global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

calcified microbial reefs in cambrian series 2 north china platform implications for the evolution of cambrian calcified microbes abstract this study focuses on the microbial reefs of the zhushadong formation cambrian series 2 in shandong province china in order to understand the evolution of calcified microbes in the north china platform during the cambrian series 2 and 3 the microbial reefs occur in a thin unit ca 3 m thick over an area of 1 km 2 they consist of three types of thrombolite based on their mesostructures rimmed grainstone patch and dendritic the thrombolites mainly occur in various coarse grained carbonate facies including crudely stratified oolitic grainstone stromatolitic grainstone and disorganized limestone conglomerate calcified microbes in the thrombolites include epiphyton kordephyton a tubiform microbe bija tarthinia renalcis amgaina and razumovskia the zhushadong thrombolites were formed within a grainstone shoal and experienced repeated burial and exposure the rimmed thrombolite and grainstone patch thrombolite experienced abundant input of carbonate grains forming grainstone patches in contrast the dendritic thrombolite formed solely by calcification of microbes that mainly include epiphyton tarthinia and the tubiform microbe the outer crusts of the rimmed thrombolite were formed by amgaina under high energy conditions the diverse calcified microbes of the zhushadong formation form the earliest assemblage of their type in the north china platform their descendants mostly epiphyton subsequently thrived forming a ca 180 m thick microbialite oolite dominated succession during the cambrian series 3 zhangxia formation although the reefs in the zhushadong formation are much smaller than those of the overlying zhangxia formation their calcified microbes are more diverse this most likely reflects changes in depositional environments e g abundant siliciclastic input and tidal effects vs those of a stable carbonate platform and or global changes within reef environments e g end cambrian series 2 extinction of archaeocyaths and calcified microbes a decrease in diversity of calcified microbes in the north china platform where archaeocyaths were absent may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence

the impact of glacier retreat from the ross sea on local climate characterization of mineral dust in the taylor dome ice core east antarctica recent declines in ice shelf and sea ice extent experienced in polar regions highlight the importance of evaluating variations in local weather patterns in response to climate change airborne mineral particles dust transported through the atmosphere and deposited on ice sheets and glaciers in antarctica and greenland can provide a robust set of tools for resolving the evolution of climatic systems through time here we present the first high time resolution radiogenic isotope strontium and neodymium data for holocene dust in a coastal east antarctic ice core accompanied by rare earth element composition dust concentration and particle size distribution during the last deglaciation we aim to use these combined ice core data to determine dust provenance with variations indicative of shifts in either dust production sources and or transport pathways we analyzed a series of 17 samples from the taylor dome 77 47 47 s 158 43 26 e ice core 113 391m in depth from 1 1 31 4ka radiogenic isotopic and rare earth element compositions of dust during the last glacial period are in good agreement with previously measured east antarctic ice core dust records in contrast the holocene dust dataset displays a broad range in isotopic and rare earth element compositions suggesting a shift from long range transported dust to a more variable local input that may be linked to the retreat of the ross ice shelf during the last deglaciation observed changes in the dust cycle inferred from a coastal east antarctic ice core can thus be used to infer an evolving local climate

variations in the zn si record over the last interglacial glacial transition we examined the zinc content of diatom frustules as an indicator of past changes in surface seawater zn2 concentration zn si data of samples from three cores located in the south atlantic sector of the southern ocean spanning the last interglacial glacial transition are presented changes in the zn si record are linked to changes in the surface water zn2 concentration the source of variation in zn2 concentration appears to be via changes in deep water upwelling and circulation we rule out changes in phytoplankton productivity and aeolian dust input as a source of variation in the zn si record likewise the zn si data are not linked to shifts in the diatom species composition of the sediment or sediment preservation effects the zn si results presented do not support the zinc hypothesis there is no link between the uptake of co2 by phytoplankton as inferred from the 13c record and the zn si record

toward constraining the long term reversing behavior of the geodynamo a new maya superchron 1 billion years ago from the magnetostratigraphy of the kartochka formation southwestern siberia abstract we present new magnetostratigraphic data obtained from the late mesoproterozoic 1 ga kartochka formation in the east angara terrane of the yenisey ridge region southwestern siberian platform a 200 m thick section encompassing the carbonate kartochka formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites paleomagnetic analyses revealed the existence of two magnetization components a low unblocking temperature component which likely has a viscous origin was first isolated below 200 c a characteristic component carried either by magnetite or by a mixture of magnetite and hematite was then isolated in the medium to high temperature range up to 565 585 c or 680 c the hematite bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening a positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition this primary origin was further verified by comparison between the paleomagnetic poles derived for the kartochka formation and other late mesoproterozoic siberian sections all data from the kartochka formation contain a single magnetic polarity assumed to be normal this long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the siberian uchur maya region the new data fortify evidence of a normal polarity superchron spanning the 1000 ma old mesoproterozoic neoproterozoic boundary which we propose to call the maya superchron these data confirm the occurrence of sharp transitions between a frequently reversing regime and a non reversing regime of the geodynamo this behavior also observed in other regions may represent a consistent property of the proterozoic geomagnetic field together with changes in the amount of time spent in the superchron regime they may testify to a different field behavior during the late precambrian than during the phanerozoic this difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core mantle boundary during the precambrian possibly as a consequence of the inner core not yet being nucleated at this time

past and recent changes in the large scale tropospheric cycles of lead and other heavy metals as documented in antarctic and greenland snow and ice a review abstract the investigation of the occurrence of lead and other heavy metals in antarctic and greenland ancient ice and recent snow is of great interest in reconstructing the past natural tropospheric cycles of these metals and determining to what extent these cycles have now been altered by man because concentrations to be measured are exceedingly low down to the sub pg g level reliable data can however be obtained only if full control of contamination is achieved from field sampling to laboratory analysis the available data show that the past natural concentrations of pb cd zn cu and hg in antarctic ancient ice are highly dependent upon climatic conditions the highest values occurring during the coldest periods of the ice ages especially during the last glacial maximum about 18 000 years ago human activity has led to an increase in lead concentrations during recent centuries both in antarctica tenfold increase and in greenland two hundredfold increase this last increase was followed by a 7 5 fold decrease during the past two decades mainly as a consequence of the rapid fall in the use of lead alkyl additives in the usa as confirmed by recent isotopic data

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth

dendroid morphology and growth patterns 3 d computed tomographic reconstruction abstract this paper analyzes the growth patterns of dendrolite in the zhangxia formation middle cambrian shandong province china using the technique of 3 d computed tomographic reconstruction dendroids are classified into v dendroids columnar dendroids and arborescent dendroids based on morphological characteristics the means of interconnection between dendroids are classified into trunks nodules shoots and fingers stacking and tiering control the gross morphology and structural framework of dendrolite stacking is a process of vertical growth in which v dendroids create a staircase like structure tiering occurs when a layer of dendroids is covered by sediment and then partially eroded allowing a new layer of dendroids to form a comprehensive blueprint of the structural divisions of dendrolite is presented according to scale being divided into micro meso macro and megastructures the mesostructure which includes individual dendroids and their combined structures is subsequently divided into primary v dendroid secondary columnar and arborescent dendroid and tertiary stair and tier structures and a basic growth model is provided for v dendroids the stages of v dendroid growth are 1 trunk extension and base expansion 2 divergence 3 expansion and convergence followed by repetition of stages 2 and 3 until 4 growth completion followed by the subsequent emergence of a new dendroid by either stacking or tiering this development of systematically ordered structures is suggestive of the reaction of microbial colonies to external environmental conditions

the middle late cambrian reef transition and related geological events a review and new view abstract the reefs of the middle late cambrian cambrian epoch 3 furongian have long been simply considered as microbialites that flourished in the aftermath of the archaeocyath extinction thorough review of the reefs in these periods shows however that the cambrian epoch 3 and the furongian actually yield different types of reefs the cambrian epoch 3 reefs are dominated by thrombolites and dendrolites largely constructed by the calcified microbes epiphyton and renalcis on the other hand the furongian reefs consist mainly of maze like maceriate reefs and columnar stromatolites the maceriate reefs most likely formed by siliceous sponges and calcified microbes including girvanella and tarthinia whereas the columnar stromatolites were mainly constructed by girvanella tarthinia and minor siliceous sponges other microbial reefs e g non columnar stromatolites persisted during the cambrian epoch 3 and the furongian mainly as small patch reefs or reefal crusts lithistid sponge microbial reefs initially formed in the cambrian epoch 3 and occurred throughout the furongian but occupied only a minor portion during these periods several geological events occurred across the boundary between the cambrian epoch 3 and the furongian including positive carbon and sulfur isotope excursions a eustatic sea level drop major faunal turnover of trilobites and diversification of new organisms the coincidence of these events and the transition in reefal fabrics suggest that reef ecosystems were significantly influenced by at least some of these events among the events it is likely that sea level change would have affected reefs although further studies are required in order to test whether and how the other events influenced the reef transition epiphyton and renalcis and thrombolites and dendrolites formed by them declined greatly most likely due to sea level fall and thus decreased in shallow marine habitat at the end of cambrian epoch 3 the cambrian epoch 3 thrombolites and dendrolites were gradually replaced by a sponge microbial association of the maze like maceriate reefs and the columnar stromatolites in the furongian coincident with a sea level rise

early recovery of sponge framework reefs after cambrian archaeocyath extinction zhangxia formation early cambrian series 3 shandong north china abstract reefs dominated by the anthaspidellid sponge rankenella zhangxianensis the calcimicrobe epiphyton and the stem group cnidarian cambroctoconus orientalis together with encrusting microstromatolites occur early in the middle cambrian series 3 late stage 5 of shandong eastern china in the zhangxia formation these in situ components created a tight framework with centimeter scale growth cavities mainly filled by fine grained matrix among them r zhangxianensis and c orientalis mutually attached and locally formed metazoan dominated frameworks these metazoan microbial reefs form thin lenses 505 ma these rankenella reefs indicate recovery of framework building metazoans within 5 million years of the archaeocyath reef decline in structure they resemble archaeocyath reefs as well as early ordovician lithistid sponge microbial reefs having conjoined conical macroskeletons thickly veneered by calcimicrobes and microbial carbonate with largely matrix filled intervening cavities in combination with other sponge microbial reefs reported from australia iran korea and the usa they demonstrate that an anthaspidellid sponge microbial reef consortium was widespread throughout the mid late cambrian

microbialite morphostratigraphy as a tool for correlating late cambrian early ordovician sequences abstract microbialite morphostratigraphy is a new tool for intrabasinal correlation using diverse microbialite structures morphotypes the recognition of the succession of morphotypes over constrained temporal intervals and broad areas is a function of the complex interactions that operate to create the structure because so many nonlinked variables e g biotic sedimentological physicochemical are involved similar morphotypes do not reoccur over long temporal intervals to demonstrate the technique the upper cambrian lowermost ordovician shelf strata of the great basin united states were correlated using both morphostratigraphy and standard lithostratigraphy six morphozones and one morphosubzone were recognized as were four main lithologic successions because the boundaries between the morphozones and lithologic successions did not coincide it is inferred that the characteristics of the various microbialite structures are not solely controlled by physical factors the principles for establi

the occurrence of zinc in antarctic ancient ice and recent snow concentrations of zinc zn have been measured in various sections of the dome c and vostok deep antarctic ice cores whose ages range from 3850 to 155 000 years bp and in several large size surface antarctic snow blocks collected in adelie land and at the geographic south pole all the samples were mechanically decontaminated and detailed outside inside variation profiles were drawn for most of them then allowing us to clearly establish the accuracy of the data obtained from the analysis of the most central parts of each individual core section or snow block natural zn concentrations are found to have strongly varied in antarctic ice during the past 155 000 years the highest values up to about 100 pg zn g being observed during the last glacial maximum and possibly during the end of the next to last ice age wind blown dust from crustal rock and soil appears to be the main natural source of zn during the glacial periods especially the last glacial maximum zn concentrations in present day antarctic snow from central east antarctica about 5 pg zn g are found to be comparable with those in holocene ice several thousand years old which evidences that the antarctic tropospheric cell is still little affected by anthropogenic zn

frequency of proterozoic geomagnetic superchrons abstract long term geodynamo evolution is expected to respond to inner core growth and changing patterns of mantle convection three geomagnetic superchrons during which earth s magnetic field maintained a near constant polarity state through tens of myr are known from the bio magnetostratigraphic record of phanerozoic time perhaps timed according to supercontinental episodicity some geodynamo simulations incorporating a much smaller inner core as would have characterized proterozoic time produce field reversals at a much lower rate here we compile polarity ratios of site means within a quality filtered global proterozoic paleomagnetic database according to recent plate kinematic models various smoothing parameters optimized to successfully identify the known phanerozoic superchrons indicate 3 10 possible proterozoic superchrons during the 1300 myr interval studied proterozoic geodynamo evolution thus appears to indicate a relatively narrow range of reversal behavior through the last two billion years implying either remarkable stability of core dynamics over this time or insensitivity of reversal rate to core evolution

structure and diversity of oldest sponge microbe reefs lower cambrian aldan river siberia the oldest sponge reef is a small early cambrian bioherm at the base of the tommotian stage 535 540 ma in southeast siberia the mainly archaeocyath construction may be a response to turbid conditions cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic archaeolynthus and calcified microbes renalcis in addition to these constructors and binders other reef guilds are present bafflers other archaeocyaths spiculate sponges and hyoliths and dwellers hyoliths mollusks and many others this is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal autotroph composition it provides a blueprint for younger phanerozoic reefs

changes in cadmium concentrations in antarctic ice and snow during the past 155 000 years changes in cd concentrations in antarctic ice and snow during the last full climatic cycle the past 155 000 yrs have been investigated by analysing various sections of the dome c and vostok deep antarctic ice cores and several blocks of recent antarctic snow each sample was mechanically decontaminated using ultraclean procedures and then analysed for cd by the new ultrasensitive laser excited atomic fluorescence technique cd concentrations are found to have been highly variable in ancient antarctic ice and therefore in the past pristine south polar atmosphere during the last climatic cycle the highest values being observed during the cold terminal stages of the last and next to last ice ages concentrations measured in recent antarctic snows are comparable with those in antarctic holocene ice several thousand years old which suggests that the anthropogenic influence is probably still negligible for this heavy metal in the south polar atmosphere for some of the samples measured cd concentrations can be simply accounted for by rock and soil dust and volcanic emissions while for others there is a significant excess over the contributions from these two sources

accumulation rates of carbonate platforms the growth potential of carbonate platforms i e the maximum rate at which they can aggrade vertically is difficult to assess to estimate the growth potential from the conservative side and search for variations with time we have compiled published accumulation rates from the phanerozoic accumulation rates vary in a pattern with maximum rates up to 200 m ma or more not corrected for compaction in the late devonian permian late triassic late jurassic and mid cretaceous separated by intervals of reduced rates e g carboniferous early jurassic and late cretaceous the most dramatic drop occurs at the permo triassic boundary after which there is no documented platform growth this pattern resembles the evolutionary rhythm of bloom and demise of reefs and reef builders the observed maximum accumulation rate of 200 m ma over 10 6 10 7 yr is probably imposed by the maximum rate of long term relative sea level rise on passive margins the most favorable site for carbonate platform deve

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

early proterozoic geomagnetic field in western laurentia implications for paleolatitudes local rotations and stratigraphy abstract the east west trending post orogenic subvertical cleaver dykes from the great bear magmatic arc are dated by the u pb baddeleyite method to be 1740 5 4 ma they have a mean direction of magnetization of d i 136 5 and 57 4 k 64 95 4 5 based on samples from 17 diabase dykes a mean location 65 7 n 118 03 w and a corresponding paleopole at 19 4 n 83 3 w a95 6 1 studies of baked contacts country rocks distant from dykes and later intrusions that cut them show that the dykes became magnetized soon after intrusion the paleopole is not significantly different from that obtained from the 1759 1 ma post orogenic jan lake granite from the hudsonian orogen in southeast saskatchewan these paleopoles fall among a cluster previously determined from post orogenic overprints in a wide variety of rocks and geological settings across laurentia their mean is 20 8 n 94 5 w a95 5 2 collectively they record a widespread remagnetization episode following the hudsonian orogeny during which the polarity of the field remained constant this single polarity interval the cleaver superchron has a mean age of approximately 1747 ma and its duration is at least 13 million years but is probably much longer with further refinement it could become a global statigraphical marker there are 18 paleopoles from red beds and igneous rocks which record the paleofield in western laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 ma during the hudsonian orogeny over this interval western laurentia remained at a low and essentially constant latitude reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency there are numerous magnetochrons which could in future assist in regional stratigraphic correlations on the basis of six studies we estimate that the reference paleopole for this interval is at 1 1 s 71 7 w a95 12 1 n 6 paleopoles from the other 12 studies are spread over a 100 arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering archean slave and superior cratons during the hudsonian orogeny these reference paleopoles show that during the hudsonian orogeny western laurentia was in paleolatitudes of 15 30 during post hudsonian time ca 1750 ma paleolatitudes were about 20 higher paleowind directions inferred by others indicate that laurentia during these intervals was in the northern hemisphere

past natural changes in cu zn and cd in vostok antarctic ice dated back to the penultimate interglacial period 1 improved ice core decontamination procedures have allowed us to obtain the first reliable data on cu zn and cd in ancient antarctic ice drilled in a fluid filled hole at vostok station dated back to the beginning of the next to last ice age 240 000 years ago cu zn and cd concentrations and fallout fluxes display very large natural variations by up to a factor of 35 with high values obtained during the coldest climatic stages and much lower values during warm climatic stages crustal dust was an important source of cu and zn during the coldest climatic stages and volcanic emissions were probably an important source of cd both during warm and cold periods

changes in natural lead copper zinc and cadmium concentrations in central greenland ice from 8250 to 149 100 years ago their association with climatic changes and resultant variations of dominant source contributions abstract we present here the first reliable time series of pb cu zn and cd in greenland ice for the last climatic cycle they were obtained by analysing various sections of the 3028 8 m grip deep ice core drilled at summit central greenland our results show that climatic changes have led to large variations in the concentrations of natural pb cu zn and cd in the high latitude troposphere of the northern hemisphere between the interglacial and glacial periods concentrations have varied by factors of 320 for pb 100 for cu 36 for zn and 13 for cd based on a good correlation between each heavy metal and al pb and cu are found to have mainly originated from soil and rock dust for both glacial and interglacial periods on the other hand continental biogenic emissions were the main source of cd and to a lesser extent zn in the arctic troposphere during the warm eemian and the pre boreal to holocene transition whereas wind blown dust was the predominant source for these two metals during the cold glacial climatic stages this characteristic change of relative cd and zn contributions from different sources in contrast to that for pb and cu is well documented in the ice from the last deglaciation period 15 000 to 8250 yrs ago after the younger dryas event ended a remarkable increase of cd al and zn al ratios occurred from 13 000 to 9300 yrs ago which is consistent with the progressive expansion of vegetation following the retreat of the north american and north eurasian ice sheets the subsequent decrease of these ratios to holocene values can be explained by the stabilization of atmospheric circulation in the northern high latitude regions which became similar to that for the holocene due to further reduction of the laurentide ice sheet finally the observed variations of the metal al ratios suggest changes in the soil and rock dust source regions in parallel with climatic changes which have altered the mean composition of the crustal particles transported to the arctic

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

variations in geomagnetic reversal frequency during the earth s middle age 1 we have obtained new magnetostratigraphic results from two precambrian sedimentary sections from eastern siberia and the southern urals dated between 1100 ma and 800 ma sample magnetizations from the uppermost mesoproterozoic talakh khaya section in the siberian uchur maya region appear to be mostly carried by a mixture of magnetite and hematite a sequence of 33 magnetic polarity intervals is recorded within the section all reversals occur in the first 24 m while the upper 160 m are characterized by a single magnetic interval of normal polarity assuming a northern hemisphere position of siberia around 1000 ma the lower neoproterozoic uralian minyar section also possesses an ancient magnetization carried by magnetite and hematite the high temperature magnetization component obtained from the minyar section establishes a sequence of 43 magnetic polarity intervals positive reversal tests obtained from the data sets presented here and from other previously analyzed proterozoic sections provide no convincing evidence for a long standing asymmetric geomagnetic field during the proterozoic that would make the precambrian field markedly less dipolar than during the phanerozoic the new data further reveal the occurrence of sharp transitions and alternations between long periods without any reversal one superchron is observed from the talakh khaya section and periods with high reversal frequencies possibly larger than 5 10 reversals per myr this characteristic may well be an important property of the precambrian field although rather sudden transitions between reversing and nonreversing states of the geodynamo may not be unique to this period

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

siluro devonian paleomagnetic results from the tuva terrane southern siberia russia implications for the paleogeography of siberia new paleomagnetic data from seven sections of siluro devonian age sedimentary sequences in the tuva terrane south siberia russia reveal the presence of up to three different components of magnetization component a is of recent or late mesozoic cenozoic origin and component b identified unambiguously in 25 samples from one section yields a sample mean direction declination inclination of 300 6 54 7 k 42 2 95 4 5 in geographic and 283 9 63 5 k 45 4 95 4 3 in stratigraphic coordinates however since the results of the fold test are inconclusive no further attempt has been made to assign an age to this magnetization component c with unblocking temperatures of 500 680 c is identified in all seven sections and with dual polarity after bedding correction the section mean directions all plot along a common small circle in stereographic projection with a mean inclination of 42 1 n 7 sites k 184 2 95 2 9 it passes the inclination only fold test the within site fold test both on the 95 significance level and the reversal test and is thus interpreted as being primary in origin and siluro devonian in age adopting the normal polarity option for component c yields a paleolatitude of 24 n 2 for the tuva terrane on the basis of geological evidence the tuva terrane was clearly accreted to siberia by early silurian times the results obtained in this study therefore indicate lower paleolatitudes for siberia than those predicted from previously published apparent polar wander paths for the siberian platform and demonstrate that siberia was positioned between the equator and 30 n during latest silurian earliest devonian times these new results are also supported by paleoecological and paleoclimatological evidence the dispersion of declinations along a small circle is indicative of clockwise block rotations within the tuva terrane of up to 72 with respect to siberia

modelling the palaeo evolution of the geodynamo although it is known that the geodynamo has been operating for at least 3 2 ga it remains difficult to infer the intensity dipolarity and stability occurrence of reversals of the precam brian magnetic field of the earth in order to assist the interpretation of palaeomagnetic data we produce models for the long term evolution of the geodynamo by combining core ther modynamics with a systematic scaling analysis of numerical dynamo simulations we update earlier dynamo scaling results by exploring a parameter space which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to earth in the precambrian our analysis highlights the central role of the convective power which is an output of core thermodynamics and the main input of our updated scalings as the thermal evolution of the earth s core is not well known two end member models of heat flow evolution at the core mantle boundary cmb are used respectively terminating at present heat flows of 11 tw high power scenario and 3 tw low power scenario the resulting models predict that until the appearance of the inner core a thermal dynamo driven only by secular cooling and without any need for radioactive heating can produce a dipole moment of strength comparable to that of the present field thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence the observed lack of strong long term trends in palaeointensity data throughout the earth s history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter specifically the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation but the dynamo becomes deeper seated in the core thus largely cancelling the increase at the core and earth surface and diminishing the prospect of observing this event in palaeointensity data our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history in the precambrian we predict a dynamo with similar dipolarity and less frequent reversals than at present times due to conditions of generally lower convective forcing quantifying the typical cmb heat flow variation needed for the geodynamo to cross the transition from a reversing to a non reversing state we find that it is unlikely that such a variation may have caused superchrons in the last 0 5 ga without shutting down dynamo action altogether

sequence sequence stratigraphy sea level changes and depositional systems in the cambro ordovician of the north china carbonate platform abstract the cambro ordovician strata in north china were deposited over a very extensive craton extending some 1500 km east west and 1000 km north south the dominantly shallow water carbonate succession reaches up to 2000 m in thickness and two megasequences transgressive regressive cycles can be distinguished lower cambrian through lower ordovician strata and middle through upper ordovician strata separated by a major palaeokarst the first megasequence consists of nine sequences which are generally 50 150 m in thickness the lower cambrian sediments consist of phosphorites and phosphatic sandstones deposited during the flooding of the craton carbonates mudrocks and evaporites were deposited in the early cambrian under an arid climate laying the foundation for the subsequent long period of shallow water carbonate deposition which lasted some 70 m y the middle and lower upper cambrian sequences consist predominantly of mudrocks and storm deposits tempestites in the lower part and oolitic grainstones and tidal flat lime mudstones in the upper part these represent outer mid and mid inner ramp depositional systems respectively of the transgressive and highstand systems tracts these sequences have strong similarities with the grand cycles in the cambro ordovician successions of north america in the upper cambrian there is a distinctive unit of glauconitic purple red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence in the upper cambrian there was a phase of tilting of the north china carbonate platform to the north and storm deposits especially intraclastic conglomerates and hummocky cross stratified grainstones packstones were very common at this time also common in upper middle and lower upper cambrian strata are stromatolitic thrombolitic bioherms several metres in diameter the upper upper cambrian through lower ordovician strata are dominantly fine grained limestones and dolomites deposited in shallow subtidal and inter to supra tidal environments on a low energy epeiric sea platform this part of the succession is the regressive part of the first megasequence so that overall the platform shows an evolution from platform initiation to platform foundation to a ramp depositional system and then an extensive epeiric platform on a small scale the succession is composed of metre scale shallowing upward cycles parasequences arranged into cycle sets of 10 30 m thick the middle upper ordovician megasequence consists mostly of shallow water carbonates with several thick evaporite units minor palaeokarsts and palaeosoils separate the sequences which are composed of metre scale cycles regional uplift affected the north china platform in the ordovician and sedimentation did not resume until the carboniferous there is a good correlation of the two transgressive regressive megasequences described here with the global 2nd order relative sea level curve for the cambro ordovician

controls on geomagnetic reversals and core evolution by mantle convection in the phanerozoic abstract numerical dynamos driven by non uniform heat flux at the core mantle boundary are used to investigate the connections between geomagnetic field structure geomagnetic reversal frequency core evolution and mantle convection through phanerozoic time polarity reversal sequences and time average magnetic field structures are calculated using dynamos driven by two representations of lower mantle history a reconstruction of mantle convection with plate motions by zhang and zhong 2011 that produces time variable core mantle boundary cmb heat flux and an irregular evolution of the core and a second model based on hotspot locations with a time independent pattern of cmb heat flux derived from the present day seismic shear wave heterogeneity of the lower mantle that produces a monotonic evolution of the core for both mantle histories present day values of the dynamo control parameters are tuned to match geomagnetic polarity time scale reversal statistics for 0 5 ma and the time dependences of the dynamo control parameters are determined from the thermal evolution of the core including time variability of cmb heat flow inner core size inner core chemical buoyancy flux and rotation rate the dynamo with time independent cmb heat flux shows minor fluctuations in reversal frequency with age whereas the dynamo with time variable cmb heat flux shows reversal rate fluctuations including stable polarity at 275 and 475 ma and frequent reversals at other times this dynamo also produces departures from geocentric axial dipole symmetry during the time of supercontinent pangaea and a heterogeneous growth history of the inner core

dust composition changes from taylor glacier east antarctica during the last glacial interglacial transition a multi proxy approach abstract mineral dust is transported in the atmosphere and deposited in oceans ice sheets and the terrestrial biosphere temporal changes in locations of dust source areas and transport pathways have implications for global climate and biogeochemical cycles the chemical and physical characterization of the dust record preserved in ice cores is useful for identifying of dust source regions dust transport dominant wind direction and storm trajectories here we present a 50 000 year geochemical characterization of mineral dust entrapped in a horizontal ice core from the taylor glacier in east antarctica strontium sr and neodymium nd isotopes grain size distribution trace and rare earth element ree concentrations and inorganic ion cl and na concentrations were measured in 38 samples corresponding to a time interval from 46 kyr before present bp to present the sr and nd isotope compositions of insoluble dust in the taylor glacier ice shows distinct changes between the last glacial period lgp in this study ranging from 46 7 15 3 kyr bp the early holocene in this study ranging from 14 5 8 7 kyr bp and zero age samples the 87sr 86sr isotopic composition of dust in the taylor glacier ice ranged from 0 708 to 0 711 during the lgp while the variability during the early holocene is higher ranging from 0 707 to 0 714 the end composition ranges from 0 1 to 3 9 during the lgp and is more variable from 1 9 to 8 2 during the early holocene the increased isotopic variability during the early holocene suggests a shift in dust provenance coinciding with the major climate transition from the lgp to the holocene the isotopic composition and multiple physical and chemical constraints support previous work attributing southern south america ssa as the main dust source to east antarctica during the lgp and a combination of both local ross sea sector dust sources and ssa after the transition into the holocene this study provides the first high time resolution data showing variations in dust provenance to east antarctic ice during a major climate regime shift and we provide evidence of changes in the atmospheric transport pathways of dust following the last deglaciation

inferred syngenetic textural evolution in holocene cryptic reefal microbialites heron reef great barrier reef australia cryptic microbialites in the heron reef framework occur as crusts of fingerlike microcolumns or branching dendrolites rarely more than 1 cm long microstructure of the most recently growing microbialite surfaces consists of coalesced 3 m scalenohedra that are indistinguishable from previously described mg calcite abiotic cement the transformation from submicrometer anhedral crystallites to 3 m scalenohedra is inferred to have occurred only during active microbialite accretion beneath a biofilm this syngenetic change from primary biologically induced microstructures to microstructures that are indistinguishable from abiotic cement has important implications for the recognition and interpretation of early marine microcrystalline carbonates and cements

a comparison of deep antarctic ice cores and their implications for climate between 65 000 and 15 000 years ago abstract three ice cores drilled in the central part of the antarctic continent extend back to the last glacial period one from west antarctica byrd and two from east antarctica vostok and dome c this period is also partly covered by a few cores from the coastal areas in these cores climatic information is mostly derived from the isotopic profiles d or 18 o from which surface temperature and more indirectly precipitation rate can be estimated the main objective has been to compare thoroughly the three deep ice cores for the main part of the last glacial period from ca 65 000 15 000 yr b p the time scales have been examined in detail and a new 40 000 yr chronology for the dome c core adopted special emphasis is placed on the link between the concentration of 10 be and past accumulation changes and on the use of peaks in the concentration of this cosmogenic isotope as stratigraphic markers elevation changes of the ice sheet derived from gas content and isotopic data bear directly on interpretations of past temperature and precipitation rate changes

lower cambrian shelf and shelf margin buildups flinders ranges south australia1 carbonate buildups in the flinders ranges of mid early cambrian age grew during a period of high archaeocyath diversity and are of two types 1 low energy archaeocyath sponge spicule mud mounds and 2 high energy archaeocyath calcimicrobe calcified microbial microfossil bioherms mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone have a fenestral fabric display distinct stromatactis contain abundant sponge spicules and form structures up to 150m wide and 80 m thick bioherms are either red or dark grey limestone and occur as isolated small structures 2 20 m in size surrounded by cross bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across red bioherms comprise masses of white epiphyton with scattered archaeocyaths and intervening areas of archaeocyath rich lime mudstone grey bioherms are complex intergrowths of archaeocyaths encrusting dark grey renalcis and thick rinds of fibrous calcite cement the bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities up to 1 5 m across lined with fibrous calcite the buildups are isolated or in contiguous vertical succession mud mounds occur alone in low energy frequently nodular limestone facies individual bioherms and bioherm complexes occur in high energy on shelf and shelf margin facies the two types also form large scale shallowing upward sequences composed of basal deep water mud mounds grading upward into archaeocyath calcimicrobe bioherm complexes and bioherms in cross bedded carbonate sands the uppermost sequence is capped by ooid grainstone and or fenestral to stromatolitic mudstone the calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout much of subsequent phanerozoic time

paleoenvironmental implications of an extensive maceriate microbialite bed in the furongian chaomidian formation shandong province china abstract this study focuses on the unique occurrence of an extensive microbialite bed 10 20 m thick in the chaomidian formation furongian in shandong province china in order to understand its paleoenvironmental implications the microbialite bed can be traced for over 6000 km2 in area the microbialites are characterized by centimeter to decimeter scale branching maze like constituents maceriae of microbial and muddy sediments with chaotic texture including tabular maceriate microbialite type 1 columnar maceriate microbialite type 2 and columnar chaotic microbialite type 3 within the bed each microbialite unit is generally well correlated for tens of kilometers the entire bed is bounded by limestone marlstone alternation at the base and an erosional surface at the top that is subsequently overlain by a grainstone bed of various thicknesses the microbialite bed formed when the seafloor reached suitable water depth for the microbial growth during sea level rise accompanied with carbonate production the maze like maceria structures formed to acquire a larger surface area under a relatively large input of lime mud the tabular maceriate microbialites initially formed in relatively deep water environments and flourished on broad and flat seafloor whereas the columnar microbialites types 2 and 3 developed under the influence of strong waves and currents the microbialite bed was eroded and terminated by frequent storm events and buried under the reworked grainstone further rise in sea level and increased metazoan activities afterwards limited the resurgence of microbes in the late furongian

the carbon isotopic composition of proterozoic carbonates riphean successions from northwestern siberia anabar massif turukhansk uplift thick carbonate dominated successions in northwestern siberia document secular variations in the c isotopic composition of seawater through mesoproterozoic and early neoproterozoic early to early late riphean time mesoproterozoic dolomites of the billyakh group anabar massif have delta 13c values that fall between 0 and 1 9 permil versus pdb with values in the upper part of the succession yusmastakh formation consistently higher than those of the lower ust il ya and kotuikan formations consistent with available biostratigraphic and radiometric data delta 13c values for billyakh carbonates compare closely with those characterizing early mesoproterozoic carbonates about 1600 1200 ma worldwide in contrast late mesoproterozoic to early neoproterozoic limestones and dolomites in the turukhansk uplift exhibit moderate levels of secular variation only the lowermost carbonates in the turukhansk succession linok formation have delta 13c values that approximate billyakh values higher in the turukhansk succession delta 13c values vary from 2 7 to 4 6 permil with outliers as low as 5 0 permil interpreted as diagentically altered again consistent with paleontological and radiometric data these values compare well with isotopic values from 1200 to 850 ma successions elsewhere five sections measured in different parts of the turukhansk basin show nearly identical patterns of variation confirming that carbonate delta 13c correlates primarily with time and not facies the siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra and interbasinal correlation of mesoproterozoic and early neoproterozoic rocks

new middle cambrian and middle ordovician palaeomagnetic data from siberia llandelian magnetostratigraphy and relative rotation between the aldan and anabar angara blocks abstract whether siberia was a coherent tectonic unit throughout the phanerozoic or whether significant rotations occurred in early palaeozoic times between the anabar angara and aldan blocks is still a matter of debate addressing this problem a detailed palaeomagnetic study of sections of mid cambrian and llandeillian age has been carried out along the river lena river maya river kulumbe and river stolbovaya with a total of 403 collected hand samples a section of llandeilian age was also investigated on the russian platform near st petersburg the studied rocks are carbonates or fine grained terrigenous sediments of reddish and greyish colors stepwise thermal demagnetization experiments reveal a rather simple directional behavior in most samples after removal of a secondary magnetizations at low to intermediate temperatures a high temperature component is isolated which is carried by magnetite and or hematite positive reversal tests and fold tests clearly demonstrate the primary character of the latter component while only reversed polarity directions are obtained in the lower two thirds of the llandeilo six magnetozones of normal polarity are found in the upper llandeilo which mark the end of the moyero reverse superchron together with previously published paleomagnetic data our new results show rather systematic difference in declinations of 20 between the aldan and anabar angara blocks this argues in favor of a relative rotation between these two blocks with an euler pole of rotation located at latitude 62 n and longitude 117 our interpretation is further supported by existing results from deep sounding seismic experiments yielding a very similar pole of rotation

growth dynamics of neoproterozoic calcimicrobial reefs mackenzie mountains northwest canada abstract neoproterozoic buildups of the little dal group grew in a deep water epicratonic basin these kilometer scale reefs display aggradational and progradational geometries comparable to those described from phanerozoic reefs four phases of reef growth correspond to four regional shale to carbonate packages in laterally equivalent off reef strata the lower shaly part of each package is interpreted to reflect transgressive to highstand deposition the upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea level lowstand reefs nucleated at the beginning of the first major transgressive event reefs typically aggraded during transgressive intervals but could also prograde or contract likely depending on the rate of relative sea level rise versus reef growth rate reefs prograded during regressions probably owing to reduction of accommodation space they shed talus at lowstand in response to increased erosion and or progradation over unstable substrates reef growth ended with return of shallow water conditions the overall architecture of aggradation and progradation demonstrates that the neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied ecologically complex and faunally diverse buildups of the phanerozoi

the 125 150 ma high resolution apparent polar wander path for adria from magnetostratigraphic sections in umbria marche northern apennines italy timing and duration of the global jurassic cretaceous hairpin turn a new high resolution apparent polar wander path apwp segment has been obtained from the magnetostratigraphy of four kimmeridgian to lower aptian sections in the northern apennines italy the use of paleomagnetic data for determination of the adria apwp was hampered by the large local rotations linked to apennine tectonics characterized by folds and thrusts developed during the neogene to overcome this problem we have computed relative rotations between time overlapping sections and realigned them in a common declination reference frame namely the bosso section we synthesized a new high resolution 150 to 125 ma apwp for adria which has a similar shape to the time equivalent segment of the synthetic apwp of africa of besse and courtillot j besse v courtillot apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr j geophys res 107 b11 2002 doi 10 1029 200jb000050 a 26 clockwise rotation of our combined adria apwp places it in almost perfect overlap with african data of same age confirming that the adria promontory moved coherently with africa during this time span whereas the counterclockwise rotation of adria with respect to africa was introduced later most probably during apennines orogenesis finally we discuss in relation with worldwide plate evolution the peculiar shape of our apwp which displays a hairpin turn during berriasian time and dates the main late jurassic early cretaceous change in plate motion at around anomaly m16

estimates of the transport of trace metals from the ocean to the atmosphere the bubble interfacial microlayer sampler bims was used in the north atlantic to determine the enrichment factors relative to sodium and the ocean efsea for a series of trace metals on bubble derived atmospheric sea salt particles no enrichment was found for k and mg within the analytical precision of 10 mean efsea values determined for al co cu fe mn pb v and zn ranged from 10 to 20 000 positive correlations were found between bubble generation depth and the efsea values for fe pb sc and zn estimates suggest that the ocean is the primary global source of atmospheric k and mg that it is potentially a nontrivial 5 20 of the total source of atmospheric cu v and possibly zn and that it is an insignificant source of al co fe mn pb and sc in the remote north pacific during certain periods of the year the ocean may be the primary source for all these elements present on aerosol particles with diameters greater than 3 m

ice core record of dust sources in the western united states over the last 300 years over the past 5000 years amplified dust generation and deposition in the american west has been linked to human activity in recent decades intensified rates of agriculture and livestock grazing have been correlated with greater dust production detected on seasonal to annual timescales the combination of land use intensification and climate change i e increased drought frequency in north america highlights the importance of characterizing the sources of dust both before and after the influence of anthropogenic activity we apply high precision geochemical and isotopic sr and nd isotopes techniques to an ice core from the upper fremont glacier wyoming usa to produce the first glacial dataset from the american west our sr nd isotopic composition data indicates the evolving dust provenance to the upper fremont glacier ufg from a long range transport of mineral dust to a local source this increasing input of dust from a local source is supported by a rise in average dust particle diameter combined with greater average dust concentration throughout the record the greater presence of dust particles smaller than 2 5 m in the most recent samples from ufg ice core record support existing satellite and sediment core data regarding the effects of anthropogenic activity upon dust sources and pathways in the american west although the sr nd isotope database in north america needs be expanded our results provide a survey of windborne dust through the past 270 years

upper cambrian to middle ordovician magnetostratigraphy from the kulumbe river section northwestern siberia abstract we present the magnetostratigraphy of an upper cambrian to middle ordovician sedimentary section sampled along the kulumbe river in northwestern siberia thermal demagnetization yields the determination of two magnetic components a component of steep inclination is interpreted as a remagnetization acquired during the emplacement of the siberian traps at the permian triassic boundary another component with a moderate inclination essentially carried by hematite and sometimes by magnetite likely represents a syndepositional or nearly syndepositional magnetization of the studied deposits a sequence of 17 magnetic intervals is observed between the upper part of the upper cambrian and the tremadoc three of them being poorly defined by single samples we also find that the llanvirn magnetostratigraphic sequence contains only one reversed polarity interval together with magnetostratigraphic results previously obtained from the moyero river section from siberia our data suggest the occurrence of a 25 30 myr long period during the lower and the middle ordovician characterized by a very low magnetic reversal frequency possibly containing a superchron and dominated by reversed polarity this geomagnetic feature would be compatible with a time constant of about 150 myr in the reversal process through the whole phanerozoic

the signature of inner core nucleation on the geodynamo abstract energy considerations indicate that the power delivered to the present day geodynamo comes mainly from the growth of the solid inner core through light element and latent heat releases the nucleation of the inner core was therefore a major transition for the geodynamo here we use numerical dynamo simulations linked by thermochemical evolution of the core to investigate the effects of inner core nucleation icn on the geodynamo and identify possible icn footprints in the palaeomagnetic field our results suggest that a strong field dipole dominated dynamo prior and after icn the uniformitarian scenario is most likely we predict little footprint of icn on the surface field intensity consistent with the observed lack of a long term trend in paleointensity data instead we find that the best proxy for inner core growth is an increase in axial octupole strength with age from present day to icn which might be resolvable in the palaeomagnetic record we confirm the existence of weak field multipolar dynamos prior to icn suggesting the dynamo might have transiently reached a weak field state the catastrophic scenario the narrow stability domain of these weak field dynamos implies that the catastrophic scenario is less probable than the uniformitarian scenario

a new siberian record of the 1 0 gyr old maya superchron abstract we report new magnetostratigraphic data from the late mesoproterozoic linok formation consisting of clayed carbonates together with paleomagnetic data from the neoproterozoic carbonate sukhaya tunguska burovaya and turukhansk formations located in the turukhansk region northwestern siberian platform we also re analyzed demagnetization data previously obtained from the derevnya formation paleomagnetic analyses revealed up to four magnetization components first a steep inclination component likely of recent origin is isolated in all sections in the low temperature range second a magnetization is recovered in most samples between 250 c and 350 400 c it shows a single magnetic polarity and is interpreted as a remagnetization acquired during the kiaman permo carboniferous reverse superchron third a magnetization component is isolated between 400 c and 500 c in samples from the burovaya and turukhansk formations this component is interpreted as a remagnetization that occurred before the end of the neoproterozoic a fourth high temperature ht magnetization component is observed up to 575 c in samples from the linok and burovaya formations and up to 680 c for those of the derevnya and turukhansk formations a series of magnetic polarity reversals is observed in the lower and middle parts of the linok formation whereas a single polarity interval is found in its upper part the syn depositional origin of this magnetization is attested by positive reversal conglomerate and fold tests the ht magnetization component isolated in the derevnya and burovaya formations shows a single magnetic polarity its primary origin is supported by the fact that the derevnya and burovaya ht mean directions are positioned in the continuity of the directional evolution observed through the linok formation it is further constrained by very consistent late mesoproterozoic to early neoproterozoic apparent polar wander paths obtained from different regions of the siberian platform a single magnetic polarity reversal is observed in a 40 m thick section of the turukhansk formation the ht directions found in the upper part of the linok formations over a stratigraphic thickness of 200 m and in the derevnya and burovaya formations strengthen the existence of the normal polarity maya superchron around 1 0 ga at the transition between the mesoproterozoic and the neoproterozoic

latest middle cambrian metazoan reef from northern iran middle and late cambrian reefs were built mainly by cyanobacterial communities a few reefs with a metazoan as well as an algal component however are known from this interval a middle cambrian reef formed primarily by spicular demosponges is described here from the mila formation in the elburz mountains northern iran the reef is enclosed within calcareous grainstones which contain terminal middle cambrian late mayan trilobites the mila formation reef was constructed by sponges of the family anthaspidellidae and bacterial algal sheaths and is the earliest metazoan reef to be documented from the interval after the demise of archaeocyath sponges the reefal community is typical of subsequent reefal communities of early middle ordovician age the ordovician examples differ only by the incorporation of additional metazoan elements

cambrian series 3 carbonate platform of korea dominated by microbial sponge reefs abstract metazoans have been considered as negligible components of cambrian series 3 and furongian microbial dominated reefs in contrast to their presence in earlier terreneuvian cambrian series 2 microbial archaeocyath reefs however recent discoveries of sponges in cambrian series 3 furongian reefs of australia china iran usa and korea have raised question regarding their contribution in terms of carbonate platform development which have never been assessed this study examines cambrian series 3 deposits of the daegi formation korea to elucidate this question the 100 m thick middle part of the daegi formation is dominated by boundstone facies which occupies 45 of the study interval as well as bioclastic wackestone to packstone bioclastic grainstone and ooid packstone to grainstone facies the daegi reefs are primarily thrombolitic in composition with 90 n 26 29 of the reefs containing an average of 9 sponges in aerial percentage calculated from thin sections lithistid sponges composed of peloidal fabrics some desma spicules and spicule networks commonly occupy the interstitial space in microbial clusters are encrusted by mesoclots and epiphyton and are surrounded by micrite subordinate non lithistid demosponges occur within clusters of microbial elements the middle daegi formation can be largely subdivided into shoal environment dominated by grainstone to packstone facies and shallow subtidal platform interior environment located behind shoal with wackestone to packstone facies the microbial sponge reefs mainly developed around platform interior as patch reefs the current study indicates that metazoans in the form of lithistid and non lithistid demosponges are nearly ubiquitously incorporated in daegi reefs and contributed greatly to the formation of microbial sponge reefs as well as carbonate platform during the time study of these microbial sponge reefs and their distribution within the carbonate platform may help us to understand how carbonate sedimentary environments responded to the extinction of archaeocyaths

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

changes in heavy metals in antarctic snow from coats land since the mid 19th to the late 20th century v cr mn cu zn co ag cd ba pb bi and u have been measured in a series of dated snow samples covering the period from 1834 to 1990 collected at remote low accumulation sites in coats land antarctica they were determined by ultrasensitive inductively coupled sector field mass spectrometry in ultraclean conditions concentrations are found to be extremely low down to 3 10 15 g g for most metals then confirming the high purity of antarctic snow the results show contrasting time trends for the different metals for mn co ba and possibly v and cd no clear time trends are observed for cr cu zn ag pb bi and u on the other hand pronounced enhancements are observed during the recent decades they are attributed to emissions of heavy metals to the atmosphere from human activities in southern america southern africa and australia especially non ferrous metal mining and smelting in chile peru zaire zambia and australia it shows that atmospheric pollution for heavy metals in the remote antarctic continent is not limited to pb and cu as previously thought but also affects several other metals it is a further indication that atmospheric pollution for heavy metals is really global

formative mechanisms depositional processes and geological implications of furongian late cambrian reefs in the north china platform abstract the cambrian series 3 furongian successions of the north china platform contain various microbial metazoan and microbial reefs this study focuses on furongian reefs of the platform in order to understand formative processes and the evolution of the reefs during cambrian epoch 3 and the furongian three types of furongian reefs were differentiated in the shandong region china maceriate reefs columnar stromatolitic reefs and small scale microbial reefs maceriate reefs show dm to m scale domal or flat bedded geometry and consist of cm scale maze like maceria structures made of siliceous sponges and microbial components microstromatolites girvanella and tarthinia columnar stromatolitic reefs are characterized by stromatolite columns of 10 100 cm in height and 5 50 cm in diameter they consist dominantly of girvanella with less conspicuous poorly preserved sponge spicule networks small scale microbial reefs commonly show cm to dm scale domal macrostructures and were constructed mainly by calcimicrobes girvanella and renalcis these three types of furongian reefs were deposited in various shallow marine settings in response to relative sea level changes the furongian reefs are markedly different in terms of macro and micro fabrics from the cambrian series 3 reefs that are dominated by thrombolites and dendrolites and were constructed mainly by epiphyton in the shandong region this difference is also recognized in the beijing region ca 500 km away the abrupt transition from the cambrian series 3 type to furongian type reefs coincidently with a decrease in calcified microbe diversity was most likely due to global euxinic oceanic conditions and a possible eustatic sea level drop rather than the highly diachronous platform wide drowning event i e drowning of the cambrian series 3 carbonate platform the abundant occurrence of sponge spicule networks in the furongian reefs suggests that metazoan reef builders i e sponges resurged and became actively involved in the reefal systems prior to the great ordovician biodiversification event this study may provide an important basis for further investigation into the evolution of reefal systems during the middle to late cambrian when metazoan reef builders were known to be scarce

natural concentrations of lead in ancient arctic and antarctic ice concentrations of pb and k were determined in a series of veneer layers chiseled in sequence from the outside toward the center of each of the five 1500 5500yr old ice core sections that had been drilled in greenland and antarctic ice they were analogs of very old ice samples analyzed earlier by herronet al 1977 and craginet al 1975 who reported high concentrations of pb in them lead contamination existing at exterior concentrations of about 10 6 ng kg ice had intruded to the centers of the cores establishing interior values of at least 1 4 ng kg ice in three electromechanically drilled camp century core sections taken from fluid filled drill holes corresponding pb concentration changes were 3 10 4 ng kg ice to 1 2 ng kg ice in two thermally drilled new byrd station core sections taken from non fluid filled drill holes contamination made the lowest center concentrations serve only as upper limits to the original concentrations of pb in the ice potassium concentrations decreased from exterior values of about 5 10 5 ng kg ice to an interior value of 2 10 3 ng kg ice in the camp century core sections and from 8 10 4 ng kg ice to 9 10 2 ng kg ice in new byrd station core sections potassium contamination effects were not large within the central portions of the cores these data verify earlier findings by murozumiet al 1969 and extend to a broader geographical significance the general validity of their observation of a 300 fold increase of pb concentrations in the greenland ice sheet during the past 3000 yr our findings refute claims by herronet al 1977 and craguinet al 1975 that 100 fold excesses of natural pb exist in 800 yr old greenland ice above levels contributed by silicate dusts our new data also show that average pb concentrations of 26 ng pb kg ice claimed by boutron and lorius 1979 to be natural and present for 60 yr in snow strata in antarctica did not exist in old antarctic ice and that pb concentrations have increased at least 10 fold in that ice during the past century virtually all of the present day 300 fold excess of pb above natural levels in greenland ice can be shown to be caused by industrial pb emissions to the atmosphere on the basis of the following factors 1 the historic increase of pb in snow strata coincides with the historic increase of industrial pb production and atmospheric emissions 2 mass inventories of industrial emissions can account for the excess pb in polar snow 3 new quantitative measurements of pb emissions from volcanic plumes by buat menard and arnold 1978 pattersonet al 1981 and buat menardet al 1981 and from sea spray by ng and patterson 1981 and settle and patterson 1981 show that these natural sources cannot account for 99 of the excess pb above contributions by silicate dusts observed today in the atmosphere and 4 the historic increase of pb in snow strata is paralleled by analogous increases of excess pb shown by isotopic tracers to be industrial in water laid sediments in a remote continental region shirahataet al 1980 it is now known however that snows display about a 10 fold greater excess of industrial pb above crustal silicate concentrations than exists in the air above the snows

magnetostratigraphy of the moyero river section north western siberia constraints on geomagnetic reversal frequency during the early palaeozoic summary we present the magnetostratigraphy of the early palaeozoic moyero river section in north western siberia the section investigated which consists mainly of reddish limestones marls and greyish limestones extends from the upper cambrian to the lower silurian palaeomagnetic analyses reveal two magnetic components one is interpreted as a remagnetization acquired during siberian trap emplacement the other is believed to be early diagenetic consistent with the established lower palaeozoic apparent polar wander path for siberia reversed polarity dominates from the upper cambrian to the middle ordovician while normal polarity dominates from the upper ordovician to the lower silurian only a few reversals which are poorly defined are observed through the section in particular during the llandeilo these data are in agreement with previous magnetostratigraphic data from the baltic platform and siberia and indicate that magnetic reversal frequency was low during the ordovician because of the time delayed process of acquisition of the magnetization carried by haematite we cannot clearly ascertain if a reversed superchron occurred during the lower to middle ordovician however our results suggest that a 15 myr long reversed interval existed during the arenig

sequence organization and growth patterns of late mesoproterozoic stromatolite reefs an example from the burovaya formation turukhansk uplift siberia abstract the latest mesoproterozoic burovaya formation is a 600 1000 m thick stromatolite mud dominated carbonate unit demarcated by an erosional unconformity into two subformations containing stromatolite reefs of variable size and geometry the lower subformation consists of two distinct stratigraphic sequences the lower sequence records shelf to ramp evolution and includes small scale stromatolite reefs developed within an upper subtidal shelf settings at the transgressive systems tract and overlying mud dominated highstand deposits accumulated in lower subtidal to peritidal environments the second sequence of muddy and fine grained sediments makes up a lowstand systems tract and documents a progressive structural and environmental differentiation of the burovaya platform the upper subformation records a third stratigraphic sequence containing transgressive and highstand systems tracts represented by reef bearing and reef cap facies complexes the reef bearing complex characteristic of the transgressive and the bulk of the highstand systems tracts contains a giant 20 25 km wide 10 km long up to 550 m thick stromatolite reef and surrounding deep water carbonate mudstones containing sporadic slump block deposits the reef was located on the distally steepened part of the ramp and contains distinct internal facies organization defined by particular stromatolite microstructure and morphology and prevailing environments late highstand development of the burovaya platform was marked by the termination of reef accretion infilling of basinal relief by shallow water carbonate and shale sediments of the cap complex and subsequent subaerial exposure the history of the burovaya reef is indicative of the interplay between reef growth potential and platform geometry defined by regional tectonic and sea level dynamics

the local and global dimensions of metalliferous pollution derived from a reconstruction of an eight thousand year record of copper smelting and mining at a desert mountain frontier in southern jordan abstract this paper establishes an eight thousand year history of anthropogenic metal pollution at one of the oldest most important and longest sustained sites of the extraction and smelting of copper ores in the old world the faynan orefield in jordan which is located between the hyper arid southern desert and the front of the wetter mountains of edom the modern land surface is shown in significant part to be a complex palimpsest of archaeological sites metal pollution of various ages and ore processing deposits quantitative and qualitative observations of the storage and cycling of heavy metals through the local natural and domestic systems have produced a body of information on processes with which the past has been interpreted heavy metal concentrations in semi continuous sedimentary bodies indicate that over the last 1500 years metals were removed by natural processes at a comparatively slow rate given the scale of the original anthropogenic metal burden the proportions of lead with respect to copper have increased as the overall metal burden has been lowered distinctive anthropogenic metal pollution signatures have been detected in ash and charcoal rich deposits that were discarded onto the banks of a perennial stream in the late neolithic at present the nature of the human activities that might have produced these pollution signatures is unknown substantial metal pollution from industrial scale smelting activity was present from the early bronze age the intensity of heavy metal pollution produced in classical times locally exceeded that recorded at major european copper smelting centres in the nineteenth century a d the pollution evidence indicates that intensive copper smelting at the immediate area took place until approximately the end of the byzantine period with the exception of one further minor episode of smelting radiocarbon dated to cal bp 530 330 an observed partial but perhaps significant parallelism is also noted between this local record and the records of the metal burdens of the northern hemisphere determined at mire or ice sheet sites at high altitudes or high latitudes the paper discusses the extent to which this parallelism might be a geochemical indicator of the actual existence of economic systems of a geographical scope and scale to have both exercised substantial pull upon the resources of such isolated and difficult locations at the wadi faynan and to have generated sufficient overall metal pollution to have materially altered the chemistry of the global atmosphere the comparative absence of quantitative information upon the nature spatial scale and impacts of human industrial and natural processes affecting metal pollution in other metal rich arid lands provides a significant impediment to such approaches that seek to research locally and think globally

relative levels of natural and anthropogenic lead in recent antarctic snow concentrations of lead have been measured by ultraclean isotope dilution mass spectrometry in large blocks of surface snow collected along a 433 km coast interior axis in east antarctica and near the geographic south pole slight contamination existed on the outside of the blocks but concentration profiles from their exteriors to their interiors indicate that lead concentrations in the innermost parts of the blocks do represent the original concentrations in present day antarctic snow geographical variations of lead concentrations appear to be mainly due to local emissions from dumont d urville and amundsen scott stations the globally significant lead concentration in present day antarctic snow is found to be about 2 pg pb g the corresponding value in antarctic air is estimated to be about 7 pg pb m 3 stp which is approximately fivefold larger than total natural lead contributed by soil dusts volcanoes and sea salts a tentative temporal curve of globally significant lead concentrations in antarctic ice and snow for the last 13 000 years is given it shows concentrations of about 0 4 pg pb g throughout most of the holocene with recent fivefold increases to about 2 pg pb g today the general picture is then that four fifths of total lead in the antarctic troposphere today is anthropogenic

no asymmetry in geomagnetic reversals recorded by 1 1 billion year old keweenawan basalts 1 1 billion year old volcanic rocks in north america are thought to record asymmetric geomagnetic reversals indicating non axial dipolar behaviour of the magnetic field high resolution data from ontario suggest that the reversals were instead symmetric and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of north america

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

opal zn si ratios as a nearshore geochemical proxy in coastal antarctica during the last 50 years the antarctic peninsula has experienced rapid warming with associated retreat of 87 of marine and tidewater glacier fronts accelerated glacial retreat and iceberg calving may have a significant impact on the freshwater and nutrient supply to the phytoplankton communities of the highly productive coastal regions however commonly used biogenic carbonate proxies for nutrient and salinity conditions are not preserved in sediments from coastal antarctica here we describe a method for the measurement of zinc to silicon ratios in diatom opal zn si opal which is a potential archive in antarctic marine sediments a core top calibration from the west antarctic peninsula shows zn si opal is a proxy for mixed layer salinity we present down core zn si opal paleosalinity records from two rapidly accumulating sites taken from nearshore environments off the west antarctic peninsula which show an increase in meltwater input in recent decades our records show that the recent melting in this region is unprecedented for over 120 years

trace elements in vostok antarctic ice during the last four climatic cycles li v cr mn co as rb sr ba bi and u were determined by inductively coupled plasma sector field mass spectrometry icp sfms in various sections of the 3623 m vostok deep antarctic ice core dated from 4600 to 410 000 years bp which corresponds to the last four climatic cycles back to isotopic stage 11 3 concentrations of all elements were found to be highly variable with low values during interglacial periods and warm interstadials and much higher values during the coldest periods of the last four ice ages crustal enrichment factors suggest various sources for the different elements rock and soil dust is the dominant source of v mn rb ba and u whatever the period and of li cr co sr and as during cold periods sea salt aerosol together with aeolian dust also contributes significantly to sr whereas volcanic emissions could provide a significant input for as and bi especially during warm periods