 85 6 o i 12 1 cuss 5 33 the results obtained from a single red bed locality from qijiang county sichuan 28 58 n 106 85 e yield a similar direction the results from the upper silurian of yunnan 25 5 n 103 73o e were obtained from only two sites and the isolated directions were towards the west with shallow inclinations this direction is opposed to the characteristic directions obtained from sichuan and are interpreted as a reversed silurian direction the paleomagnetic pole position determined from all sites given equal weight n 15 falls at 4 9 n lat 194 7 e long with ass 5 61 o

pb pb ages of neoproterozoic doushantuo phosphorites in south china constraints on early metazoan evolution and glaciation events the pb pb isochron ages of 576 14 ma for the upper part of the upper phosphorite bed upb this study and 599 4 ma for the lower part of the upb earth planet sci lett 201 2002 203 in the doushantuo formation in the weng an area south china suggest that the deposition of the upb lasted at least 5 ma and indicate that the observed microfossil assemblage animal remains multicellular algae and giant process bearing acritarchs in the doushantuo formation predates all well dated diverse ediacaran assemblages and represents the earlier paleontological record of eukaryotes radiation on earth the 576 14 ma age in the upper part of the upb approximates the age of a negative delta 13 c excursion near the boundary between the doushantuo and the overlying dengying formation if the doushantuo dengying boundary at the weng an area is equivalent to that in the yangtze gorges area this negative excursion might correlate to a post marinoan glaciation in south china suggesting that the ice age following doushantuo sediment deposition is younger than 590 ma the 599 4 ma age in the lower part of the upb earth planet sci lett 201 2002 203 represents the minimum age of the nantuo diamictites which are generally correlated with the marinoan glaciation c 2004 elsevier b v all rights reserved

carbon isotope chemostratigraphy and sedimentary facies evolution of the ediacaran doushantuo formation in western hubei south china abstract the doushantuo formation of south china consisting of intercalated siliciclastic and carbonate rocks which exhibit considerable variation in thickness and sedimentary facies is one of the key sedimentary units in the world for understanding ediacaran geobiological processes however stratigraphic correlation and sedimentary facies evolution of the doushantuo formation remain controversial sedimentary facies analysis of the well known sections of the yangtze gorges area and new sections in northern yichang western hubei indicate that the doushantuo succession in the yangtze gorges area was deposited in an offshore intra shelf basin within the yangtze platform in contrast the doushantuo formation in the northern yichang area which is characterized by one or a few phosphate rich horizons represents deposition on the subtidal or intertidal shelf and so is similar to the succession in the weng an area central guizhou province the carbon isotope profiles of the different sections exhibit significant variation which is interpreted to result from facies differences sequence and high resolution carbon isotope stratigraphy have clarified problematic correlations and the temporal relationship between fossil assemblages of the doushantuo formation on the yangtze platform the results confirm that a prominent negative 13ccarb excursion baince nadir down to 5 in the middle portion of the second sequence of the doushantuo formation is a widely traceable event baince together with three other 13ccarb excursions namely cance at the base wance near the first sequence boundary near the middle and dounce at the top of the doushantuo formation constitute a complete carbon isotope profile for the doushantuo formation that reflects contemporaneous seawater and so serves as a reference chemostratigraphic profile for global stratigraphic correlation and for evaluating the chemical evolution of seawater during the ediacaran period

a new age constraint on the onset of the neoproterozoic glaciations in the yangtze platform south china abstract a sample from the top of the banxi group early neoproterozoic in hunan province south china was dated by the shrimp zircon u pb method the resulting age of 725 10 ma probably solves the debate between the synglacial and the preglacial correlation of the liantuo formation and indicates that the liantuo formation most likely correlates with the preglacial banxi group since the banxi danzhou group is upwardly transitional to the glacial jiangkou group the age becomes a new maximum age constraint on the onset of the jiangkou glaciation which therefore most likely correlates with the sturtian glaciation

rapid precise and high sensitivity acquisition of paleomagnetic and rock magnetic data development of a low noise automatic sample changing system for superconducting rock magnetometers among earth sciences paleomagnetism is particularly linked to the statistics of large sample sets as a matter of historical development and logistical necessity because the geomagnetic field varies over timescales relevant to sedimentary deposition and igneous intrusion while the fidelity of recorded magnetization is modulated by original properties of rock units and by alteration histories ideal paleomagnetic results measure remanent magnetizations of hundreds of samples at dozens of progressive demagnetization levels accompanied by tests of magnetic composition on representative sister specimens we present an inexpensive open source system for automating paleomagnetic and rock magnetic measurements using vacuum pick and place technology and a quartz glass sample holder the system can in one hour measure remanent magnetizations as weak as a few pam2 of 30 specimens in two vertical orientations with measurement errors comparable to those of the best manual systems the system reduces the number of manual manipulations required per specimen 8 fold

late neoproterozoic paleomagnetic results from the sugetbrak formation of the aksu area tarim basin nw china and their implications to paleogeographic reconstructions and the snowball earth hypothesis abstract in order to better constrain the neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball earth hypothesis paleomagnetic investigations were carried out in the aksu area of the northwestern tarim basin forty eight sites of samples were collected from the sugetbrak and chigebrak formations twenty four sites of sandstone and volcanic rock from the sugetbrak formation revealed stable characteristic remanent components chrm isolated between 500 and 680 c the computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages both normal and reverse polarities have been observed and a positive fold test is revealed after bedding corrections at 95 confidence level anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees a paleomagnetic pole is calculated at 19 1 n 149 7 e k 11 2 a95 9 3 with n 24 yielding a paleolatitude of 27 n for the sampling area the chemo stratigraphic correlation of this section with reference ones reveals an average age of 595 ma a new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the australia south china and tarim blocks at about 600 ma this observation provides new evidences for the snowball earth hypothesis

the biostratigraphic succession of acanthomorphic acritarchs of the ediacaran doushantuo formation in the yangtze gorges area south china and its biostratigraphic correlation with australia abstract abundant large acanthomorphic acritarchs from the ediacaran doushantuo formation in the yangtze gorges area south china are becoming useful tools for biostratigraphic subdivision and correlation of the ediacaran system despite limitations caused by facies control taphonomic bias and taxonomic problems based on thin section examination of chert samples collected from six sections of the doushantuo formation a clear stratigraphic distribution of acritarchs has allowed two assemblages of acanthomorphic acritarchs lower tianzhushania spinosa assemblage and upper tanarium anozos tanarium conoideum assemblage to be established these two assemblages which are found in the lower member ii and the upper member iii doushantuo formation respectively are constrained stratigraphically to the first and the second positive 13c excursions ep1 and ep2 respectively and are separated by a second negative 13c excursion en2 the lower t spinosa assemblage is dominated by the taxon t spinosa by comparison with the lower assemblage the upper t anozos t conoideum assemblage preserves more acanthomorphic acritarchs both in terms of number of individuals and in the diversity of the forms fifteen forms eotylotopalla delicata knollisphaeridium maximum echinosphaeridium maximum meghystrichosphaeridium magnificum schizofusa zangwenlongii sinosphaera rupina t anozos t conoideum tanarium irregulare variomargosphaeridium litoschum as well as the unnamed b c d e f and g forms are the predominant species in upper t anozos t conoideum assemblage among them the taxa t anozos and t conoideum occur in most great number and throughout the assemblage whereas the species t spinosa is presumed to have become extinct the upper t anozos t conoideum assemblage shares a number of species with the ediacaran complex acritarch palynoflora ecap in south and central australia including ceratosphaeridium glaberosum gyalosphaeridium pulchrum s zangwenlongii t conoideum t irregulare and v litoschum in addition the species t spinosa is not present in the ecap of australia hence the upper t anozos t conoideum assemblage of the doushantuo formation can be correlated with the ecap of south and central australia while the lower tianzhushania assemblage appears to be missing in south and central australia

further paleomagnetic results from the silurian of the yangtze block and their implications abstract silurian redbeds were sampled at two localities in northeastern guizhou and yunnan provinces of china for paleomagnetic study progressive thermal demagnetization revealed multiple magnetic components from these samples a low unblocking temperature component a of recent origin a post folding middle to high unblocking temperature component b probably acquired during late mesozoic time and a pre folding high unblocking temperature component c two statistically distinguishable groups of directions were further recognized from the c component with one c1 having more easterly declination than the other c2 and both having a of shallow inclination the paleomagnetic pole from the c1 magnetization coincides with a pre folding pole reported from the coeval rocks in neighboring southern sichuan province its relation to the apparent polar wander path apwp for the yangtze block yb suggests that the c1 magnetization was probably acquired during the silurian by the same argument the c2 magnetization was probably acquired sometime between the silurian and carboniferous these results reconfirm the equatorial position for the yb during mid paleozoic time and clockwise rotation of the yb since at least the mid cambrian