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

the role of fungi in the diagenetic alteration of spar calcite fungi may play a far more important role in carbonate diagenesis than has been previously suspected this can be demonstrated by placing known species of fungi on crystals of iceland spar calcite such experiments show that fungi can 1 cause sparmicritization through dissolution 2 produce spiky calcite through dissolution 3 trap and bind loose particles of calcite and 4 promote the growth of new calcite crystals although not observed in this series of experiments fungi can bore into substrates and cause sparmicritization the activity of fungi involves a complex interplay of substrate destruction and construction without the intervention of vast quantities of fluid

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

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

late cambrian steptoean sedimentation and responses to sea level change along the northeastern laurentian margin insights from carbon isotope stratigraphy carbon isotopes are applied as tools for stratigraphic correlation of poorly fossiliferous upper cambrian carbonate strata in the northern u s appalachians upper cambrian steptoean marine carbonate rocks record a significant global positive carbon isotope excursion 13c 4 5 vienna peedee belemnite vpdb the timing of which is well documented in fossiliferous sections elsewhere the steptoean excursion peaks at a sea level lowstand that produced the sauk ii sauk iii sequence boundary on the north american craton in this study this excursion is documented for the first time in the northern u s appalachians in poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the schodack formation these deposits contain the only reported pre elvinia zone steptoean fauna in new york and record 13c values of up to 3 the slope carbonate sediment was mainly derived from the shelf margin and is mixed with common coarse grained silici clastic material these deposits reflect a seaward migration of the siliciclastic source area exposed craton suppressed carbonate platform sedimentation and shelf bypassing during the sauk ii sauk iii sea level fall nonfossiliferous dolostones and dolomitic marbles of the proposed carbonate platform correlatives the pine plains formation in southeastern new york and the stockbridge formation from western massachusetts also contain common coarse siliciclastics however sampled sections do not record elevated 13c values indicating that these strata are probably not of steptoean age this suggests that steptoean time is represented in the carbonate platform to slope succession of the northeastern present day laurentian margin by an extremely condensed stratigraphic interval or even a hiatus

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

sr isotopic compositions of cold seep carbonates from the south china sea and the panoche hills california usa and their significance in palaeooceanography 87sr 86sr ratios 13c and 18o values and concentrations of sr and mn in cold seep carbonates collected from the south china sea and the panoche hills california usa are determined in this study the carbon isotopic values of carbonates reflect different carbon sources carbonate samples with high sr concentrations and low mn sr ratios are considered to have very little or no influence by diagenetic alteration sr isotopes of seep carbonates from both the south china sea and the panoche hills record the information on the original seawater or pore water together with the published sr isotopic data of cold seep carbonates in italy we applied the sr isotope stratigraphy sis and sis lowess version 3 curve to calibrate the ages for the cold seep carbonates these ages are consistent with their sedimentary strata indicating that the cold seep carbonates with high sr concentration and low mn sr ratio are good candidates for dating by their sr isotopic compositions

a chemostratigraphic overview of the late cryogenian interglacial sequence in the adelaide fold thrust belt south australia within the neoproterozoic sequence of the adelaide fold thrust belt south australia two of the most severe ice ages in earth history sturtian and marinoan are recorded in the glacigenic rocks which mark the base and top of the 4 5 km thick umberatana group the focus of the present chemostratigraphic study rock units from three drillholes and ten measured outcrop sections located in the southern and central flinders ranges and on the stuart shelf were sampled for isotopic analysis of their co existing calcite 13ccal dolomite 13cdol and kerogen 13corg strontium and sulphur isotopic analyses were also run on selected samples the sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen the resulting data when integrated with the lithostratigraphy of the umberatana group yield one of the most complete records of secular variation in the c isotope signature of the late cryogenian ocean between 750 and 680 ma its composite 13ccarb curve begins with a sharp positive excursion 3 to 1 5 in the dolomitic tindelpina shale which caps sturtian glacial diamictites wilyerpa formation then follows a steady climb through the shoaling upward tapley hill formation reaching 4 in the brighton limestone and culminating in a plateau of 13c enrichment within the platformal carbonates of the etina formation 8 to 10 the upper part of the temporal trend shows a decline to 6 to 7 in the stromatolite reef facies of the enorama shale and a dramatic drop to 8 to 9 5 followed by a recovery to 3 in the youngest carbonate unit trezona formation beneath the marinoan glacigenic elatina formation the latter is capped by a thin dolostone the nuccaleena formation which records a consistent up section decrease in 13c 1 to 3 superimposed on this overall pattern of c isotopic variation are second and third order fluctuations attributable to eustatic sea level change pyritic sulphur in the lower tapley hill formation 34s 9 to 40 is distinctly heavier than that in the underlying wilyerpa formation 4 to 20 minimum 87sr 86sr values obtained from the brighton limestone 0 7071 and etina formation 0 7076 are consistent with their late cryogenian age this sturtian to marinoan interglacial succession is more complete than equivalent sequences elsewhere and thus provides a useful reference for global chemostratigraphic correlation notable features of its c isotope curve are the strong post wilyerpa positive excursion also seen in key sections from svalbard and western mongolia the etina plateau of 13c enrichment which is remarkably similar to maxima in the terminal proterozoic c isotope records of western mongolia north western canada and brazil and the spectacular depletion of 13c in the lower trezona relative to similar carbonate facies in the underlying etina formation and enorama shale a preglacial offset in 13ccarb values of such proportions 14 is almost unprecedented except perhaps for that recently documented in the upper ombaatjie formation beneath tillites of the ghaub formation otavi group namibia it represents a key piece of evidence in the case presented by the proponents of a neoproterozoic snowball earth

isotopic evidence for the paleoenvironmental evolution of the mesoproterozoic helena formation belt supergroup montana usa high resolution isotopic and geochemical data constrain long debated marine vs lacustrine depositional models for the helena formation a carbonate unit within the mesoproterozoic belt supergroup montana paired carbon and oxygen isotope and minor element analyses of microsamples from a diverse array of petrographic components effectively distinguish between primary and secondary isotopic signatures 13c values reflecting primary depositional conditions increase progressively from an average of 0 near the base of the helena formation to an average of 1 4 near the top over a stratigraphic thickness of 100 200 m because the base of this 1 6 shift coincides with a basin wide facies change marked by the appearance of stromatolitic units and an increased occurrence of features indicative of evaporative conditions and subaerial exposure the shift toward more positive 13c values is interpreted to reflect an increase in biologic productivity coupled with increasingly evaporative conditions in the eastern part of the belt basin 13c values from the lower part of the helena formation are similar to those from other marine carbonates of similar age and suggest that the belt basin was marine during deposition of the helena formation taken as a whole these relations suggest that helena sediments were deposited in a shallow marine basin that was increasingly isolated from the open mesoproterozoic ocean original 13o values have been retained only in syndepositional and early diagenetic intergranular cements in oolitic grainstones that occur in the upper part of the helena formation the most positive 13o values derived from these cements 6 1 pdb are up to 3 higher than previous estimates for the 13o value of mesoproterozoic marine calcite however because these phases are associated with facies deposited under restricted marine conditions interpretation of 13o values from these cements with regard to the composition of the open mesoproterozoic ocean awaits study of additional coeval marine carbonate successions

strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation an example from high grade rocks of the norwegian caledonides isotope stratigraphy has been applied to an undated high grade carbonate sequence from the norwegian caledonides in order to constrain the depositional age sokumfjell group marbles of the beiarn nappe complex in the uppermost allochthon of the nordland caledonides were subjected to several phases of deformation culminating in amphibolite facies metamorphism the ca 1000 m thick marble sequence studied is composed of five different lithofacies lithofacies 1 4 are calcite marbles and lithofacies 5 is dolomite marble all the calcite marble lithofacies exhibit throughout rhythmic bedding and textural grading on different scales suggesting that carbonate material was deposited in a pelagic turbidite system 123 samples were analysed for major and trace elements and acid soluble mg and ca of which 42 samples of calcite marbles and four samples of dolomite marbles were selected for 13 c 18 oa nd 87 sr 86 sr analyses all selected calcite marbles have high sr contents averaging 2000 ppm they are characterised by 87 sr 86 sr ratios ranging between 0 70642 and 0 70999 and 13 c values ranging between 1 3 and 8 4 v pdb 18 o values are highly variable 14 1 24 8 v smow the best preserved samples selected on the basis of mn sr and rb concentration and mn sr and mg ca ratios yield average 87 sr 86 sr values of 0 70669 0 70697 0 70727 0 70749 and 0 70737 for lithofacies 1 2 3 4 and 5 respectively post sedimentary alterations including amphibolite facies metamorphism resulted in only a slight increase of 87 sr 86 sr values in the marbles studied apparently due to aragonite precursor high sr concentration and a very early and fast diagenetic alteration caused by fluids isotopically comparable with that of seawater projection of these values along with 13 c measurements onto 87 sr 86 sr and 13 c calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 635 and 600 ma 2001 elsevier science b v all rights reserved

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

carbon isotopic profile across the bilara group rocks of trans aravalli marwar supergroup in western india implications for neoproterozoic cambrian transition abstract the rocks of marwar supergroup in the trans aravalli sector in western india are presumed to span the time interval between neoproterozoic and early cambrian this predominantly unfossiliferous marine sedimentary sequence is characterized by a lower arenaceous facies jodhpur group middle carbonate facies bilara group and upper argillaceous arenaceous facies nagaur group rocks the sedimentation has been essentially in a shallow basin described either as the fore land slope of the rising aravalli mountains or a sag basin which developed and evolved due to subsidence of the updomed crust during neoproterozoic malani magmatism that failed to open rifts the carbon isotopic profile for the bilara group carbonate rocks in the lower part shows marked oscillations and broadly negative 13 c character with negative anomalies as low as pdb observed near the base of dhanapa formation lower unit and pdb in the overlying gotan formation middle unit the upper part of the profile shows a gradual positive shift the carbon isotopic signatures of the bilara group rocks can be correlated with the end neoproterozoic early cambrian vendian tommotian carbon isotopic evolution curve extremely low 13 c values indicate the glaciation related cold climatic postulates of the end neoproterozoic followed by the warmer climatic conditions as indicated by the positive shift the carbon isotopic data for gotan formation carbonates at variance with the globally observed 13 c trends for early tertiary do not support the recently proposed tertiary age for the bilara group

biogenic structures and micrite in stalactites from grand cayman island british west indies stalactites from modern and old caves developed in the bluff formation of grand cayman island contain laminae and bulbous masses of micrite intercalated with sparry calcite the micrite as well as some of the sparry calcite around it contains small up to 25 m long ovate to spherical bodies that have a high concentration of either manganese or iron such bodies may be of bacterial origin the micrite contains numerous calcified filaments that are probably of algal origin calcification of the filaments occurred either during life or shortly after death of the algae the algae played an important role in trapping and binding the micrite furthermore the algae may be directly or indirectly responsible for the formation of much of the micrite

the nature and rate of weathering by lichens on lava flows on lanzarote abstract samples of lichen covered and bare lava surfaces from lanzarote dating from the 1730 1736 eruption have been analysed using scanning electron microscopy sem energy dispersive spectroscopy eds and x ray diffraction xrd to investigate the relative roles of biological and inorganic weathering processes lichens here grow preferentially on n and ne facing surfaces and create a range of nanomorphological features as a consequence of their weathering activities the fruticose lichen stereocaulon vesuvianum and a mixture of crustose lichen species are found to be particularly effective agents of weathering comparison of the thickness of the weathering rind on bare and lichen covered samples mean thickness 15 7 and 253 9 m respectively shows a significant difference at the 99 confidence level following previous studies in hawaii by jackson and keller am j sci 269 1970 466 these results are used to suggest that lichens on lanzarote lava flows cause a 16 times increase in weathering rates over those found on bare surfaces comparison of these results with those from similar lava flows in hawaii indicates that under the wetter climate of hawaii both biological and inorganic rates are over double those found in lanzarote

neoproterozoic stratigraphy of the centralian superbasin australia abstract the basement of central australia includes the arunta block and the 1200 1100 ma musgrave block succeeded by 1075 1000 ma bimodal volcanic rifts and dolerite dykes a regional hiatus at 1000 800 ma interpreted as reflecting a central post volcanic underplated upland was followed at 800 ma by a second swarm of dykes in the musgrave block gawler craton and stuart shelf associated with the crustal sagging that initiated the centralian superbasin the centralian superbasin is defined here to encompass the neoproterozoic fill of the amadeus georgina ngalia officer and savory basins the intra cratonic superbasin was disrupted internally 540 600 ma ago petermann ranges orogeny by a central uplift and associated thrusts and by mid carboniferous 320 ma tectonism alice springs orogeny to form the structural basins listed above the stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions new results from acritarch biostratigraphy and isotope chemostratigraphy in conjunction with conventional lithostratigraphy and sequence analysis allow better stratigraphic resolution than previously possible field observations in the recently discovered savory basin in western australia provide a basis for correlations with the adjacent basins

oceanic redox conditions in the late mesoproterozoic recorded in the upper vazante group carbonates of sao francisco basin brazil evidence from stable isotopes and rees the vazante group consists of a late mesoproterozoic 1 0 1 1 ga carbonate dominated marine platform sequence in east central brazil the upper part of the sequence consists of a glaciomarine diamictite unit overlain by a cap carbonate the 13c profile of the upper vazante shows significant negative plunges one preglacial drop of 5 5 vpdb and two postglacial drops of 9 and 5 vpdb respectively the c isotope plunge in the preglacial carbonates is correlated with low th u ratios 0 1 1 4 and a negative ce ce shift 0 4 suggesting deposition under relative reducing conditions in contrast the c isotope plunges in the postglacial carbonates are associated with high th u ratios 2 and positive ce ce shifts up to 1 5 thus reflecting oxidizing conditions variations in the redox conditions of the late mesoproterozoic ocean reflected by changes in the th u and ce ce ratios are likely attributable to a combination of both global and local climatic and oceanographic changes similar to what has been inferred for the neoproterozoic oceans

origin of dolomite in the middle devonian guanwushan formation of the western sichuan basin western china abstract the carbonates in the middle devonian guanwushan formation d2g of the western sichuan basin are extensively dolomitized three types of replacement dolomite can be distinguished 1 micritic microcrystalline dolomite 100 m with anhedral subhedral dolomite crystals type 3 the anhedral to subhedral fine crystalline crystals in type 3 dolomite are truncated by a stylolite which indicates that the formation of type 3 dolomite most likely occurred simultaneously or predated the stylolite formation stratigraphic petrographic and geochemical data indicate that the 87sr 86sr ratios 0 70839 to 0 71117 of the d2g dolomites which are higher than those of the contemporaneous seawater 0 70780 0 70830 were affected by the underlying siliciclastic rocks of the jinbaoshi formation d2j pingyipu formation d1p and silurian strata the dolomitizing fluids incorporated elemental sr which tends to be abundant in crustal fluids the rare earth element ree compositions of the d2g dolomites are characterized by enrichment in light rees and depletion in heavy rees ce and eu indicating that the dolomites of the d2g inherited the characteristics of the contemporaneous seawater and formed in a low temperature relatively oxic environment the 18o values of type 1 and type 2 dolomites 6 3 to 0 43 and 7 2 to 1 69 respectively are slightly higher than those of seawater derived dolomite suggesting that these dolomites may be related to the evaporation of seawater during the penecontemporaneous stage the 18o values of type 3 dolomite 8 32 to 0 83 are slightly lower than those of the seawater derived dolomites indicating that this dolomite may have formed at a higher temperature during shallow moderate burial type 1 dolomite has higher 18o values sr2 concentrations and total ree ree concentrations and slightly lower mn2 concentrations than the other two types of dolomite suggesting that type 1 dolomite formed via capillary concentration dolomitization from evaporated seawater type 2 dolomite has lower 18o values and ree contents and slightly higher mn2 and sr2 concentrations than type 1 dolomite suggesting that type 2 dolomite formed via the seepage reflux process from denser seawater type 3 dolomite has a stylolite lower 18o values sr2 concentrations and ree concentrations and slightly higher mn2 concentrations and mg ca values suggesting that type 3 dolomite formed during shallow moderate burial

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

micro erosion meter measurements of rock wear on the otway coast of southeast australia abstract the use of the micro erosion meter mem has been refined by providing 1 a base plate on which to check the relationship of legs and probe and 2 a micrometer level to check the relationships of the three studs countersunk in the rock at each site along 50 km of the otway coast lines of mem stations were installed across shore platforms normal to the shore and surveyed to mean low water readings were made bimonthly from 1978 to 1980 average site downwear ranged from 0 02 to 1 8 mm yr 1 according to rock type and the amount of exposure to the almost continuous swell waves the results of this study combined with other information available are used to discuss the origin and evolution of otway coastal morphologies

an upper turonian fine grained shallow marine stromatolite bed from the munecas formation northern iberian ranges spain abstract a fine grained stromatolite bed laterally continuous on the kilometer scale and with small synoptic relief crops out in the munecas formation in the northern iberian ranges the munecas fm was deposited during the late turonian on a shallow water platform in the upper cretaceous intracratonic iberian basin the stromatolite bed has a tabular to domed biostromal macrostructure its internal mesostructure consists of planar wavy to hemispherical stromatoids that display a broad spectrum of microstructures including dense micrite bahamite peloids peloidal to clotted microfabrics irregular micritic wall tubes which are suggestive of algae and filamentous microframeworks which are suggestive of filamentous cyanobacteria various stromatolite growth stages have been linked to the dominance of different accretion processes the accretion of the entire fine grained stromatolite involves a complex mosaic of processes trapping and binding of quartz silt grains and bahamites which form the agglutinated parts of some laminae and microbially induced precipitation which forms spongiostromic and micritic laminae tubiform and filamentous microframeworks resembling porostromatate or skeletal stromatolitic growth were also recognized laser icp ms measurements of al si mg mn sr s and fe were analyzed to detect the influence of siliciclastic inputs and major trends during stromatolite accretion carbon and oxygen isotopic compositions from the stromatolite and associated facies were used to identify possible microbial signatures these data describes a unique and well preserved example of a shallow marine upper turonian fine grained stromatolite

insights from isotope stratigraphy a symposium on precambrian isotope stratigraphy was heldwithintheframeofthe33rdigcheldatoslo norway august2008 this symposium spawned the idea for a special issue on this themeand ultimately to the ten papers assembled here this special issueprovides the results of some of the latest cutting edge research ontraditional c sr s isotopechemostratigraphyandintroducesomenewer isotope systems ree ca and hg chemostratigraphy as apotential tracer for glacial events and estimate of the buildup ofvolcanicgases includingco

the micro erosion meter use and limitations detection of anomalies in measurements of low rates of limestone surface lowering with a traversing micro erosion meter mem led to temperature controlled laboratory and field investigations of some sources of error in the method regressions for the correction of temperature change in the instrument and in the stud rock interface were obtained probe erosion tests permitted a correction to be devised and also provided information whereby a measure of operational irregularity was adopted corrections and error terms were applied to field measurements from two southern new south wales karsts and a collection of australian limestone slabs exposed at one site examination of these results and of published mem rates from other parts of the world validated the mem method marine platforms are lowered at such gross rates that these error sources may be neglected there however limestone lowering in terrestrial environments cannot be assumed to proceed at rates which permit this and recommendations are made about procedures to ensure that investigations are prosecuted within the limitations of the method

neoproterozoic fossils from the franklin mountains northwestern canada stratigraphic and palaeobiological implications abstract shales from the lone land formation of the cap mountain inlier southern franklin mountains and pre palaeozoic horizons of a nearby drillhole shell blackwater lake g 52 have yielded two distinct fossil assemblages the presence of trachyhystrichosphaera aimika in g 52 and chuaria circularis in the lone land identifies both sections as neoproterozoic previous lithostratigraphic correlations with mesoproterozoic strata on the coppermine homocline are not supported marked differences in thermal alteration of the fossils suggest that the two successions are not related despite their present geographic proximity whereas the g 52 fossils are medium orange in colour those in the lone land are brown we argue that the cap mountain succession is allochthonous and originated probably to the west of its present position by contrast the g 52 assemblage shows marked similarity to a fossil in the wynniatt formation shaler supergroup of victoria island compound and or coiled filaments are abundant in the g 52 assemblage and appear to be associated generally with exceptional fossil biotas in the neoproterozoic insofar as these are likely to document particular environmental conditions they may help to identify the locus of early eukaryotic diversification the tendency of some prokaryotes to aggregate and form compound structures also has implications for interpreting various problematic macrofossils including putative seaweeds from the palaeoproterozoic negaunee iron formation we erect a new genus and species gemmuloides doncookii n gen n sp from the lone land formation

weathering rates on a caribbean reef limestone results and implications abstract weathering of presently raised reef islands provides an analogue for the behaviour of pleistocene reefs made emergent by glacio eustatic falls in sea level micro erosion meter measurements on grand cayman island west indies have shown that high mean weathering rates 1 00 mm yr 1 are restricted to the marine environment large scale landform development is unlikely to have been the product of the last period of exposure but rather the result of successive phases of weathering and reef growth increasing the differential relief of the reef base throughout the pleistocene

surface swelling of coastal bedrock on inter tidal shore platforms kaikoura peninsula south island new zealand this paper reports the occurrence of swelling of coastal bedrock measured with a traversing micro erosion meter tmem on inter tidal mudstone and limestone shore platforms while numerous micro erosion meter mem studies have been made only a small number have reported surface swelling during this study 24 055 measurements were made with a tmem of those 4047 indicated swelling or 17 of the total number of measurements vertical swelling of up to 8 9 mm was measured and values of 3 and 4 mm were common swelling displayed a complex pattern with minor swelling being superimposed on longer period episodes duration of swelling events in excess of 697 days were recorded while other events were commonly in the order of 3 to 4 months there is a distinct seasonal variation with greater and more frequent swelling occurring during summer months compared with winter explanation for swelling includes the growth of salt crystals in rock lattices and expansion from wetting and drying warm summer temperatures provide the best conditions for both processes swelling appears to be an important precursor to erosion of material from shore platform surfaces the conclusion drawn is that subaerial weathering makes an important contribution to the lowering of the shore platforms on the kaikoura peninsula

rock temperatures from southeast morocco and their significance for experimental rock weathering studies surface and subsurface rock temperature measurements are presented from three rock types chalk granite and sandstone from a hot desert climate the temperature regimes undergone by each rock type and the differences between them can be best explained by reference to the values of albedo and thermal conductivity for the rocks the significance of thermal properties in influencing rock temperatures suggests that these properties should be taken into account when attempting to simulate desert weathering and also when trying to explain variations in the weathering behavior of different rock types

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

carbon sulfur and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the ediacaran hankalchough glaciation in order to understand spatial variations of stable isotope geochemistry in the quruqtagh basin northwestern china in the aftermath of an ediacaran glaciation we analyzed carbonate carbon isotopes 13ccarb carbonate oxygen isotopes 18ocarb carbonate associated sulfate sulfur 34scas and oxygen isotopes 18ocas and pyrite sulfur isotopes 34spy of a cap dolostone atop the ediacaran hankalchough glacial diamictite at four sections the four studied sections ykg mk h and zbs represent an onshore offshore transect in the quruqtagh basin our data show a strong paleobathymetry dependent isotopic gradient from the onshore to offshore sections 13ccarb values decrease from 2 to 16 vpdb whereas 18ocarb values increase from 4 to 1 vpdb both 34scas and 34spy show stratigraphic variations in the two onshore sections mk and ykg but are more stable in the two offshore sections h and zbs 18ocas values of onshore samples are consistent with terrestrial oxidative weathering of pyrite we propose that following the hankalchough glaciation seawater in the quruqtagh basin was characterized by a strong isotopic gradient the isotopic data may be interpreted using a three component mixing model that involves three reservoirs deep basin water surface water and terrestrial weathering input in this model the negative 13ccarb values in the offshore sections are related to the upwelling of deep basin water where anaerobic oxidation of dissolved organic carbon resulted in 13c depleted dic whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite the new data provide evidence for the oceanic oxidation following the hankalchough glaciation

chemostratigraphy of predominantly siliciclastic neoproterozoic successions a case study of the pocatello formation and lower brigham group idaho usa abstract isotopic chemostratigraphy has proven successful in the correlation of carbonate rich neoproterozoic successions in successions dominated by siliciclastic rocks chemostratigraphy can be problematic but if thin carbonates punctuate siliciclastic strata useful isotopic data may be obtained the upper pocatello formation and lower brigham group of southeastern idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions the 5000 m thick succession consists predominantly of siliciclastic lithologies with only three intervals that contain thin intercalated carbonates its depositional age is only broadly constrained by existing biostratigraphic sequence stratigraphic and geochronometric data the lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the pocatello formation the delta 13c values of these carbonates are distintly negative 5 to 3 similar to carbonates that overlie neoproterozoic glaciogenic rocks worldwide stratigraphically higher carbonates record a major positive delta 13c excursion to values as high as 8 8 within the carbonate member of the caddy canyon quartzite the magnitude of this excusion is consistent with post sturtian secular variation recorded elsewhere in the north american cordillera australia svalbard brazil and nambia and exceeds the magnitude of any post varanger delta 13c excursion documented to date in most samples sr isotopic abundances have been altered by diagenesis and greenschist facies metamorphism but a least altered value of approximately 0 7076 supports a post sturtian and pre marinoan varanger age for upper pocatello and lower brigham rocks that lie above the pocatello diamictite thus even though available chemostratigraphic data are limited they corroborate correlations of pocatello formation diamictites and overlying units with sturtian glaciogenic rocks and immediately post sturtian successions in western north america and elsewhere

isotope stratigraphy of the lapa formation sao francisco basin brazil implications for late neoproterozoic glacial events in south america the lapa formation is a thick carbonate sequence 900 m that constitutes the upper part of the vazante group on the s francisco craton brazil it conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age the sequence above the glacial unconformity consists predominantly of organic rich shale subtidal rhythmic dolomicrites and microbialaminites and intertidal stromatolites four boreholes spanning different depositional settings were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation the 13 c and 18 o values of well preserved lapa carbonate microsamples range from 8 2 to 3 3 vpdb and from 13 6 to 0 9 vpdb respectively each of the 13 c profiles of the investigated cores reveals two strong negative excursions of up to 8 an event in post glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic rich shale and a second near the top of the sequence associated with a shale interval based on the observation of dropstones and sedimentary iron formation in the underlying diamictite as well as the distinguishable carbon isotope trends the lapa formation is considered as a cap carbonate lithofacies the age of the lapa formation is presently unknown but the least radiogenic 87 sr 86 sr value 0 7068 associated with a negative carbon isotope excursion matches that from the rasthof formation in namiba on the congo craton which is radiometrically constrained to be younger than ca 750 ma

the weathering effects of the lichen lecidea aff sarcogynoides koerb on magaliesberg quartzite the effect of the weathering processes generated by lecidea aff sarcogynoides koerb on the substrate was studied by means of a scanning electron microscope the elements present in the substrate magaliesberg quartzite and in the lichen thallus were determined by x ray fluorescence spectrometry for the purpose of comparison the elements present were mostly similar although a few were present in the thallus which were not observed in the quartzite it is possible that those elements present in the lichen thallus which were not present in the substrate may have been extracted from the atmosphere the occurrence of small hollows weathering pits in which the early stages of plant development occurs and the disintegration of the rock indicate that lecidea aff sarcogynoides koerb contributes to the chemical weathering processes by chelation and mechanically by the penetration and expansion of hyphae a model is proposed in which a possible mechanism for these weathering processes is suggested

rock coasts with particular emphasis on shore platforms rock coasts are one of the most common elements of the world s littoral zone and they are often important sources of sediment for estuaries and beaches economic development growing populations and the potential effects of rising sea level provide a practical need to understand the dynamics of these coasts although there have been significant advances in our understanding of rocky coastal systems further progress has been hindered by a lack of researchers and by the often imperceptible changes that generally occur within human time scales this paper reviews rock coast processes and landforms in a variety of morphogenic environments one of our most fundamental challenges is to determine the degree to which rock coasts are contemporary rather than inherited features from the quaternary when changes in sea level and climate were responsible for marked variations in the nature intensity and elevational distribution of the marine and subaerial processes that sculpture rock coasts

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

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

trace elements and sr and c isotopic signatures in late neoproterozoic and earliest cambrian sedimentary organic matter from siliciclastic successions in the east european platform sedimentary organic matter deriving from tubes of sabelliditids vendotaenids sapropelic films and kerogens was extracted by acid processing from upper vendian siliciclastic successions in the east european platform elemental composition obtained by ins

micro topographic change on calcarenites grand cayman island west indies changes in micro topography as a result of varying erosion rates have been monitored on a calcarenite ridge on grand cayman island west indies using a traversing model of the micro erosion meter erosion takes place through the solution of a calcitic cement and solutional disintegration results in a rapid change of micro relief where a protective crust is present surface lowering is slower and also more uniform thus the original relative relief is maintained

sr and nd isotopes of cold seep carbonates from the northern south china sea as proxies for fluid sources abstract in this study sr isotopes combined with nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids we presented sr and nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the south china sea scs based on the evidence for negligible detrital contamination and diagenetic alteration carbonate samples are considered dependable archives which record sr and nd isotope signals of the original pore waters from which carbonates have precipitated excellent correlation between 87sr 86sr ratios and 1 sr observed in carbonate samples indicates two end members with distinct sr isotopic signatures including modern seawater as a major source and a lesser contribution from methane rich seep fluid driven from a deep source besides these two end members release of nd from organic matter and fe mn oxyhydroxide is an additional contribution to pore water nd furthermore sr and nd isotopic compositions suggest water rock interactions between upward seep fluid and detrital component in sediments which resulted in the isotopic shift towards pronounced 143nd depleted and slightly 87sr enriched in the fluid phase this study provides insight into the knowledge for tracing seepage fluid circulation by sr nd isotope system at modern seep systems

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

limestone weathering in eastern australia part 2 surface micromorphology study results from long term microerosion meter mem studies indicate that microenvironments are characterized by different erosion rates a complementary study carried out on the same sites examines the microscale morphology in each environment optical and scanning electron microscopy were used to identify a range of features thought to reflect different processes under subaerial conditions micromorphology is dominated by biological weathering features owing to the presence of microflora sites permanently covered by soil or stream water have only dissolution etch features though stream sites may exhibit microfractures sites with periodic exposure exhibit both dissolution and biological etch features the natural environment is more accurately reflected by allowing some microflora colonization of mem sites rather than preventing colonization by artificial means

systematic variations in c o and sr isotopes and elemental concentrations in neoproterozoic carbonates in namibia implications for a glacial to interglacial transition abstract we analyzed c o and sr isotopic compositions of neoproterozoic cap carbonates overlying a glacial deposit in namibia to trace environmental changes following the glaciation the three isotopic records showed coherent and phased changes that corresponded to the stratigraphic variation a lower rhythmite unit had three intervals which were distinguished by isotopic variations and mineral type in the basal interval 13 c increased exponentially from a negative value 4 3 pee dee belemnite pdb 13 c values were relatively constant in the middle interval and increased again to positive values in the top interval indicating a phased removal of 13 c depleted carbon from the surface ocean the gradual increase to larger values 6 in the upper stromatolite unit was interpreted to reflect a change in seawater composition resulting from biological activity after environmental recovery from the glacial episode 18 o values and sr isotopic ratios decreased in the basal interval and approached stable values in the middle interval they increased in the upper interval and fluctuated in the stromatolite unit on the basis of geochemical screening criteria for detecting diagenetic alteration of sr isotopes the middle interval of the rhythmite unit with a minimum 87 sr 86 sr ratio of 0 70685 was interpreted to have retained the primary value or to have the least altered sr isotopic composition the basal interval which had ratios 0 718 at its base as high as those of old sialic rocks was interpreted as having altered sr isotopic ratios the high sr isotopic ratios at the base of this interval gradually decreased to near the minimum value at the top of the interval and other isotopic compositions and trace element concentrations showed systematic variations such systematic variations except for the variation in na concentration can be explained by a progressive diagenetic fluid rock interaction alternatively they might reflect a compositional transition from a local transport limited water following the glaciation to global well mixed seawater

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

strontium isotope stratigraphy and paleomagnetic age constraints on the evolution history of coral reef islands northern south china sea understanding the history of the response of coral reefs to past climate changes can provide valuable information for predicting the future response of modern reefs however dating such ancient biotic carbonate is still challenging because of its sensitivity to diagenetic alteration processes scarcity of well preserved fossils and low magnetic mineral content there have been a long debates about the origin and evolutionary history of coral reefs in the northern south china sea mainly due to the lack of direct and reliable age constraints this provides us with a good opportunity to verify the practicability of different dating approaches especially the strontium sr isotope analysis of bulk carbonate here we retrieved a 972 55 m long core from the xisha islands to provide a credible chronologic constraint on the carbonate platform evolution the lithostratigraphy strontium isotope stratigraphy and magnetostratigraphy were analyzed throughout the whole reef sequence the lithostratigraphic results show that the 873 55 m reef sequence developed on an ancient volcaniclastic basement and experienced multiple evolutionary phases the sr 87 sr 86 results of all 100 bulk carbonate samples vary from 0 708506 to 0 709168 and show a monotonic increase with decreasing depth except for a few outliers trace element criteria and stable isotope delta o 18 and delta c 13 methods were applied to these bulk carbonate samples and results imply that the primary or near primary seawater sr 87 sr 86 values were likely preserved although different degrees of diagenetic alteration occurred in addition the paleomagnetic results indicate 10 normal polarity and eight reversed polarity magnetozones based on the sr 87 sr 86 ratios of the selected 58 samples and paleomagnetic reconstruction of polarity reversals the bottom of the reef sequence is dated to 19 6 ma and the observed polarity chronozones extend from chron c6 19 722 18 748 ma at 866 60 m to present at the top based on the new data we propose a new chronologic framework for the evolutionary history of the reef islands where 1 the reefs initiated in the early miocene 19 6 ma and were drowned until 16 26 ma 2 during 16 26 10 66 ma lagoon to lagoon slope environments prevailed 3 the lagoon environment progressively transformed into a reef crest environment from 10 66 to 4 36 ma and 4 36 to 1 59 ma and 4 the reef started to be drowned again during 1 59 0 ma our study provides a new and reliable chronologic constraint on the general evolutionary history of the reef islands in the northern south china sea furthermore the sr 87 sr 86 results from bulk carbonate indicate that strontium isotope stratigraphy is a powerful dating tool only when rigid sample selection sequential leaching procedures and strict trace element and isotopic criteria are applied

two hourly surface change on supra tidal rock marengo victoria australia a traversing micro erosion meter was used to measure rock surface micro topography over 40 cm2 on a supra tidal cliff face from early morning to late evening in late spring from 06 00 hours to 22 00 hours the relative heights of 188 coordinates were obtained using the meter at 2 hour intervals resulting in a data set of 1607 readings monitoring shows that rock surfaces are dynamic entities with significant rise and fall relative to the first measurement at shorter timescales than previously reported the maximum positive rise between readings was 0 261 mm and lowering was 0 126 mm the pattern of change did not relate as expected to environmental variables such as temperature or insolation rather the surface showed greater surface change in the early morning and late afternoon it is hypothesized that this pattern relates to the expansion and contraction of lichen thalli as moisture is absorbed during higher humidity in the morning and late afternoon the implications of these results for weathering studies are considered copyright 2006 john wiley sons ltd

measuring erosion rates using the micro erosion meter 20 years of data from shore platforms kaikoura peninsula south island new zealand abstract fifteen micro erosion meter sites giving 42 individual readings of erosion were remeasured twenty years after installation on mudstone and limestone shore platforms on the kaikoura peninsula south island new zealand the mean annual surface lowering rate was calculated to be 1 43 mm yr longer term data were compared to shorter term data collected over a two year period between 1973 and 1975 to test the validity of extrapolating average erosion rates from shorter term data it was found that the average lowering rate for the shorter term data were in statistically acceptable agreement with those for the longer term data extrapolation of short term micro erosion meter data is therefore acceptable in this particular environment in previous work it has been suggested that measurements from as few as thirty individual micro erosion meter positions are required to calculate a mean annual lowering rate on shore platforms measurements from 30 mem positions provided an statistically acceptable mean annual lowering rate for shore platforms on the kaikoura peninsula

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

c o isotope and whole rock geochemistry of proterozoic jahazpur carbonates nw indian craton abstract the jahazpur group nw indian craton meta pelite carbonate quartzite sequence of presumed palaeoproterozoic age unconformably overlies the volcano sedimentary rocks of hindoli group and a tonalitic gneiss leucogranite complex jahazpur granite basement leucogranite the younger component of the jahazpur granite complex has been considered to be coeval with the ca 2 5 ga berach granite the lowermost unit of the jahazpur group the basal conglomerate which unconformably overlies the hindoli group is conformably overlain by a phyllite dolomite phyllite sequence this sedimentary package is developed in two belts designated as the eastern and western jahazpur belts preliminary c and o isotopic results and relevant whole rock geochemical data on representative dolomitic limestone samples are presented and discussed in this paper the lower dolomite unit shows significantly negative to near zero d13c values 4 8 to 0 9 v pdb while the upper dolomite developed in the western belt only shows near zero values various geochemical parameters mn sr and ca mg ratios and non correlative d13c and d18o values indicate well preserved isotopic characteristics the isotopic signatures of jahazpur dolomites are significantly different from the highly positive d13c values reported from the palaeoproterozoic aravalli supergroup rocks occurring around udaipur jhamarkotra formation the jahazpur carbonates show some similarity in the c isotopic characteristics with the delwara formation carbonates which underlie the jhamarkotra formation however the observed negative d13c values in the delwara formation carbonates have possibly resulted from effects of metamorphic decarbonation