global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

a basin redox transect at the dawn of animal life multiple eukaryotic clades make their first appearance in the fossil record between 810 and 715 ma molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation animals require oxygen to fuel their metabolism and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the cambrian radiation of large ecologically diverse animals here paleoredox conditions were investigated in the fifteenmile group ogilvie mountains yukon canada which hosts an 811 ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals iron based redox proxies redox sensitive trace elements organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects these data suggest that for this basin oxygenated shelf waters overlay generally anoxic deeper waters the anoxic water column was dominantly ferruginous but brief periods of euxinia likely occurred these oscillations coincide with changes in total organic carbon suggesting euxinia was primarily driven by increased organic carbon loading overall these data are consistent with proposed quantitative constraints on proterozoic atmospheric oxygen being greater than 1 of modern levels but less than present levels comparing these oxygen levels against the likely oxygen requirements of the earliest animals both theoretical considerations and the ecology of modern oxygen deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges eumetazoans or bilaterians thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized neoproterozoic oceans although these inferred levels would constrain animals to very small sizes and low metabolic rates

coupled re os and u pb geochronology of the tonian chuar group grand canyon well preserved strata of the late tonian chuar group exposed in the grand canyon host fossil evidence for the development of eukaryotic predation the presence of unique biomarkers and large changes in c s and mo isotope chemostratigraphy despite the importance of this critical succession few radioisotopic age constraints are available to place these records into a global context here we couple high precision u pb chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims on zircon crystals with the rhenium osmium re os sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of earth history zircons recovered from a tuff within the uppermost walcott member of the kwagunt formation yield a weighted mean 206pb 238u age of 729 0 0 9 ma mean square of weighted deviates mswd 0 86 differing significantly from the previous air abrasion upper intercept age of 742 6 ma on zircons from this same horizon organic rich carbonates from the carbon canyon member of the galeros formation yield a model 1 re os age of 757 0 6 8 ma mswd 0 47 n 8 and an initial os isotope 187os 188os osi composition of 1 13 0 02 the radiogenic osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the carbon canyon member in agreement with sedimentological and stratigraphic data the re os geochronology of marcasite fes2 nodules from the awatubi member of the kwagunt formation yield a model 1 age of 751 0 7 6 ma mswd 0 37 n 5 with an osi of 0 44 0 01 this re os date is interpreted to constrain the growth of the marcasite nodules in the awatubi member during deposition the formation of sulfides and the less radiogenic osi value are consistent with an influx of sulfate laden seawater to the basin during deposition of the kwagunt formation attempts to apply the re os geochronometer to the walcott and tanner members of the chuar group failed to yield meaningful ages despite elevated re enrichments 20 ng g the re os data from these units yielded negative osi values which suggest disturbance to the re os system the low os abundances typically 100 pg g relative to the amount expected based on the elevated re abundances suggest leaching of os due to oxidative weathering on geologically recent time scales finally the carbon canyon member provides a useful case study for quantifying how input uncertainties in the re os geochronometer propagate into the resulting age uncertainty and we discuss the protocols that will yield the best improvement in age precision for future studies the u pb and re os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods our analysis suggests that for our data the most efficient way of reducing uncertainties in the presented re os dates is through improved precision of measured os values

tonian cryogenian boundary sections of argyll scotland abstract the tonian cryogenian system boundary is to be defined at a gssp global boundary stratigraphic section and point beneath the first evidence of widespread glaciation a candidate lies within the dalradian supergroup of scotland and ireland which is least deformed and metamorphosed in argyll western scotland we present new stratigraphic profiles and interpretations from the isle of islay and the garvellach islands update the chemostratigraphy of the appin group tonian carbonates underlying the thick ca 1 km glacigenic port askaig formation paf and demonstrate an environmental transition at the contact the appin group forms a regionally extensive 4 km thick succession of limestones shales and sandstones deposited on a marine shelf on islay the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure oolite stromatolitic dolomite and intraclastic microbial mounds significantly deeper erosion at the unconformity at the base of the overlying paf is demonstrated in the southern section carbonate facies show a gradual decline in 13 c vpdb from 5 to 2 upwards in ne garbh eileach garvellach islands a continuously exposed section of appin group carbonates 70 m thick here designated the garbh eileach formation gef lies conformably beneath the paf the gef and the gef paf boundary relationships are re described with new sedimentological logs petrological and stable isotope data interstratified limestone and dolomicrosparite with 13 c of 4 to 7 a feature named the garvellach anomaly replacing the term islay anomaly are overlain by dolomite in which the isotope signature becomes weakly positive up to 1 upwards shallow subtidal conditions become peritidal upwards with evidence of wave and storm activity gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the gef the basal diamictite d1 of the paf is rich in carbonate clasts similar to slightly deeper water parts of the underlying succession d1 is typically several metres thick with interstratified sandstone and conglomerate but dies out laterally scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft sediment deformation is interbedded with carbonate below and above d1 dolomite beds with derived intraclasts and gypsum pseudomorphs are found above d1 or equivalent position where d1 is absent published and new sr isotope studies including successive leach data demonstrate primary tonian 87 sr 86 sr values of 0 7066 0 7069 on islay decreasing to 0 7064 0 7066 in the younger gef limestones on the garvellachs with 1700 2700 ppm sr other typically tonian characteristics of the carbonates are the sr rich nature of limestones molar tooth structure and dolomitized peritidal facies with evidence of aridity seabed surveys suggesting uniformly dipping strata and shallow borehole core material illustrate the potential for extending the tonian record offshore of the garvellachs a candidate tonian cryogenian gssp is proposed on garbh eileach within the smooth 13 c profile at the cross over to positive 13 c signatures 4 m below the first occurrence of ice rafted sediment and 9 m below the first diamictite although lacking radiometric constraints or stratigraphically significant biotas or biomarkers the scottish succession has a thick and relatively complete sedimentary record of glaciation coherent carbon and strontium chemostratigraphy lateral continuity of outcrops and 100 exposure at the proposed boundary interval

changing tectonic controls on the long term carbon cycle from mesozoic to present tectonic drivers of degassing and weathering processes are key long term controls on atmospheric co2 however there is considerable debate over the changing relative importance of different carbon sources and sinks existing geochemical models have tended to rely on indirect methods to derive tectonic drivers such as inversion of the seawater 87sr 86sr curve to estimate uplift or continental basalt area here we use improving geologic data to update the representation of tectonic drivers in the copse biogeochemical model the resulting model distinguishes co2 sinks from terrestrial granite weathering total basalt weathering and seafloor alteration it also distinguishes co2 sources from subduction zone metamorphism and from igneous intrusions we reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing we adopt a recently published reconstruction of subduction related degassing and relate seafloor weathering to ocean crust creation rate revised degassing alone tends to produce unrealistically high co2 but this is counteracted by the inclusion of seafloor alteration and global basalt weathering producing a good overall fit to mesozoic cenozoic proxy co2 estimates and a good fit to 87sr 86sr data the model predicts that seafloor alteration and terrestrial weathering made similar contributions to co2 removal through the triassic and jurassic after which terrestrial weathering increased and seafloor weathering declined we predict that basalts made a greater contribution to silicate weathering than granites through the mesozoic before the contribution of basalt weathering declined over the cenozoic due to decreasing global basaltic area

regulation of atmospheric oxygen during the proterozoic abstract many studies suggest that oxygen has remained near modern levels throughout the phanerozoic but was much less abundant from the great oxygenation event around 2 4 ga until the late neoproterozoic around 600 ma kump 2008 using a simple model we show that the maintenance of atmospheric po2 at 1 of present atmospheric levels pal is inconsistent with modern biogeochemical cycling of carbon sulfur and iron unless new feedbacks are included low oxygen conditions are stable in our model if the flux of phosphorus to the oceans was greatly reduced during the proterozoic we propose a mechanism to reduce this flux through the scavenging of phosphate ions with an iron trap driven by greater surface mobility of ferrous iron in a low po2 world incorporating this feedback leads to two stable equilibria for atmospheric oxygen the first quantitative hypothesis to explain both proterozoic and phanerozoic o2 concentrations

nd isotopic variations of phanerozoic paleoceans abstract the size of various phanerozoic paleoceans determined from paleogeographic maps provides a framework for assigningend measurements of primary authigenic marine minerals to individual paleoceans and for establishing theirend versus age curves we report newend data in the range from 80 to 750 ma for the largest phanerozoic and late precambrian paleocean the pacific panthalassa ocean ppo and limited data on some small paleozoic paleoceans these new data and all other published data on paleoceanend values are used to determine the trends through time ofend in seven phanerozoic paleoceans the variations in paleoceanend curves are due to changes in the rate of crustal additions and in paleoceanic geometry from these data we can calculate a meanend curve and a mean nd model age curve for phanerozoic seawater these curves show large variations with age and reflect the variations ofend in erosion products from the continents as a function of age the high erosion rates of young uplifted areas enhanceend and nd model age variations of phanerozoic seawater this makes it difficult to quantitatively recover crustal addition rates from these curves however it makes the episodic nature of continental additions more evident

linking the bitter springs carbon isotope anomaly and early neoproterozoic oxygenation through i ca mg ratios abstract the ca 810 ma bitter springs carbon isotope anomaly is an abrupt and long lived 5 10 myr departure from the positive carbon isotope values 13 c carb that otherwise characterize early middle neoproterozoic carbonate rocks the onset of this isotope anomaly is manifested by a globally expressed 8 negative shift in 13 c values in shallow marine carbonate strata given increasing evidence that metazoan diversification began well before the ediacaran period the bitter springs anomaly is a logical interval to explore the potential relationship between biological innovation perturbations to the carbon cycle and oxygenation the iodine to calcium magnesium ratio i ca mg in marine carbonates is a sensitive proxy for seawater redox conditions that is increasingly being applied to reconstruct the oxygenation of the global surface ocean through earth s history we report i ca mg ratios along with carbon and oxygen isotope ratios and major and minor element concentrations from carbonate sections spanning the bitter springs anomaly in svalbard east greenland and the mackenzie mountains northwestern canada the results from svalbard and greenland collectively show two prominent features the early stage of the bitter springs anomaly is characterized by a negative 13 c shift coupled to low i ca mg ratios compared to carbonates pre and post dating the bitter springs anomaly the last stage of the bitter springs anomaly displays a positive excursion in i ca mg with the highest values at 7 mol mol yet documented in rocks older than 580 ma in contrast carbonates from the mackenzie mountains are uniformly low and display no variation in i ca mg across the bitter springs anomaly which we interpret to be the consequence of alteration of primary signatures during diagenesis the observed geochemical variations in our svalbard and greenland datasets are interpreted to be the result of a shift from pre bitter springs ocean conditions defined by well oxygenated surface waters and anoxic ferruginous bottom waters to syn bitter springs ocean conditions characterized by expanded euxinia

stratigraphic and tectonic implications of a newly discovered glacial diamictite cap carbonate couplet in southwestern mongolia we report here the discovery of a new end cryogenian glacial diamictite and an overlying basal ediacaran cap carbonate within the tsagaan oloom formation of southwestern mongolia the identifi cation of the cryogenian ediacaran boundary coupled with new 13 c chemostratigraphic profi les facilitates the integration of the neoproterozoic stratigraphy of mongolia with records elsewhere these correlations indicate that a previously unrecognized 16 cryogenian 13 c anomaly is present in the newly defi ned tayshir member informal of the tsagaan oloom formation furthermore chemostratigraphic and lithostratigraphic relationships suggest an 35 m y depositional hiatus below the phosphorite bearing zunne arts member informal of the upper tsagaan oloom formation and that subsidence renewed in the latest ediacaran early cambrian we propose that the lower 1500 m of the tsagaan oloom formation was deposited on a thermally subsiding passive margin after rodiniaage rifting whereas the zunne arts member and the overlying 1600 m of early cambrian strata were deposited in a foredeep basin that formed as the southern margin of the dzabkhan terrane was subducted beneath the khantayshir dariv arc

enhanced chemical weathering triggered an expansion of euxinic seawater in the aftermath of the sturtian glaciation abstract the cryogenian period comprised two episodes of global glaciation sturtian and marinoan glaciations separated by a non glacial interval which was characterized by early radiations of eukaryotic algae and putative metazoans geochemical data indicate that the non glacial interval might be marked by a transient marine oxygenation nevertheless oceanic redox conditions varied both in time and space further the links between non glacial climate and marine redox variations are not well constrained here we present high resolution lithium isotope 7 li fe speciation and trace element mo and u data for clastic sedimentary rocks from the cryogenian interglacial datangpo formation south china in order to track the evolution of continental chemical silicate weathering and driving factors behind marine redox variability during the cryogenian non glacial interval a significantly negative 7 li excursion of 5 is observed in the basal datangpo formation suggesting a dramatic increase in chemical silicate weathering intensity in the aftermath of the sturtian glaciation expansion and contraction of anoxic sulfidic conditions as demonstrated by fe speciation and trace element mo and u data mirror changes in silicate weathering intensity our study provides evidence that greater nutrient and sulfate availability due to high silicate weathering intensity associated with increased exposure of fresh rocks and a warm climate facilitated the spread of euxinic waters over the continental margins of the otherwise ferruginous cryogenian ocean

chemostratigraphic evidence of deccan volcanism from the marine osmium isotope record continental flood basalt cfb volcanism is hypothesized to have played a causative role in global climate change and mass extinctions uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of cfb volcanism our results document a 25 decline in the marine 187 os 188 os record that predates the cretaceous tertiary boundary ktb and coincides with late maastrichtian warming we argue that this decline provides a chemostratigraphic marker of deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of deccan volcanism was a transient global warming event of 3 to 5 c that is fully resolved from the ktb mass extinction

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

climate an exceptionally long interglacial ahead today9s comparatively warm climate has been the exception more than the rule during the last 500 000 years or more if recent warm periods or interglacials are a guide then we may soon slip into another glacial period but berger and loutre argue in their perspective that with or without human perturbations the current warm climate may last another 50 000 years the reason is a minimum in the eccentricity of earth9s orbit around the sun

milankovitch forcing of the last interglacial sea level during the last interglacial sea level was as high as present 4000 to 6000 years before peak northern hemisphere insolation receipt 126 000 years ago the sea level results are shown to be consistent with climate models which simulate a 3 to 4 c july temperature increase from 140 000 to 130 000 years ago in high latitudes with all northern hemisphere land areas being warmer than present by 130 000 years ago the early warming occurs because obliquity peaked earlier than precession and because precession values were greater than present before peak precessional forcing occurred these results indicate that a fuller understanding of the milankovitch climate connection requires consideration of fields other than just insolation forcing at 65 n

a one dimensional model of continental ice volume fluctuations through the pleistocene implications for the origin of the mid pleistocene climate transition abstract a new one dimensional zonally averaged high resolution spherical model of the accumulation and flow of continental ice sheets in response to variations in insolation forcing has been constructed the model includes a detailed representation of the influence of the isostatic adjustment of the earth under the weight of the ice at a rate determined by the viscosity of the planetary mantle with this model we address the issue as to whether the combination of physical processes that it includes is sufficient to allow us to reconcile the variations of continental ice volume that are inferred to have dominated the pleistocene epoch on the basis of the analysis of 18o data from deep sea sedimentary cores we use high resolution data from the newly analyzed odp site 677 and from dsdp site 607 as principal observational constraints these datasets reveal a marked transition in climate to have occurred at mid pleistocene time about 900 000 years bp in order to understand this transition we find that we

tracking the onset of phanerozoic style redox sensitive trace metal enrichments new results from basal ediacaran post glacial strata in nw canada abstract a global rise in oxygen levels has been proposed to coincide with the ediacaran to cambrian radiation of animals yet the precise timing and nature of this change remains unresolved one hypothesis is that the ocean atmosphere system became temporarily well oxygenated in the earliest ediacaran directly following the marinoan snowball earth glaciation 635 ma the evidence for oxygenation is based on large enrichments of redox sensitive trace elements in black shale from south china presumably deposited under a euxinic water column these enrichments meet or exceed the bulk concentrations of redox sensitive trace elements found in phanerozoic shale deposited under euxinic water columns here we test the early ediacaran post snowball oxygenation hypothesis with new data from a high resolution multi proxy geochemical and sedimentological study of three stratigraphic sections in the earliest ediacaran sheepbed formation of the mackenzie and wernecke mountains nw canada iron speciation data from all sections suggest that the local water column was dominantly ferruginous with a notable exception of probable euxinic conditions recorded in part of one section redox sensitive elements show no appreciable enrichments in the basal sheepbed formation with maximum concentrations of molybdenum vanadium uranium and chromium only slightly above world average shale values the lack of substantial elemental enrichments within the sheepbed formation is consistent with results from coeval ferruginous strata in svalbard the dominance of local ferruginous conditions in nw canada cannot alone account for the muted redox sensitive element enrichments as v u and cr are thought to be enriched under these conditions and samples deposited under euxinic conditions also lack phanerozoic style enrichments these contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution

cryogenian glaciation and the onset of carbon isotope decoupling global carbon cycle perturbations throughout earth history are frequently linked to changing paleogeography glaciation ocean oxygenation and biological innovation a pronounced carbonate carbon isotope excursion during the ediacaran period 635 to 542 million years ago accompanied by invariant or decoupled organic carbon isotope values has been explained with a model that relies on a large oceanic reservoir of organic carbon we present carbonate and organic matter carbon isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the sturtian and marinoan glacial events of the cryogenian period approximately 720 to 635 million years ago growth of the organic carbon pool may be related to iron rich and sulfate poor deep ocean conditions facilitated by an increase in the fe s ratio of the riverine flux after sturtian glacial removal of a long lived continental regolith

740 ma vase shaped microfossils from yukon canada implications for neoproterozoic chronology and biostratigraphy biostratigraphy underpins the phanerozoic time scale but its application to pre ediacaran strata has remained limited because proterozoic taxa commonly have long or unknown stratigraphic ranges poorly understood taphonomic constraints and or inadequate geochronological context here we report the discovery of abundant vase shaped microfossils from the callison lake dolostone of the coal creek inlier yukon canada that highlight the potential for biostratigraphic correlation of neoproterozoic successions using species level assemblage zones of limited duration the fossiliferous horizon dated here by re os geochronology at 739 9 6 1 ma shares multiple species level taxa with a well characterized assemblage from the chuar group of the grand canyon arizona usa dated by u pb on zircon from an interbedded tuff at 742 6 ma the overlapping age and species assemblages from these two deposits suggest biostratigraphic utility at least within neoproterozoic basins of laurentia and perhaps globally the new re os age also confirms the timing of the islay 13ccarbonate anomaly in northwestern canada which predates the onset of the sturtian glaciation by 15 m y together these data provide global calibration of sedimentary paleontological and geochemical records on the eve of profound environmental and evolutionary change

cryogenian of yukon abstract cryogenian strata of the windermere supergroup were deposited in tectonically active basins throughout the north american cordillera from alaska to mexico the windermere supergroup of yukon canada hosts key geochronological constraints on the start of the cryogenian period and the onset of the sturtian snowball earth glaciation at ca 717 ma a 57 million year duration for the sturtian glaciation has been suggested by correlations of the glacigenic rapitan group in yukon with equivalent strata in the mackenzie mountains of northwest territories where it is overlain by organic rich argillaceous carbonate of the ca 660 ma twitya formation however syn depositional faulting and large lateral facies variation complicate these stratigraphic correlations here we describe cryogenian strata of the northern wernecke mountains yukon where correlations with the rapitan and hay creek groups of the mackenzie mountains have been previously established these units are then traced south across cryogenian structures to the southern wernecke mountains and west through the hart river inlier to the coal creek and tatonduk inliers of the ogilvie mountains in order to construct a new regional stratigraphic framework the coal creek inlier preserves volcanic rocks in the mount harper group and the eagle creek formation of the rapitan group for which we provide new u pb ca idtims zircon ages we further formalize the glacigenic cryogenian eagle creek formation together the new correlations and geochronological data allow us to more precisely integrate geological data from northwest canada with the rest of the cordillera and beyond particularly these data refine the onset of the sturtian glaciation in northwestern canada to between 716 9 0 4 and 717 4 0 2 ma and demonstrate that syn sedimentary tectonism resulted in unconformities throughout northwest laurentia between the end of the sturtian glaciation at 660 ma and the end of the marinoan glaciation at 635 ma these unconformities confound correlations throughout the cordillera by commonly preserving only one of the two cryogenian glacial deposits

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

was the antarctic glaciation delayed by a high degassing rate during the early cenozoic abstract the cenozoic is a period of major climatic changes marked by the formation of the antarctic ice sheet at the eocene oligocene e o boundary the opening of the southern ocean seaways and the decrease in atmospheric co2 are two processes generally evoked to explain this e o cooling the debate is still ongoing but modeling studies tend to demonstrate that the decrease in atmospheric co2 is the main driver of the cooling however uncertainties persist on what drove the decrease in atmospheric co2 during the cenozoic in this study we investigate the impact of continental drift lithology distribution and volcanic degassing rates on the atmospheric carbon dioxide concentration over the cenozoic within a coupled climate carbon model geoclim in the model the continental drift results in driving low atmospheric co2 levels during the eocene and the oligocene the dispersed configuration and the location of a large continental area north africa northern south america within the inter tropical convergence zone itcz promote co2 consumption by weathering forcing co2 to remain low icehouse conditions are also promoted by the drifting of india and the weathering of the deccan basalts in the itcz during the eocene and by the weathering of the ethiopian traps during the oligocene to prevent the building up of the antarctic ice sheet at the eocene the model needs enhanced solid earth degassing flux by 50 so that atmospheric co2 levels stay above the glacial threshold 750 ppm we find that the decrease in atmospheric co2 from the eocene to the oligocene is probably due to a reduction in the source of volcanic co2 rather than an increase in silicate weathering the model results furthermore suggest that during the miocene period the northward drifting of both the african plate and india including the deccan traps might have decreased the continental surface exposed to chemical weathering therefore generating higher co2 values finally the uplift of the tibetan plateau from the miocene to the present day induces in the model an increase in silicate weathering through the intensification of the south eastern asian monsoon causing atmospheric co2 to come back to a pre industrial value

heterogeneity in southern central indian ridge morb implications for ridge hot spot interaction between the rodrigues triple junction and the marie celeste fracture zone basalts from the central indian ridge cir exhibit an enrichment in incompatible elements that increases in intensity northward in addition h2o tio2 al 8 and dy yb ratios increase while na 8 remains unchanged and fe 8 decreases evolution of the enriched magma appears to be affected by elevated water contents which lower the mantle solidus thereby increasing the initial depth of melting as well as delaying plagioclase crystallization however the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization instead of trending toward a small melt fraction from the mantle as predicted by hydrous melting models the cir samples lie on a mixing line between n morb and a source component that closely resembles present day reunion hot spot lavas thus it appears that while hydrous melting and crystallization affect the cir the enriched and wet mantle originates from the reunion hot spot where it migrates eastward toward the cir against the direction of motion of the lithosphere

clues from mis 11 to predict the future climate a modelling point of view simulations performed with the lln two dimensional northern hemisphere climate model have confirmed that climate is largely triggered by changes in insolation forcing although atmospheric co concentration also plays an important role in particular in the amplitude of the simulated variations marine isotope stage 11 mis 11 some 400 kyr ago and the future share a common feature related to climate forcing i e the insolation at these times displays small similar variations mis 11 can be considered an analogue for future natural climate changes different simulations were performed to identify the conditions constraining the length of the mis 11 simulated interglacial clearly its length strongly depends on the phase relationship between insolation and co2 variations it is only when insolation and co act together towards a cooling i e they both decrease together that the climate enters quickly into glaciation and that the interglacial may be short otherwise each forcing alone is not able to drive the system into glaciation and the climate remains in an interglacial state the same situation applies for the future however we already know that co2 and insolation do not play together indeed insolation has been decreasing since 11 kyr bp and co2 concentration remains above 260 ppmv with a general increasing trend over the last 8000 yr therefore we conclude that the long interglacial simulated for the future is a robust feature and the earth will not enter naturally into glaciation before 50 kyr ap c 2003 elsevier science b v all rights reserved

continental flood basalt weathering as a trigger for neoproterozoic snowball earth abstract atmospheric co2 levels and global climate are regulated on geological timescales by the silicate weathering feedback however this thermostat has failed multiple times in earth s history most spectacularly during the cryogenian c 720 635 ma snowball earth episodes the unique middle neoproterozoic paleogeography of a rifting low latitude supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo under these primed conditions the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios 13 c of neoproterozoic seawater due to their elevated p contents in order to test these hypotheses we have compiled new and previously published neoproterozoic nd isotope data from mudstones in northern rodinia north america australia svalbard and south china and sr isotope data from carbonate rocks the nd isotope data are used to model the mafic detrital input into sedimentary basins in northern rodinia the results reveal a dominant contribution from continental flood basalt weathering during the ca 130 m y preceding the onset of cryogenian glaciation followed by a precipitous decline afterwards these data are mirrored by the sr isotope record which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early middle neoproterozoic ocean including a pulse of unradiogenic sr input into the oceans just prior to the onset of cyrogenian glaciation hence our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average 13 c of seawater in the neoproterozoic and to the initiation of the first sturtian snowball earth

early wuchiapingian cooling linked to emeishan basaltic weathering abstract the last glaciation during the late paleozoic ice age commenced at around the guadalupian lopingian g l boundary and is synchronous with the emplacement of the emeishan large igneous province using ca tims zircon u pb dating we obtained an age of 259 51 0 21 ma for the uppermost tuff from the puan volcanic sequence in the eastern emeishan large igneous province constraining the timing of emeishan volcanism and providing another candidate age for the g l boundary in addition we determined an age of 259 69 0 72 ma for a basal claystone in the immediately overlying longtan formation from a drill core section in southwest south china these ages along with source weathering trends of mudstones from the lower longtan formation and compiled paleotemperature records indicate an earliest wuchiapingian cooling coinciding with the onset of the last permian glaciation this global cooling is associated with positive shifts in both organic and carbonate carbon isotopic records and likely a decrease in atmospheric pco2 a hypothesised causal linkage is proposed in which the rapid post eruptive basaltic weathering of the emeishan province in an equatorial humid belt may accelerate the atmospheric co2 consumption and lead to climate cooling our work supports the long term climate cooling effects of large igneous provinces

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

searching for an oxygenation event in the fossiliferous ediacaran of northwestern canada abstract late neoproterozoic ediacaran strata from northwestern canada provide a thick and rich sedimentological record preserving intercalated carbonates and shale extending from the 635 million year old marinoan glacial deposits up through the 541 million year old precambrian cambrian boundary this region also holds one of the classic localities for the study of early animal life with the ensuing suggestion that this temporal interval captures a gross change in the o 2 content of the earth s atmosphere to test this hypothesis and bring records of northwestern canada into line with other ediacaran fossil bearing basins we provide a detailed geochemical reconstruction from the wernecke mountains of the yukon where possible we also extend these records to the ogilvie mountains to the west and previously published data from the mackenzie mountains to the east our work in the wernecke mountains is set against a composite 13 c record for carbonate that preserves three distinct ediacaran isotope excursions the lowermost of which preserved in the gametrail formation is a putative shuram excursion equivalent what emerges from a multi proxy fe speciation sulfur isotopes major and trace element analyses reconstruction is a picture of a persistently anoxic and ferruginous ediacaran ocean notably absent is geochemical evidence for a prominent oxygenation event an expectation given the appearance of animals and large swings in 13 c the new insight gained through these data challenges the idea of an ediacaran jump in atmospheric oxygen which in turn muddles the link between animal evolution and local geochemical environments

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

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

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

constraints on 87sr 86sr of late ediacaran seawater insight from siberian high sr limestones abstract in se siberia carbonate formations with 13 c carb values ranging between 12 and 7 v pdb and sr concentrations of up to 2 5 occupy an area of 40 000 km 2 several successions exceed 1000 m in thickness and represent the world9s largest known exposures of sedimentary carbonates exhibiting extreme depletion in 13 c the carbonates were deposited on a carbonate platform evolving from a mixed carbonate siliciclastic ramp to a carbonate ramp and then from a peritidal rimmed shelf to a deep water open shelf all sequences reveal a facies independent upward rise in marine 13 c carb from 12 to 7 the trend and magnitude of the values mimic those that are characteristic of the 600 550 ma shuram wonoka isotope event a coincident stratigraphic rise in 87 sr 86 sr from 0 70802 to 0 70862 in several sections of limestones containing 4400 g g 1 sr on average is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the late ediacaran if the greatest temporal rate of change in seawater 87 sr 86 sr observed in the cenozoic is applied to the siberian sections the calculated minimum duration for the suram wonoka event is 10 ma supplementary material xrf and icp aes analysis c o and sr isotopic data are available at http www geolsoc org uk sup18324

modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans the increase in atmospheric oxygen ca 2 4 ga had a significant impact on the geochemical cycling of fe the history of environmental oxygenation may be recorded in the fe isotope composition of archean and proterozoic sediments but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations here we present fe isotope data and iron aluminum ratios from the black sea oxic shelf and euxinic basin the isotope data demonstrate that shelf fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin we propose that there is net transport of isotopically light fe from sediments of the shelf to those of the distal anoxic basin consistent with enrichments in reactive fe seen in the deep basin the low 56 fe benthic fe flux is generated during the coupling of microbial fe iii reduction or sulfidization with fe 2 aq oxidation low 56 fe values reported previously from late archean sedimentary pyrites may be an isotopic fingerprint of analogous fe redox cycling in the late archean oceans this interpretation implies shallow water fe redox recycling in the late archean we predict that the light isotopic compositions of the late archean will prove to be distinct from those of the early archean before fe redox cycling became an important process and we infer that this difference may be related to the presence of oxygen in the surface ocean

thick sulfate evaporite accumulations marking a mid neoproterozoic oxygenation event ten stone formation northwest territories canada thick sulfate evaporite accumulations are absent from proterozoic strata between ca 2000 and ca 1000 ma and detailed sedimentologic stratigraphic and geochemical data for the oldest neoproterozoic thick marine sulfate evaporite successions are largely lacking the middle neoproterozoic ten stone formation little dal group northwest territories canada consists of 500 m of pelagic lagoonal gypsite and anhydritite rocks consisting of the minerals gypsum and anhydrite deposited shortly before the ca 811 ma bitter springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of rodinia the thickness of regional stratigraphic subdivisions of this formation defined by subtle silt and carbonate bearing intervals indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest deposition of the ten stone formation began with abrupt tectonically triggered subsidence and restriction and ended equally abruptly as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply with no evidence for hiatus or erosion stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom hugging dense lagoonal brine from overlying waters deposition of the ten stone formation in a basin that experienced intermittent basin scale tectonic adjustments as recorded by details of its stratigraphy supports the interpretation that the mackenzie mountains supergroup accumulated in an extensional tectonically active intracratonic basin whose structure resembled a lower plate extensional system the absence of halite from the ten stone formation contrasts with its abundance in the stratigraphically lower gypsum free dodo creek formation suggesting that deposition of the lower to middle little dal group spanned a major oxygenation event during which the sequence of evaporite mineral precipitation from seawater changed from halite first to sulfate first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir the limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid neoproterozoic atmosphere an abundance of sulfate in seawater and marine oxygenation extending below storm wave base the mineralogy sedimentology stratigraphy and geochemistry of the ten stone formation are virtually indistinguishable from those of thick phanerozoic deep water below wave base evaporite successions and indicate that the tectonic climatic and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca 811 ma thick sulfate evaporite successions in equivalent stratigraphic positions just below the bitter springs carbon isotope excursion elsewhere in laurentia as well as on the congo craton and in south australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to rodinia s breakup high relative burial rates of organic matter prevailed before the breakup of rodinia and led to oxygenation of the atmosphere ocean system growth of the seawater sulfate reservoir and in association with a warm and arid climate deposition for the first time in earth s history of thick sulfate evaporites in the middle neoproterozoic 100 m y before the first cryogenian glacial episode the neoproterozoic oxygenation event may have taken place in several steps the first of which preceded the bitter springs anomaly

the enigma of neoproterozoic giant ooids fingerprints of extreme climate geologists have documented at least fourteen occurrences of giant ooids a geologically rare type of carbonate allochem in neoproterozoic successions at low paleo latitudes recent experiments a

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

pacific plate motion and undulations in geoid and bathymetry abstract previous studies have shown that the pacific geoid and gravity fields exhibit lineated anomalies trending approximately in the direction of absolute plate motion over the underlying mantle because the undulations obliquely cross fracture zones they have often been attributed a convective origin recently lithospheric boudinage caused by diffuse extension has been proposed as a possible mechanism we have examined the undulations in the free air anomalies geoid and bathymetry over a portion of the pacific plate to determine quantitatively how the undulations are related to plate motion we compare the observed data to an axisymmetric sinusoidal undulation defined in an arbitrary frame of reference in particular we seek the north pole of this reference frame that maximizes the correlation between data and model poles that are close to the pacific hotspot pole represent copolar undulations possibly related to plate motion the distance between the best fitting poles and the hotspot pole is determined as a function of undulation wavelength and reveals several minima with distance

summer solstice solar radiation the 100 kyr ice age cycle and the next ice age modeling studies suggest that the summer solstice solar radiation is more important than the caloric half year solar radiation in producing glacial interglacial cycles because it is more representative of the energy available to melt ice during the short melt season here it is shown that the correlation between the summer solstice solar radiation and the rate of change of the oxygen isotope record is generally greater than that between the caloric half year radiation and the rate of change of the oxygen isotope record these results also suggest that the sawtoothed nature of the 100 kyr cycle may be produced by periods of relatively slow changes in ice volume punctuated by periods of rapid growth that are initiated at times of extremely low summer solstice radiation and that it is unlikely that an ice age will begin in the next 70 kyr

strontium isotope 87sr 86sr stratigraphy of ordovician bulk carbonate implications for preservation of primary seawater values the present study on bulk carbonate 87 sr 86 sr stratigraphy represents a companion work to earlier research that presented a conodont apatite based ordovician seawater 87 sr 86 sr curve for the tremadocian katian stages 485 445 ma here we directly compare the curve based on conodont apatite including some new data not published in earlier work with a new curve based on 87 sr 86 sr results from bulk carbonate from the tremadocian sandbian stages we sampled eight lower to upper ordovician carbonate successions in north america to assess the reliability of bulk carbonate to preserve seawater 87 sr 86 sr and its utility for 87 sr 86 sr chemostratigraphy a high resolution 87 sr 86 sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 sr 86 sr 87 sr 86 sr seawater in total 230 bulk carbonate samples that are paired to conodont samples were measured for 87 sr 86 sr in order to determine the conditions under which 87 sr 86 sr seawater is preserved in bulk carbonate results indicate that well preserved bulk carbonate can faithfully record the 87 sr 86 sr seawater trend but that its 87 sr 86 sr values are commonly more variable than those of conodont apatite on average bulk carbonate samples of the same age vary by 10 20 10 5 compared to 5 10 10 5 for conodont apatite the amount of isotopic alteration of bulk carbonate from seawater 87 sr 86 sr 87 sr 86 sr was determined by taking the difference between 87 sr 86 sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 sr 86 sr values cross plots comparing 87 sr 86 sr values to bulk carbonate sr concentration sr and conodont color alteration indices cai an estimate of the thermal history of a rock body indicate that bulk carbonate is most likely to preserve 87 sr 86 sr seawater minimally altered when either 1 bulk carbonate sr is greater than 300 ppm or 2 carbonate rocks experienced minimal thermal alteration with burial temperatures less than 150 c carbonates with intermediate sr e g between 130 and 300 ppm can also yield 87 sr 86 sr seawater values but results are less predictable and local diagenetic conditions may play a greater role modeling results support the argument that seawater 87 sr 86 sr can be preserved in bulk carbonates with low sr if pore water rock ratios are low 87 sr 86 sr is similar to the seawater 87 sr 86 sr value preserved in limestone bulk carbonate samples that meet these criteria can be useful for high resolution measurements of 87 sr 86 sr seawater with a sample variation on par with fossil materials 5 particularly for successions where well preserved fossil material i e conodonts or brachiopods is not available such as precambrian strata sequences recording mass extinction events or otherwise fossil barren facies these criteria and model predictions based on bulk carbonate sr must be considered in the context of whether a limestone accumulated under calcite seas e g ordovician with relatively high seawater sr ca or aragonite seas in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non seawater sr

the impact of milankovitch solar radiation variations on sea ice and air temperature in a coupled energy balance climate sea ice model the sensitivity of thermodynamicall y varying sea ice and surface air temperature to variations in solar radiation on the 104 to 105 time scales is examined in this study model simulation results show the mean annual sea ice thickness is very sensitive to the magnitude of mid summer solar radiation during periods of high mid summer solar radiation between 115 ka b p and the present

a continental weathering control on orbitally driven redox nutrient cycling during cretaceous oceanic anoxic event 2 the cretaceous period 145 65 m y ago was characterized by intervals of enhanced organic carbon burial associated with increased primary production under greenhouse conditions the global consequences of these perturbations oceanic anoxic events oaes lasted up to 1 m y but short term nutrient and climatic controls on widespread anoxia are poorly understood here we present a high resolution reconstruction of oceanic redox and nutrient cycling as recorded in subtropical shelf sediments from tarfaya morocco spanning the initiation of oae2 iron sulfur systematics and biomarker evidence demonstrate previously undescribed redox cyclicity on orbital time scales from sulfidic to anoxic ferruginous fe rich water column conditions bulk geochemical data and sulfur isotope modeling suggest that ferruginous conditions were not a consequence of nutrient or sulfate limitation despite overall low sulfate concentrations in the proto north atlantic instead fluctuations in the weathering influxes of sulfur and reactive iron linked to a dynamic hydrological cycle likely drove the redox cyclicity despite the potential for elevated phosphorus burial in association with fe oxides under ferruginous conditions on the tarfaya shelf porewater sulfide generation drove extensive phosphorus recycling back to the water column thus maintaining widespread open ocean anoxia

atmospheric pco2 perturbations associated with the central atlantic magmatic province the effects of a large igneous province on the concentration of atmospheric carbon dioxide pco2 are mostly unknown in this study we estimate pco2 from stable isotopic values of pedogenic carbonates interbedded with volcanics of the central atlantic magmatic province camp in the newark basin eastern north america we find pre camp pco2 values of 2000 parts per million ppm increasing to 4400 ppm immediately after the first volcanic unit followed by a steady decrease toward pre eruptive levels over the subsequent 300 thousand years a pattern that is repeated after the second and third flow units we interpret each pco2 increase as a direct response to magmatic activity primary outgassing or contact metamorphism the systematic decreases in pco2 after each magmatic episode probably reflect consumption of atmospheric co2 by weathering of silicates stimulated by fresh camp volcanics

paleomagnetism and u pb geochronology of the clarence head dykes arctic canada orthogonal emplacement of mafic dykes in a large igneous province the north south trending clarence head dyke swarm located on devon and ellesmere islands in the canadian high arctic has a trend orthogonal to that of the neoproterozoic franklin swarm that surrounds it the clarence head dykes are dated by the u pb method on baddeleyite to between 716 1 and 713 1 ma ages apparently younger than but within the published age range of the franklin dykes alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages but a comparison of the u pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the clarence head dykes are indeed a distinctly younger event within the period of franklin magmatism the clarence head dykes represent a large swarm tangential to and cogenetic with a giant radiating dyke swarm 800 km from the indicated source the preferred mechanism for the emplacement of the clarence head dykes is the exploitation of concentric zones of e

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

the initiation of ice sheet growth milankovitch solar radiation variations and the 100 ky ice age cycle much work has gone into deciphering the causes of the large scale glacial interglacial variations in the climate system over the last 900 000 years while variations on the 41 thousand year ky and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere milankovitch solar radiation variations the causes of the dominant 100 ky cycle in the geologic record are still unknown one of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky this is done through the use of a coupled energy balance climate thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation and a land surface energy balance which determines the net accumulation of snow and ice results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season

chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in earth history the neoproterozoic shuram formation nafun group oman abstract oman records in its neoproterozoic nafun group huqf supergroup an essentially complete carbonate rich ediacaran succession the nafun group overlies the presumed marinoan rift related fiq member ca 635 ma and ends just below the precambrian cambrian boundary 542 ma tectonic subsidence caused by thermal contraction following fiq aged rifting allowed the preservation of about 1 km of postrift stratigraphy with no major stratigraphic breaks the nafun group above the marinoan cap carbonate hadash formation represents two siliciclastic to carbonate grand cycles both initiated by significant transgressions these cycles comprise the masirah bay khufai formations and the shuram buah formations the khufai shuram boundary is associated with the start of a major carbon isotope perturbation the uppermost ramp carbonates of the khufai formation record a smooth decrease in 13c from about 4 to values around 0 followed by two descending steps across which values plunge to a nadir of 12 in the overlying red siltstones and shales interbedded with thin limestones of the shuram formation this fall in isotopic values is temporally rapid and coincident in both shallow and deep water sections in the time span of a single parasequence the 13c nadir is then followed by 50 million years of monotonic recovery the shuram shift represents the largest 13c inorganic carbon negative excursion in earth history although the snowball earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation the non glaciated context of the shuram formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non unique although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic the long term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate

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

a cryogenian chronology two long lasting synchronous neoproterozoic glaciations the snowball earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons here we present four new re os geochronology age constraints on sturtian 717 660 ma and marinoan 635 ma termination glacial deposits from three different paleocontinents a 752 7 5 5 ma age from the base of the callison lake formation in yukon canada confirms nonglacial sedimentation on the western margin of laurentia between ca 753 and 717 ma coupled with a new 727 3 4 9 ma age directly below the glacigenic deposits of the grand conglomerate on the congo craton africa these data refute the notion of a global ca 740 ma kaigas glaciation a 659 0 4 5 ma age directly above the maikhan uul diamictite in mongolia confirms previous constraints on a long duration for the 717 660 ma sturtian glacial epoch and a relatively short nonglacial interlude in addition we provide the first direct radiometric age constraint for the termination of the marinoan glaciation in laurentia with an age of 632 3 5 9 ma from the basal sheepbed formation of northwest canada which is identical within uncertainty to u pb zircon ages from china australia and namibia together these data unite re os and u pb geochronological constraints and provide a refined temporal framework for cryogenian earth history

nd isotopic and geochemical constraints on provenance of sedimentary rocks in the eastern officer basin australia implications for the duration of the intracratonic petermann orogeny sm nd isotopic and geochemical data from neoproterozoic to cambrian sedimentary rocks in the intracratonic eastern officer basin in central australia highlight the evolving provenance roles of the basement complexes that underlie and bound the basin initial e nd values of around 12 for the basal units indicate that both were largely derived from the late archaean to mesoproterozoic gawler craton which bounds the basin to the south at c 720 ma an influx of juvenile glacially derived sediment indicates partial uplift of the mesoproterozoic musgrave block along the basin9s northern margin in a regime interpreted to be broadly extensional at around 600 ma synchronous with the development of a foreland architecture there was a large influx of musgrave block derived sediments this is interpreted to mark the onset of the intracratonic petermann orogeny which was a long lived event or series of events spanning more than 70 ma subsequent to c 600 ma the nd isotopic composition of sequences within the officer basin indicates an increasing contribution from the gawler craton despite up to 45 km of denudation of part of the musgrave block this suggests that the majority of sediment derived from the petermann orogen bypassed the eastern officer basin for much of the history of the petermann orogeny

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr