sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

conditions of formation and diagenetic evolution of upper proterozoic phosphate nodules from southern sweden evidence from petrology mineral chemistry and isotopes abstract dark gray to black cryptocrystalline francolite occurs as nodules 2 8 cm in diameter which are embedded in organic matter rich mudstones of the visingso group upper proterozoic southern sweden the 13cpdb values 18 4 to 0 98 of structural co2 complemented by studies of the petrographic character and strontium isotopic composition of francolite indicate precipitation in the suboxic and sulfate reduction zones francolite is intergrown with berthierine that has formed in the suboxic fe reduction zone calcite enriched in mn and to a lesser extent fe occurs as fracture and void fillings and as replacement of francolite mn and mg rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis the carbon 13 c pdb 0 75 to 7 04 and oxygen 18 o pdb 14 21 to 10 58 isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60 to 80 c some of the void and fracture filling calcites 13 c 20 8 to 18 63 18 o 5 46 to 5 21 have however precipitated at lower temperatures 25 30 c in the sulfate reduction zone

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

ediacaran radiation of organic walled microbiota recorded in the ura formation patom uplift east siberia a new assemblage of morphologically complex ornamented and large organic walled microfossils acritarchs from the ura formation in the patom uplift east siberia is identified and taxonomically

a tonian age for the visingso group in sweden constrained by detrital zircon dating and biochronology implications for evolutionary events detrital zircon u pb ages from samples of the neoproterozoic visingso group sweden yield a maximum depositional age of 886 9 ma 2 a minimum depositional age is established biochronologically using organic walled and vase shaped microfossils present in the upper formation of the visingso group the upper formation correlates with the kwagunt formation of the 780 740 ma chuar group in arizona usa and the lower mount harper group yukon canada that is older than 740 ma mineralized scale microfossils of the type recorded from the upper fifteenmile group yukon canada where they occur in a narrow stratigraphic range and are younger than 788 ma are recognized for the first time outside laurentia the mineralized scale microfossils in the upper formation of the visingso group seem to have a wider stratigraphic range and are older than c 740 ma the inferred age range of mineralized scale microfossils is 788 740 ma this time interval coincides with the vase shaped microfossil range because both microfossil groups co occur the combined isotopic and biochronologic ages constrain the visingso group to between 886 and 740 ma thus tonian in age this is the first robust age determination for the visingso group which preserves a rich microfossil assemblage of worldwide distribution the organic and mineralized microorganisms preserved in the visingso group and coeval successions elsewhere document global evolutionary events of auto and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the neoproterozoic chronostratigraphic subdivision

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

organic walled microfossils from earliest cambrian or latest proterozoic tindir group rocks northwest canada abstract uppermost tindir group limestones exposed in the headwaters of tindir creek yukon territory canada contain a diverse and abundant microbiota of mineralized and organic walled microfossils preserved primarily in chert modules and chert beds the organic walled forms include mat building cyanobacterial filaments and coccoids accompanied by less common bacteria fungi and forms of uncertain affinity as well as planktonic coccoidal cyanobacteria and acritarchs several lines of evidence support interpretation of an early cambrian age whereas others suggest a late proterozoic age of the 54 organic walled taxa recognized 12 are newly described yukonosphaeridion interior awramik n gen n sp microagglomeratus borealis awramik n gen n sp phacelogeminus lineatus awramik n gen n sp palaeoanacystis magna awramik n sp cephalophytarion majesticum allison n sp heliconema bulbosa allison n sp trachyhystrichosphaera magna allison n sp fusilabellum brevistriatum awramik n gen n sp sphaeranasillos irregularis allison n gen n sp hyalocyrillium clardyi allison n gen n sp eophycomyces herkoides allison n gen n sp and archeomyces dimakeloides allison n gen n sp

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

possible early foraminiferans in post sturtian 716 635 ma cap carbonates foraminifera are an ecologically important group of modern heterotrophic amoeboid eukaryotes whose naked and testate ancestors are thought to have evolved 1 ga ago however the single chambered agglutinated tests of these protists appear in the fossil record only after ca 580 ma coinciding with the appearance of macroscopic and mineralized animals here we report the discovery of small slender tubular microfossils in the sturtian ca 716 635 ma cap carbonate of the rasthof formation in namibia the tubes are 200 1300 m long and 20 70 m wide and preserve apertures and variably wide lumens folds constrictions and ridges their sometimes flexible walls are composed of carbonaceous material and detrital minerals this combination of morphologic and compositional characters is also present in some species of modern single chambered agglutinated tubular foraminiferans and is not found in other agglutinated eukaryotes the preservation of possible early foraminifera in the carbonate rocks deposited in the immediate aftermath of sturtian low latitude glaciation indicates that various morphologically modern protists thrived in microbially dominated ecosystems and contributed to the cycling of carbon in neoproterozoic oceans much before the rise of complex animals

extremely low oxygen concentration in mid proterozoic shallow seawaters abstract the mid proterozoic 1 8 0 8 ga witnessed the first appearance but unusually low diversification of eukaryotes the stagnant biotic evolution during this billion year long period commonly referred to as the boring billion was arguably ascribed to low oxygen levels in atmosphere and ocean however evidence supporting low oxygen in shallow marine environments where early eukaryotes first evolved is generally lacking or insufficient here we report rare earth element and yttrium ree y data particularly cerium ce anomalies from a suite of mid proterozoic sedimentary rocks of the north china platform the new data from north china in combination with available ce anomaly data from other proterozoic successions demonstrate that during mid proterozoic negative ce anomalies did not occur until 1 54 ga and after 1 54 ga only episodic negative ce anomalies were present in shallow water carbonates trace element enrichments u ef v ef and mo ef remained at the average continental crust level before 1 54 ga but showed a sudden increase at 1 54 ga the data suggest that oxygen concentration in shallow marine environments of the mid proterozoic ocean was extremely low probably

biotic turnover driven by eutrophication before the sturtian low latitude glaciation a dramatic oceanic biotic shift from eukaryotic phytoplankton to bacteria occurred about 740 million years ago microfossil and geochemical data from the chuar group in the southwestern united states link this biotic turnover to widespread eutrophication of surface waters reconstructions of the diversity of precambrian microorganisms suggest a pronounced biotic turnover coinciding with the onset of neoproterozoic low latitude glaciation in which diverse assemblages of organic walled microfossils known as acritarchs were replaced by assemblages of simple smooth walled forms called leiosphaerids and the remnants of bacterial blooms1 2 3 4 this turnover has been interpreted as the mass extinction of eukaryotic phytoplankton1 2 3 4 and the subsequent proliferation of bacteria5 however the causes of this mass extinction and its exact temporal relationship to the glaciations remain unclear1 2 3 4 here we present palaeontological data from the 742 6 myr old chuar group from arizona which indicate that the biotic turnover occurred before the first low latitude sturtian glaciation constrained to be between 726 and 660 myr in age6 in our record the turnover is associated with the appearance of abundant and diverse protozoan fossils and a shift to rising total organic carbon suggestive of increased primary productivity spurred by the influx of nutrients this is followed by an increase in the ratio of highly reactive iron to total iron which we interpret as persistent water column anoxia we therefore conclude that the biotic turnover recorded in the chuar group was driven by widespread eutrophication of surface waters rather than low latitude glaciation

high resolution δ13c stratigraphy of the chuar group ca 770 742 ma grand canyon implications for mid neoproterozoic climate change a high resolution c isotope record based on 13 c org from organic rich shales and 13 c carb from dolomites in the ca 770 742 ma chuar group provides important new data for evaluating the signifi cance of large magnitude c isotope anomalies in neoproterozoic climate change three successive large magnitude isotopic excursions 8 15 are interpreted to represent primary seawater values based on a series of diagenetic tests and they are not associated with evidence of signifi cant long term 10 6 10 7 m y sea level change nor glaciomarine deposits intrabasinal correlation of 13 c org values suggests that most chuar shales record primary values and is consistent with previously reported h c ratios of 0 49 indicating that chuar shales experienced minimal thermal alteration although some chuar dolomites reveal early diagenetic alteration their 13 c dol values typically fall near those of coeval least altered dolomites or organic rich shales relative to dolomite values the chuar carbon record is interpreted to refl ect predominantly primary organic carbon 13 c values and contains suf

a jurassic tongue of the ocean infilled with oolitic sands the belluno trough venetian alps italy abstract the belluno trough is a narrow and elongate basin on the northwestern corner of the apulian plate continental margin it acquired topographic identity in early liassic time during the breakup of a widespread upper triassic carbonate shelf at the beginning liassic the belluno trough was a starved depression accumulating euxinic mudstone and bounded by two shallow water carbonate banks trento and friuli platforms this typical bahamian physiography lasted until late liassic time when the shallow water trento platform collapsed tectonically and suddenly drowned tilting northward and breaking along an old tectonic line it became therefore a submerged pelagic plateau in the meantime the western edge of the friuli platform now the windward margin of an oceanic platform turned into a prolific super producing oolite factory quite soon the oolite sands of the friuli margin started to flow down the slope reaching the basin through numerous entry points and gradually building a complex of coalescent deep sea fans which prograding westward infilled the belluno trough the fans terminated westward against the faulted edge of the trento plateau on which above a karst like surface locally encrusted with manganese red nodular ammonitic limestone was slowly accumulating northward where the topographic relief decreased the oolite fans could spill over the escarpment onto the edge of the plateau by the end of middle jurassic time the belluno trough was no longer a submarine depression but a gentle slope connecting the friuli platform to the deep pelagic trento plateau in callovian time probably because of a short drop in sea level large parts of the friuli platform were exposed to subaerial diagenesis during the subsequent oxfordian transgression the resulting rocky bottom became the substratum for an extensive growth of reefal organisms hydrozoans corals rudists which fringed the friuli platform for the whole malm and cretaceous times

variations in the abundance of photosynthetic oxygen through precambrian and paleozoic time in relation to biotic evolution and mass extinctions evidence from mn fe ratios abstract this paper reports new information about variations in the abundance of photosynthetic oxygen through precambrian and paleozoic time non detrital marine sediments cherts limestones and dolomite were analysed for nh2oh hcl acetic acid extractable mn and fe and the mn fe ratio a proxy for the oxidation reduction potential of the sediment at the time of deposition was plotted against geologic age the method has never before been applied to ancient sediments but previously published data produced independently by other methods confirmed its applicability and underlying assumptions the mn fe ratio was unexpectedly high ca 3 416 ga implying localised oxidation due to oxygen production by cyanobacteria but fell dramatically over the interval 3 416 3 298 ga suggesting mass mortality or mass extinction of early archaean cyanobacteria owing to asteroid impacts however the ratio increased continuously though at episodically varying rates from a minimum at 1 8783 ga to a maximum at 0 680 ga signifying accumulation of oxygen in the atmosphere and hydrosphere the rate of increase was relatively high at first but dropped abruptly at some point during the interval 1 8783 1 6 ga possibly signalling the appearance of eukaryotic herbivores the ratio increased exponentially from 1 6 to 0 8 ga and then rose more rapidly from 0 8 to 0 680 ga indicating a late proterozoic upsurge of oxygen production whereupon it fell catastrophically to a minimum in the cambrian reflecting widespread anoxia due to mass extinction of ediacaran organisms the crisis at the precambrian cambrian boundary was followed by a logarithmic increase from the cambrian to the permian indicating a resurgence of photosynthetic activity

origin of quartz cement in the lower ordovician dongjeom formation korea abstract this study deals with the origin of quartz cement developed in deeply buried mature sandstones intercalated in a thick carbonate sequence the dongjeom formation early ordovician korea is composed of three coarsening upward sequences and is divided into two facies according to depositional conditions with respect to changes in relative sea level transgressive facies and regressive facies the transgressive facies are characterized by calcite cemented allochem bearing sandstones whereas the regressive facies is characterized by non calcareous quartzose to arkosic sandstones quartz overgrowths occupy significant amounts of pore space average 16 in the regressive facies sandstones fluid inclusions reveal that quartz overgrowths precipitated between 110 and 165 c petrographical study and homogenization temperatures indicate that quartz overgrowths precipitated mostly during and after chemical compaction during the late diagenetic stage considering the 18 o values of detrital quartz grains and the volumes of quartz overgrowth the 18 o values of quartz overgrowth are estimated to range from 14 4 to 19 0 smow on the basis of mass balance silica for quartz overgrowths was principally derived from chemical compaction pressure solution of detrital quartz quartz overgrowths in the dongjeom sandstones precipitated from formation waters 2 1 to 2 5 smow that evolved from contemporary meteoric waters by diffusion

microbiotas from the late proterozoic chuar group northern arizona and uinta mountain group utah and their chronostratigraphic implications abstract samples of shales from the dominantly clastic upper proterozoic chuar and uinta mountain groups in northern arizona and utah yielded relatively abundant assemblages of acritarchs these are taxonomically varied and are interpreted as consisting of openwater cosmopolitan eukaryotic plankton the assemblages are compared with late proterozoic late riphean and early vendian c 800 700 ma sequences in the southern urals of the ussr the russian platform northern and southern scandinavia svalbard and east and north west greenland in all instances both the assemblages and discrete form taxa predate the varangerian vendian glacial event of the north atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western north america one new acritarch species vandalosphaeridium walcottii is erected and the partly megascopic species chuaria circularis is emended

marine redox conditions in the middle proterozoic ocean and isotopic constraints on authigenic carbonate formation insights from the chuanlinggou formation yanshan basin north china abstract to improve our understanding of ocean chemistry and biogeochemical cycling following the termination of large scale paleoproterozoic iron formation if deposition 1 85 billion years ago ga we conducted a fe s c mo geochemical study of the 1 65 ga chuanlinggou formation yanshan basin north china despite the cessation of if deposition our results suggest the presence of anoxic but non euxinic ferruginous conditions persisted below the surface mixed layer for the deepest portion of the continental rifting basin and that this pattern is apparently independent of the local organic carbon content however our paired s isotope data of carbonate associated sulfate and pyrite suggest presence of sulfate in pore fluids which is not consistent with insufficient sulfate for bacterial sulfate reduction in the water column despite evidence for deposition under anoxic conditions sedimentary molybdenum mo concentrations are mostly not enriched relative to average continental crust this relationship is consistent with the notion that sulfide dominated conditions in the water column and or the sediments are required for mo enrichment and validates past assertions that mo enrichment patterns in ancient shales track both the local presence and global distribution of euxinia specifically in addition we identified extensive diagenetic carbonate precipitation in the upper chuanlinggou formation with only moderately negative 13 c values 3 4 1 4 we propose with support from a numerical model that these diagenetic carbon isotope values were most likely derived from precipitation of carbonates dominantly in the methanic zone within the sediments diagenetic carbonate precipitation in the methanic zone is likely to have been more extensive in the proterozoic than the phanerozoic due to porewater oxidant limitation

a complex microbiota from snowball earth times microfossils from the neoproterozoic kingston peak formation death valley usa a thin carbonate unit associated with a sturtian age 750 700 million years ago glaciogenic diamictite of the neoproterozoic kingston peak formation eastern california contains microfossil evidence of a once thriving prokaryotic and eukaryotic microbial community preserved in chert and carbonate stratiform stromatolites oncoids and rare columnar stromatolites also occur the microbial fossils which include putative autotrophic and heterotrophic eukaryotes are similar to those found in chert in the underlying preglacial units they indicate that microbial life adapted to shallow water carbonate environments did not suffer the significant extinction postulated for this phase of low latitude glaciation and that trophic complexity survived through snowball earth times

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

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

early neoproterozoic scale microfossils in the lower tindir group of alaska and the yukon territory the tindir group is a 4 km thick neoproterozoic succession exposed in the tatonduk inlier of east central alaska and the western yukon territory the tindir group is informally divided into the lower tindir group which consists of 2 km of mixed carbonate and clastic rocks and the overlying upper tindir group which contains two cryogenian glacial deposits and an additional ediacaran succession of mixed carbonate and clastic strata unique mineralized scale microfossils have been recovered from sections previously correlated with the upper tindir group and interpreted variously as cryogenian to early cambrian in age our remapping of the area indicates that these sections are stratigraphically below an early cryogenian glacial diamictite unit 2 of the upper tindir group and are actually part of the lower tindir group carbon and strontium isotope correlations further suggest that the fossiliferous lower tindir group is correlative with early neoproterozoic strata of the northwestern canadian cordillera this new age model is consistent with the accompanying microfossil assemblage and indicates that the diverse microfossils in the lower tindir group can be added to the early neoproterozoic record of eukaryotic evolution

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

phosphate biomineralization in mid neoproterozoic protists the origin and expansion of biomineralization in eukaryotes played a critical role in earth history linking biological and geochemical processes however the onset of this phenomenon is poorly constrained due to a limited early fossil record of biomineralization although macroscopic evidence for biomineralization is not known until the late ediacaran we here report biologically controlled phosphatic biomineralization of scale microfossils from mid neoproterozoic pre sturtian strata of northwest canada primary biological control on mineralization is supported by the identification of apatite in both chert hosted and limestone hosted specimens the conspicuously rigid original morphology of the scale microfossils relative to co occurring organic walled cyanobacteria and acritarchs and the microstructure of the constituent phosphate cell enveloping mineralized scales occur in a wide range of extant protists but the apparent restriction of phosphate scales to one modern taxon of green algae suggests a possible affiliation for these fossils documentation of primary phosphate biomineralization in fifteenmile group yukon territory canada microfossils greatly extends the known record of biologically controlled mineralization and provides a unique window into the diversity of early eukaryotes

the effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies in the neoproterozoic johnnie formation death valley usa abstract carbonates within the rainstorm member in the terminal neoproterozoic johnnie formation of death valley california record a remarkable negative carbon isotope anomaly to a nadir of near 11 that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate 34 s values the carbonates including the laterally extensive johnnie oolite were deposited during marine flooding atop a sequence boundary best observed in cratonward sections the 12 c rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric o 2 which resulted in 1 the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides and 2 the ventilation of the oceans similar carbon isotope anomalies are recorded in broadly equivalent successions that post date known marinoan glacial deposits and pre date the precambrian cambrian boundary in oman india china australia and namibia supporting the view that the oxygenation event was global thus the biogeochemical anomaly post dates the last widely recognized neoproterozoic ice age but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin the radiation of a high diversity microbiota and the first appearance of large metazoan fossils ediacaran animals follow this carbon isotope anomaly suggesting that a critical threshold with respect to atmospheric o 2 had been crossed by this time a negative excursion of similar magnitude occurs in overlying strata at the precambrian cambrian boundary which likely reflects similar processes

benthic perspective on earth s oldest evidence for oxygenic photosynthesis the great oxidation event goe is currently viewed as a protracted process during which atmospheric oxygen increased above 10 5 times the present atmospheric level pal this threshold represents an estimated upper limit for sulfur isotope mass independent fractionation s mif an archean signature of atmospheric anoxia that begins to disappear from the rock record at 2 45 ga however an increasing number of papers have suggested that the timing for oxidative continental weathering and by conventional thinking the onset of atmospheric oxygenation was hundreds of million years earlier than previously thought despite the presence of s mif we suggest that this apparent discrepancy can be resolved by the earliest oxidative weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere such as biological soil crusts and freshwater microbial mats covering riverbed lacustrine and estuarine sediments we calculate that oxygenic photosynthesis in these millimeter thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant s mif signature as continental freeboard increased significantly between 3 0 and 2 5 ga the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass balance perspective these observations help reconcile evidence for pre goe oxidative weathering with the history of atmospheric chemistry and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the goe