a proton and electron microprobe study of gallium nickel and zinc distribution in chromian spinel abstract chromian spinels from a number of localities were analyzed by proton and electron microprobes to examine the variation of trace elements in different geological environments chromian spinels studied include those representing ophiolitic complexes layered intrusions in continental areas alaskan type complexes komatiitic subvolcanic sills ni sulfide ores and metamorphosed mafic and ultramafic rocks nickel concentrations range from 30 to 4690 ppm proton microprobe with a mean value of 943 ppm nickel favouring the octahedral sites in the spinel structure displays an overall increase with a decrease in the cr cr al fe 3 ratio nickel partitioning between chromian spinel and mantle silicates appears to be primarily dependent upon the major element composition of the chromian spinel zinc concentrations range from 230 to 9810 ppm proton microprobe with a cluster around 500 ppm ophiolitic chromian spinels display the lowest overall zn values zinc levels appear to discriminate between chromian spinels in chromitite samples from those occurring as accessory chromian spinel in dunites or other rock types chromian spinels that are enclosed in or adjacent to sulfide grains display higher concentrations of zn zinc strongly favouring the tetrahedral sites displays a crude negative correlation with the mg mg fe 2 ratio and appears to vary as a function of temperature all the chromian spinel grains analyzed contain measurable quantities of ga by proton microprobe ranging from 10 to 208 ppm there is a reciprocal relationship between the ga concentration and the cr cr al fe 3 ratio and a crude positive correlation between the ni and ga concentrations measured ga concentrations in chromian spinel and whole rock samples are supportive of the experimental mineral melt partition coefficient of malvin and drake 1987 similar to the peridotite xenoliths mid ocean ridge basalts and oceanic island basalts ga correlates well with al 2 o 3 in the case of the ophiolitic chromian spinels and most of those representing layered complexes the ophiolitic chromian spinels seem to have crystallized from melts whose compositions are similar to those of partial melts of the upper mantle

the platinum group mineralogy of the merensky reef at the impala platinum mines bophuthatswana the platinum group mineralogy of the merensky reef at the impala platinum mines south africa has been investigated polished sections were prepared from pyroxenitic and pegmatoidal reef samples collected underground microscope studies were followed up by the identification and analysis of platinum group minerals by electron microprobe the most abundant base metal sulfides occurring in the merensky reef in this area are pyrrhotite 41 pentlandite 37 chalcopyrite 18 and pyrite 4 the core of a typical base metal sulfide bleb consists of pyrrhotite surrounded by a mosaic of pentlandite grains which in turn may be enclosed by a rim of chalcopyrite the relative volumetric abundances of the platinum group minerals were determined to be as follows coopetite 44 laurite 21 moncheite 17 and braggite 12 thirteen additional platinum group minerals comprised the remaining 6 percent cooperite and laurite occur preferentially in the chromitite layers whereas moncheite was most commonly found in the silicate part of the merensky reef observed textures include the marked tendency of platinum group minerals to occur with chalcopyrite the exsolution of laurite and moncheite from pyrrhotite and stringers of mon cheite and sperrylite cutting across silicate and base metal sulfide grain boundaries pt fe alloy commonly forms the core with cooperite as a rim of compound zoned platinum group mineral grains the textural features observed indicate that the platinum group minerals formed at a relatively late stage in the crystallization history of the merensky reef the preferential association of platinum group minerals with chalcopyrite is important chalcopyrite formed from the cu rich sulfide liquid of the original monosulfide solid solution which separated from the silicate magma it is suggested that the platinum group elements concentrated in this liquid and that the platinum group minerals crystallized only after the chalcopyrite as they occur mostly on the chalcopyrite margins a part of the platinum group element fraction remained in the fe ni monosulfide liquid and crystallized as platinum group minerals associated with pyrrhotite and pentlandite the remaining platinum group element fraction especially ru and pd remained in solid solution in the fe ni monosulfide the ru exsolved at lower temperatures as laurite but the pd remained in solid solution in the pentlandite this is concluded from the work of other authors ross and keays 1979 watkinson and dunnington 1979 and by the paucity of pd minerals although pd is the second most abundant platinum group element in the merensky reef

trace fossils and substrates of the terminal proterozoic cambrian transition implications for the record of early bilaterians and sediment mixing the trace fossil record is important in determining the timing of the appearance of bilaterian animals a conservative estimate puts this time at 555 million years ago the preservational potential of traces made close to the sediment water interface is crucial to detecting early benthic activity our studies on earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic which can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments overall trace fossils can provide a sound record of the onset of bilaterian benthic activity

platinum group element abundances in the lower and the lower critical zones of the eastern bushveld complex forty whole rock samples representing the principal silicate rock types of the lower and the lower critical zones of the bushveld complex were analyzed for ir ru rh pt pd and au ir concentration ranges from 0 1 to 2 2 ppb with sporadic values greater than 4 ppb there is no readily evident trend of ir enrichment or depletion up the sequence dunites 85 98 olivine have less ir 1 ppb than the pyroxenites ru ranges from 2 to 15 ppb with the lower part of the sequence having on average less than 5 ppb and the upper part more than 5 ppb rh ranges from 1 5 to 5 ppb with sporadic values exceeding 5 ppb the majority of the pt values are less than 30 ppb but range from 10 to 130 ppb pd was detected in seven of the samples and in these it correlates with the higher au levels au is generally less than 2 ppb but ranges up 13 ppb the platinum group element contents of the bushveld layered rocks normalized to c1 chondrite exhibit trends that parallel the merensky reef except for pt and the trends of the magnesian basaltic bushveld related sills interelement relationships are consistent with platinum group mineral control on platinum group element distribution in the nonmineralized layered rocks the parallel trends for the contrasting sets of data show that in the absence of abundant sulfides which concentrate pt and pd due to their greatly enhanced chalcophile character over the other platinum group elements the ratios are constant throughout the development of the complex to the stage of the merensky reef

comment on terreneuvian small shelly faunas of east yunnan south china and their biostratigraphic implications by b yang et al palaeogeography palaeoclimatology palaeoecology 398 2014 28 58 the recent article by yang et al 2014 provides a very useful summary of the litho and biostratigraphy of the sub trilobitic lower cambrian on the tropical shelf of south china our concern with this article is that it did not draw on or utilize existing reports that followed iugs protocols in the proposal of a gssp horizon and locality for informal stage 2 of the lower cambrian indeed a proposal of an integrated bio and chemostratigraphic signature i e the peak of the l4 carbon isotope excursion within the watsonella crosbyi assemblage zone in south china for the base of stage 10 and the location of a stage 2 gssp 9 m below the top of the dahai member at the laolin roadcut offers the potential for correlation of a stage 2 base across southern china and globally i e into siberla and avalonia

complex trace fossils from the terminal proterozoic of namibia we document for the first time the occurrence of the complex trace fossil treptichnus from the terminal proterozoic nama group namibia these traces differ in detail from the cambrian index fossil treptichnus pedum but document precursors with the same basic behavior the producer of the trace is not known in its complexity however this trace fossil provides strong evidence for advanced latest proterozoic bilaterians

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

the oldest evidence of bioturbation on earth detailed study of the khatyspyt formation in arctic siberia has shown that contrary to common expectations the earliest ichnofabric is of late ediacaran age records a food seeking behavior and is found in a distal carbonate ramp setting where it co occurs with fossils of avalon type ediacaran soft bodied organisms preserved depth of bioturbation reaches 5 cm the ichnofabric solely comprises nenoxites the oldest known meniscate trace fossil which is interpreted as burrows with terminal backfill structure formed as a result of active displacement of sediment to form a tunnel within the substrate followed by emplacement of the material behind the animal as it progressed through the sediment in addition to being the most reliable paleontological evidence for the existence of bilaterians at ca 555 ma the late ediacaran bioturbation is regarded as a key step in the escalatory engineering of phanerozoic ecospace leading to sudden diversification of macroscopic organisms and macrocommunities

vendobionta and psammocorallia lost constructions of precambrian evolution the non availability of biomineralized skeletons and low levels of predation led vendian evolution along strange avenues the ediacara type vendobionta appear to represent a kingdom in which foliate shapes large sizes and the necessary compartmentalization were achieved by quilting of the skin rather than by multicellularity psammocorallia in contrast are interpreted as coelenterates that constructed an internal sand skeleton both were immobile soft bottom dwellers that had high population densities and both became preserved by obrutional accidents thus they render fossil snap shots in which the original distributional patterns age structures and standing biomasses of populations are accurately recorded

earth s oldest jellyfish strandings a unique taphonomic window or just another day at the beach discoidal macrofossils reported herein from the lower cambrian zabriskie quartzite great basin western united states record the oldest phanerozoic medusozoan body fossils as well as the oldest medusozoan stranding event on record moreover these fossils provide evidence of a significant shift in the taphonomic mode characteristic of preservation of nonmineralized taxa in coarse grained siliciclastic successions near the onset of the phanerozoic taphonomic and sedimentological evidence recorded by these and younger examples of stranded cambrian medusae is consistent in suggesting that several of the requirements for preservation of these fossils were holdovers from the ediacaran period including the presence of microbial mats and a lack of carcass disturbance by scavenging and or bioturbating taxa to shed further light upon the taphonomic factors necessary for the preservation of cambrian medusae we compared the biostratinomy and sedimentology of cambrian medusa strandings to those of ediacara biota assemblages from lithologically similar successions we find key secular disparities in the taphonomic histories of these two types of fossil assemblage inconsistencies between the preservational styles characteristic of fossil assemblages preserved in sandstone lithofacies on each side of the precambrian cambrian boundary are explained by a considerable change in the preferred depositional setting in which these macrofossil assemblages are preserved thus rather than documenting a single taphonomic continuum through the precambrian cambrian transition the zabriskie and younger medusozoan body fossil assemblages record the advent of an entirely new yet still very rarely exploited taphonomic window exclusive to the cambrian period

neoproterozoic substrate condition vis a vis microbial mat structure and its implications sonia sandstone rajasthan india dst fast track project young scientist scheme no sr ftp es 03 2011 cas phase v upe ii programme of jadavpur university national research foundation and university of pretoria south africa

trace fossil evidence for ediacaran bilaterian animals with complex behaviors abstract distinguishing ediacaran trace fossils from tubular body fossils can be a challenge and several ediacaran fossils previously interpreted as animal traces have been shown to be tubular body fossils nonetheless true ediacaran trace fossils are present but they are relatively few and morphologically simple dominated by horizontal trails and shallow burrows such simple morphologies have been taken as evidence for a modest behavioral complexity and limited geobiological impact of animal bioturbators before the cambrian explosion here we report three types of trace fossils horizontal tunnels surface tracks trails and vertical traces from the latest ediacaran dengying formation 551 541 ma in the yangtze gorges area of south china cross cutting tunnels and the presence of scratch marks indicate that these traces are unlikely tubular body fossils instead these three types of traces likely represent animal activities related to under mat feeding epibenthic locomotion and temporary dwelling respectively we show that these three types of traces were constructed by the same bilaterian animals that had moderately complex interactions with microbial mats to exploit nutrients and o 2 resources these animals heralded a new age in ecosystem engineering animal sediment interaction and biogeochemical cycling

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

trends in depth and extent of bioturbation in cambrian carbonate marine environments western united states cambrian carbonate strata of the great basin deposited below fairweather wave base but above mean storm wave base inner shelf and below mean storm wave base but above maximum storm wave base middle shelf were examined for extent and depth of bioturbation data indicate that initial extensive colonization of marine infaunal habitats occurred between tommotian and atdabanian time throughout the remaining 65 m y of cambrian time bioturbation increased although depth of bioturbation remained relatively constant at less than 6 cm during this time the amount of bioturbation in the inner shelf was greater than that in the middle shelf

diminutive trace fossils in the chengjiang lagerstatte diminutive trace fossils described here have been well preserved in connection with certain pliable carapaces or skeletons of the chengjiang animals taphonomic evidence suggests that these traces were formed in situ the peculiar preservation indicates that some bilaterians of uncertain affinities seem to have been active at the burial site constituting an important group of the hidden majority of cambrian marine community this may be an essential feature closely associated with the palaeoecology and exceptional preservation of the cambrian fossil lagerstatten the rare preservation of such millimetre sized trace fossils raises further doubts about the timing of the initial benthic bilaterians by applying trace fossil evidence

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

widespread delayed mixing in early to middle cambrian marine shelfal settings abstract the radiation of burrowing metazoans in the early phanerozoic dramatically altered the properties of marine sediment an event commonly referred to as the cambrian substrate revolution or agronomic revolution the advent of infaunalization and especially biogenically mediated sediment mixing profoundly impacted the development of phanerozoic biogeochemical cycling including nutrient fluxes organic carbon burial seafloor oxygenation and sediment ecology however the timing of the development of mixed seafloor sediment has historically not been well constrained mixing has been assumed in the absence of data to occur at the precambrian cambrian boundary with the appearance of the index fossil and three dimensional burrow treptichnus pedum herein we present new ichnological stratigraphic and taphonomic data suggesting that although significant developments in infaunalization paleobiologically complex animal substrate interactions particularly burrow construction occurred during the early and middle cambrian mixing remained suppressed throughout this interval we demonstrate using a novel multi proxy approach that shelfal sediment in the earliest cambrian was essentially unmixed moreover our findings indicate that even as late as the middle cambrian 30 million years after the precambrian cambrian transition and well after the appearance of supposedly deposit feeding trilobites seafloor heterolithic shelfal sediment remained largely unmixed on a global scale these findings challenge the current assumption that mixing occurred with the first appearance of three dimensional burrows and hold important implications for the advent and development of modern style biogeochemical cycling

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

preservation of early and middle cambrian soft bodied arthropods from the pioche shale nevada usa abstract the early middle cambrian pioche shale of lincoln county nevada preserves a diverse array of soft bodied ecdysozoans that are well known from the burgess shale soft bodied fauna of british columbia canada at pioche certain genera occur both above and below the middle cambrian boundary allowing a comparison of the nature of their preservation across this interval in order to investigate the mineralization of this fauna sem analysis of mineral textures and element mapping of anomalocaris tuzoia and canadaspis specimens using energy dispersive x ray spectroscopy eds was undertaken results of these analyses show that early cambrian specimens are generally preserved in botryoidal hematite which is dark red in hand specimen while specimens from the middle cambrian are preserved as kerogenized carbon films with associated oxidized pyrite crystals and framboids comparison with other cambrian soft bodied biotas reveals that the nature of preservation of the middle cambrian biota at pioche is remarkably similar to that described for the chengjiang soft bodied fauna china there does not seem to be an adequate analogue for the early cambrian preservation of soft bodied organisms at pioche the preservation of the fauna is discussed in relation to changing environmental conditions at the early middle cambrian boundary

abundance and distribution of palladium platinum iridium and gold in some oxide minerals abstract palladium platinum iridium and gold were determined in chrome spinel ilmenite and magnetite samples from mafic and ultramafic layered complexes bushveld stillwater rhum rhochrome mine in zimbabwe and chambers mine in the u s a and in alpine type deposits greece pakistan and turkey purified mineral samples 50 100 mg were analysed by neutron activation after a radiochemical ion exchange separation the abundances of the individual platinum group elements were similar in all the chrome spinels pd and pt 50 500 ppb ir 10 100 ppb au 0 5 5 0 ppb with the exception of the alpine type deposits where the chrome spinels were depleted in pd and pt often analyses of coexisting chrome spinel and silicate in the rhum layered intrusion showed that pd pt and au are concentrated at the mineral grain boundaries there is a two fold enrichment of ir in the spinel phase it is proposed that the platinum group elements do not substitute for base metals in the chrome spinel or other oxide phases but that they tend to remain in the magmatic fluid until they finally crystallise at overgrowths on grain boundaries of the cumulus oxide

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

interactions between ediacaran animals and microbial mats insights from lamonte trevallis a new trace fossil from the dengying formation of south china abstract a new ichnogenus and ichnospecies lamonte trevallis is formally described from the shibantan member limestone of the upper ediacaran dengying formation yangtze gorges area south china it is characterized by horizontal tunnels connected with short vertical burrows and surface trails the horizontal burrows are elliptical or bilobed in transverse cross section preserved in full relief and filled with carbonate intraclasts micrites as well as calcite and silica cements they occur exclusively in silty crinkled and microlaminated layers that are interpreted as amalgamated cyanobacterial microbial mats no burrows have been found in intraclastic layers adjacent to the microlaminated layers the vertical traces are filled with the same material as the burrows but they typically project through the crinkled microlaminae and are exposed on the bedding surface the surface tracks are always preserved in negative epirelief or positive hyporelief and consist of two parallel series of either sharp scratch marks or small knobs the burrow infill has 18ocarb and 13ccarb values distinct from but intermediate between microlaminated and intraclastic layers consistent with petrographic observation that burrow infill consists of a mixture of early carbonate cements intraclasts and micrites bedding plane bioturbation intensity 20 40 measured as percentage of bedding plane area covered by l trevallis traces is comparable to similar measurements in pre trilobite cambrian carbonates the exclusive occurrence of l trevallis within microbial mats may have both taphonomic and ecological significance these mats may have provided firm substrates and localized geochemical conditions that contributed to the structural integrity of the burrows and they may have also facilitated early diagenetic cementation of burrow infill thus facilitating burrow preservation the close association of these burrows with microbial mats implies that the trace producers actively mined cyanobacterial mats to exploit oxygen or nutrient resources the trace makers of l trevallis were better able to utilize the resources around them than many other ediacaran trace makers and provide an ichnological record of a flourishing benthic ecology in late ediacaran oceans at the dawn of the agronomic revolution

sulphur saturation in the lower and critical zones of the eastern bushveld complex abstract a suite of ultramafic and mafic rocks from the lower critical and lower portion of the main zones of the bushveld complex has been analysed for th cs zr ni cr and au by inaa and xrf spectrometry the incompatible elements th cs and zr correlate positively and show a gradual upward increase in abundance assuming constant average proportion of intercumulus material this upward increase implies that the zones of the complex studied represent crystallization of a single magma type some 3600 m thick pyroxenites dominate the lower portion of the section studied and their ni content shows an initial rapid decrease from 850 ppm in the lowermost rocks to around 500 ppm with considerable scatter this distribution is most likely to have resulted from bottom crystallization with superimposed convective overturn near the transient floor of the chamber gold abundances are generally higher in chromitites and correlate positively with ni indicating the presence of significant amounts of cumulus immiscible sulphide in the silicate rocks au does not correlate with any of the analysed elements and it is concluded that au was trapped in small quantities of immiscible sulphide which precipitated continuously during crystallization there is an upward increase in the amount of cumulus immiscible sulphide indicating a progressive increase in sulphur solubility in the magma

the early paleozoic development of bioturbation evolutionary and geobiological consequences abstract bioturbation the physical and chemical mixing of sediments by burrowing animals is a critical engineering process in modern seafloor environments and exerts an important control on not only benthic ecology and sediment properties but also ocean wide biogeochemical cycling well mixed sediments have long been assumed to appear at the precambrian cambrian boundary with the first occurrence of the index fossil and three dimensional burrow treptichnus pedum however field based analyses synthesizing ichnological stratigraphic sedimentological and taphonomic data collected from lower paleozoic successions worldwide indicate that sediment mixing in marine shelf environments remained limited until at least the late silurian 120 million years after the precambrian cambrian transition in spite of early advances in complex modes of infaunalization mixed layer development had a much more gradual trajectory and did not occur in step with the first appearance of three dimensional burrows the delayed appearance of modern style intensities of sediment reworking postdating both the cambrian explosion and the great ordovician biodiversification event may be due to the late stage radiation of mobile infaunal deposit feeders biological bulldozers the organisms which are in modern marine settings the most efficient bioturbators the protracted development of the sediment mixed layer holds important implications for contemporaneous biogeochemical cycling and oxygenation of the ocean atmosphere system the delayed development of intensive sediment mixing may have also mediated exceptional preservation of both fossilized soft tissues and shallow tier trace fossils and may therefore have contributed to the anomalous preponderance of konservat lagerstatten and exceptionally preserved shallow tier trace fossils and bioglyphs characteristic of the lower paleozoic stratigraphic record

depositional and preservational environments of the ediacara member rawnsley quartzite south australia assessment of paleoenvironmental proxies and the timing of ferruginization abstract the paleoenvironmental setting in which the ediacara biota lived died and was preserved in the eponymous ediacara member of the rawnsley quartzite of south australia is an issue of longstanding interest and recent debate over the past few decades interpretations have ranged from deep marine to shelfal to intertidal to terrestrial here we examine the evidence in support of and against various paleoenvironmental interpretations of the fossiliferous ediacara member as well as exploring in depth a range of paleoenvironmental proxies that have historically been employed in such studies we emphasize the importance of reconciling paleoenvironmental analysis with an understanding of sedimentological processes and outline which proxies are consistent with this approach a careful assessment of paleoenvironmental parameters is essential to the reconstruction of the depositional and early diagenetic history of the ediacara biota and thus the physical chemical and biological factors that shaped the development and the fossilization of these earliest examples of complex life we find no compelling evidence for a terrestrial setting for the ediacara member and strong support for a shallow marine depositional environment

burrowing below the basal cambrian gssp fortune head newfoundland the range of treptichnus pedum the index trace fossil for the treptichnus pedum zone extends some 4 m below the global standard stratotype section and point for the base of the cambrian period at fortune head on the burin peninsula in southeastern newfoundland the identification of zigzag traces of treptichnus isp even further below the gssp than t pedum in the fortune head section and in other terminal proterozoic successions around the globe supports the concept of a gradational onset of three dimensional burrowing across the proterozoic cambrian boundary although t pedum remains a reasonable indicator for the base of the cambrian period greater precision in the stratotype section can be achieved by a detailed re evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the t pedum zone

au ir ni and co in some chromitites of the eastern bushveld complex south africa abstract instrumental neutron activation analyses of the lower group middle group and upper group chromitite layers of the eastern bushveld complex are reported the elements analysed were ir 300 ppb au 240 ppb ni 540 to 2240 ppm co 101 to 370 ppm ir and au are higher in the upper critical zone compared to the lower critical zone the relationship of the element abundance to the amount and composition of the chromite in the sample and the possible controls on the distribution of the elements are discussed at low levels of concentration ir is most likely controlled by discrete primary ir bearing platinum group minerals and au is related to small amounts of base metal sulphide both being sympathetic with the volume percent chromite in the sample higher ir and au levels are possibly related to sulphides present in even greater amounts

platinum group mineral inclusions in chromite from the bird river sill manitoba platinum group minerals pgm have been identified as inclusions in chromite from the bird river sill manitoba the inclusions are small 20 microns and are commonly euhedral the pgm inclusions are ru os ir s2 laurite and os ir ru alloy rutheniridosmine laurites contain up to 2 99 wt palladium arsenic content is negligible and no platinum or rhodium has been detected one platinum group element alloy contains 0 96 wt rhodium but neither platinum nor palladium has been detected laurite inclusions in chromite from the ultramafic zone record two compositional trends first increasing and then decreasing ru ru os ir up section pgm inclusions and other solid inclusions occur as discrete phases in chromite and are part of the chromite precipitation event increasing oxygen fugacity by wall rock assimilation or new magma injection initiates chromite precipitation locally increasing the sulphur content of the magma to convert pge alloys to sulphides

the behavior of gold in unaltered island arc tholeiitic rocks from izu oshima fuji and osoreyama volcanic areas japan high au concentrations 4 25 ppb in titanomagnetite concentrates from basalts and basaltic andesites of the izu oshima and fuji volcanic areas in the east japan arc indicate that au is strongly partitioned into titanomagnetite relative to the bulk rock the concentration of an in volcanic rocks increases during the early stages of fractional crystallization dominated by olivine pyroxenes and plagioclase but decreases with subsequent fractional crystallization involving titanomagnetite a delay in titanomagnetite formation during fractional crystallization subsequently results in comparatively high concentrations of au in island arc tholeftes wide variations in the au concentration are observed in rocks derived from primary magmas across a single volcanic area and these can be positively correlated with the concentrations of incompatible elements such as rb and zr these features are inherited from a s depleted mantle source since variable degrees of partial melting do not significantly affect the au concentrations of the primary magmas

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

impact of particle transfer on flow properties of crushed mudstones karst collapse pillar kcp is widespread in north china coalfields where coal extraction above the ordovician limestone aquifer is threatened by the abundant supply of water and a very high hydraulic pressure kcp is composed of rock skeleton and fine fillings which can be transferred under the effect of water flow thus kcp usually functions as a channel for groundwater inrush in order to investigate the mechanism of mining induced groundwater inrush of kcp which was caused by fine fillings transfer a self designed particle transfer permeability testing system was used to test the crushed mudstones flow properties which included fine filling particle transfer rate porosity increase rate and permeability under the conditions of varying pore pressure particle size mixture and compaction level initial porosity the tests indicate fine fillings transfer is the essential reason for mining induced groundwater inrush flow instability of kcp the flow properties changeability during fine particle transfer could be divided into four stages i e initial flow stage flow inrush stage continued particle flow stage and stable flow stage and flow inrush stage which is the key point to cause water inrush due to the crushing of edges and corners and the adjustment of the structure the fluctuation of permeability time relationship mainly distributed in the first two stages which make a change to flow channel moreover with the increasing of pore pressure particle size mixture and initial porosity the water inrush time and stable seepage time decreased more rapidly the fine fillings porosity and permeability increased gradually the efficiency criteria analysis between measurement and calculation permeability shows an empirical equation can fit the relationship between porosity and permeability well and not all of the pore structures were in flow this means there was a part of pores that were invalid but the effective porosity in flow can be treated as calculation value approximately

when the worm turned concordance of early cambrian ichnofabric and trace fossil record in siliciclastic rocks of south australia studies of sedimentary fabrics and trace fossils across the precambrian cambrian interval in south australia demonstrate that the appearance of treptichnus pedum which defines the base of the cambrian also marks the initial development of preservable infaunal activity there is a significant shift in the amount and nature of bioturbation at this time these results support the broad scale utility of t pedum as the indicator of the base of the cambrian

platinum group minerals in alpine chromitites from southwestern oregon electron microprobe and scanning electron microscopy the platinum group minerals found in the chromitites are laurite porous ru rich alloys os ir and pt rich alloys with highly variable compositions an ir cu rich sulfide a pt arsenide and three relatively rare platinum group sulfides the platinum group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains some of these inclusions also contain silicates and millerite base metal alloys largely ni fe occur in cracks in the chromitites along with magnetite ferritchromite chromian chlorite and serpentine the primary platinum group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite modified journal abstract

high resolution geochemical evidence for oxic bottom waters in three cambrian burgess shale type deposits abstract the cambrian radiation of complex animals is a fundamental event in the history of life on earth much of our understanding of this event is made possible through the study of exceptionally preserved fossils in burgess shale type bst deposits based on bioturbation levels in bst deposits they are often interpreted as representing restricted oxygen settings this study tests the low oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three bst deposits the early cambrian maotianshan shale china the middle cambrian wheeler shale usa and the middle cambrian spence shale usa results from 96 samples show fine scale geochemical evidence for oxic bottom water conditions during the deposition of these three bst deposits trace element paleoredox indices pi give consistent oxic signals in all three shales in addition total organic carbon toc and total sulfur ts levels in the spence shale are extremely low the average c s ratio is 3 1 comparable to values for modern oxic marine water bioturbation levels in all three shales are low with rare development of minimal to moderate horizontal bioturbation while short term oxygen fluctuations or poikiloaerobic conditions could still have sporadically existed in these settings this study shows that these bst deposits were deposited in a dominantly oxic paleoredox setting evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions with an oxic water column positioned above oxygen depleted sediment pore waters may have existed in many cambrian bst deposit paleoenvironments