the paleoproterozoic snowball earth a climate disaster triggered by the evolution of oxygenic photosynthesis although biomarker trace element and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2 8 giga annum before present ga and perhaps as early as 3 7 ga a skeptical examination raises considerable doubt about the presence of oxygen producers at these times geological features suggestive of oxygen such as red beds lateritic paleosols and the return of sedimentary sulfate deposits after a 900 million year hiatus occur shortly before the 2 3 2 2 ga makganyene snowball earth global glaciation the massive deposition of mn which has a high redox potential practically requires the presence of environmental oxygen after the snowball new age constraints from the transvaal supergroup of south africa suggest that all three glaciations in the huronian supergroup of canada predate the snowball event a simple cyanobacterial growth model incorporating the range of c fe and p fluxes expected during a partial glaciation in an anoxic world with high fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years as the geological evidence requiring oxygen does not appear during the pongola glaciation at 2 9 ga or during the huronian glaciations we argue that oxygenic cyanobacteria evolved and radiated shortly before the makganyene snowball

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

modeling phosphorus cycling and carbon burial during cretaceous oceanic anoxic events article i nfo in the greenhouse world of the cretaceous there were episodes of oxygen depletion in the deep ocean associated with enhanced organic carbon burial in sediments black shale formation in this study we use a box model of the oceanic phosphorus p organic carbon orgc and oxygen cycles to explore the hypothesis that variations in marine p availability control deep ocean oxygen depletion anoxia and the formation of black shales under cretaceous ocean conditions we find that for the cretaceous ocean with large continental shelves slow oceanic overturning and high sea surface temperatures oceanic anoxic events oaes can be triggered by enhanced p supply from land and that the system is particularly sensitive to oceanic mixing in our baseline scenario the deep sea becomes completely anoxic while the shelves attain only partial anoxia sedimentary burial differs between the shelves and open ocean while organic carbon burial is enhanced in both regions deep sea reactive p burial decreases dramatically under anoxia but not on the shelves where oxygen depletion is not complete furthermore our model results imply that oaes can be sustained by p recycling from sediments under low oxygen conditions ultimately however the feedbacks which result in the accumulation of atmospheric oxygen terminate the anoxic event atmospheric oxygen is modulated by land processes such as forest fires and oxidative weathering which limit the rise of atmospheric o2 our model findings are corroborated by p burial data from the geological record for oae2 approximately 94 myr bp through a sensitivity analysis we identify two necessary criteria for oaes low mixing of surface and deep waters poor ocean ventilation and enhanced sedimentary p recycling under low oxygen conditions when these criteria are met ocean anoxia is a robust result to a mild increase of continental supply of phosphorus under a wide range of environmental conditions

nutrients as the dominant control on the spread of anoxia and euxinia across the cenomanian turonian oceanic anoxic event oae2 model data comparison 1 the cenomanian turonian oceanic anoxic event oae2 is characterized by large perturbations in the oxygen and sulfur cycles of the ocean potentially resulting from changes in oxygen supply via oxygen solubility and ocean circulation and in marine productivity we assess the relative impact of these mechanisms comparing model experiments with a new compilation of observations for seafloor dysoxia anoxia and photic zone euxinia the model employed is an intermediate complexity earth system model which accounts for the main ocean dynamics and biogeochemistry of the cretaceous climate the impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations the model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with oae2 observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content responsible for 85 of the change higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation the model also shows that the presence of seafloor anoxia as suggested by black shale deposition in the proto north atlantic ocean before the event might be the result of the silled shape and lack of deep water formation of this basin at the late cretaceous overall our model data comparison shows that oae2 anoxia was quasi global spreading from 5 of the ocean volume before the event to at least 50 during oae2

a new mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of poc with ballast minerals abstract in simulation studies of the ocean s role in the global carbon cycle predicting the depth distribution for remineralization of particulate organic carbon poc is of particular importance following sarmiento et al global biogeochemical cycles 7 1993 417 most simulation models have the power law curve of martin et al deep sea research 34 1987 267 for this purpose the martin et al curve is an empirical fit to data most of which is from shallow floating sediment traps using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself so that the organic carbon flux f oc z at any depth z can be predicted from the flux of organic carbon f oc z 0 at some near surface depth z 0 here we challenge this basic premise arguing that fluxes of ballast minerals silicate and carbonate biominerals and dust determine deep water poc fluxes so that a mechanism based model of poc flux must simultaneously predict fluxes of both poc and ballast minerals this assertion is based on the empirical observation that poc fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean here we develop a model structure that incorporates this observation and fit this model to us jgofs eqpac data this model structure plus the preliminary parameter estimates we have obtained can be used to explore the implications of our model in studies of the ocean carbon cycle

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

atmospheric oxygenation caused by a change in volcanic degassing pressure around two and a half billion years ago following the end of the archaean eon the atmosphere turned from anoxic to weakly oxic in what is known as the great oxidation event using a model of volcanic degassing gaillard et al demonstrate that a preceding period of continental crust formation may have been the trigger they propose that as continents emerged and volcanoes became increasingly subaerial rather than submarine magmatic volatiles were degassed at lower pressures leading to a progressive oxidation of the gases released this shift to a release of sulphur as sulphur dioxide rather than as hydrogen sulphide could then have fed marine sulphate reduction and the eventual oxygenation of earth s atmosphere the precambrian history of our planet is marked by two major events a pulse of continental crust formation at the end of the archaean eon and a weak oxygenation of the atmosphere the great oxidation event that followed at 2 45 billion years ago this oxygenation has been linked to the emergence of oxygenic cyanobacteria1 2 and to changes in the compositions of volcanic gases3 4 but not to the composition of erupting lavas geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3 5 billion years ago5 6 here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation using thermodynamic calculations simulating gas melt equilibria in erupting magmas we suggest that mostly submarine archaean volcanoes produced gases with so2 h2s 1 and low sulphur content emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late archaean then led to widespread subaerial volcanism which in turn yielded gases much richer in sulphur and dominated by so2 dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

direct observations of the upper layer circulation in the southern gulf of mexico abstract the upper layer circulation in the bay of campeche is analyzed with three years of data recorded by surface drifters current meter moorings and satellite altimetry the measurements show that the mean cyclonic circulation observed by previous authors extends below 1000 m and that its size and location are delimited by the particular topography of the region a deep basin to the west and a shallower and gentle sloping submarine fan to the east an empirical orthogonal function analysis and large correlations of the surface flow with the deeper currents suggest that the topographic constraint is the result of potential vorticity conservation for an equivalent barotropic flow the variability of the surface currents in the western basin is mostly due to changes in the size form position and intensity of the cyclonic gyre due to its interaction with northern gulf of mexico eddies particularly loop current eddies traveling the southern route towards the western boundary by contrast the eastern basin is characterized by a weak northward drift with the occasional generation of anticyclones in the southeastern boundary the genesis of which remains to be understood this suggests that the variability in the eastern basin is mostly driven by locally generated disturbances rather than by an influx of northern gulf of mexico eddies strong northward flows in the central and eastern basins result from the flow convergence between locally generated anticyclones and the cyclonic gyre

