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

facies and sequence controls on the appearance of the cambrian biota in southwestern mongolia implications for the precambrian cambrian boundary neoproterozoic cambrian rocks of the zavkhan basin govi altay western mongolia com prise large scale alternations of siliciclastic and carbonate dominated units cf grand cycles analysis of such dcpositional sequences near the base of the cambrian confirms that the distribution of trace fossils small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy corre sponding to specific points in a sea level cycle evolution of the cambrian biota is thus viewed through a series of narrow time windows once only for each dcpositional cycle correlation of the precambrian cambrian boundary level on the basis of the first appearance of the phycodes pedum assem blage is also fraught with difficulty since stratigraphic resolution may be limited to a single sea level cycle c 1 5 ma it is suggested that in many cases basin analysis will need to be undertaken before this bound ary can be drawn

gold bearing mesothermal veins from the gubong mine cheongyang gold district republic of korea fluid inclusion and stable isotope studies the gubong deposit consists of five massive gold bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in precambrian metasedimentary rocks of the gyeonggi massif the veins are divided into three groups based on their orientation and location they have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones the vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized separated by a major faulting event the ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes wall rock alteration minerals during stage i the main ore stage include sericite chlorite and minor pyrite and carbonates sulfide minerals deposited along with electrum during this stage include arsenopyrite pyrite pyrrhotite sphalerite marcasite chalcopyrite galena and argentite electrum also was deposited during stage ii mineralization in the one group of veins along with pyrite sphalerite chalcopyrite and galena but the second stage of deposition in other veins was barren petrographic examination of textural relationships among sulfides fluid inclusions microfracturing and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment systematic studies of fluid inclusions in stage i vein indicate two contrasting events a relatively high temperature 203 432 c and pressure 943 2 098 bars event related to early sulfide deposition and associated with h2o co2 ch4 nacl n2 fluids less than about 13 4 wt nacl and a lower temperature 202 399 c and pressure 670 850 bars late sulfide depositional event involving h2o nacl fluids 3 9 17 3 wt nacl the h2o nacl fluid involved in ore related stage ii mineralization had a salinity of 0 4 to 4 2 wt percent nacl and a homogenization temperature of 201 to 378 c the calculated sulfur isotope compositions of hydrothermal fluids from the stage i veins 34sh2s 3 5 10 5 indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks the calculated and measured oxygen and hydrogen isotope compositions of the ore forming fluids stage i 18oh2o 1 1 9 0 d 92 to 21 stage ii 18oh2o 0 1 to 0 3 d 95 to 93 indicate that the fluids were derived from magmatic and or deep seated metasedimentary rocks stage i and evolved by mixing with local meteoric water stage ii by limited water rock exchange and by degassing during mineralization in uplift zones the h2o nacl fluids involved in the stage i development of these veins represent fluids that evolved either through unmixing of h2o co2 ch4 nacl n2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters possibly as a result of uplift and or unloading during mineralization the h2o nacl fluid involved in stage ii was derived from meteoric water early deposition of gold in stage i was caused by a decrease in sulfur fugacity h2s loss that accompanied the immiscible separation of carbonic vapor from h2o co2 ch4 nacl n2 fluids gold in late stage i and stage ii veins was precipitated from h2o nacl fluids by cooling and by dilution caused by mixing with meteoric water

biostratigraphic and chemostratigraphic correlation of neoproterozoic sedimentary successions upper tindir group northwestern canada as a test case recent advances in proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of neoproterozoic 1000 540 ma successions is possible because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available the upper tindir group of northwestern canada provides an opportunity to test this hypothesis the age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc shaped scale microfossils that may provide insights into the early evolution of biomineralization a reinterpretation of upper tindir microfossil assemblages suggests a late riphean age although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates least altered samples indicate that delta 13c of contemporaneous seawater was at least 4 7 typical of neoproterozoic but not cambrian carbonates strontium isotopic compositions of the least altered samples yield values of approximately 0 7065 which can be uniquely correlated with late riphean seawater together micropaleontology and the isotopic tracers of c and sr constrain the upper tindir carbonates and their unique fossils to be late riphean likely between 620 and 780 ma

gold mineralization in proterozoic black shales example from the haoyaoerhudong gold deposit northern margin of the north china craton abstract the haoyaoerhudong gold deposit in the northern margin of the north china craton ncc is a large tonnage lower grade deposit with a reserve of about 148 tons of gold gold mineralization is characterized by pyrite and pyrrhotite films and thin veins on the schistosity plane of the proterozoic black shales the orebodies strictly controlled by a near ew trending shear zone are stratabound within carbonaceous phyllite and andalusite garnet schist of the bilute formation hydrogen and oxygen isotopic data show that the ore forming fluid was derived from a magmatic source and mixed with meteoric water sulfur and carbon isotope data indicate that most of the sulfur and carbon came from the black shale strata well defined biotite ar ar plateau age and inverse isochron age show that the deposit formed at ca 270ma which suggests a probable link between hercynian magmatism and gold mineralization studies on regional geology ore geology isotope geochemistry and ore forming age substantiate a complex evolutionary history of the deposit the proterozoic black shales rich in gold sulfur and organic matter which were deposited in the proterozoic continental margin rifts comprised the source bed for gold mineralization ew ene structures products of palaeozoic orogenic process provided pathways and mineralization space for ore forming fluids hercynian tectono magmatism and subsequent hydrothermal events remobilized gold and drove the ore forming fluids to dilatational fracture zones related to postcollisional magmatic hydrothermal events the haoyaoerhudong gold deposit is considered as a special type of orogenic gold deposit formed in the compression extension transition stage

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

sequence stratigraphy and depositional controls in late proterozoic early cambrian sediments of amadeus basin central australia the amadeus basin is an isolated intracratonic basin at the center of the australian continent which because of its location and geometry provides an ideal opportunity to investigate depositional controls to this end more than 6 000 km of seismic data in conjunction with a field and well log program have been used in a study of the late proterozoic early cambrian arumbera sandstone the arumbera sandstone was deposited as two major depositional sequences in a shallow marine and deltaic or coastal plain setting similar to that of the devonian catskill delta of north america sediment was supplied from the southwest by heavily laden braided streams during the deposition of the first sequence when sea level was at its maximum sedimentation appears to have been largely aggradational as sediment supply balanced or exceeded basin subsidence the sediments were probably deposited in a coastal or delta plain setting where small scale deltas prograded across the carbonate platform sediments of the julie formation during the accumulation of the second sequence sea level fell rapidly exposing large areas of sediment deposited during the first cycle and restricting edimentation to two steep sided rapidly subsiding subbasins deposition was largely progradational as major deltaic complexes developed on the southern and southwestern margins of the subbasins between the major delta complexes a depositional environment similar to that of the first sequence predominated in the deep subbasins the arumbera sandstone forms a continuous depositional unit across the proterozoic cambrian boundary the unconformable nature of that boundary over much of the earth possibly could be the result of a major eustatic sea level drop the final geometry of the arumbera sandstone and the nature of its sediments were determined largely by local subsidence rates subsidence briefly outstripped sediment supply during a stage of crustal extension as the basin evolved during deposition of sequence 1 depositional space and facies distribution were however fine tuned by salt tectonics and major sea level cycles as at passive margins sequence boundaries appear to be controlled by sea level cycles other variables such as climate the lack of vascular plants and the uplift of the source area were also factors in controlling deposition

evaluation of δ13c chemostratigraphy for intrabasinal correlation vendian strata of northeast siberia integrated sedimentologic stratigraphic and geochemical 13 c 18 o fe mn and sr data from vendian cambrian carbonate ramp deposits of the northeast siberian platform olenek uplift and kharaulakh mountains region are used to constrain primary time dependent oscillations in the carbon isotope record and to evaluate 13 c chemostratigraphy for high resolution intrabasinal correlation the vendian 13 c record of northeast siberia reflects global variations seen elsewhere by displaying in ascending order a strong positive isotopic shift to values near 6 herein named the p interval an intermediate interval of relatively little isotopic change i interval which shows in some sections a monotonic decrease in 13c from 2 at the base to near 0 at the top and a negative excursion to 4 n interval just beneath the vendian cambrian boundary in addition to secular isotopic shifts these strata exhibit local small scale signals related to intrabasinal variations in subsidence erosion and diagenetic alteration e g degradation of organic matter dolomitization and hydrothermal and burial effects these local intrabasinal processes in some cases have modified both the magnitude and form of the primary isotopic excursions which are used as the basis for correlation in all studies carbon isotope profiles provide an important method to evaluate intrabasinal variations in subsidence erosion and stratigraphic completeness these profiles reflect increasing subsidence along the platform to basin transition whereas progressive truncation of isotopic profiles along this trend illustrate pronounced uplift and erosion of ramp strata along unconformity surfaces

outcrop gamma ray logging and its applications examples from the german triassic abstract gamma ray logging of outcrops is a quick and simple yet powerful technique to better correlate well data with surface geology it is particularly useful for integrated sequence stratigraphic analysis and for reservoir and aquifer characterisation using outcrop analogues three examples from the triassic of the cratonic german basin are discussed 1 shallowing upward cycles in the upper muschelkalk carbonate ramp with retrogradational and progradational stacking patterns 2 evaporite to red bed cycles of the gipskeuper 3 fluvial architecture across the terminal alluvial plain system of the stubensandstein such outcrop analog studies help 1 to calibrate subsurface well log data 2 to evaluate the heterogeneity of subsurface reservoirs and aquifers and 3 to provide improved data for quantitative reservoir modelling

apparent polar wander and reversal stratigraphy of the palaeo mesoproterozoic southeastern mcarthur basin australia abstract approximately 1800 samples from volcanic clastic and carbonate sequences of the southeastern mcarthur basin in northern australia were analysed palaeomagnetically to define in detail the australian apparent polar wander path apwp for a period centred at 1670 ma and to develop a proterozoic reversal stratigraphy for correlation and dating twelve palaeomagnetic poles are interpreted as primary including two that are preliminary these increase the australian palaeomagnetic database for the proterozoic by 30 another ten poles record three or possibly four periods of overprinting the study also yields one of the oldest reversal records reported so far the ten better established primary poles were obtained from the lower three of the four main stratigraphic sub divisions of the basin the tawallah mcarthur and nathan groups these poles define an angular trajectory of about 120 for a time span of at least 75 m y and possibly exceeding 100 m y they confirm earlier preliminary results that suggested a major revision of the proterozoic apwp for australia the two preliminary poles were obtained from the youngest sub division of the basin the roper group and confirm a large age gap between the nathan and roper groups the poles for the oldest overprints were from igneous units in the tawallah group and appear to record metasomatism at the end of the last major period of volcanism in the basin the pole for the next overprint falls on the path near the apex of a hairpin bend it coincides with a likely period of rifting and high heat flow that accompanied the deposition of the upper mcarthur group the third overprint apparently acquired during the long time break separating the nathan and roper groups was found mostly in red dolomitic rocks the geomagnetic reversal record from this study represents an accumulated stratigraphic thickness of 2100 m excluding gaps where the polarity remained undetermined because of overprinting unstable remanences intervals not sampled or other causes reversal patterns from duplicate sections at different localities were compared for two formations and were found to be consistent several parts of the reversal column appear to be distinctive enough for use in stratigraphic correlation

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

petroleum geology and geochemistry of middle proterozoic mcarthur basin northern australia ii assessment of source rock potential five potential source rocks have been discovered in the middle proterozoic of the mcarthur basin the lacustrine barney creek formation mcarthur group and the marine velkerri formation roper group compare favorably in thickness and potential with demonstrated petroleum source rocks in the phanerozoic the former contains up to 7 total organic carbon toc and kerogen types i and ii toc values in the latter range up to 6 5 and the kerogen is type ii petrographic examination shows the organic matter is mostly lamalginite which on maturation yields nonfluorescent lamalginite and bitumen extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred tmax values derived from rock eval analysis may be used to define maturation levels in much the same way as in phanerozoic sediments at low maturation levels the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices but converge in the mature zone this convergence reflects both the inherent lower reflectivity of hydrogen rich macerals and the source control on methylphenanthrene indices at low maturity maturation levels in mcarthur group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults hydrocarbon generation occurred prior to the deposition of the younger proterozoic sediments of the roper group in contrast the source rocks in the roper group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the roper group although an early paleozoic timing cannot be excluded

geodynamic evolution of the proterozoic mount isa terrain the proterozoic mount isa terrain records the effects of four periods of intraplate tectonism the c 1870 ma barramundi orogeny was characterized by a massive felsic magmatic event and global correlations suggest a physical link between australia and laurentia at this time thereafter the terrain underwent an extensional history spanning 200 ma involving repeated episoded of rifting post rift subsidence and associated depositional and magmatic phases this protracted rifting history resulted in a cumulative stratigraphic thickness of up to 25km above attenuated continental crust rifting was interrupted prior to the formation of ocean crust by the compressional isan orogeny 1590 1500 ma the isan orogeny was synchronous with low pressure high temperature metamorphism and widespread metasomatism in the waning stages of shortening the mount isa terrain evolved into a wrench system characterized by an extensive network of strike slip faults the current level of exposure in this terrain provides spectacular examples of superimposed rifts basin inversion and wrench geometries developed at middle to upper crustal levels

surface gamma ray logs a correlation tool for frontier areas surface gamma ray logs or scintillometer profiles of measured sections provide an excellent correlation tool in frontier areas where well control is sparse but outcrops are abundant subtle changes in lithology that might have been missed during customary section measuring are commonly detected by surface gamma ray measurements furthermore gamma ray measurements across concealed strata may give clues to the lithology of the buried rock a portable scintillation counter is used in conjunction with lithologic descriptions five foot stratigraphic intervals provide optimum results surface gamma ray logs have been successfully used to correlate and trace paleozoic and mesozoic units in nevada utah and kentucky

a billion years of environmental stability and the emergence of eukaryotes new data from northern australia carbon isotopes through 6km of fully cored drill holes in 1 7 to 1 5 ga carbonates of the mount isa and mcarthur basins australia which host the earliest known eukaryote biomarkers provide the most comprehensive and best dated delta 13c stratigraphy yet obtained from such ancient rocks both basins reveal remarkably stable temporal delta 13c trends mean of 0 6 2 pdb peedee belemnite and confirm the impression of delta 13c stasis between 2 0 and 1 0 ga which together with other evidence suggest a prolonged period of stability in crustal dynamics redox state of surface environments and planetary climate this delta 13c stasis is consistent with great stability in the carbon cycle controlled we suggest by p limitation of primary productivity recent evidence shows that p depletion is a major factor in obligate associations between photosymbionts and host cells we argue that a billion years of stability in the carbon and nutrient cycles may have been the driving force that propelled prokaryotes toward photosymbiosis and the emergence of the autotrophic eukaryote cell

age of the barramundi orogeny in northern australia by means of ion microprobe and conventional u pb zircon studies abstract u pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust forming event in early proterozoic terranes of northern australia magmatism and tectonic processes associated with this event the barramundi orogeny appear similar over very wide areas about one third of the australian continent the barramundi event marks the deformation metamorphism and cratonization of the earliest proterozoic 2 0 1 9 ga basinal successions and it was followed by and closely linked to extensive orogenic felsic magmatism the yaringa metamorphics are an inlier of paragneiss and schist that pre dates 1865 3 ma leichhardt felsic magmatism and younger basin formation in the mount isa inlier from an ion microprobe u pb zircon study of massive and migmatitic gneiss the time of yaringa metamorphism is established at 1890 8 ma recognized younger proterozoic deformations in the interval 1610 1550 ma are generally not evident in zircon u pb systems but have totally reset local rb sr whole rock values conventional u pb work on multi grain zircon samples of yaringa gneiss gives a geologically meaningless age 2090 40 ma that is an artifact of complex zircon growth part of which was initially inherited from a 2 55 to 2 2 ga provenance and the other major part of which was formed in situ during the 1890 ma high grade metamorphism predictably for simple zircon systems in low grade metavolcanic terranes conventional and ion microprobe zircon ages are in close agreement dating of volcaniclastic sequences above and below the barramundi tectonic break in the pine creek inlier brackets the orogeny in this terrane to a similar time frame 1885 1870 ma some zircons which crystallized at this time form selvedges around older cores whose age of 2100 2000 ma may be relics of an earlier deep crustal underplating the uniform chemistry of post barramundi felsic magmatism and the similar tectonic setting of the areally extensive barramundi event are mirrored by their temporal coherency cycles of geosynclinal deposition burial metamorphism and uplift took place during intervals of several million years or less implying comparable rates for early proterozoic orogenesis to those established for tertiary and quaternary settings

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

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

latest proterozoic stratigraphy and earth history the end of the proterozoic eon was a time of pronounced biological biogeochemical climatic and tectonic change new bio and chemostratigraphic data provide an improved framework for stratigraphic correlation making possible a deeper understanding of latest proterozoic earth history and providing tools for a chronostratigraphic division of late proterozoic time

carbon isotope geochemistry and palaeontology of neoproterozoic to early cambrian siliciclastic successions in the east european platform poland a well preserved stratigraphic record of the organic carbon isotopic composition across the precambrian cambrian transition has been obtained from a sequence of entirely detrital rocks underlying the lublin slope east european platform poland the observed 13 c pattern for organic carbon is comparable to isotope data determined for coeval carbonate platform sequences elsewhere these results underline the truly global nature of these secular variations within the carbon cycle during the terminal proterozoic and early cambrian largely independent of host lithology and facies the direct combination of micropalaeontological and geochemical results from sedimentary organic matter allows a detailed interpretation of the isotope record as probably caused by secular changes in the net balance of burial and erosion rates of repositories of organic carbon furthermore these variations appear to be directly correlatable with changes in the diversity and abundance of the biota and radiation extinction events

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

occurrence of nitrate in groundwater grand forks british columbia the regional sand and gravel aquifer at grand forks supplies groundwater to the city of grand forks and three surrounding water districts the aquifer believed to be of late pleistocene age and glaciofluvial origin is vulnerable to groundwater quality degradation from human activities initial field survey and sampling from 1989 90 indicate three areas of nitrate nitrogen no3 n contamination 1 south of the airport 2 in the nursery area and 3 near north fork road along the western limits of the city of grand forks nitrate concentrations in these areas are elevated and locally exceed the maximum canadian drinking water guideline cdwg of 10 mg l no3 n levels in groundwater at grand forks are associated with high specific conductance and total dissolved solids tds chloride cl sulfate so42 and hardness calcium ca2 and magnesium mg2 nitrate decreases with increasing well depth and appears to be derived from the land surface areas of elevated no3 n appear associated with areas of major vegetable farming nitrate cl so42 ca2 and mg2 in inorganic fertilizers appear to be leaching into the aquifer further investigation and monitoring is required to better understand no3 n occurrence and behavior in the groundwater describe land use practices and define the groundwater flow system for developing effective strategies to protect the aquifer

mantle flow mechanisms for the large scale subsidence of continental interiors evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large scale subsidence we propose that mantle flow associated with the descent of cold plumes and slabs and their interaction with the endothermic phase change at 660 km depth may provide a plausible mechanism for these epeirogenic events simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth and temperature dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary we find that the plume slab flow scenarios are capable of supporting topographic deflections of amplitudes of 1 km and horizontal wavelengths of 1000 km that persist over time scales of 100 150 m y

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

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago

petroleum geology and geochemistry of the middle proterozoic mcarthur basin northern australia iii composition of extractable hydrocarbons abstract middle proterozoic organic rich sediments from the mcarthur basin in northern australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence are representative of the organic remains of microorganisms living at that time the major classes of hydrocarbons identified were n alkanes monomethyl branched alkanes cyclohexyl alkanes and acyclic isoprenoids there were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes low concentrations of steranes were present in most samples but like the triterpanes they were only easily detected in the least thermally altered sediments and hence were of limited use in detailed assessments of thermal maturity the presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 ma although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes