radiation of meso neoproterozoic and early cambrian protists inferred from the microfossil record of china abstract marine protists can be affected by and reflect changes in the ecological environment based on available microfossil evidence from late mesoproterozoic early cambrian rocks of china radiations of protists and related changes in ecological environments are discussed in the paper remarkable development of protists had occurred by as early as the mesoproterozoic protist diversity significantly decreased in the middle neoproterozoic due to disruption of the ecological environment that may have been caused by global glaciations the doushantuo microfossil assemblage records high diversity and radiation of protists during the early to middle ediacaran an acritarch assemblage of asteridium comasphaeridium heliosphaeridium megathrix which is contemporaneous with the lowermost meishucunian small shelly fossil assemblage anabarites trisulcatus protohertzina anabarica is widely distributed in the lowermost cambrian strata of south china and the tarim basin and is useful for biostratigraphic correlation with baltica and other areas

a carbon isotope reference curve for ca 1700 1575 ma mcarthur and mount isa basins northern australia abstract shallow marine paleo and mesoproterozoic sedimentary successions are widely distributed in several major basins across northern australia the successions are only gently deformed and their stratigraphy is relatively continuous thus offering an ideal opportunity to document secular variations in carbon isotopes marine carbonate intervals from two of these major basins the mcarthur and mount isa basins have been sampled to document secular variation in 13ccarb from approximately 1700 to 1575 ma in all cases the samples have been tied to a well established sequence stratigraphy which along with u pb shrimp zircon dates provides a time resolution of the order of 1 m y the data presented here thus provide the most comprehensive and best dated 13ccarb stratigraphy yet obtained from such ancient rocks diagenesis occurred early in the carbonate rocks from both basins with the result that fluid movements were restricted and primary carbon isotopic signatures were retained the 13ccarb values from both basins vary within a very narrow range around a mean of 0 6 with extreme values seldom lying further than 1 from the mean that is the curves are essentially flat the results of this study combined with earlier studies on younger rocks imply that the global ocean reached a state of equilibrium in the mid paleoproterozoic and remained stable for much of the following billion years current models of the ocean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required which in turn suggests that the availability of nutrients such as phosphorus was stable and low prolonged nutrient stability may therefore have exerted a major influence upon the evolution of the biosphere over this time interval

biostratigraphic significance of stromatolites in upward shallowing sequences of the early proterozoic duck creek dolomite western australia abstract the branching columnar stromatolite pilbaria perplexa walter 1972 and the microdigitate asperia ashburtonia grey 1985 occur together in upward shallowing sedimentary sequences in the 2 ga duck creek dolomite the distribution of stromatolite taxa is controlled principally by sedimentary environment p perplexa grew in shallow lagoons with the larger p cf perplexa extending into the low intertidal zone a ashburtonia occupied a high intertidal or supratidal setting probably in laterally restricted pools throughout the world pilbariform and asperiform stromatolites occur in stratigraphic units of different ages within each group the taxa are not identical although morphologies are broadly similar the taxonomic variations may be due to evolutionary development rather than environmental factors

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

shallow stratification prevailed for 1700 to 1300 ma ocean evidence from organic carbon isotopes in the north china craton abstract the late paleoproterozoic to early mesoproterozoic from 1700 ma to 1300 ma was highlighted by the assembly of the nuna supercontinent expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes the isotopic composition of carbonate carbon 13ccarb was surprisingly constant during this interval but little is known about the secular variation in the organic carbon isotopic composition 13corg here we report 13corg data from the latest paleoproterozoic 1650 ma to early mesoproterozoic 1300 ma succession in north china the 13corg values range from 25 to 34 and are dependent on sedimentary facies in subtidal and deeper environments 13corg values are low and constant ca 32 but relatively enriched and more variable in shallower intertidal and supratidal environments we attribute the facies dependent variation in 13corg to the presence of a shallow chemocline a probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and or anaerobic photoautotrophic microbes in relatively deep environments from the latest paleoproterozoic to early mesoproterozoic continental shelf of north china

protists of the upper mesoproterozoic ruyang group in shanxi province china abstract organic walled microfossils described in this paper have been obtained from the beidajian formation of the mesoproterozoic ruyang group ca 1300 ma in southern shanxi province china they are characterized by many eukaryotic organisms such as acanthomorphic acritarchs sphaeromorphic acritarchs with a distinct circular excystment acritarchs with multilayered walls and ornamentation and multicellular filaments and algal thallose recently more eukaryotic microfossils have been found and the triaromatic dinosteroid biomarker was extracted from the whole rock sample containing abundant acritarch specimens of dictyosphaera delicata and shuiyousphaeridium one acritarch form spiromorpha segmentata shows morphological similarities to the extant green alga spirotaenia it means that sexual reproduction by conjugation probably arose in the late mesoproterozoic a new genus spiromorpha is proposed here in combination with known mesoproterozoic microfossil assemblages from the roper group of australia and the bahraich group of ganga basin india we infer that marine protests had a rapid development during the mesoproterozoic era

biostratigraphic and detrital zircon age constraints on the basement of the himalayan foreland basin implications for a proterozoic link to the lesser himalaya and cratonic india the himalayan foreland basin in the ganga valley is key to assessing the pre collision relationship between cratonic india and the himalaya the world s largest mountain chain the subsurface ganga supergroup representing the sedimentary basement of the ganga valley has been interpreted as a northern extension of the proterozoic vindhyan supergroup in cratonic india this interpretation is contentious because the depositional age of the ganga supergroup is not resolved whereas the lower ganga supergroup is widely regarded as proterozoic the upper ganga supergroup has been variously inferred to include neoproterozoic lower palaeozoic or cretaceous strata here we integrate biostratigraphic and detrital zircon data from drill cores to show that the entire ganga supergroup is likely proterozoic and can be correlated with proterozoic successions on the northern indian craton and in the lesser himalaya this helps redefine the first order stratigraphic architecture and indicates broad depositional continuity along the northern indian margin during the proterozoic this article is protected by copyright all rights reserved

genesis of carbonate precipitate patterns and associated microfossils in mesoproterozoic formations of india and russia a comparative study abstract the jaradag fawn limestone formation 1 6 ga semri group vindhyan supergroup india contains abundant microfossils and precipitates in early diagenetic bedded and stromatolitic cherts the silicified carbonate precipitates and associated micritic event laminae were formed at or near the sediment water interface and presumably lithified very rapidly the precipitates deposited inorganically without active participation of cyanobacterial mats can be grouped into three morphological categories radial fibrous fans microlaminated stratiform laminae and poorly differentiated laminated stratiform laminae a diverse assemblage of cyanobacteria is preserved both within precipitates where preservation is excellent and in other synsedimentary textures precipitates and microfossil assemblages from four mesoproterozoic formations of siberia are described and compared with those from jaradag fawn limestone comparison of the indian and the siberian precipitates and associated microfossil assemblages reveals an almost identical pattern for contemporaneous mesoproterozoic carbonate precipitates and microbiotas in cherts the microbial assemblages almost exclusively composed of the remnants of cyanobacteria are dominated by entophysalidacean members and short trichomes and can be termed as typical mesoproterozoic microbiotas co occurrence of these microbiotas and precipitates is probably related to the depositional environment of the mesoproterozoic tidal flats with high carbonate saturation neoproterozoic depositional conditions changed drastically as did the composition of microbial communities

carbonate platform development in a paleoproterozoic extensional basin vempalle formation cuddapah basin india abstract sedimentological investigation of the late paleoproterozoic orosirian vempalle formation of the cuddapah basin dharwar craton india reveals three facies association that range from supratidal to deep subtidal sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone intraformational carbonate conglomerate and a variety of columnar domal and stratiform microbialite facies deposition occurred in an extensional regime during development of a low gradient ramp where the distribution of microbialite facies is distinctly depth partitioned a gradual increase in synoptic relief of columnar stromatolites through the section and the upward transition from stratiform to columnar microbialites record a prolonged marine transgression with little or no influx of terrigenous detritus siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory earthquake induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep seated mantle plume that resulted in thermal doming of the dharwar crust prior to the onset of cuddapah deposition isotopic and elemental chemistry of a selection of vempalle formation carbonate rocks record elevated mn2 and fe2 concentrations and depleted carbon isotope values in inner ramp lagoonal facies relative to more open marine stromatolitic facies patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses either within the water column or within substrate pore fluids that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments these data suggest that even in the aftermath of early paleoproterozoic oxygenation that ocean chemistry was heterogeneous and strongly affected by local basin conditions

glacial sediments and erosional pavements near the cambrian precambrian boundary in western henan province china exceptionally well preserved glaciated pavements and a variety of glacigenic sediments comprising the luoquan formation in henan province central china are believed to have formed close to the cambrian precambrian boundary they may therefore be younger than the better known sinian tillites representing the changan and nantuo ice ages they perhaps belong to a distinct glacial epoch that is recorded elsewhere in china and in other continents but for which there is less evidence than for earlier late proterozoic glacial periods lithofacies represented in the luoquan formation are massive diamictite signifying deposition as lodgement or melt out till bedded diamictite waterlain till or glaciomarine glaciolacustrine sediments with dropstones rhythmites with dropstones glaciolacustrine or fjord deposits possibly varves tabular conglomerates and sandstones outwash deltaic or sub aqueous mass flow deposits lens shaped breccia deposits sub aqueous mass flow deposits and gravel sand wedges frost contraction crack infillings the glaciated pavements reveal well preserved striations grooves often intersecting 9plastically moulded9 forms crescentic gouges and steps on a regional scale the linear features are of variable orientation but they indicate a dominant ice flow direction towards the southeast all this evidence points to upland glaciation with erosion followed by deposition in a terrestrial glacial and glaciolacustrine environment

new sedimentological and biostratigraphic data in the kwahu group meso to neo proterozoic southern margin of the volta basin ghana stratigraphic constraints and implications on regional lithostratigraphic correlations abstract since several decades geologists disagree about the lithostratigraphic subdivision of the volta basin sedimentary infilling correlations at the scale of the volta basin were up to now largely limited by the discontinuity of outcrops along the basin margin and by the scarcity of stratigraphic constraints within the sedimentary succession even if a subdivision of the volta basin infilling into three main groups bombouaka oti and obosum groups is now generally accepted there is no agreement for lithostratigraphic subdivision at the formation scale the bombouaka group which represents first deposits of the volta basin sedimentary infilling is particularly concerned during mapping project in ghana authors proceeded to a sedimentological study of the kwahu group deposits on the kwahu plateau area some samples collected during field mapping were analyzed on a palynological point of view and revealed several layers rich in palynomorphs acritarchs siliciclastic sediments composing the kwahu group are distinguished in 12 main facies associations which are interpreted to be representative of both marine and continental environments sedimentological study of these facies associations leads to identify a cyclic vertical evolution of depositional settings at different scales which are interpreted as sequences the kwahu group is thus composed of three distinct macrosequences about 300 500 m thick interpreted as deltaic progradational sequences each macrosequence has been used to define formations for mapping sequence stratigraphy key surfaces being considered as limits for cartographic units the acritarch content of the analyzed samples allows rather coarse but nonetheless useful biostratigraphic constraints which in association with data from sedimentological and sequential analysis are used to improve correlations into the bombouaka and kwahu groups at the scale of the volta basin the present data complemented with the few recently published studies on voltaian deposits lead to a new regional lithostratigraphic synthesis of the bombouaka and kwahu groups based on stratigraphic constrains and sequence stratigraphy

stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of proterozoic lesser himalayan rocks in central nepal abstract in the himalaya of central nepal uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the proterozoic part of the lesser himalayan series has hindered structural mapping east west correlation of units and structures and interpretations of basin architecture during both the proterozoic and paleozoic at the stratigraphic base of the lesser himalayan series in central nepal 441 new u pb isotopic ages of detrital zircons from the previously defined type kuncha kushma and fagfog formations constrain the maximum possible depositional ages for these clastic units to be c 1900 1770 and 1810 ma respectively 182 detrital zircon u pb isotopic ages confirm the identity of another sample of the fagfog formation and field relations indicate that this unit rests depositionally on the kushma formation with no obvious unconformity or lithologic difference this interpretation combined with the recognition that the term kushma formation historically has been applied incongruously to apparently different units in different parts of nepal leads us to recommend abandoning the term kushma formation and replacing it with lower fagfog formation the overlying part of the fagfog then becomes the upper fagfog formation because the 1770 ma constraint on the maximum depositional age comes from the lower part of the unit the entire fagfog formation must have been deposited after 1770 ma the depositional ages of both the lower and upper parts of the fagfog must be younger than the kuncha formation because it was intruded by a granite at 1878 22 ma 2 sigma thus an unconformity lasting at least 86 m y probably separates the kuncha formation deposited between 1900 and 1856 ma considering uncertainties and the fagfog formation deposited after 1770 ma the lower fagfog formation is present in the kali gandaki region but apparently pinches out to the east because it is not present structurally beneath the kathmandu nappe suggestions about the depositional age of the carbonate dominated malekhu formation at the top of the proterozoic part of the lesser himalayan series have ranged from mesoproterozoic to late paleozoic these age estimates largely stem from correlation with parts of the krol succession some 500 km to the west in northwest india and changing interpretations of the depositional age of the krol rocks 104 new measurements of carbon isotope abundances in carbonate from a 300 m measured section through type exposures of the malekhu formation reveal a narrow range of 13c values between 1 7 and 0 2 vpdb and a mean of 0 9 0 4 1 standard deviation these values are inconsistent with correlation with the krol succession which preserves large amplitude positive and negative excursions in 13c values comparison with known marine carbonate 13c values through time suggests deposition of the malekhu formation prior to c 1300 ma upper and lower bounds for deposition of the proterozoic part of the lesser himalayan series thus are c 1300 and c 1900 ma though actual deposition likely occurred during only a portion of this interval further a profound unconformity representing at least 900 m y separates the malekhu formation from the depositionally overlying carboniferous permian sisne formation no upper mesoproterozoic neoproterozoic or lower paleozoic rocks have been found in the lesser himalayan series in central nepal the carbon isotope stratigraphy of the malekhu formation in central nepal matches that of the buxa formation exposed in the ranjit window of sikkim to the east but does not match the chemostratigraphy of rocks correlated with the buxa formation in eastern bhutan and arunachal pradesh northeast india

secular variations in carbon isotopic compositions of carbonates from proterozoic successions in the ming tombs section of the north china platform abstract thick proterozoic unmetamorphosed carbonate dominated successions are widely distributed in the north china platform the ming tombs section near beijing is a well studied stratigraphic section and has been selected for the study of secular variations in the carbon isotopic composition of marine carbonates in ascending order this section includes the changzhougou chuanlinggou tuanshanzi dahongyu and gaoyuzhuang formations in the changcheng group the yangzhuang wumishan hongshuizhung and tieling formations in the jixian group and the jing eryu formation in the qingbaikou group whereas the depositional ages of the changzhougou chuanlinggou and tuanshanzi formations of the changcheng group are older than 1600 ma the gaoyuzhuang yangzhuang and wumishan formations belong to the mesoproterozoic era with ages evidently greater than 1300 1200 ma and the jing eryu formation of the qingbaikou group were deposits of the early neoproterozoic era 13c values vary only within a narrow range between 1 6 and 1 5 permil in the early mesoproterzoic gaoyuzhuang yangzhuang and wumishan formations secular variation in 13c of carbonates from the early neoproterozoic jing eryu formation is more significant showing values from 0 5 to 3 3 permil the secular variations in 13c values of carbonates from the proterozoic successions in the ming tombs section can be broadly correlated with those in the jixian section of the north china platform they compare well with the established global isotopic curve of the mesoproterozoic and neoproterozoic eras

organic walled microfossils from the tonian gouhou formation huaibei region north china craton and their biostratigraphic implications abstract the meso and neoproterozoic huaibei group in the huaibei region north china craton ncc is emerging as a target for geobiological and tectonic studies due to its thick unmetamorphosed sediments with well preserved organic walled fossils however the lack of accurate age constraints for this sedimentary sequence dramatically hampers our ability to take full advantage of the rich geological and paleontological history recorded in the huaibei group particularly the depositional age of the uppermost unit of the huaibei group the gouhou formation is a controversial issue although it is widely accepted that there is an unconformity between the gouhou formation and the overlying early cambrian houjiashan formation the magnitude of this unconformity and hence the scale of the underlying tectonic event are unconstrained due to the uncertainty of the sedimentary age for the gouhou formation which has been variously interpreted as cambrian cryogenian ediacaran 720 ma to 541 ma and tonian 1000 ma to 720 ma to improve our knowledge about this sedimentary gap we carried out a biostratigraphic study of the gouhou formation our investigation using a low manipulation maceration technique revealed a diverse organic walled microfossil assemblage a total of 22 taxa have been revealed including a new species dictyosphaera tacita n sp importantly the co occurrence of trachyhystrichosphaera valeria and dictyosphaera in the gouhou formation suggests a likely tonian age the biostratigraphic data indicate an infra cambrian depositional gap of 200 myr between the tonian gouhou formation and cambrian houjiashan formation perhaps driven by a major tectonostratigraphic event

sinian ice ages and glacial sedimentary facies areas in china abstract glacial rocks of the sinian period are widespread in china on the bases of stratigraphy and geochronology three sinian ice ages can be discerned in ascending order these are the gucheng nantuo and luoquan ice ages the gucheng and nantuo ice ages are assigned to the lower sinian 740 700 ma and the luoquan ice age is tentatively placed between about 640 ma and 580 ma this paper is largely concerned with the distribution of glacial sedimentary facies areas a glacial sedimentary facies area is an area over which the same or similar facies are developed terrestrial transitional and marine glacial sedimentary facies areas have been differentiated nine glacial sedimentary facies have been recognized in the sinian of china

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

new record of organic walled morphologically distinct microfossils from the late paleoproterozoic changcheng group in the yanshan range north china abstract eukaryotic life has likely existed since the late paleoproterozoic yet little is known about its early diversity and phylogenetic relationships organic walled microfossils owms with conspicuous morphology provide a unique material to investigate the deep evolution of eukaryotic and prokaryotic microbial clades here we report a diverse assemblage of owms from the lower changcheng group c 1673 1638 ma changzhougou and chuanlinggou formations in the yanshan range north china which consists of 15 species including 2 that are newly described the fossil assemblage is dominated by spheromorphs with less numerous process bearing vesicles as are colonial and filamentous forms among these 6 morphologically complex taxa dictyosphaera 2 species of germinosphaera pterospermopsimorpha simia and valeria are identified as unambiguous unicellular eukaryotes four species cucumiforma navifusa schizofusa and large leiosphaeridia with relatively simple morphology but having large size thick wall and some showing median split excystment structures are of probable eukaryotic affinity however various colonial microfossils could be either eukaryotes or prokaryotes the new record of morphologically disparate owms represents one of the earliest occurrences of eukaryotes in both china and the world and indicates that the eukaryotic life was already well established in the late paleoproterozoic and was of moderate diversity similar to that of the mesoproterozoic

tappania bearing association of the siberian platform biodiversity stratigraphic position and geochronological constraints abstract a new fossil microbiota yielding eukaryotic tappania is described from the mesoproterozoic kamo group of the central angara basin siberian craton the assemblage also includes valeria satka lophosphaeridium pulvinosphaeridium miroedichia osculosphaera tasmanites obruchevella glomovertella as well as branching and anastomosing filamentous microfossils of putative fungal affinities and peculiar short trichomes that together substantially increase the biodiversity of the tappania association stratigraphic position and geochronological constraints indicate a relatively young late mesoproterozoic age of the tappania bearing sediments there appears to be a continuity between the tappania association and the neoproterozoic microbiota the latter being firmly rooted in the late mesoproterozoic

c isotope composition of carbonates from indravati basin india implications for regional stratigraphic correlation abstract the indravati basin represents an unmetamorphosed and unfossiliferous sequence and shows a broad similarity in lithological association with other purana basins of central india the carbonates belonging to indravati basin have on average moderately positive 13 c values up to 4 5 numerous successions reported from other parts of world show similar 13 c values and contain evidence to suggest that the latest mesoproterozoic to early neoproterozoic period may represent an isotopically recognizable interval globally the elevated 13 c values of indravati basin are also correlatable with other purana basins in india including upper vindhyan supergroup

biostratigraphic and chemostratigraphic constraints on the age of early neoproterozoic carbonate successions in north china abstract thick carbonate successions of proterozoic age were deposited in the huaibei and huainan regions along the southern margin of the north china block these successions including the huaibei group in the huaibei region and the huainan and feishui groups in the huainan region have been correlated with the qingbaikou group which outcrops in the jixian area of north china and is the eponymous succession of the qingbaikouan system traditionally regarded as 1000 800 ma recently it has been shown that lower qingbaikouan strata in the jixian area may be as old as 1300 1400 ma by correlation the huaibei huainan and feishui groups may include mesoproterozoic strata but these units have also been interpreted as tonian or cryogenian ediacaran cambrian in age these contradictory age interpretations hamper our ability to explore the rich paleontological and sedimentary records preserved in these rocks and to assess paleogeographic reconstructions in this report we describe new microfossils and stable isotope data 13ccarb 18ocarb 34scas from the huaibei huainan and feishui groups possible vase shaped microfossils from the jiayuan formation of the lower huaibei group and characteristic early neoproterozoic acritarchs particularly trachyhystrichosphaera aimika from the gouhou and liulaobei formations are consistent with a tonian age for the huaibei and huainan groups similarly carbon isotope trends in the huaibei huainan and feishui groups are comparable with tonian 13ccarb patterns in south australia and laurentia with a plateau of moderately positive 1 5 values followed by pronounced negative 13ccarb excursion in the gouhou formation which may be equivalent to the bitter springs anomaly in south australia 34scas values of the huaibei group are mostly 25 40 similar to values from the stratigraphically equivalent jingeryu formation in the jixian area overall the combination of biostratigraphic chemostratigraphic and available radiometric data suggests that the huaibei huainan and feishui groups are early neoproterozoic tonian in age

meso neoproterozoic isotope stratigraphy on carbonates platforms in the brasilia belt of brazil abstract carbonate platforms were present worldwide during the proterozoic eon and variations in their c and sr isotope ratios are commonly used as a correlation tool particularly through the neoproterozoic when rapid secular change in marine c and sr isotope values permit distinction between cryogenian glacial events in central brazil the late mesoproterozoic and neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited and later deformed along the eastern margin of the sao francisco craton these strata are divided into the three major groups 1 the paranoa group which consists of a succession of sandstone siltstone rhythmite and selected intervals of carbonate 2 the macaubas group which consists of a glacial diamictite jequitai formation and 3 the bambui group with an important carbonate bearing succession that includes characteristic cap carbonate facies in its lower strata carbonate facies of paranoa and bambui groups typically occur in unconformable contact and when the diamictite of jequitai formation is absent it can be difficult to determine the stratigraphic position of these lithological similar groups furthermore uncertainties in the age of the bambui group has lead to several distinct interpretations regarding the age of the jequitai glacial diamictites we investigated the c o and sr isotopes and chemical composition of carbonate rocks in five measured sections including both pre and post glaciation carbonate successions the 13 c pdb values in the upper paranoa group occur in a narrow interval between 0 6 and 3 6 whereas the post glacial bambui group begins with substantially negative values as low as 5 7 in cap dolomite facies and rises to values up to 11 permil in limestone of the upper sete lagoas formation similarly carbonate rocks of the paranoa and bambui groups are distinct in terms of their 87 sr 86 sr ratio generally non radiogenic ratios between 0 7056 and 0 7068 are recorded in the upper paranoa group and ratios between 0 7074 and 0 7080 occurring within the bambui group the stratigraphic pattern of the c and sr ratios indicates distinctive isotopic characteristics for these two carbonate successions the isotopic data for the cabonates in the paranoa group are consistent with a sedimentation age in upper mesoproterozoic or lowermost neoproterozoic preceding the first cryogenian glaciation carbonate facies and the isotopic data for the lower bambui group suggest a relationship with the second cryogenian glaciation

1 3 billion years of acritarch history an empirical morphospace approach abstract acritarchs are a group of organic walled vesicular microfossils interpreted as protists and are among the first eukaryotes preserved in the fossil record taxonomic inconsistencies amongst acritarch workers have made it difficult to address the evolutionary history of this group through more traditional methods i e biodiversity through species counts we have constructed an empirical morphospace to examine the first 1 3 billion years of acritarch evolution we show that protist morphologic evolution is broadly correlated with major environmental and biologic revolutions in earth history such as late neoproterozoic global glaciations the first appearance of the ediacaran metazoans and the cambrian explosion our results also show that protist morphologic expansion precedes their taxonomic diversification this pattern similar to that seen in phanerozoic animal clades suggests that early morphospace saturation and convergence are common occurrences in eukaryote macroevolution in addition our data do not support a monotonic increase in maximum diameter of acritarch vesicles through the proterozoic instead maximum vesicle diameter appears to fluctuate in the proterozoic before decreasing significantly in the early cambrian

a density dependant finite element model for analysis of saltwater intrusion in coastal aquifers summary saltwater intrusion is a serious problem in coastal regions all over the world it is one of the processes that degrade water quality by raising salinity to levels exceeding acceptable drinking water standards it may occur due to human activities and or by natural events over abstraction is considered the main cause of saltwater intrusion moreover climate change and sea level rise speed up saltwater intrusion this paper presents the development and validation of a coupled transient finite element model for simulation of fluid flow and solute transport in saturate and unsaturated soils with application to study saltwater intrusion in coastal aquifers the model includes coupling of water flow air flow heat flow and solute transport furthermore transient density dependent flow is included in the model and the dependency of dispersion on velocity is considered different mechanisms that govern solute transport in porous media including advection diffusion dispersion adsorption chemical reactions and biological degradation are included in the model the governing equation of the solute transport is solved together with three balance equations for water flow air flow and heat transfer the nonlinear system of governing differential equations is solved using the finite element method in the space domain and a finite difference scheme in the time domain the model is validated by application to a standard case study from the literature henry s problem and then applied to predict saltwater intrusion in a coastal aquifer the results of the model predictions are presented and discussed

stratigraphic variations in carbon and oxygen isotopes in the dolostone of the carswell formation proterozoic of northern saskatchewan petrographic and stable isotopic analyses of stromatolitic sediments deposited in nearshore environments provides us with some of the best information available on ancient environments diamond dri

contributions of u th pb dating on the diagenesis and sediment sources of the lower group bi of the mbuji mayi supergroup democratic republic of congo abstract in this paper we present new age constraints for the lower part of the meso neoproterozoic sedimentary mbuji mayi supergroup democratic republic of congo drc this supergroup preserves a large diversity of organic walled microfossils evidencing the diversification of early eukaryotes for the first time in central africa we use different methods such as in situ u pb geochronology by la icp ms and u th pb chemical datings by electron microprobe on diagenetic and detrital minerals such as xenotimes monazites and zircons we attempt to better constrain the provenance of the mbuji mayi sediments and the minimum age of the mbuji mayi supergroup to constrain the age of the microfossils results with la icp ms and emp provide new ages between 1030 and 1065 ma for the diagenesis of the lower part of the sedimentary sequence these results are consistent with data on biostratigraphy supporting the occurrence of worldwide changes at the mesoproterozoic neoproterozoic boundary

combined micro fourier transform infrared ftir spectroscopy and micro raman spectroscopy of proterozoic acritarchs a new approach to palaeobiology abstract micro scale analytical techniques permit correlation of chemistry with morphology of individual proterozoic acritarchs organic walled microfossils and thus provide new approaches for elucidating their biological affinities a combination of micro fourier transform infrared ftir spectroscopy and laser micro raman spectroscopy was used to investigate the organic structure and composition of individual acritarchs well preserved neoproterozoic acritarchs from the tanana formation australia ca 590 565 ma and mesoproterozoic acritarchs from the roper group 1 5 1 4 ga australia and ruyang group china 1 4 1 3 ga age poorly resolved but certainly 1000 ma and h stretching bands in the 2900 cm 1 region relative to the c c aromatic ring stretching band at 1600 cm 1 this ftir spectrum is consistent with the ftir spectra obtained from algaenans isolated from extant chlorophyte and eustigmatophyte microalgae ftir spectra of leiosphaeridia sp from the tanana formation contain a less intense aliphatic c h stretching band relative to the c c aromatic ring stretching band by comparison the spectra acquired from the mesoproterozoic acritarchs were dominated by c c aromatic ring stretching bands at 1600 cm 1 relative to moderate weak ch3 terminal groups 1345 cm 1 c h aliphatic stretching 3000 2700 cm 1 and c o 1710 cm 1 although some differences in biopolymer composition occurred between species curve fitting of the aliphatic c hx stretching region provides greater insight into the aliphatic structures of the acritarchs the ch2 ch3 intensity ratio can be used to assess the relative chain length and degree of branching organic material in the tanarium conoideum consists of straight long chain hydrocarbons while the other acritarchs contain hydrocarbons consisting of short chains that are highly branched in this study it was found that raman spectroscopy does not provide additional information about biopolymer composition of proterozoic acritarchs but rather offers complementary data regarding the aromaticity and degree of saturation of the macromolecular structure of acritarch cysts

synsedimentary cements as paleoproterozoic platform building blocks pethei group northwestern canada peritidal and subtidal carbonates of the pethei group are a complex arrangement of fibrous and micritic synsedimentary precipitates carbonate grains and detrital micrite synsedimentary precipitates present largely as stromatolitic laminae constitute the bulk of the pethei platform providing about 75 of the total carbonate they occur as stromatolitic precipitates precipitates closely associated with stromatolitic laminae which constitute about 60 of the rock and encrusting precipitates precipitates outside stromatolites and between grains i e cement which constitute about 15 of the rock the most abundant precipitates are fibrous in the form of stromatolitic microdigitate stalks tufa and masses of radial fans as well as encrusting pore filling fans isopachous rinds and laminar crusts micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds the ratio of fibrous to micritic precipitates is 2 1 stromatolitic precipitates are arranged in three distinct microfabrics 1 ubiquitous spar micrite couplets of fibrous and clotted micrite laminae 2 deep water vertical fibrous masses that precipitated over micritic precipitate cores and 3 supratidal microdigitate crusts of branching fibrous stalks micritic cements are interpreted to be of biogenic origin having precipitated somehow in association with microorganisms fibrous precipitates are interpreted to be largely abiotic since stromatolitic and encrusting precipitates are petrographicaily similar they probably share similar origins there is little evidence of sediment trapping and binding as a mechanism for stromatolite formation the abundance of synsedimentary precipitates varies with platform geometry subtidal ramps subtidal rimmed shelves subtidal open shelves peritidal rimmed shelves while the basic building blocks of the pethei platform synsedimentary precipitates carbonate grains and detrital micrite are the same as those in phanerozoic platforms their distribution and relative proportions are different these differences reflect a proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation

kotuikan formation assemblage a diverse organic walled microbiota in the mesoproterozoic anabar succession northern siberia abstract abundant and diverse microfossils from shales of the lower member of the 1500 ma old kotuikan formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the kotuikan assemblage is dominated by prokaryotic cyanobacteria both filamentous oscillatorialeans and nostocaleans which are represented primarily by cellular trichomes and by empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens however unlike mesoproterozoic microbiota of shallow water settings the kotuikan compressed organic walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans as well as other morphologically complex microorganisms the kotuikan assemblage contains numerous taxa previously recorded from neoproterozoic successions e g elatera eosolena palaeastrum pterospermopsimorpha rugosoopsis and navifusa but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits the kotuikan assemblage documents the status of the marine biosphere at an early mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic microbiota as reported in this study 35 taxa were identified in the kotuikan assemblage of which five forms are described informally that are assigned to 18 genera of microscopic prokaryotes and eukaryotes two new genera and five new species are proposed hirudiforma lancetica gen and sp nov h simmetrica gen and sp nov lineaforma elongata gen and sp nov elatera minor sp nov eosolena minuta sp nov

deposition and deformation of late archaean sediments and preservation of microfossils in the harris greenstone domain gawler craton south australia abstract late archaean sediments felsic mafic and ultramafic volcanics in the harris greenstone domain central gawler craton south australia are interpreted to have been deposited at 2520 ma in back arc settings and were metamorphosed at 2440 ma sedimentary evidence suggests that the metasediments in the domain might have been deposited in fluvial estuarine to deltaic shelf environments and deformed by intermediate amphibolite facies metamorphism accompanying the sleafordian orogeny at 2440 ma microfossils are preserved in metachert layers that had been boudinaged into lenses these microfossils including spinose acritarchs are organic walled as evidenced by laser raman microscopy fossil bearing off cuts were then treated by hf and similar microfossils degraded organic matter and acid resistant minerals such as zircon were exposed in situ on the etched surface their morphologically complex forms and detailed wall structures suggest that these microfossils are of primary biological origin the microfossils in foliated metachert lenses either from original deposition or from post depositional quartz veins are older than the 2440 ma age of the sleafordian orogeny which provides a minimum age for the host rocks in either case these spinose acritarchs are the oldest protists known to date

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

diversity of organic walled microfossils from the early mesoproterozoic ruyang group north china craton a window into the early eukaryote evolution mesoproterozoic era was an important time for the initial diversification of eukaryotic groups and the appearance of the first complex morphologies while eukaryotes evolved around 2 4 ga the firs

textural transition in an aragonite travertine formed under various flow conditions at pancuran pitu central java indonesia abstract a large travertine mound at pancuran pitu central java indonesia displays various types of lamination reflecting different hydrological conditions to understand the geomicrobiological processes of the laminated travertines we investigated the mound s hydrology water chemistry texture microbial composition and volume percentage of carbonate mineral vpc we identified four types of travertines dense travertine 80 vpc flow rate 200 cm s lithified microbial travertine 60 vpc flow rate 70 cm s fragile microbial travertine 30 vpc flow rate 10 cm s and poorly lithified microbial mat

proterozoic ministromatolites with radial fibrous fabric small digitate stromatolites with diameters in the range of 0 2 20 mm ministromatolites are common in early and middle proterozoic carbonate sequences and extend stratigraphically from the archaean to the holocene an occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial fibrous fabric and microscopically crinkled microcrenate lamination is described from the 1 9 gyr old belcher supergroup mcleary formation in southeastern hudson bay canada the structures which can be considered to be a variety of tufa are unusually well preserved because of early diagenetic silicification columnar types are referable to pseudogymnosolen asperia and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern canada where the fabric is normally preserved by a secondary mosaic of dolomite the textural evidence of angulate cross sections and rectilinear divergent patterns indicates that the radial fibrous fabric represents primary or very early diagenetic precipitation and that pseudogymnosolenids with mosaic dolomite originally also had radial fibrous structure the precipitation may have been within or on microbial mats

stromatolite assemblages of the late precambrian in china abstract the late precambrian as used in this paper is represented by the sections in the jixian chihsien and yangtze gorges these rocks correspond to the middle late proterozoic of some authors and range in age from c 1900 to 600 ma the succession can be subdivided into four systems the changcheng jixian qingbaikou and sinian systems all of which yield abundant stromatolites based on the latest studies we suggest a new sixfold classification scheme supertype type subtype family genus and species and refer the stromatolites to 13 families and 50 genera including some material published already in china the stromatolites of the late precambrian of china may be represented by elements from yanshan southern liaodong peninsula and kaiyang of guizhou etc they are subdivided into eight stromatolite assemblages and correlated throughout most of china

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

a molecular and isotopic geochemical study of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china abstract this paper systematically describes the molecular and isotopic characteristics of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china a correlation is made between molecular fingerprints and paleoenvironmental and paleontologic data acquired from these strata the agreement between these parameters suggest that the biomarkers preserved in these proterozoic sediments are good indicators of the evolution of the paleoenvironment and early life in the yanshan basin the fact that extended regular isoprenoids c20 were detected in most of the studied samples suggest a sustained lipid input to sedimentary organic matter from halophiles and other possible archaebacteria through meso to neoproterozoic two dehydroabietin isomers i e 18 and 19 norabieta 8 11 13 trienes were unexpectedly present in these samples which provides evidence suggesting that the compounds with abieta structure could be derived from primitive organisms such as algae the stable carbon isotope characteristics of kerogen and coexisting carbonate show a rapid decline in atmospheric co2 concentration from 1 73 to1 58 ga and a stable concentration afterwards the difference c in isotopic composition between carbonate and associated kerogen lies in zonal variations along the stratigraphic sequence which can be attributed to the alternations of marine transgressions and regressions that occurred in the yanshan basin during the proterozoic the carbon isotope relationships between kerogen and alkanes from these strata are consistent with the hypothesis that before 1 4 ga both n alkanes and isoprenoids had been derived mainly from heterotrophic halophiles and other archaebacteria but after that only isoprenoids originated from archaebacteria whereas n alkanes were possibly derived from photosynthetic organisms changes in the source of n alkanes around 1 4 ga may suggest a decline of heterotrophic reworking to primary lipids which can be related to the formation of a quiet shallow water environment of an epicontinental sea at this stage in the yanshan basin

stable isotopic compositions of carbonates from the mesoproterozoic bangemall group northwestern australia marine carbonate rocks from the mesoproterozoic bangemall group of northwestern australia show little deviation 1 3 in whole rock delta 13c carb values about a mean of 0 5 this narrow range persists despite close sampling every 10 20 m through long sections up to 2500 m that are geographically widespread up to 250 km apart over many depositional environments supralittoral to outer shelf sediment sources stromatolitic bioherms to detrital calcilutites and rock types pure limestones to dolomitic shales the only major excursions from the norm seem related to unusual environmental or post depositional processes as they are correlated with large enrichments to 3 or depletions to 16 in 18o relatively heavy delta 13c values up to 2 5 occur in a single bed of brecciated ferruginous dolostone at a single locality these abnormal values may result from local evaporitic conditions limey and shaley nodular dolostones have delta 13c values as low as 4 3 probably caused by remineralization of organic matter during late and patchy dolomitization most notably sharp negative excursions in delta 13c up to 8 4 occur in bleached kerogen free rocks with mineral assemblages of dolomite quartz calcite tremolite talc reflecting isotopic re equilibration in thick metamorphic aureoles around dolerite intrusions general environmental variations are minor with delta 13c values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative there are no strong secular trends but subtle fluctuations within the range 2 to l can be correlated along the northwestern margin of the basin this resembles the pattern seen in other mesoproterozoic successions but is markedly unlike the heavy background 5 and extreme variations up to l0 in delta 13c evident in neoproterozoic successions of similar thickness and environmental setting hence in contrast to the neoproterozoic the global rate of organic carbon burial was probably fairly constant during deposition of the bangemall group and perhaps generally during the mesoproterozoic as was the redox state of the atmosphere and hydrosphere

organic walled microfossils from the proterozoic thule supergroup northwest greenland abstract the proterozoic rocks of the intracratonic thule basin on the northern margin of the canadian greenlandic shield have recently been referred to as the thule supergroup the succession comprises unmetamorphosed unfolded sedimentary and magmatic rocks which are with few exceptions not deformed five groups have been defined and fine grained siliciclastic samples from the uppermost three baffin bay dundas and narss rssuk groups yielded moderately well preserved acanthomorphic and non acanthomorphic acritarchs prasinophytes and filamentous microfossils the age of the supergroup is quite poorly constrained at the base by a 1268 ma basaltic sill and at the top by a basic dyke swarm with a k ar age range between 630 and 725 ma that cuts the entire succession biostratigraphical studies further constrain the age of the thule supergroup and suggest that the succession is of a late mesoproterozoic earliest neoproterozoic age this age assignment is corroborated by independent stable isotope data and correlation with the bylot supergroup to the south this leads us to propose that the thule supergroup was deposited between c 1300 and c 1200 ma microfossil taxa previously interpreted as exclusively occurring in pre varangerian neoproterozoic rocks i e simia annulare jankauskas tasmanites rifejicus jankauskas and possibly vandalosphaeridium varangeri vidal are consequently re evaluated process bearing acritarchs herein attributed under open nomenclature to comasphaeridium sp trachyhystrichosphaera truncata and vandalosphaeridium sp are of special palaeobiological importance since acanthomorphic acritarchs have rarely been reported from pre neoproterozoic successions

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

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

highly 13c enriched carbonate and organic matter in the neoproterozoic sediments of the bambuigroup brazil abstract paleontological and radiometric dating methods rb sr k ar and pb pb suggest neoproterozoic ages for sedimentary carbonate rocks of the bambuigroup in the southern part of the sa o francisco basin of central brazil the 13c values for carbonates fall within two ranges 2 to 2 and 10 to 16 for both groups coexisting organic carbon is depleted in 13c by 28 2 relative to the carbonates similar to numerous neoproterozoic examples worldwide secular variations in 13c in the bambuigroup show a significant positive excursion beginning with values near 0 in carbonates immediately above a diamictite and rising to values as high as 16 upsection the sr isotope data for carbonates from the bambuigroup agree with the age inferred from the secular variation in oceanic 13c values the striking feature in this study is that both the carbonate and total organic carbon are considerably more enriched in 13c than previously reported for neoproterozoic carbonates elsewhere it is not clear if these unusual 13c values are caused by primary or secondary processes

isotopic composition of organic and inorganic carbon from the mesoproterozoic jixian group north china implications for biological and oceanic evolution a b s t r a c t analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of earth surface environments particularly in the latter portion of the proterozoic eon earlier mesoproterozoic successions however have received comparatively little attention due to the relatively subdued nature of carbon isotope variation in this study we present high resolution isotopic profiles from three sections in the yanshan basin north china craton that combined comprise the entirety of the early mesoproterozoic 1600 1400 ma calymmian period jixian group high resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con strain both the pattern and origin of isotopic variation in the mesoproterozoic marine carbonate rocks of the jixian group show generally muted isotopic variation with average values near 0 consistent with previous observations from the early mesoproterozoic data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long term decrease in pco2 and conse quently in the isotopic buffering capacity of marine dissolved inorganic carbon dic by contrast the isotopic composition of marine organic matter suggests facies dependent differences in carbon cycling organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom position in shallow water environments and increased remineralization by anaerobic heterotrophs in deeper water environments correlation between organic carbon composition and depositional environ ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters

problems in applying preservation facies and sequence models to sinian neoproterozoic glacial sequences in australia and asia abstract the sinian 800 530 ma is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate eustasy and possibly tectonics understanding sinian stratigraphy may help interpret the co evolution of life and environment during the period though sinian successions are poorly dated and difficult to correlate preservation facies and sequence models based on mesozoic recent successions may be used to interpret them preservation models based on basin subsidence permit estimates of how thick and complete successions can be in different tectonic settings and how long it took for them to be deposited facies models for rift craton shelf slope and basin environments help in identification of these different tectonic settings sequence models facilitate interpretation of the interplay of tectonic subsidence eustasy rates of sedimentation and climate such models applied to the sinian successions of australia and asia indicate the following great shallowing upwards megacycles occur in sinian successions and each ends with regional glaciations deep rift basins formed around 850 800 ma preserve the oldest megacycles but these basins soon were filled with rapidly deposited glaciogenic sediments relatively complete sequences are found only in deep basins subject to repeated rifting cratons and shallow shelves preserve only the youngest megacycles older deposits have been removed by younger glacial advances deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types but these sequences are not developed to the same extent in all blocks problems in correlating the four great megacycles recognized elsewhere with those of asia will persist until the requisite facies analysis and u pb dating of interbedded volcanics are done

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

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

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

sulfur and carbon isotope records from 1700 to 800 ma carbonates of the jixian section northern china implications for secular isotope variations in proterozoic seawater and relationships to global supercontinental events abstract this study is a comprehensive stable isotope survey of the marine carbonate dominated upper paleo to lower neoproterozoic stratigraphy of jixian county china carbonate associated sulfate cas was extracted and measured for 34 s cas using the same samples analyzed for 13 c carbonate this integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of proterozoic seawater we specifically sampled marine carbonate intervals from the lower section of the chuanlinggou formation changcheng group ca 1700 ma to the top of the jingeryu formation qingbaikou group ca 800 ma 13 c carbonate values are mostly negative in the upper paleoproterozoic changcheng group with an ascending trend from 3 to 0 we observed variation of approximately 0 1 in the mesoproterozoic jixian group and positive values of 2 2 characterize the lower neoproterozoic qingbaikou group stratigraphic variations in 34 s cas are more remarkable in their ranges and magnitudes including conspicuously high values exceeding 30 in the three intervals at ca 1700 ma 1300 1100 ma and 1000 900 ma in the changcheng group 34 s cas values are typically higher than 25 with only a few values of less than 15 in contrast most of the data spanning from the mesoproterozoic tieling formation of the jixian group to the lower neoproterozoic jingeryu formation of the qingbaikou group are highly variable between 10 and 25 with some values exceeding 25 in the late paleoproterozoic 1700 1600 ma a 10 decrease in 34 s cas and 3 increase in 13 c carbonate are coincident with and likely related to the breakup of columbia a supercontinent that predated rodinia carbon and sulfur isotope data from the mesoproterozoic when global tectonic activity was comparatively weaker fall mostly in the ranges of 15 10 and 0 1 respectively but fluctuations of 20 for 34 s cas and 3 for the 13 c carbonate at ca 1450 1400 ma may reflect subduction and large scale magmatic activity in island arcs marking the end of columbia breakup from the late mesoproterozoic ca 1300 1100 ma to the early neoproterozoic ca 800 ma the 13 c and 34 s of seawater increased gradually with increasing variability most impressive are 34 s cas values that exceed 30 in two intervals at ca 1300 1100 ma and ca 1000 900 ma which may reflect the assembly and early breakup of rodinia although gaps in the record remain and studies of even higher resolution are warranted our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of c and s furthermore periods of the proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity

macroscopic worm like body fossils from the upper precambrian 900 700 ma huainan district anhui china and their stratigraphic and evolutionary significance abstract abundant and varied macroscopic worm like body fossils from the late precambrian liulaobei formation huainan group and the jiuliqiao formation feishui group in the huainan district northern anhui province china are further investigated through this progress report after the initial discovery by zheng wenwu and a recent study by wang guixiang sinosabellidites zheng in the liulaobei formation is a worm like organism of questionable metazoan origin it has merely a superficial resemblance with the tubes of late vendian early cambrian true sabelliditida but has much in common with the accompanying macroscopic algal remains of tawuia hofmann both pararenicola wang and protoarenicola wang are representatives of primitive worm like animals in the jiuliqiao formation their macroscopic size elongate cylindrical body anterior aperture and apparatus prominent and elastic annulations are characteristic for their metazoan origin although they lack sufficient evidence for their definite relationship with any known kind of living worms the liulaobei formation and the jiuliqiao formation are dated at about 850 ma and 740 ma respectively regional geology and stratigraphic correlation and the occurrence of the distinctive chuaria tawuia assemblage suggest a time range between less than 900 ma and over 700 ma for the whole shallow water marine sedimentary sequence of the huainan and feishui groups this latter group is disconformably covered by the drop stone facies diamictites and tillites of the late precambrian luoquan glaciation apart from the enigmatic sinosabellidites both pararenicola and protoarenicola are claimed to be the oldest multicellular animals so far discovered in the world and the first reliable evidence for the pre ediacarian evolutionary history of manifest metazoan life

simplified physically based models for free surface flow in karst systems abstract most karst aquifers are characterized as a dual flow system comprised of a highly conductive conduit network embedded in a low porosity matrix the conduits are hydraulically connected to the matrix and behave either as a drain or a source depending on the recharge conditions simplified physically based models are herein employed to simulate the spring outflow for such aquifer systems the processes consist of a free surface flow in the conduit that is interacting laterally with a laminar groundwater flow in the surrounding matrix the conduit is subject to a concentrated recharge at its upstream end while the groundwater aquifer is subject to a diffuse recharge over its contributing surface area the flow system is described by a coupled system of partial differential equations the conduit flow is approximated by the kinematic wave equation and the groundwater flow by the linearized boussinesq equation the governing equations are solved using the laplace transform method after an appropriate linearization of the nonlinear coefficient the derived spring discharge models are a function of three dimensionless parameters the time lag parameter the lumped conduit parameter and the aquifer parameter the simulation results highlight the contrast between pressure driven and gravity driven flows and the importance of the conduit matrix interaction on the response of the karst system application of the models on real karst aquifers demonstrates their effectiveness in simulating the observed spring hydrograph using lumped physical parameters of the karst system

microfossils from the lower mesoproterozoic kaltasy formation east european platform abstract basinal shales of the lower mesoproterozoic kaltasy formation sampled from three boreholes drilled into the southeastern east european platform russia contain abundant and moderately well preserved microfossils 34 distinct entities have been identified most assigned to simple sphaeromorphic or small filamentous taxa found widely and characterized by long stratigraphic ranges ornamented microfossils found in coastal successions of other lower mesoproterozoic basins are absent but large filamentous microfossils interpreted as possible benthic photosynthetic eukaryotes are recorded drawing comparisons to relatively deep water shales in siberia in overall aspect the kaltasy microfossils are consistent with other broadly coeval assemblages but they highlight the importance of environment as well as age in determining the distributions of remains that record the early diversification of marine eukaryotes rectia magna is described as a new species

chemostratigraphy of the late mesoproterozoic atar group taoudeni basin mauritania muted isotopic variability facies correlation and global isotopic trends a b s t r a c t sedimentary strata of the atar el mreiti groups taoudeni basin mauritania are characterized by car bon isotope values that fall largely between 2 5 and 4 0 and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from 1 2 ga to 1 1 ga chemostratigraphic results are consistent with recent re os dates of 1 1 ga for atar el mreiti group strata rooney et al 2010 and support an emerging dataset that identifies the late mesoproterozoic as an isotopically distinctive interval in earth history the relatively low amplitude carbon isotope variabil ity limits the use of chemostratigraphy in detailed correlation during this time interval in this study we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of deposi tional facies to construct a rigorous and testable model for the intrabasinal correlation of atar and el mreiti group strata data support a depositional model that results in broadly uniform stratigraphic thicknesses across the west african craton with predominantly stromatolitic facies of the atar group mauritania and the hank and dar cheikh groups algeria representing deposition on shallow water craton margins and clayey carbonate and shale facies of the el mreiti group mauritania representing deposition on a broad shallow water epicratonic platform within this stratigraphic framework regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely asso ciated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean similarly distinct differences in trace element composition recorded in multilayered marine cements within deeper water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox gradients in the water column we use this multifaceted approach to test a recent hypothesis that suggests correlation of atar el mreiti group strata with glaciogenic strata of the vazante group sao francisco craton critically preservation of shallow water carbonate and evaporite strata across more than 1200 km of the predominantly epicra tonic taoudeni basin with minimal evidence for high amplitude sea level change is consistent with inferences of late mesoproterozoic highstand conditions in a global greenhouse climate such charac teristics are inconsistent with suggestions of a time equivalency with glaciogenic strata of the vazante group sao francisco craton despite broad similarities in re os geochronology and carbon isotope com position together these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution 2012 elsevier b v all rights reserved

geochemical and stable isotope signatures of proterozoic stromatolitic carbonates from the vempalle and tadpatri formations cuddapah supergroup india implications on paleoenvironment and depositional conditions abstract spectacular stromatolitic dolomites with varied morphological features are preserved in the vempalle and tadpatri formations of the proterozoic cuddapah basin eastern dharwar craton they exhibit stratifera columnar conical domal and spheroidal lobate morphologies in which columns are branched at some places in tadpatri formation based on their trace elements such as rey and stable isotope compositions carbon and oxygen the vempalle stromatolites have been classified as three types vempalle i stromatolites exhibit pronounced positive eu anomalies with small scale positive ce anomalies whereas vempalle ii counterparts have slight negative to positive eu feeble positive ce and positive gd anomalies vemaplle iii stromatolites display positive ce eu and gd anomalies the tadpatri formation stromatolites have pronounced gd anomalies with low order positive ce and eu anomalies significant depletion in 18o is observed in vempalle ranging from 10 70 to 7 07 and 20 73 to 11 87 in tadpatri formations the tadpatri stromatolites display negative 13c varying from 4 56 to 1 26 contrarily the vempalle stromatolites recorded both positive and negative 13c ranging from 0 24 to 2 00 thereby reflecting their deviation from the popular belief of the boring billion concept of displaying midproterozoic 13c stasis the flat ree patterns along with ce eu and gd anomalies chondritic to superchondritic y ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic terrigenous materials and hydrothermal signatures these cherty dolomitic stromatolites were deposited in subtidal intertidal supratidal zones of a shallow shelf of proterozoic open ocean under warm alkaline and anoxic conditions

neoproterozoic fossils in mesoproterozoic rocks chemostratigraphic resolution of a biostratigraphic conundrum from the north china platform abstract siliciclastic rocks of the ruyang group southern shanxi and the broadly equivalent gaoshanhe group shaanxi contain exceptionally well preserved large o 150 m acanthomorphic acritarchs recently interpreted as late neoproterozoic sinian c 800 544 ma in age this biostratigraphic interpretation is based on the presence of large acanthomorphs in sinian successions of south china and elsewhere and the perceived absence of comparable forms in older rocks it casts doubt on the long accepted interpretation of ruyang and correlative rocks as mesoproterozoic in age 1600 1000 ma in contrast thick marine dolomites in overlying units contain abundant radial fibrous fabrics and a narrow range of 13c values c 0 1 vs pdb features which characterize unambiguously mesoproterozoic carbonates elsewhere on the north china platform and on other continents age estimates based on petrofabrics and chemostratigraphy are corroborated by a u pb zircon age of 999 ma no recorded error for granites which intrude overlying carbonates thus in combination the available data constrain the ruyang siliciclastics and overlying carbonates to be older than about one billion years making shuiyousphaeridium yan and other large process bearing acritarchs from these units among the oldest known distinctly ornamented eukaryotic microfossils

the biostratigraphic conundrum of siberia do true tonian cryogenian microfossils occur in mesoproterozoic rocks abstract abundant and diverse microfossils from shales of the 1500 ma old ust il ya formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the ust il ya assemblage is dominated by cyanobacteria both filamentous oscillatorialeans and nostocaleans represented primarily by cellular trichomes and empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens microbiotas from shallow water settings that contain morphologically simple and small microfossils were originally taken to be representative of mesoproterozoic life however the ust il ya and other compressed organic walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms the ust il ya assemblage contains numerous taxa previously reported from tonian and cryogenian successions e g elatera pterospermopsimorpha rugosoopsis and chuaria but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a few taxa reported from the calymmian microfossil assemblages e g lineaforma aff l elongata satka favosa and elatera aff e minor the ust il ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic world as reported in this study 38 taxa were identified in the ust il ya assemblage of which six forms are described informly that are assigned to 17 genera of microscopic eukaryotes presumably 17 20 entities and prokaryotes a new species is proposed heliconema ustilense

first record of carbonates with spherulites and cone in cone structures from the precambrian of arctic norway and their palaeoenvironmental significance abstract we report for the first time carbonates from the upper ediacaran sedimentary succession of finnmark arctic norway carbonates occur as calcareous siliciclastic beds lenses and concretions some with calcite spherulites and cone in cone cic calcite in a mudrock to fine grained sandstone succession from approximately 3 m to 26 m above the base of the 2nd cycle of the manndrapselva member of the stahpogieddi formation vestertana group they occur c 40 m below the ediacaran cambrian boundary which is well defined by trace fossils thin section petrography and scanning micro x ray fluorescence elemental mapping reveal a layered composition of the calcareous sedimentary rocks in some of those well developed nested cones of cic calcite form the outer layer thin clay coatings outline individual cones the inner layers are composed of 1 carbonate with calcite spherulites grainstone and 2 thinly laminated fine grained calcareous siliciclastics mudstone and wackestone indicated by elevated concentrations of al si fe and ti the inner siliciclastic layers contain framboidal pyrite and probably organic matter formation of calcite spherulites took place probably at the sediment water interface either in a coastal littoral environment or in situ in the sublittoral zone under high alkaline conditions whereas cic calcite formed during burial diagenesis and clearly in pre caledonian time before metamorphism and cleavage formation this new record of carbonates with calcite spherulites and cic structures from the ediacaran of arctic norway adds to their rare occurrences in the geological record

heterogeneous redox conditions and a shallow chemocline in the mesoproterozoic ocean evidence from carbon sulfur iron relationships abstract oxygenation of earth s oceans in the proterozoic was a protracted process that began prior to the great oxidation event goe and culminated in deep ocean ventilation following neoproterozoic glacial episodes with both of these major thresholds accompanied by profound changes in the evolution of earth s biosphere our understanding of the redox structure of mesoproterozoic oceans however is considerably less certain limited proxy investigations of oceanic redox in the mesoproterozoic have focused largely on early mesoproterozoic shelf to basinal settings with comparatively little attention paid to epeiric sea environments that dominate the late mesoproterozoic sedimentary record here we report multi proxy shale redox data from epicratonic and pericratonic environments of the 1 1 ga atar and el mreiti groups taoudeni basin mauritania these strata were deposited across the west african craton during global sea level highstand and provide insight into the redox structure of mesoproterozoic epeiric seas iron speciation pyrite sulfur isotope and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments we suggest that a protracted increase in biospheric oxygenation in the late mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow nearshore areas contributing to the fragmentary pattern of early eukaryotic evolution and diversification

evidence for primary aragonite precipitation lower proterozoic 1 9 ga rocknest dolomite wopmay orogen northwest canada abundant precipitation of aragonite is suggested by well preserved fabrics in dolostones of the lower proterozoic 1 9 ga rocknest formation fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans and cement crusts in cryptalgal tufas aragonite was common on the 1 9 ga rocknest shelf as it is on modern tropical shelves a search for similar marine cement fabrics in other precambrian carbonates should help resolve whether aragonite was an important precipitate from precambrian tropical seas implying an ocean carbonate chemistry similar to today or if aragonite precipitation was restricted to rare times in the past calcite and even dolomite being the main carbonates precipitated as recently suggested by some authors

spatial and temporal variations in carbon and sulfur isotopic compositions of sinian sedimentary rocks in the yangtze platform south china abstract eight stratigraphic sections from the yangtze platform south china were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of sinian sedimentary rocks carbon isotope compositions of carbonate from lower sinian strata deposited between pulses of glacial diamictite range from 3 9 to 9 9 the more negative values recorded from mn carbonates suggest that in part the carbon was derived from the oxidation of organic matter during early diagenesis in the upper sinian strata 13c varies with the facies they are positive for the platform carbonates whereas they are negative from about 3 to 12 for the carbonates formed in or near basin environments it is evident that post depositional alteration accounts for the strongly negative 13c values however some information on primary isotopic compositions can be obtained by evaluating sample quality the primary 13c values of carbonates from the doushantuo formation are ca 3 to 4 and from the dengying formation as low as 6 34s values for pyrite from the lower sinian rocks are highly positive and range from values near the coeval seawater sulfate up to ca 60 in contrast 34s values for pyrite from the upper sinina rocks are negative down to 27 to 30 the feature of sulfur isotopic composition for the lower sinian rocks is consistent with the late proterozoic sulfur isotopic record it might be explained by the supercontinent model many basins on the yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher 34s values than the coeval seawater for a geologically long period an alternative explanation is based on the sulfate minimum zone smz modal because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower sinian shallow water environment particularly in the smz the residual sulfate might have been significantly enriched in 34s so that pyrite produced in closed diagenetic system would yield very positive 34s values

can crystal morphology indicate different generations of dolomites evidence from magnesium isotopes abstract among various geochemical and petrographic approaches dolomite crystal morphology and dolostone fabric have been widely applied in the study of ancient dolostones it is proposed that dolomite crystal morphologies and the rock fabric may reflect the formation temperature and thus can be used to distinguish different generations of dolomite however this scenario has also been challenged by some researchers in order to test whether the dolomite crystal morphology can be used to differentiate different generations of dolomite in this study we measured the mg isotopic compositions 26mg of dolomite with different crystal morphologies 26mg of dolomite is controlled by a variety of factors including temperature magnesium isotopic composition of dolomitization fluids and the flow rate of dolomitization fluids if dolomite with distinct crystal morphologies were derived from different dolomitization processes it is highly plausible that they would have different 26mg five types of dolomite with distinct crystal morphologies and rock fabric were recognized from three sampling intervals s1 s2 and s3 in the middle ordovician majiagou formation in north china different types of dolomite in the same sampling interval have similar 26mg values suggesting that these dolomites might have derived during the same dolomitization event our study indicates that the crystal morphology alone may not unambiguously differentiate the generations of dolomites we propose the following reasons 1 the dolomite crystal morphology might be controlled by various factors rather than the formation temperature alone or 2 the dolomite crystal morphology might be modified in diagenesis but 26mg remains unchanged

diagenetic xenotime age constraints on the sanjiaotang formation luoyu group southern margin of the north china craton implications for regional stratigraphic correlation and early evolution of eukaryotes abstract eukaryote fossil bearing ruyang and luoyu groups are extensively exposed along the southern margin of the north china craton but their age has long been a matter of controversy due to the absence of reliable radiometric dates by dating detrital zircons and diagenetic xenotimes from the sanjiaotang formation of the luoyu group this paper confirms that the ruyang and luoyu groups were deposited during the period of 1750 1400 ma the new age constraints suggest that the sanjiaotang formation is coeval with the xiamaling formation of the jixian group on the northern margin of the north china craton correlation of the luoyu group with the jixian group is thus indicated the ruyang and luoyu groups contain abundant fossils of eukaryotic organisms such as acritarch and macroalgae accordingly eukaryotic proliferation extends back to the mesoproterozoic consistent with results of molecular phylogenetic analysis that a rapid morphological diversification of eukaryotic organisms occurred prior to 1400 ma

a new paleoproterozoic tectonic history of the eastern capricorn orogen western australia revealed by u pb zircon dating of micro tuffs abstract the earaheedy basin is one of several paleoproterozoic siliciclastic basins in the eastern part of the capricorn orogen in western australia in the absence of an adequate chronostratigraphic framework multiple conflicting models have been proposed for the tectonic history of the basin we present in situ shrimp u pb zircon dates of 1971 11 ma 1954 5 ma and 1949 10 ma from three thin tuff horizons micro tuffs in the yelma formation at the base of the basin succession these micro tuffs are only slightly younger than the 1990 6 ma imbin porphyry which has been interpreted as a basement inlier our new dates combined with field mapping suggest that the imbin porphyry is a volcanic subvolcanic complex within the basin micro tuffs in the yelma formation are more than 50 m y older than c 1890 ma micro tuffs within the overlying frere formation deposition of the yelma formation was coeval with continent continent collision farther west in the orogen marked by the 2005 1950 ma glenburgh orogeny and the formation is possibly correlative with peripheral foreland deposits of the padbury group the frere formation was deposited in shallow marine conditions within an intracratonic basin about 60 m y after the glenburgh orogeny the frere formation is unconformably or disconformably overlain by the miningarra group the youngest unit of which is the mulgarra sandstone this sandstone was deposited after c 1865 ma and derived either from exposed frere formation or from a now buried source within the capricorn orogen that was uplifted by inception of the 1820 1770 ma capricorn orogeny therefore the earaheedy basin comprises at least three distinct unconformity bound successions deposited in contrasting tectonic settings

raman spectroscopy and structural heterogeneity of carbonaceous material in proterozoic organic walled microfossils in the north china craton abstract raman spectroscopy is a minimally intrusive and minimally destructive technique that has played an important role in analyzing precambrian carbonaceous microfossils and microstructures previous studies have shown that heterogeneities in structural order of carbonaceous material cm as revealed by raman spectroscopy can be preserved in proterozoic silicified cyanobacterial fossils however such heterogeneities have not been documented in proterozoic eukaryotic microfossils preserved as carbonaceous compressions in this study we used raman spectroscopy to investigate the structural characteristics of organic walled microfossils including eukaryotes preserved as carbonaceous compressions from the late paleoproterozoic ruyang group and the early neoproterozoic tonian liulaobei formation in the north china craton raman geothermometers indicate that cm experienced low grade metamorphism with apparent peak metamorphic temperatures of 200 c for both the ruyang and liulaobei microfossils heterogeneities in structural characteristics of cm in eukaryotic microfossils were found among different subcellular structures of shuiyousphaeridium macroreticulatum and among different taxa from the same stratigraphic horizon we suggest that these heterogeneities can be attributed to differences in precursor organic compounds thus the importance of organic precursors should be considered in cm raman geothermometry studies especially in environments where heating events occurred quickly the raman data presented here are also consistent with hypotheses that the ruyang microfossils dictyosphaera delicata and s macroreticulatum are biologically conspecific taxa with their vesicle walls having similar cm raman features and that the liulaobei microfossils leiosphaeridia jacutica and l tenuissima are distinct taxa despite their similarities in morphologies other than vesicle wall thickness although more data are needed to verify the consistency of interspecies intraspecies and intraspecimen variability this study indicates that raman spectroscopy may have the potential to provide an independent test of early eukaryote taxonomy which has been traditionally based only on morphological features and to reveal heterogeneities of cm of early eukaryotic organisms