multiple sulfur isotopes from paleoproterozoic huronian interglacial sediments and the rise of atmospheric oxygen mass dependently fractionated mdf sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate in conjunction with these proxies mass independently fractionated mif sulfur isotopes can be used to track the rise of atmospheric oxygen the anoxic oxic transition of the surface environment in the paleoproterozoic has been constrained by mif sulfur in 2 47 ga sulfides from western australian banded iron formations and the absence of mif in diagenetic sulfides from 2 32 ga black shales in the transvaal supergroup south africa we report new multicollector ion microprobe data for individual sulfides from water lain sedimentary units in the 2 45 2 22 ga huronian supergroup ontario canada the authigenecity of sulfides in these units is interpreted on the basis of host lithology mode of distribution sulfide chemistry and mdf sulfur isotopes sulfides interpreted to have an authigenicsedimentary origin from the mckim fm have a range of 0 7 in 33 s values and those in the pecors fm preserve small magnitude mif 33 s values up to 0 88 which we propose represents the last remnants of an anoxic atmosphere high 34 s values up to 31 2 and near zero 33 s values in authigenic sulfides from the stratigraphically higher espanola and gordon lake fms suggest local variations of seawater sulfate concentrations and or 34 ssulfate consistent with increased atmospheric o2 levels mif sulfur is absent from the marine record after the second huronian glaciation which is analogous to the record of the transvaal supergroup where mif is absent from the upper part of the interglacial sequence our results suggest that the possible simultaneous demise of mif sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate paleoproterozoic supergroups we propose a new biogeochemical model where enhanced weathering rates during paleoproterozoic post glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis the demise of methane and ultimately to the irreversible rise in atmospheric oxygen

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

apatite weathering and the phanerozoic phosphorus cycle despite widespread debate in the literature there is still considerable uncertainty concerning which nutrient ultimately controls marine net ecosystem production nep over geologic time geochemical arguments suggest that phosphorus is the culprit the weathering of apatite the primary phosphorus sink in earth s exosphere controls long term phosphorus availability if phosphorus is the ultimate controlling nutrient over geologic time scales then long term marine nep is coupled to the release of phosphorus from apatite weathering the most abundant apatite compositions found in nature are igneous fluorapatite and marine sedimentary carbonate fluorapatite sparse data exist on how these compositions dissolve under earth s surface conditions to demonstrate a need for these data and their application we present a kinetic treatment of existing data augmented by new results we then use these results in a weathering model designed to illustrate the control exerted by temperature via activation energy and surface area on the phosphorus flux from apatite dissolution during the phanerozoic our conclusion is that activation energy and hence temperature and apatite surface area are important parameters governing the phosphorus flux from apatite weathering and therefore marine nep during phanerozoic time

middle late triassic 230 5 ma singularity in the stratigraphic and magmatic history of the pangean heat anomaly the phanerozoic history of continental dispersal and coalescence contains an epoch of radical change at about the middle late triassic boundary 230 5 ma marked by 1 the final coalescence of pangea by the contact of south china and cimmeria with laurasia and terminal compressive deformation in australia and southern africa and 2 the incipient dispersal of pangea by the onset of continental rifting in eastern north america and northwest africa argentina east africa and western australia this epoch of coalescence and incipient dispersal correlates with a change from 1 an icehouse state to a greenhouse state and 2 changes from a fall to a rise in the sea level curve in the emplacement of north american granite and in the cumulative curve of reversed to normal magnetic polarity these changes are interpreted as indicating the first major loss from the heat accumulated beneath pangea now represented by the atlantic african geoid high

lower and upper neoproterozoic magmatic records in itaiacoca belt parana brazil zircon ages and lithostratigraphy studies abstract the itaiacoca belt is a sequence of metavolcanic and metasedimentary rocks that crop out east of parana and southeast of sao paulo states in southern brazil this geologic geochronologic study supports division of the itaiacoca belt into two major lithologic sequences the older is a carbonate platform sequence dolomitic meta limestones metamarls calc phyllites carbonate phyllites with minimum deposition ages related to the end of the mesoproterozoic beginning of the neoproterozoic 1030 908 ma u pb zircon of metabasic rocks the younger sequence contains mainly clastics deposits meta arkoses metavolcanics metaconglomerates metapelites with deposition ages related to the neoproterozoic 645 628 ma u pb zircon of metavolcanic rocks these ages are quite close to k ar ages fine fraction of the 628 610 ma interval associated with metamorphism and cooling of the itaiacoca belt the contact between the dolomitic meta limestones and meta arkoses is marked by intense stretching and high angle foliation suggesting that the discontinuity between these associations resulted from shearing it is proposed here that the term itaiacoca sequence should represent the dolomitic meta limestones and the term abapa sequence represents the meta arkoses metavolcanics phyllites in a major tectonic context these periods are related to the break up of rodinia supercontinent 1030 908 ma and the amalgamation of the gondwana supercontinent 645 628 ma

enhanced productivity led to increased organic carbon burial in the euxinic north atlantic basin during the late cenomanian oceanic anoxic event 1 three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in 13c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins

biogeochemical effects of atmospheric oxygen concentration phosphorus weathering and sea level stand on oceanic redox chemistry implications for greenhouse climates abstract understanding the key factors influencing the global oceanic redox system is crucial to fully explaining the variations in oceanic chemical dynamics that have occurred throughout the earth s history in order to elucidate the mechanisms behind these variations on geological timescales numerical sensitivity experiments were conducted with respect to the partial pressure of atmospheric molecular oxygen p o 2 the continental shelf area a cs and the riverine input rate of reactive phosphorus to the oceans r p the sensitivity experiment for atmospheric p o 2 indicates that pervasive oceanic anoxia and euxinia appear when p o 2 p o 2 are higher than a previous estimate of 50 pal present atmospheric level due to redox dependent phosphorus cycling the sensitivity experiment regarding the shelf area demonstrates that changes in the shelf area during the phanerozoic significantly affected oceanic oxygenation states by changing marine biogeochemical cycling a large continental shelf acts as an efficient buffer against oceanic eutrophication and prevents the appearance of ocean anoxia euxinia we also found that an enhanced r p is an important mechanism for generation of widespread anoxia euxinia via expansion of both the oxygen minimum zone and coastal deoxygenation although the critical r p value depends significantly on p o 2 a cs and the redox dependent burial efficiency of phosphorus at the sediment water interface our systematic examination of the oceanic redox state under cretaceous greenhouse climatic conditions also supports the above results

a linear equivalent barotropic model of the antarctic circumpolar current with realistic coastlines and bottom topography abstract a linear equivalent barotropic eb model is applied to study the effects of the bottom topography h and baroclinicity on the total transport and the position of the antarctic circumpolar current acc the model is based on the observation of killworth that the time mean velocity field of the fram model is self similar in the vertical a realistic large scale topography h is constructed by filtering 5 minute resolution data with an appropriate smoothing kernel it is shown that the asymptotic behavior of the solution of the barotropic model a particular case of the eb model in the limit of very small bottom friction depends on subtle details of topography and basin geometry given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography the authors found that the eb model with a vertical profile function similar to that of killworth reproduces the major features of the time and depth ave

seasonal patterns of wind stress and wind stress curl over the gulf of mexico meteorological observations from an array of stations deployed along the periphery of the gulf of mexico between 1990 and 1993 are used to describe the seasonal fluctuations in patterns of atmospheric variables from a contemporary set of measurements seasonal maps of wind stress based on these measurements resemble wind stress maps based on ship observations as published by elliott 1979 rather than maps based on analyses of numerical weather forecasts as published by rhodes et al 1989 particularly near the western boundary of the gulf seasonal maps of wind stress curl are characterized by positive curls over the western and southwestern gulf the central result of this study is to document the important role of the mountain chain which extends along the southwestern section of the gulf in channeling the wind toward the isthmus of tehuantepec

intraplate seismicity in mid plate south america correlations with geophysical lithospheric parameters mid plate south america remains one of the least studied regions of intraplate seismicity little is known about the origin and controlling factors that make this area the least seismically active intraplate region in the world we analysed the distribution of intraplate seismicity and its correlation with several geophysical lithospheric parameters in an attempt to establish which factors might promote or inhibit the occurrence of intraplate earthquakes we found that above average seismicity occurs mostly in neoproterozoic fold belts associated with areas having a positive gravity anomaly lower elastic thickness higher heat flow thinned crust and a negative s wave anomaly at 100 km depth associated with non cratonic crust cratonic areas with a higher elastic thickness and lower heat flow are associated with low rates of seismicity our study suggests that the most important controlling factors are elastic thickness and heat flow we propose that earthquake prone areas with these favourable conditions correspond to regions of weakened lithosphere where most of the regional lithospheric stresses are supported by the overlying brittle upper crust these areas act as local concentrators of the regional compressional stress field with the stress build up then leading to the occurrence of intraplate seismicity

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

an equivalent barotropic mode in the fine resolution antarctic model abstract both the 6 year time mean flow and the eddy kinetic energy in the fine resolution antarctic model are found to be approximately self similar in the vertical with the velocity showing a rapid decay with depth a simple argument is given as to why this would be expected to occur in this and other numerical models and an analysis is given as to the degree of steering of the mean flow by topography

oxygen dynamics in the aftermath of the great oxidation of earth s atmosphere the oxygen content of earth s atmosphere has varied greatly through time progressing from exceptionally low levels before about 2 3 billion years ago to much higher levels afterward in the absence of better information we usually view the progress in earth s oxygenation as a series of steps followed by periods of relative stasis in contrast to this view and as reported here a dynamic evolution of earth s oxygenation is recorded in ancient sediments from the republic of gabon from between about 2 150 and 2 080 million years ago the oldest sediments in this sequence were deposited in well oxygenated deep waters whereas the youngest were deposited in euxinic waters which were globally extensive these fluctuations in oxygenation were likely driven by the comings and goings of the lomagundi carbon isotope excursion the longest lived positive 13c excursion in earth history generating a huge oxygen source to the atmosphere as the lomagundi event waned the oxygen source became a net oxygen sink as lomagundi organic matter became oxidized driving oxygen to low levels this state may have persisted for 200 million years

strontium isotope stratigraphy in the upper cenomanian shallow water carbonates of the southern apennines short term perturbations of marine 87sr 86sr during the oceanic anoxic event 2 abstract strontium isotope stratigraphy of shallow water carbonates from the southern apennines italy indicates that the late cenomanian early turonian evolution of marine 87sr 86sr is not accurately depicted by the marine reference curve using the low mg biotic calcite of well preserved rudist shells as a study material and carbon isotope stratigraphy for correlation to the standard ammonite biozonal scheme of northern europe new 87sr 86sr values are proposed for four different stratigraphic levels the middle and uppermost part of the guerangeri zone the lower and upper part of the geslinianum zone the southern apennines data suggest that a sharp positive shift at the onset of oceanic anoxic event 2 precedes the well known sr isotope negative shift the positive shift is interpreted in terms of enhanced rates of chemical weathering driven by global warming forced by volcanogenic co2 outgassing a stratified ocean is invoked to reconcile the high gradient and the short lag time of the perturbation with estimates of present day total mass and residence time of strontium in the ocean the sharp switch to the negative shift is related to the collapse of water column density gradient driven by thermal instability during a late cenomanian cool event more active ocean circulation suddenly delivered to surface waters the signal of increased submarine volcanism that had accumulated in deep waters

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

modelling oceanic carbon and phosphorus fluxes implications for the cause of the late cenomanian oceanic anoxic event oae2 a two box ocean model with a coupled carbon and phosphorus cycle is used to test which processes can realistically generate a rapid positive 2 13c excursion in the global carbon reservoir internal oceanic processes enhanced preservation of organic matter under anoxic conditions enhanced upwelling cannot produce a substantial 13c excursion under the assumption that burial rates of c and p are coupled loss of nutrients to the sediment acts as a negative feedback mechanism that prevents a substantial perturbation of the carbon cycle a large and renewable source of nutrients is needed either from increased continental weathering or from enhanced regeneration of sedimentary organic phosphorus under anoxic conditions measurements of total phosphorus and organic carbon in sediments from the tarfaya basin morocco suggest that a large decrease in p burial occurred relative to organic carbon at the base of the cenomanian turonian oceanic anoxic event oae2 with the right magnitude to quantitatively explain a 2 shift in 13c it is concluded that the main underlying mechanism that allowed and sustained enhanced carbon burial during the mid cretaceous was a perturbation of the oceanic phosphorus cycle

nonlinear vorticity balance of the antarctic circumpolar current 1 using a recently published global ocean mean surface dynamic topography based predominantly on drifter data the near surface vorticity balance of the antarctic circumpolar current acc is calculated with a little smoothing the dynamic topography is found to produce good estimates of even such highly differentiated quantities as the relative vorticity advection two clear modes of flow are found in the acc meanders of wavelength 300 500 km in which the nonlinear term is important resulting in a balance between advection of relative and planetary vorticity as in a stationary equivalent barotropic rossby wave this implies a surface divergence which is at least partly balanced by an opposite divergence at depth and a flow with divergence associated with topographic features it is tentatively concluded that the divergence of this latter flow is a scaled measure of bottom pressure torque the inferred bottom pressure torque shows large scale topographic interactions as well as a strong influence of some sharp topographic features such as the fracture zones and macquarie ridge system south of the tasman sea and the narrow ridges in drake passage

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

early cretaceous ostracod biostratigraphy of eastern brazil and western africa an overview petroleum exploration in the offshore basins of eastern brazil and western africa has been aided for the last fifty years by an understanding of the early cretaceous ostracod biostratigraphy of the proto south atlantic this review charts the development of the presently accepted zonation from its early development in the reconcavo tucano basin and recognition as palaeontological evidence for continental drift in the 1960s to its application over the following five decades in the brazilian sergipe alagoas araripe and campos basins and the western african congo gabon and kwanza basins the distribution of ostracod species in the lakes which formed during the early stages of the opening of the atlantic during the early cretaceous was often extensive and this has resulted in effectively south atlantic wide application for many species and certainly for the zonation as a whole indeed petroleum companies continue to exploit the predictability of the ostracods in south atlantic operations due to the fact that they are often able to provide accurate constraints on stratigraphic position for a relatively low cost however the ostracod biozonations of the pre salt section for several eastern brazilian basins such as the reconcavo tucano sergipe alagoas and campos have not been thoroughly revised since the 1980s and the taxonomy of these remains somewhat outdated furthermore ostracod biozonations for many basins on both coasts e g the brazilian espirito santo and the western african namibe basins have never been published an updated and revised taxonomy of the ostracods of the basins of eastern brazil and western africa is presented appendix i as well as a revision of the biozonation on both sides of the modern day atlantic ocean it is hoped that this paper will act as a catalyst for further work in the basins surrounding the modern day south atlantic which hold much interest for petroleum companies and micropalaeontologists alike

main stages of the development of the sedimentary basins of south america and their relationship with the tectonics of supercontinents abstract twelve main stages of sedimentary basins of the south american platform have been identified as follows one neoarchean seven proterozoic and four phanerozoic basin forming tectonics are related to the accretion fusion and fission of the major continental landmasses of the earth s history such as atlantica nena rodinia gondwana pannotia and pangea the first proposed stage of basin development precedes the formation of these major landmasses and was developed on mesoarchean microcontinents the last stage succeeds the fission of pangea and is still in progress

solute flux and mineral mass balance approaches to the quantification of plant effects on silicate weathering the weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric co 2 over geologic time and vascular plants may have played an important role in this process we have examined the role of plants in weathering in western iceland by two methods a solute flux approach and a mineral mass balance approach using the chemistry of weathering products and of waters draining adjacent areas of basaltic rocks that are either barren with a partial cover of mosses and lichens or populated by trees the study area was chosen to maximize vegetational differences and to minimize differences in microclimate slope and lithology while avoiding hydrothermal waters and anthropogenic acid rain solute flux results including data on cation uptake by growing trees indicate that the rate of weathering release of ca and mg to streams is about 4 times higher in vegetated areas than in bare areas mineral mass balance results show that trees increase plagioclase weathering by a factor of 2 and pyroxene weathering by a factor of 10 possibly as a result of preferential weathering of pyroxene by the vegetation biomass normalized weathering fluxes from the birch trees are greater than those from the conifers suggesting that angiosperms enhance weathering to a greater degree than gymnosperms results indicate a perpetual short term sink for atmospheric c as tree induced leakage of hco super 3 from the forest ecosystem to rivers and streams in addition to the finite sink of c in forest biomass the findings also suggest a major role for vascular plants as they invaded upland portions of the continents between 410 and 360 my ago in accelerating weathering and thereby lowering atmospheric co 2

horizontal and vertical motions of barotropic vortices over a submarine mountain the evolution of barotropic vortices over a topographic axisymmetric mountain in a homogeneous rotating fluid is studied experimentally the aim is to identify the main physical processes observed in i a horizontal plane of motion perpendicular to the rotation axis of the system and ii a vertical plane across the diameter of the mountain the vortices are monopolar cyclones initially generated near or over the topography initially the vortices drift towards the mountain due to the effect associated with the topographic slope on arriving they turn around the obstacle in an anticyclonic direction whilst anticyclonic vorticity is generated over the summit the long term vorticity distribution is dominated by the original cyclone elongated around the topographic contours and the generated anticyclone over the tip of the topography in the vertical plane an oscillatory uphill downhill flow is generated which is directly related to the drift of the cyclone around the mountain depending on the vortex characteristics the period of the oscillation ranges from 4 to 10 times the rotation period of the system the horizontal and vertical flow fields are reproduced numerically by using a shallow water formulation which allows a detailed view of the vertical motions hence facilitating the interpretation of the experimental results in addition the cyclone anticyclone pair over the mountain is compared with analytical solutions of topographically trapped waves a general conclusion is that vertical motions persist for several days or rotation periods which implies that this mechanism might be potentially important for the vertical transport over seamounts

pervasive oxygenation along late archaean ocean margins the photosynthetic production of oxygen in the ocean is thought to have begun at least 2 7 billion years ago the geochemistry of marine sediments deposited 2 6 billion years ago suggests that ocean margins were oxygenated at least 100 million years before the first significant increase in atmospheric oxygen concentrations

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

modeling organic carbon burial during sea level rise with reference to the cretaceous after three decades of research on the correlation between sea level and various proxies there has been very little quantification of the first order influence of sea level change on nutrient inventory marine productivity and burial of organic carbon we present a model aimed at quantifying the burial of organic carbon as a function of sea level rise the biogeochemical model explicitly considers the mean surface area distribution of the earth as a function of elevation also included is dissolved inorganic phosphate dip liberated from coastal sediments during transgression we quantify how sea level rise of magnitudes inferred for the mid cretaceous influences the phosphorus cycle and burial of organic carbon the burial efficiency is greater on the shelf therefore in the model the larger shelf area under high sea level results in more efficient burial of marine organic carbon for a given dip concentration and associated new production of organic matter np with a late cretaceous model shelf area the residence time of dip decreases by 60 relative to the present day reference thus in the final steady state dip and np in the open ocean are reduced compared to today when the biologically reactive phosphorus preac fluxes to the ocean from land are held constant the lower new production reduces the oxygen demand for respiration and results in a significant oxygenation of the global ocean finally the global organic carbon burial decreases by 30 relative to today in the model this decrease results from feedbacks between the flux of organic carbon to the seafloor and the ratio of organic carbon to preac in buried sediments in contrast during sea level rise coastal erosion may increase the preac flux to the ocean by up to 20 relative to today and cause a temporary increase in dip concentration in the ocean the resulting increase in organic carbon burial is equivalent to a carbon isotope event of 0 5 to 1 larger carbon isotope events can be triggered by sea level rise only when the ocean is sufficiently close to anoxia in the oxygen minimum zone just prior to the sea level rise

thermal history of potential gas reservoir rocks in the eastern parnaiba basin brazil the parnaiba basin is a major intracratonic sedimentary basin in brazil with unconventional petroleum systems as a potential natural resource formed by the influence of igneous intrusions to constrain the thermal history of unexplored potential reservoir rocks in the eastern part of the parnaiba basin sedimentary rocks near intrusions were analyzed by petrography and thermochronology apatite fission track aft and zircon fission track zft dating petrography shows grain dissolution and carbonate pore filling generated by thermal destabilization of feldspars the aft results indicate partial annealing by the last magmatic event in the basin and zft results show ages of maximum paleotemperature compatible with the sardinha magmatic event 120 130 ma in thermal history models rocks close to intrusions experienced maximum paleotemperatures above 300 c which is higher than what is considered favorable for reservoir rocks hydrothermal fluids modified the diagenetic evolution of the succession by dissolving and precipitating carbonate cement in pore spaces in the studied area at distances greater than 50 m 164 ft from the intrusion the rocks were not substantially altered and in terms of paleotemperature they can be considered potentially viable reservoirs the thermal history analysis of potential tight gas sandstone reservoirs affected by intrusive bodies is valuable for characterizing the thermal enhancement or destruction of the reservoir s qualities

paleoproterozoic crustal evolution of the sao luis craton brazil evidence from zircon geochronology and sm nd isotopes abstract the sao luis craton northern brazil is composed of a few granitoid suites and a metavolcano sedimentary succession new single zircon pb evaporation ages and nd isotope data combined with other available information show that the metavolcano sedimentary succession developed from 2240 ma to approximately 2200 2180 ma from juvenile protoliths the subduction related calc alkaline suites of granitoids spatially associated with the metavolcano sedimentary sequence formed in an oceanic island arc setting between 2168 2147 ma most of these granitoids are tonalitic and formed from juvenile mantle or oceanic plate derived protoliths whereas minor true granites are the product of the reworking of the juvenile island arc material these arc related successions represent an accretionary event around 2 20 0 05 ga which is coincident with one of the main periods of crustal growth in the south american platform this accretionary orogen has subsequently been involved in a collision episode at ca 2100 2080 ma which is mainly recorded in the nearby gurupi belt the rock associations inferred geological settings and the crustal evolution detected in the sao luis craton are similar to what is described in paleoproterozoic domains of major geotectonic units of the south american platform such as part of the sao francisco craton southeastern guyana shield and of the west african craton

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

rise in seawater sulphate concentration associated with the paleoproterozoic positive carbon isotope excursion evidence from sulphate evaporites in the 2 2 2 1 gyr shallow marine lucknow formation south africa past oceanic sulphate concentration is important for under standing how the oceansredox state responded to atmospheric oxygen levels the absence of extensive marine sulphate evaporites before 1 2 gyr probably reflects low seawater sulphate and or higher carbonate concentrations sulphate evaporites formed locally during the 2 22 2 06 gyr lomagundi positive d 13 c excursion however the 2 2 2 1 gyr lucknow formation south africa provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input these marginal marine deposits contain evidence for evaporite molds pseudomorphs after selenite gypsum and solid inclusions of ca sulphate in quartz carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines the apparent precipitation of gypsum before halite requires 2 5 mm l 1 sulphate concentration higher than cur rent estimates for the paleoproterozoic during the lomagundi event which postdates the 2 32 gyr initial rise in atmospheric oxygen seawater sulphate concentration rose from archean values of 200 lm l 1 but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state

transition to an oxygen rich atmosphere with an extensive overshoot triggered by the paleoproterozoic snowball earth abstract the earth is thought to have multiple stable steady states regarding climate modes and atmospheric oxygen levels the paleoproterozoic is a remarkable period in earth s history because of the simultaneous occurrence of large climatic and redox transitions between steady states i e snowball earth glaciation and the rise of oxygen geochemical evidence suggests that the oxygen rise was a dynamic transition with an extensive long term overshoot however previous models have not explained the dynamics of the oxygen rise leaving its causal mechanism poorly understood in the present study we suggest that the oxygen transition with an overshoot occurred in response to a climatic transition at the termination of the snowball glaciation biogeochemical cycle modelling indicates that prolonged super greenhouse conditions and effective nutrient input to the ocean after the glaciation lead to high levels of primary productivity and burial of organic carbon this causes an abrupt jump of oxygen levels within 10 4 yr after the glaciation and an extensive oxygen overshoot to the present atmospheric level the overshoot persists for 10 8 yr because net consumption of oxygen in the atmosphere ocean system is inefficient we show that the post glacial jump of oxygen levels occurred in biologically short timescale which may have stimulated the ecological shift and or biological innovations toward the prosperity of oxygen dependent life

geology of the northern borborema province ne brazil and its correlation with nigeria nw africa the borborema and benin nigeria provinces of ne brazil and nw africa respectively are key areas in the amalgamation of west gondwana by continental collision during the brasiliano pan african orogenies both are underlain by complex basement nigeria has c 3 05 ga archaean crust but no known palaeoproterozoic rocks 2 0 ga in ne brazil 2 6 3 5 ga archaean rocks form small cores within palaeoproterozoic gneiss terrains affected by plutonism at c 2 17 ga both regions exhibit late palaeoproterozoic c 1 8 ga rift related magmatism and metasedimentary sequences overlying the basement the serido group of ne brazil 0 65 ga is similar to the igarra sequence in sw nigeria the ceara group which may date back to c 0 85 ga is a passive margin deposit on crust thinned during initiation of an oceanic domain in both provinces basement and sedimentary cover were involved in tangential tectonics that resulted in crust thickening by nappe stacking associated with closure of this ocean frontal collision between c 0 66 and 0 60 ga later evolved to an oblique collision generating north south continental strike slip shear zones at c 0 59 ga in ne brazil the main pan african suture is probably buried beneath the parnaiba basin the transbrasiliano lineament interpreted as the prolongation of the kandi 4 50 lineament in hoggar may represent a cryptic suture

periodic mid cretaceous oceanic anoxic events linked by oscillations of the phosphorus and oxygen biogeochemical cycles 1 a series of oceanic anoxic events oaes occurred in the mid cretaceous warm period 120 80 ma that have been linked with high rates of organic carbon burial warm high and low latitude temperatures and sea level changes oaes have been studied individually but a causal mechanism that connects them has been lacking we show that peaks in phosphorus accumulation in marine sediments broadly coincide with oaes 1a 1b 1d 2 and 3 and exhibit a 5 6 myr periodicity which for reactive p is prominent over 100 80 ma oxic anoxic oscillations of this frequency are also found in a model of the coupled n p c and o2 biogeochemical cycles these oscillations are maintained by positive feedbacks between phosphate concentration biological productivity and anoxia in the global ocean and counteracting but slower negative feedbacks involving changes in atmospheric oxygen an increase in phosphorus weathering rate above a critical threshold can shift the system into self sustaining oscillation this could have been caused by tectonic and volcanic forcing increasing atmospheric co2 and global warmth 120 80 ma augmented by the rise of flowering plants around 100 ma with a plausible forcing scenario we are able to reproduce the approximate timing of oaes 1a 1b 1d 2 and 3 in the model

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

geology and geochronology of the betara region in south southeastern brazil evidence for possible statherian 1 80 1 75 ga and calymmian 1 50 1 45 ga extension events abstract the results of geological mapping chemical analysis and radiometric dating of metabasic rocks of betara formation and mapping and dating of those present in the betara basement nucleus together with mylonitic granodiorite and syenogranite are reported here u pb analysis of bulk zircon fractions from the metabasic rocks of the basement nucleus yielded a statherian age of 1790 22 ma while the metabasic rocks from the upper part of the betara formation yielded a calymmian age between 1500 and 1450 ma this age is a minimum for the deposition of the betara formation the older metabasic rocks are associated with post tectonic possibly anorogenic syenogranite while the younger ones are gabbro or very porphyritic ankaramite whose ree patterns are consistent with crystallization from an n morb parent magma the observations and data point to the probable events associated with extensional processes of the end of paleoproterozoic and early mesoproterozoic similar registers of statherian 1 80 1 75 ga and calymmian 1 50 1 45 ga extensional events are recorded in other parts of the south american and african continents the neoproterozoic witnessed the formation and junction of the tectonic slices which formed the apiai domain during the assemblage of western gondwana

redox conditions in the atmosphere and shallow marine environments during the first huronian deglaciation insights from os isotopes and redox sensitive elements article history the paleoproterozoic 2 5 2 0 ga is one of the most important periods in earth s history and was characterized by a rise in atmospheric oxygen levels and repeated at least three severe glaciations the huronian glaciations in this study we investigate redox conditions in the atmosphere and in shallow marine environments immediately after the first huronian glaciation based on the isotopic composition of os and the abundance of redox sensitive elements os re and mo in sedimentary rocks from the huronian supergroup canada we found no significant authigenic enrichment of os in the sedimentary rocks deposited during the first huronian deglaciation the initial isotopic composition of os in the sediments was close to that of chondrite at the time of deposition 187 os

eddy energy and shelf interactions in the gulf of mexico sea surface height anomaly data from satellite are continuously available for the entire gulf of mexico surface current velocities derived from these remotely sensed data are compared with surface velocities from drifting buoys the comparison shows that satellite altimetry does an excellent job resolving gulf eddies over the shelf rise depths between 200 and 2000 m if the proper length scale is used correlations between altimeter and drifter derived velocities are statistically significant r 0 5 when the surface slope is computed over 125 km indicating that remotely sensed sea surface height anomaly data can be used to aid the understanding of circulation over the shelf rise velocity variance over the shelf rise from the altimetry data shows regions of pronounced eddy energy south of the mississippi outflow south of the texas louisiana shelf and in the northwest and northeast corners of the gulf these are the same locations where surface drifters are most likely to cross the shelf rise suggesting gulf eddies promote cross shore flows this is clearly exemplified with both warm and cold eddies finally the contribution of gulf eddies and wind stress to changes in the mean circulation are compared results indicate that the eddy generated vorticity flux to the mean flow is greater than the contribution from the surface wind stress curl especially in the region of the loop current and along the shelf rise base in the western gulf future modeling efforts must not neglect the role of eddies in driving gulf circulation over the shelf rise

ocean stagnation and end permian anoxia ocean stagnation has been invoked to explain the widespread occurrence of organic carbon rich laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end permian mass extinction however to a first approximation stagnation would severely reduce the upwelling supply of nutrients to the photic zone reducing productivity moreover it is not obvious that ocean stagnation can be achieved numerical experiments performed with a three dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the late permian ocean we find that polar warming and tropical cooling of sea surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization buildup of quantities of h2s and co2 in the late permian ocean sufficient to directly cause a mass extinction however would have required large increases in the oceanic nutrient inventory

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

dynamics of anoxia abstract a simple steady state one dimensional advective diffusive model of the ocean s coupled carbon oxygen and phosphorous system is developed the model is an extension of wyrtki s 1962 model of the oxygen distribution the occurrence of anoxia requires two conditions 1 sufficiently high productivity and 2 sufficiently low oxygen concentration in the bottom water it is shown that anoxic layers are most likely to occur fairly high in the water column anoxic events may be associated with timesof vigorous thermohaline circulation in contrast to stagnation arguments often invoked the model results are used to interpret both modern and ancient oceanic anoxic events

theoretical constraints on oxygen and carbon dioxide concentrations in the precambrian atmosphere simple one dimensional climate models suggest that carbon dioxide concentrations during the archean must have been at least 100 1000 times the present level to keep the earth s surface temperature above freezing in the face of decreased solar luminosity such models provide only lower bounds on co2 so it is possible that co2 levels were substantially higher than this and that the archean climate was much warmer than today periods of extensive glaciation during the early and late proterozoic on the other hand indicate that the climate at these times was relatively cool to be consistent with climate models co2 partial pressures must have declined from approximately 0 03 to 0 3 bar around 2 5 ga ago to between 10 3 and 10 2 bar at 0 8 ga ago this steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data which implies that pco2 did not change appreciably during that time oxygen was essentially absent from the earth s atmosphere and oceans prior to the emergence of a photosynthetic source probably during the late archean during the early proterozoic the atmosphere and surface ocean were apparently oxidizing while the deep ocean remained reducing an upper limit of 6 x 10 3 bar for po2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean the establishment of oxidizing conditions in the deep ocean marked by the disappearance of banded iron formations approximately 1 7 ga ago permitted atmospheric oxygen to climb to its present level o2 concentrations may have remained substantially lower than today however until well into the phanerozoic

possible atmospheric co2 extremes of the middle cretaceous late albian turonian 1 atmospheric carbon dioxide co2 estimates for the middle cretaceous mk have a range of 4000 ppm which presents considerable uncertainty in understanding the possible causes of warmth for this interval this paper examines the problem of mk greenhouse forcing from an inverse perspective we estimate upper ocean water temperatures from oxygen isotope measurements of well preserved late albian turonian planktonic foraminifera and compare these against temperatures predicted by general circulation model gcm experiments with co2 concentrations of 500 7500 ppm at least 4500 ppm co2 is required to match maximum temperatures inferred from well preserved planktonic foraminifera approximately 900 ppm co2 produces a good match between the model and the minimum temperature estimates for the mk an ocean model forced by these two extremes in surface conditions brackets nearly all available bottom water temperature estimates for this interval the climate model results support nearly the entire range of mk co2 estimates from proxy data the ocean model suggests possible mk oceanographic changes from deep water formation in the high latitude region of one hemisphere to the other hemisphere in response to changes in atmospheric temperatures and hydrologic cycle strength we suggest that rather than contradicting one another the various proxy co2 techniques especially those with high temporal resolution may capture true variability in co2 concentrations and that mk co2 could have varied by several thousand ppm through this interval

ekman effects in a rotating flow over bottom topography this paper presents a general two dimensional model for rotating barotropic flows over topography the model incorporates in a vorticity stream function formulation both inviscid topography effects associated with stretching and squeezing of fluid columns enforced by their motion over variable topography and viscous effects due to the ekman boundary layer at the solid bottom from the present formulation conventional two dimensional models can be recovered the model is tested by means of laboratory experiments on homogeneous vortices encountering irregular topographies the experimental observations are then compared with the corresponding numerical simulations based on the general model the results suggest that such a formulation incorporates both inviscid and viscous topography effects correctly

why western boundary currents in realistic oceans are inviscid a link between form stress and bottom pressure torques it is shown that wind stress curl is balanced by bottom pressure torque in a zonal integral over any strip wide enough to smooth out the effect of nonlinear terms typically about 3 of latitude the derivation is completely general as long as the zonal wind stress is balanced by form stress at each latitude as is known to be the case in the ocean this implies that viscous torques are not important in western boundary currents their place being taken by bottom pressure torques the prediction is confirmed in the context of a global eddy permitting numerical ocean model this link between form stress and bottom pressure torques makes it easier to consider southern ocean dynamics and subtropical gyre dynamics in the same conceptual framework with topographic interactions being important in both cases

the expression of the cenomanian turonian oceanic anoxic event in tibet the gongzha section of tibet china is located at the northern margin of the indian plate se tethys and is characterized by hemipelagic grey marls and marly limestones light grey limestones and silty limestones but no organic rich sediments high resolution biostratigraphy reveals an expanded cenomanian turonian ct boundary interval and the 13c record includes the main features of the classical positive carbon isotope excursion that characterizes the ct oceanic anoxic event the biotic response inferred from the foraminifera suggests that oxic to dysoxic conditions prevailed except for a short interval marked by peak abundance of heterohelix that indicates a significantly dysoxic environment during the 13c b peak excursion the overall decreasing trend in redox sensitive trace elements rste during the maximum 13c excursion confirms the absence of significant longer lasting anoxia in the gongzha section enrichments in rste are linked to phases of increased detrital input chemical weathering indices suggest that the upper cenomanian sediments accumulated under an increasingly hot and humid climate that culminated near the ct boundary in the early turonian lower weathering indices suggest a warm drier climatic regime with reduced continental runoff phosphorus mass accumulation rates show a significant peak at the onset of the positive 13c excursion followed by a decrease up to the basal turonian this pattern is positively correlated with the long term decrease in detrital index as also observed in numerous other ct boundary sections e g eastbourne pueblo and whadi el ghaib sinai long term phosphorus accumulation in the gongzha section is therefore associated with changes in detrital input the overall decreased detrital input can be explained by the increasingly remote continental sources due to the major transgression at the end of cenomanian coupled with changes in continental weathering intensity linked to increasingly more arid climate conditions

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

inclusion of the weathering of volcanic rocks in the geocarbsulf model the carbon cycle model geocarbsulf is extended by dividing the weathering of silicates into volcanic and non volcanic rocks the proportion of volcanic weathering is calculated as a function of time from the oceanic record of 87sr 86sr the volcanic proportion is then used to modify the equations for calculating atmospheric co2 by the addition of a new non dimensional volcanic weathering factor the effect of uplift and physical erosion on weathering is also modified by using only the distribution over time of the abundance of sandstones and shales and not sr isotopic data that had been used previously results indicate moderate change from geocarbsulf in the distribution of atmospheric co2 over phanerozoic time this includes an increased minimum in co2 during the late ordovician in agreement with the presence of a continental glaciation at that time and a shift of maximum mesozoic values from the jurassic to the early cretaceous

pan african strike slip tectonics in eastern cameroon magnetic fabrics ams and structure in the lom basin and its gneissic basement abstract field microstructural and anisotropy of magnetic susceptibility ams or magnetic fabric studies were applied to identify the sequence and character of the pan african structures in the basement of eastern cameroon at both sides of the regional scale betare oya shear zone bosz the ne sw trending bosz separates older gneisses and migmatites towards se domain i from the younger rocks of the lom meta volcano sedimentary basin towards nw domain ii in domain i early ductile compressional deformation occurred in two events d1 and d2 under relatively high t conditions during subsequent cooling strain partitioned between the competent basement gneisses with only mild compression and the bordering shear zone bosz with intense simple shear wrenching d3 strain in the less competent rocks of domain ii is dominated by simple shear strike slip wrenching d3 with an earlier stage of compressional deformation preserved only in some low strain pods magnetic fabrics ams document a progressive change from oblate ellipsoids towards prolate ellipsoids in domain i when proceeding from the south towards the bosz foliations are mostly steep but define a girdle with a pole plunging gently towards wsw the magnetic lineations also plunge mostly towards wsw at shallow angles these fabrics indicate a compression approximately normal to the bosz which is also the se margin of the lom basin in the lom metasediments domain ii ams ellipsoids are typically oblate foliations trend ne sw with mostly steep dips magnetic lineations plunge gently ne or sw this fabric with foliations mostly steep and subparallel with the major bosz combined with generally subhorizontal lineations implies the bosz as a pan african strike slip shear zone with a subordinate component of compression at a larger scale the area is part of a continent scale shear zone separating external pan african domains of compression along the northern margin of the congo craton from internal domains dominated by high angle strike slip and transpressional deformation together with published data the present study thus demonstrates that transpression is a regional phenomenon in the pan african orogen of central and eastern cameroon

faint young sun and the carbon cycle implication for the proterozoic global glaciations the earth may have been globally ice covered several times during the proterozoic while the neoproterozoic and the paleoproterozoic glaciations may have been snowball earth events there is no evidence for such glaciation during the phanerozoic it might be hypothesized that a dimmer sun earlier in earth s history may have made the earth more susceptible to global glaciation in this paper the roles of solar flux and soil biological activity in the carbon cycle and the climate during the proterozoic are investigated using a simple carbon geochemical cycle model with a one dimensional energy balance climate model the results indicate perhaps counterintuitively that the proterozoic earth with its dimmer sun was not more susceptible to snowball glaciation metamorphic and volcanic co2 fluxes accumulate in the atmosphere and ocean until such time that those inputs are balanced by silicate weathering followed by carbonate precipitation and net organic carbon burial because of the dependence of weathering rates on climatic conditions changes in geologic co2 inputs have a large influence on climatic conditions in contrast slow variation in solar flux has relatively little long term impact on climate because of large compensating changes in atmospheric co2 level a reduction in co2 inputs lowers atmospheric co2 level which finally initiates global glaciation the atmospheric co2 level at the critical condition for a globally ice covered state would have been high during the proterozoic however roughly the same amount of co2 flux reduction is required for both the proterozoic and the phanerozoic this is essentially because the temperatures at the critical condition are very low hence the silicate weathering rate which should balance with a net co2 input rate in a steady state is also very low regardless of the variation in solar flux furthermore the effect of the lower solar flux on the co2 input rate at the critical condition would have been largely canceled by a lower efficiency of the silicate weathering rate due to lower soil biological activity during the proterozoic as a result co2 flux conditions for initiating the global glaciation may be similar during both the proterozoic and the phanerozoic therefore the explanation for the susceptibility of the proterozoic earth to snowball conditions cannot hinge simply on the dimmer sun we must look to other differences in behaviors of the carbon cycle and the climate between these two ages

topographic effects on current variability in the caspian sea 1 satellite derived surface height fields reveal that variability in the central and southern basins of the caspian sea is correlated with topography consistently empirical orthogonal functions from current meter data from the southern basin are aligned with the isobaths in addition the gravest mode which accounts for over 80 of the variance has an equivalent barotropic structure in the vertical to what extent this variability can be modeled using a linear analytical model is examined the latter assumes equivalent barotropic flow aligned with the geostrophic contours which in turn are dominated by the topography with ecmwf winds and etopo2 topography the model yields surface height deviations which are significantly correlated with satellite derived estimates on seasonal and longer time scales in the central basin the model is somewhat less successful in the southern basin where the stratification is stronger nevertheless the results are encouraging given the extreme simplicity of the model

late miocene onset of the amazon river and the amazon deep sea fan evidence from the foz do amazonas basin new biostratigraphic isotopic and well log data from exploration wells on the outer continental shelf and uppermost amazon deep sea fan brazil reveal that the amazon river was initiated as a transcontinental river between 11 8 and 11 3 ma ago middle to late miocene and reached its present shape and size during the late pliocene prior to the late miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the proterozoic basement in eastern amazonia average sedimentation rates on the amazon fan show three stages of development 1 11 8 6 8 ma ago low sedimentation rates 0 05 m ka prevailed on the fan because the amazon river was not yet entrenched and some sediments were partially trapped in continental basins 2 6 8 2 4 ma ago sedimentation rates 0 3 m ka increased the river entrenched and deposition fully migrated onto the amazon fan 3 2 4 ma ago to the present very high sedimentation rates 1 22 m ka with peaks of 11 m ka prevailed on the fan and the modern amazon river developed all these paleogeographic and depositional events are closely related to andean tectonism late miocene pliocene and were exacerbated by global cooling and sea level fall during the late miocene

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

interaction of barotropic vortices with coastal topography laboratory experiments and numerical simulations the interaction of a barotropic cyclonic vortex on a b plane with a strong topographic slope is studied by means of laboratory experiments and numerical simulations in the laboratory the vortex is produced in a rectangular rotating tank with a weak uniformly sloping bottom slope angle 3 58 in order to simulate the b effect the cyclonic vortex moves in the northwestward direction and interacts with an additional pronounced linear topography slope angle 358 along the western boundary of the tank the laboratory experiments revealed that the original northwestward trajectory changes to the south until the vortex is dissipated by viscous effects and bottom friction as it moves upslope the exterior ring of the vortex forms a strong current to the northeast from this current a new cyclonic vortex is created repeating approximately the behavior of the original one later two more vortices are formed in the same way a finite difference numerical model is used to solve the nondivergent barotropic equation despite the presence of the strong slope agreement between laboratory results and numerical runs suggests that conservation of potential vorticity is indeed the basic mechanism involved in the southward motion of the vortex and the northeastward meandering current

post rift reactivation of the onshore margin of southeast brazil evidence from apatite u th he and fission track data abstract on the continental margin of southeast brazil there is good geological evidence for post rift tectonic reactivation in the form of onshore tertiary basins up to 800 m deep and cretaceous to paleocene alkaline intrusive bodies at the surface with the aim of constraining the mesozoic and cenozoic exhumation history of this region we present new apatite u th he ages on basement samples which one of us previously used for apatite fission track aft analysis the new he ages range from 115 5 2 2 ma to 47 0 6 4 ma whereas the aft ages range from 330 3 22 ma to 64 5 3 ma inverse thermal history modelling of all the data provides more detail on the post rift cooling history than do the aft data alone furthermore models incorporating radiation damage effects on he diffusion produce thermal histories that are more consistent with independent geological evidence than models without these effects the results imply that only a few samples record an early cooling phase around the time of opening of the south atlantic ocean the new u th he data also imply a late cretaceous phase of exhumation between 90 and 60 ma subsequently the region underwent slow cooling i e erosion rate two or three samples seem to indicate a period of rapid cooling during the neogene although this is at the limit of resolution of the methods the post rift cooling phases imply exhumation from depths between 2 and 5 km this activity we attribute to compression in the south american plate between the andean subduction zone and the mid atlantic ridge in southeast brazil compression acted on a crust that was thermally or mechanically susceptible to reactivation during the late cretaceous deformation in the study area was of regional extent whereas during the tertiary it concentrated along pre existing shear zones of the precambrian brasiliano orogeny

the atuba complex southern south american platform archean components and paleoproterozoic to neoproterozoic tectonothermal events abstract the atuba complex consists of an archean terrane containing rocks differentiated from the mantle and incorporated into the continental crust between 3 1 2 7 ga this terrane was later reworked several times the main metamorphism occurred in the paleoproterozoic at about 2 1 ga and can be related to the amalgamation of the paleoproterozoic supercontinent atlantica ages around 1 7 ga may be related to the break up of atlantica during the mesoproterozoic there was little activity in the atuba complex and also in the entire south american platform isolated u pb zircon ages of 1 4 ga and sm nd model ages of about 1 2 ga may be associated to the assembly of rodinia the evidence from the south american platform indicates that the rodinia rifting pulses started as early as the latest collisional events of the grenville cycle instead of at ca 0 7 ga as commonly believed finally in the neopropterozoic at 0 52 0 65 ga u pb rb sr sm nd and k ar dates there were high grade tectonothermal events associated with gondwana assembly

global basin classification system a proposed system classifies sedimentary basins worldwide into specific as well as general categories the geologic history of each basin may be subdivided into cycles using three parameters basin forming tectonics depositional sequences and basin modifying tectonics sedimentary basins may be simple with one or two tectonic sedimentary cycles or they may be complex polyhistory basins with many different cycles and events there are eight simple cycle types in this classification which cover continental continental margin and oceanic areas the eight basic cycle types their depositional fills and tectonic modifiers have been given letter and number symbols so that the specific geologic history of each basin may be written as a formula these formulas may then e compared and similarities or differences between basins noted

thermal evolution of cretaceous tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels 1 the evolution of subtropical 30 35 n upper ocean temperatures through the cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth enamel coming from the western tethyan platform mean 18o values of 22 at the berriasian valanginian boundary decrease with oscillations to 18 5 around the cenomanian turonian boundary and progressively increase to 21 5 by the end of the cretaceous the similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals major cooling events at the million year scale can be distinguished 1 at the berriasian valanginian boundary and 2 during the earliest late valanginian a third cooling event is detected during the earliest aptian these events already proposed as icehouse interludes during the lower cretaceous are also recorded at subtropical latitudes a progressive warming is identified from the aptian to the cenomanian turonian interval that corresponds to a thermal optimum and then upper ocean temperatures decreased to the maastrichtian minimum isotopic temperatures range from 15 c to 28 c assuming a 18oseawater of 1 for an ice free world taking more realistic 18oseawater values of 0 for tropical waters during glacial periods within the berriasian valanginian interval and earliest aptian or with above average salinities possibly the maastrichtian temperatures are increased by 4 5 c temperature differences between climatic extremes of the valanginian and cenomanian turonian are estimated to have been 10 c latitudinal thermal gradients for the albian cenomanian turonian and maastrichtian were 0 2 0 3 c latitude and thus weaker than modern oceanic values at about 0 4 c latitude

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

trace element abundances in the rock canyon anticline pueblo colorado marine sedimentary section and their relationship to caribbean plateau construction and oxygen anoxic event 2 1 rapid eruption of submarine lava flows during formation of the caribbean plateau correlates closely with ocean anoxic event 2 oae2 which bracketed the cenomanian turonian boundary 93 5 ma these events also correspond with a positive excursion in carbon isotopic composition of seawater hydrothermal activity associated with large scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry we determined the distribution of major minor and trace element abundances in the rock canyon marine sedimentary section pueblo colorado after normalizing element concentrations to zr to remove the variable contribution of terrigenous material to these sediments we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive 13c isotope excursion the metal abundance anomalies indicate that intermittent hydrothermal activity in the form of both water rock exchange and magmatic degassing introduced large concentrations of trace metals into the cretaceous ocean at the same time that extinctions of benthic species turnover in plankton communities and increases in isotopically light organic carbon burial occurred the stratigraphic position of this interval of trace metal anomalies matches events associated with oae2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents

timing and magnitude of early aptian extreme warming unraveling primary δ18o variation in indurated pelagic carbonates at deep sea drilling project site 463 central pacific ocean abstract in order to elucidate early aptian marine paleotemperature evolution across the period of enhanced organic carbon corg burial oceanic anoxic event oae 1a stable isotope analyses were performed on pelagic limestones at deep sea drilling project site 463 central pacific ocean the 18o data exhibit a distinct anomaly by 2 spanning the oae 1a interval i e a 6 m thick phytoplanktonic corg rich unit constrained by magneto bio and 13c stratigraphy elucidation of paleotemperature significance of the 18o shift is made by taking account of recent sr ca evidence at the same section which revealed that geochemical signals in carbonate poor lithologies are relatively unaltered against burial diagenesis by discriminating 18o values from carbonate poor samples caco3 contents 5 30 wt it appears that an abrupt rise in sea surface temperatures ssts by 8 c 1 7 shift in 18o occurred immediately before oae 1a whereas a cooling mode likely prevailed during the peak corg burial in terms of its stratigraphic relationship as to the corg rich interval and to a pronounced negative 13c excursion as well as its timescale the observed sst rise resembles those associated with the paleocene eocene thermal maximum and more strikingly jurassic toarcian oae this observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism which was likely initiated by the greenhouse effect via massive release of ch4 or co2 from the isotopically light carbon reservoir and terminated by a negative productivity feedback

conditions required for oceanic anoxia euxinia constraints from a one dimensional ocean biogeochemical cycle model article i nfo widespread black shale depositional intervals termed oceanic anoxic events oaes occurred repeatedly during the phanerozoic eon here we developed a new vertical one dimensional ocean biogeochemical cycle model that involves several chemical reactions in an oxic anoxic sulfidic water column to explore the theoretical constraints for global oceanic anoxia euxinia quantitatively and systematically we conducted sensitivity analyses of the proposed causal mechanisms including elevated rates of riverine phosphorus p input ocean stagnation and lowered oxygen solubility due to climate warming we gave special attention to the vertical chemical structure of the ocean and also to the characteristic behaviors of the marine p cycle under anoxic conditions because the relationship between the depth of anoxia and the benthic phosphorus flux could be important for the occurrence of oceanic anoxia euxinia steady state simulations indicated that 1 a decrease in ocean stagnation or oxygen solubility is not enough by itself to achieve widespread anoxia with the present reactive p river input rate and 2 shallow water anoxia followed by massive p liberation from surface sediments can lead to widespread eutrophication and anoxia euxinia we conclude that elevated riverine flux of reactive p is the most important factor for triggering global anoxic events via a positive feedback loop among ocean anoxia phosphorus regeneration and surface biological productivity

surface relative dispersion in the southwestern gulf of mexico abstractsurface dispersion properties in the southwestern gulf of mexico are studied by using a set of 441 drifters released during a 7 yr period and tracked for 2 months on average the drifters have a drogue below the surface ekman layer so they approximately follow oceanic currents this study follows two different approaches first two particle or pair statistics are calculated relative dispersion and finite scale lyapunov exponents fsles relative dispersion estimates are consistent with theoretical dispersion regimes of two dimensional turbulence an exponential growth during the first 3 days a richardson like regime between 3 and 20 days in which relative dispersion grows as a power law in time and standard dispersion linear growth for longer times the fsles yield a power law regime for scales between 10 and 150 km but do not detect an exponential regime for short separations less than 10 km robust estimates of diffusivities based on both relative dispersion and fsles are provided

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions