kinetic constraints on illitization reactions and the effects of organic diagenesis in sandstone shale sequences abstract based on water rock interaction modelling and kinetic considerations the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones 1 clay mineral conversion to illite with k feldspar being a local potential source of k and 2 dissolution precipitation processes produced in sandstones by the action of water soluble organic species derived from adjacent shales assuming firstly that the chemical reactions take place in closed systems the nature and timing of diagenetic reactions in marine sandstone shale formations were modelled for a 50 120 c temperature range in order to improve understanding of the factors that control the illitization reaction with k feldspar coexisting with aluminous clay illite is modelled here as a muscovite type mineral we tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite illite precipitation reaction to occur while using several illitization reaction rates we also compared the stable mineral parageneses predicted for organic material free systems with those predicted in the case of organic diagenesis release of co 2 ch 4 acetic and oxalic acids similarities and discrepancies between numerical results and natural mineral assemblages suggest that the illitization reaction depends on the nature of the reacting clays kaolinite conversion to end member illite involves high energy conditions 2 kcal mol 1 which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system to overcome this barrier the fluid should be oversaturated with respect to k feldspar an external source of k or a ph increase in an open system is necessary for this reaction no particular effect of organic diagenesis on this reaction was found in the present study on the other hand smectite to illite conversion involves a lower energy barrier and can operate in closed systems where k feldspars are the source of k the maturation of organic matter may speed up the smectite to illite conversion rate by increasing the gibbs free energy of illite growth interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle mainly because organic protons and ligands were neutralized in the source rock however the diffusion of shale derived cations through sandstone as organometallic complexes mg ca fe al appears to be an efficient process during carbonate cementation in sandstones where oxidizing conditions enhance the decomposition of such complexes

coastal lee waves on ers 1 sar images alternative dark bright patterns on ers 1 synthetic aperture radar sar images of the west side of the taiwan strait taken on december 8 1994 were recognized to be the sea surface signature of a coastal lee wave such waves are called coastal lee waves because they occur along the lee side of the coast the coastal lee waves appeared in the form of a wave packet distributed within an offshore band 20 40 km wide the first packet which occurred in the northern portion of the observed area contained six waves with variable wavelengths defined as the spatial separation between two waves from 1 7 to 2 7 km the second packet in the middle contained 10 waves with a relatively uniform wavelength of 4 2 km the third packet in the southern portion contained 17 waves with an average wavelength of 2 0 km the crest lengths were from 20 to 80 km local meteorologic parameters observed simultaneously at fuzhou china close to the imaged area showed an offshore wind of 1 5 3 5 m s and a land surface air temperature of 19 c which was 4 c lower than the sea surface temperature sst thus the lower atmospheric boundary conditions at imaging time were very favorable both for generating the land breeze circulation and small wind waves on the sea surface which are in the bragg scattering wavelength band of the c band ers 1 sar a physical model of a three layer atmosphere was developed in order to explain how the land breeze circulation can generate the coastal lee waves the results showed that the vertical velocity disturbance caused by the wind convergence at the land breeze frontal zone is of vital importance for the generation of coastal lee waves and the model gave very good estimates of the processes observed the sar imaging mechanisms of the waves were analyzed in detail the differences between coastal lee waves and ocean internal waves which appear as similar alternative dark bright patterns on sar images were also discussed

two step deoxygenation at the end of the paleoproterozoic lomagundi event abstract the ca 2 1 ga francevillian group of gabon was deposited in the aftermath of the great oxidation event goe and records the lomagundi event le which is the most pronounced and long lived carbon isotope excursion in the geologic record moreover the sedimentary succession contains putative evidence for the earliest appearance of macro eukaryotes an emerging paradigm is that the end of the le was accompanied by a deoxygenation event that preceded the apparent stability of environmental and redox conditions as well as the carbon cycle characteristic of the mesoproterozoic however the processes that led to deoxygenation some 300 to 200 ma after the beginning of the goe are not well understood here we present a multi proxy stable isotope 34 s 33 s 36 s 98 mo 13 c org 13 c carb and 18 o carb study of the francevillian group we suggest that sedimentation of the lower part of the francevillian group took place during the le in oxygenated shallow waters with elevated sulfate concentrations two episodes of anoxic water shoaling during deposition of the upper francevillian group correspond with broader marine deoxygenation and a contraction of the seawater sulfate reservoir this shoaling of anoxic conditions may be linked to intense submarine hydrothermal and volcanic activity that led to sedimentary manganese deposits we propose that increased concentrations of aqueous hydrothermally sourced reductants drove oxygen consumption during the first deoxygenation event and established a sulfidic oxygen minimum zone at the margin of the shallow shelf carbonates with positive 13 c carb values characteristic of the le precipitated during this first stage of deoxygenation the second deoxygenation separated from the previous event by a period of well oxygenated conditions was marked by a stronger contraction of the seawater sulfate reservoir and coincided with the end of the le during this time widespread euxinic conditions were established in shallow above storm wave base marine environments the presence of a shallow water redoxcline points to a generally low oxygen atmosphere ocean system further the negative co variation between 34 s and 13 c values in sediments of the francevillian group and other sedimentary successions of similar age worldwide suggests that the inferred two step deoxygenation corresponding to the end of the le reflects global rather than local events that likely occurred between 2 1 and 2 05 ga ago

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

the early precambrian atmosphere and hydrosphere thermodynamic constraints from mineral deposits a chemical equilibrium model for the early precambrian atmosphere and hydrosphere is presented thermodynamic calculations based on observed stable minerals and mineral assemblages in banded iron formation bif pyritic uraniferous and auriferous quartz pebble conglomerates gold bearing conglomerates of the tarkwaian in ghana and red bed sediments yield the following conditions before approx 2350 ma the atmosphere was oxygen free p sub o 2 leftpointingdoubleanglequotationmark 10 super 10 bars consisted mainly of co 2 p sub co 2 2 s 10 super 7 5 bars over continents to 10 super 9 bars over oceans and hydrogen p sub h 2 10 super 9 bars over continents to 10 super 7 5 bars over oceans the high co 2 partial pressures generated acid rain and surface waters ph or 3 9 and buffered the ocean water to values around ph 6 5 to 7 in the oceans the iron minerals of the various banded iron formation facies goethite pyrite siderite in conjunction with the atmospheric co 2 partial pressure acted as a mineral buffer for p sub h 2 s and for the redox state p sub h 2 in continental weathering environments pyrite formation from ferrous iron and atmospheric h 2 s and oxidation of h 2 s to so 4 super 2 are considered the prime redox controlling mechanisms river waters which previously equilibrated with pyrite precipitated banded iron formation upon mixing with seawater mainly due to ph changes between 2350 and 2100 ma transition from pyritic to iron oxide bearing conglomerates indicates that photosynthetic oxygen production rates began to exceed oxygen consumption rates and that the redox buffers involving hydrogen sulfide and ferrous iron were overwhelmed increased oxidation potentials and increased surface water ph due to declining atmospheric co 2 partial pressures led to immobilization of iron and consequently to continental red bed formation and cessation of banded iron formation deposition

sar derived gap jet characteristics in the lee of the philippine archipelago abstract gap jet characteristics were analyzed in the lee of the philippine archipelago during the 2004 2009 winter monsoon seasons november march using wind fields derived from envisat synthetic aperture radar sar images jet characteristics were derived for the philippine island gap jets and compared to those from theoretical literature and experimental studies it was found that 18 of 31 jets characterized from 13 sar images followed the defined power laws or classic case and were similar to the defined scale equations the remaining 13 jets did not follow the classic case and resulted from jet interaction with islands adjacent jets and variable atmospheric conditions these non classic jets were grouped based upon similarities and referred to as decreasing increasing and reverse classic cases distinct deviations from the classic case included downstream increases in velocity and decreases in jet width it was found that the orientation of the wind field relative to topography significantly impacted jet classification and thereby made it difficult to characterize jets by region for all 31 jets the initial jet width dictated the distance at which a jet became self similar wherein a larger initial jet width required a greater distance for self similarity

comparison of pelagic and nepheloid layer marine snow implications for carbon cycling abstract marine snow from upper and mid water i e pelagic depths on the california margin is texturally and compositionally different from that traveling in the nepheloid layer transmission electron microscopy shows that pelagic marine snow consists primarily of bioclasts e g diatom frustules foram tests organic matter and microbes these components are entrained as discrete particles or small aggregates 10 m in diameter in a loose network of exocellular muco polysaccharide material clays are infrequent but when present are constituents of comparatively compact organic rich microaggregates microbes are abundant and appear to decrease in number with increasing water depth in contrast marine snow aggregates collected from just above the sea floor in the nepheloid layer are assemblages of clay particles clay flocs and relatively dense clay organic rich microaggregates in an exocellular organic matrix bioclasts and microorganisms occur only rarely the prevalence of clay organic rich aggregates in the nepheloid layer suggests that prior to final deposition and burial marine snow from the pelagic zone is subject to disaggregation and recombination with terrigenous detrital material near or at the sea floor results have significant implications for the accumulation and burial rates of organic carbon on continental margins and the aging and bioavailability of sedimentary organic matter samples examined were collected offshore of northern and central california

temporal constraints on the paleoproterozoic lomagundi jatuli carbon isotopic event the paleoproterozoic lomagundi jatuli positive 13c excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in earth history although precise absolute chronology of this event remains poorly constrained in northeastern fennoscandia an 1300 m thick sedimentary volcanic succession of the pechenga greenstone belt records decline of this isotopic excursion zircons from sedimentary rocks that occur within the decline have yielded 207pb 206pb dates at 2058 2 ma 6 ma including u decay constant uncertainties and provide the first maximum age constraint on the termination of the lomagundi jatuli event combined with existing constraints these data indicate an 140 m y interval characterized by 13c rich carbonate accumulation

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

early diagenesis impact of organic matter on mass physical properties and processes california continental margin abstract high resolution scales of 2 5 mm to submicrometer interdisciplinary studies of california continental slope sediments 0 80 cm reveal the importance of marine organic material and mineralogy on mass physical properties in fine grained sediments during very early diagenesis the combination of organic material 2 wt with smectite 5 is associated with high depositional porosities low effective stresses low porosity reduction and dewatering and the impedance of interstitial fluid flow living microbiota and nonliving mucopolysaccharides organic matter affecting the physical properties was revealed by transmission electron microscopy and carbon isotope studies calculations using the redfield ratio show that sediments containing high organic carbon 5 wt have hydrated organic material in the pore spaces that can occupy a volume that is up to 20 of the total sediment porosity pore water chemistry and x ray diffraction show evidence of early diagenetic processes that may significantly influence sediment fabric an important geochemical process near the sediment water interface that affects sediment fabric is the rapid dissolution of biogenic opal

carbon isotope excursions and the oxidant budget of the ediacaran atmosphere and ocean a possible global drop in marine carbon isotope values to as low as 12 peedee belemnite pdb recorded in the ediacaran shuram formation of oman has been attributed to the non steady state oxidation of oceanic dissolved organic carbon doc resulting from the rise in atmospheric oxygen to near modern values at the end of the precambrian geologic constraints indicate that the excursion lasted between 25 and 50 m y requiring a doc pool thousands of times to 10 000 times the modern inventory to conform with carbon isotope mass balance calculations for a 12 excursion at the consequent rates of doc oxidation oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of 800 k y oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the shuram excursion furthermore a doc driven excursion does not explain strong covariation between the carbon and oxygen isotope record these indicators show that negative isotope excursions recorded in the shuram and other ediacaran sections are unlikely to represent a global ocean signal

evaluation of jers 1 sar images from a coastal wind retrieval point of view wind retrieval from japanese earth resources satellite 1 jers 1 synthetic aperture radar sar using an l band model function in coastal regions is evaluated it is known that jers 1 sar has excessive ambiguities this paper also gives a quantitative evaluation of excessive ambiguities in coastal scenes of jers 1 sar first focusing on the cases where wind blows from the shore in sagami bay we investigate phenomena of wind speed increase with offshore distance using european remote sensing satellite 1 ers 1 sar derived wind speeds the relation between wind speed and offshore distance is well formulated which indicates the transition of the atmospheric boundary layer from land to sea surface wind speeds derived from jers 1 sar should be overestimated due to the excessive ambiguity then for observation time of each jers 1 sar capturing the cases that wind blows from the shore in sagami bay the expected wind speed growth profile is derived from the wind speed growth formula and an in situ wind observation of hiratsuka experiment station we convert the wind speed profile into the sigma 0 profile by an l band model function finally the profiles of jers 1 sar observed and the estimated sigma 0 are compared and the excessive ambiguity is estimated as the difference between them as a result the dynamic range of first azimuth ambiguity is as large as that of the wind relating signal from the ocean surface moreover higher order azimuth ambiguities and range ambiguity also may have a significant impact on near shore wind retrieval

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

coupled organic and inorganic carbon cycling in the deep subseafloor sediment of the northeastern bering sea slope iodp exp 323 abstract we studied microbially mediated diagenetic processes driven by carbon mineralization in subseafloor sediment of the northeastern bering sea slope to a depth of 745 meters below seafloor mbsf sites u1343 u1344 and u1345 were drilled during integrated ocean drilling program iodp expedition 323 at water depths of 1008 to 3172 m they are situated in the high productivity green belt region with organic carbon burial rates typical of the high productivity upwelling domains on western continental margins the three sites show strong geochemical similarities the downward sequence of microbially mediated processes in the sediment encompasses 1 organoclastic sulfate reduction 2 anaerobic oxidation of methane aom coupled to sulfate reduction and 3 methanogenesis the sediment contains two distinct zones of diagenetic carbonate formation located at the sulfate methane transition zone smtz and between 300 and 400 mbsf the smtz at the three sites is located between 6 and 9 mbsf the upward methane fluxes into the smtz are similar to fluxes in smtzs underlying high productivity surface waters off chile and namibia our bering sea results show that intense organic carbon mineralization drives high ammonium and dissolved inorganic carbon dic production rates 4 2 mmol m 3 y 1 in the uppermost 10 mbsf and strongly imprints on the stable carbon isotope composition of dic driving it to a minimum value of 27 vpdb at the smtz pore water calcium and magnesium profiles demonstrate formation of diagenetic mg rich calcite in the smtz below the smtz methanogenesis results in 13c enrichment of pore water dic with a maximum value of 11 9 the imprint of methanogenesis on the dic carbon isotope composition is evident down to a depth of 150 mbsf below this depth slow or absent microbially mediated carbon mineralization leaves dic isotope composition unaffected ongoing carbonate formation between 300 and 400 mbsf strongly influences pore water dic and magnesium concentration profiles the linked succession of organic carbon mineralization and carbonate dissolution and precipitation patterns that we observe in the bering sea slope sediment may be representative of passive continental margin settings in high productivity areas of the world s ocean

coexistence of atmospheric gravity waves and boundary layer rolls observed by sar abstractboth atmospheric gravity waves agw and marine atmospheric boundary layer mabl rolls are simultaneously observed on an environmental satellite envisat advanced synthetic aperture radar asar image acquired along the china coast on 22 may 2005 the synthetic aperture radar sar image covers about 400 km 400 km of a coastal area of the yellow sea the sea surface imprints of agw show the patterns of both a transverse wave along the coastal plain and a diverging wave in the lee of mount laoshan 1133 m peak which indicate that terrain forcing affects the formation of agw the agw have a wavelength of 8 10 km and extend about 100 km offshore model simulation shows that these waves have an amplitude over 3 km finer scale 2 km brushlike roughness features perpendicular to the coast are also observed and they are interpreted as mabl rolls the fft analysis shows that the roll wavelengths vary spatially the two way interactive triply nested grid 9 3 1 km weather research and forecasti

textural and paleo fluid flow control on diagenesis in the paleoproterozoic franceville basin south eastern gabon abstract the paleoproterozoic 2 15 ga franceville basin south eastern gabon is a continental sedimentary basin that host unmetamorphosed sediments this study involve detailed mineralogy sedimentology and petrography of the basal sedimentary units of fa and lower fb fb1 formations from the basin margin to centre in relation to mineral paragenesis and fluid flow the fa formation conglomerate sandstone and mudstone consists lithofacies of mixed fluvial and fluvio deltaic transitional origin while the overlying fb1 formation includes alternating organic rich black shale and sandstone of marine deposit the medium to coarse grained fluvio deltaic quartz arenite in upper part of fa formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history this provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis in contrast the clay and unstable detrital grains rich fluvial arkosic to sub arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts these arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities from the proximal to distal basin and within sample suites there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore fluid the predominance of 1mt illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid rock ratio dolomite anhydrite barite and fe oxides are the main crystallized authigenic minerals aside illite and fe rich chlorite clay minerals the mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the fa formation in the franceville basin are mainly controlled by depositional facies

early quartz cements and evolution of paleohydraulic properties of basal sandstones in three paleoproterozoic continental basins evidence from in situ δ18o analysis of quartz cements quartz cement microstratigraphy and high precision in situ 18 o values obtained by secondary ion mass spectrometry sims from m size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant paleoproterozoic basins the athabasca and thelon basins canada and the mcarthur basin australia in these examples the earliest quartz cements have the highest 18 o values up to 33 mean 26 0 v smow indicative of low temperature precipitation at depths of 0 to 1 1 km some paragenetically early cement phases however have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures subsequent generations of quartz cements have progressively lower 18 o values mean 16 1 3 consistent with precipitation at higher temperatures these data combined with petrographic observations indicate quartz cementation began in near surface vadose and phreatic zones the next stage of cementation is characterized by syntaxial burial cement overgrowths regionally and stratigraphically well sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation which resulted in cement bridged pore throats at relatively shallow depths of 0 2 km and very early in their burial history transforming these units into diagenetic aquitards as a result these units did not experience mineralization compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement but are instead marked by minor syntaxial overgrowth cement that make upb5 of the intergranular volume formed at depths of 3 5 km and were variably replaced by illite and chlorite during burial these lithologies were still open to fluid movement at critical times in the basin when u and

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

from detrital heritage to diagenetic transformations the message of clay minerals contained within shales of the palaeoproterozoic francevillian basin gabon unmetamorphosed and undeformed marine siliciclastics rocks of the fb fc and fd of the francevillian series gabon were deposited in an epicontinental basin clay minerals found in black shale siltstone and sandstone are dominantly illite and chlorite except in two levels of the fb formation which contain smectite rich randomly ordered mixed layers their survival in a 2 1 ga old sedimentary series is not related to the abundance of organic matter total organic carbon or toc nor redox conditions at the time of deposition as indicated by the fe speciation fehr fet and fepy fehr ratios rather it results from an incomplete illitization reaction that reflects potassium deficiency the k2o al2o3 ratio of shale siltstone and sandstone vary along the series and appear to conserve the signature of the original chemical composition of the rocks k feldspars which are present in the fc and fd formations are missing in the fb formation consequently the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the illitization reaction reached during diagenesis

an ers 1 synthetic aperture radar image of atmospheric lee waves an ers 1 synthetic aperture radar sar image of the island hopen shows a distinct 7 6 km wavelength wave phenomenon near the island this wave phenomenon is interpreted as the surface imprint on open water of atmospheric lee waves the pattern is visible in the sar image since the lee waves modulate the horizontal wind speed near the ocean surface which in turn modulates the surface roughness and the radar cross section the physical setting for the observation is presented and discussed the lower bound on horizontal wind speed modulation is estimated to range from 3 2 ms 1 for the wind speed minima to 12 2 ms 1 for the wind speed maxima based upon the observed radar cross section modulation and the ers 1 scatterometer wind retrieval model cmod4 the wavelength and wind speed modulation are consistent with linear lee wave model predications the model uses an atmosphere with an exponential profile of the scorer parameter ratio of buoyancy frequency to wind speed to represent a shallow ground based inversion layer observed at bear island and a bell shaped barrier to represent the forcing effects of hopen

constraining provenance for the uraniferous paleoproterozoic francevillian group sediments gabon with detrital zircon geochronology and geochemistry abstract the paleoproterozoic sandstones of the ca 2 1 ga francevillian group southeast gabon host the oldest known redox controlled high grade uranium u deposits uranium bearing detrital minerals e g monazite in the francevillian group sediments were derived by erosion of archean basement rocks from the east gabonian block of the northwestern congo craton surrounding the francevillian basin they are viewed as primary sources for u but the nature of these sources remains poorly constrained here u pb ages and hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the archean basement to characterize the nature age and origin of the provenance for the francevillian basin and the ultimate source of u for the high grade uranium deposits detrital zircon ages range from ca 3 1 to 2 6 ga with a dominant population between ca 2 90 and 2 80 ga suggesting a major contribution from ca 2 88 to 2 81 ga high k granitoids with a limited contribution from ca 3 1 to 2 84 ga ttgs and 2 75 to 2 7 ga granitoids of the east gabonian block most detrital zircon grains share similar depleted mantle hf model ages between ca 3 08 and 3 30 ga indicating significant recycling of a paleoarchean to mesoarchean crust we suggest that ca 2 9 to 2 8 ga high k u and th granitoids which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of u to the francevillian basin and ultimately for the u deposits we propose that ca 2 9 to 2 8 ga detrital zircons with hf tdm ages between ca 3 1 and 3 3 ga may thus allow for vectoring u deposits in sedimentary basins across the congo craton

nature of mixed layer clays and mechanisms of their formation and alteration abstract mixed layer clay minerals are intermediate products of reactions involving pure end member clays they come from natural environments ranging from surface to low grade metamorphic and hydrothermal conditions most often mixed layering is essentially two component but more complicated interstratifications have also been documented variable tendency to form regular 1 1 interstratifications has been observed and explanations of this phenomenon have been proposed mixed layer clays are either di or trioctahedral di trioctahedral interstratifications are rare most mixed layer clays contain smectite or vermiculte as a swelling component exceptions are all trioctahedral serpentine chlorite in low temperature environments and mica chlorite and talc chlorite at high temperatures solid state transformation and dissolution crystallization are the two mechanisms responsible for the formation of different mixed layer clays in general the weathering reactions that produce mixed layering are revers

formation of macromolecules with peptide bonds via the thermal evolution of amino acids in the presence of montmorillonite insight into prebiotic geochemistry on the early earth abstract the formation of macromolecules is an indispensable step in the process leading to the appearance of life on earth as their constituents were widely distributed over geological time and space on the surface of the primitive earth clay minerals are regarded as an essential factor influencing the formation of macromolecules due to their high reactivity and their close association with organic matter that could be the precursors of macromolecules over the years many researchers have studied the roles of clay minerals in macromolecules formation through the condensation of amino acids aas in aqueous systems in order to understand the thermal evolution of organic matter om in pristine oceans however the condensation of aas in non aqueous systems has received limited attention in particular the stability and evolution of aas in volcanic eruption environments that are rich in clay minerals have rarely been investigated in this work the influence of clay mineral on the thermal evolution of aas was studied two types of clay aa complexes i e clay aa interlayer complexes and clay aa external complexes where aas exist outside the interlayer structure were used an expandable clay mineral montmorillonite mt was chosen as the typical clay mineral and l arginine arg and glycine gly were used as the model aas for the preparation of clay aa complexes in situ diffuse reflectance infrared fourier transform spectroscopy in situ drift was used to monitor the thermal evolution behaviors of the clay aa complexes such as the formation of macromolecules with peptide bonds and thermogravimetry tg and tg coupled with fourier transform infrared spectroscopy tg ftir analyses were used to investigate the decomposition of the produced macromolecules the results show that mt strongly affects macromolecules formation during the thermal evolution of aas which is mainly due to that the inherent solid acidity of the different structural sites of mt varies when aas are present on the external surfaces of mt the interaction between aas and clay minerals substantially decreases the initial formation temperature of the macromolecules with peptide bonds for arg and strongly affects the types of condensation products for gly when aas exist in the interlayer structure of mt the interlayer structure has a steric effect on the thermal evolution of these monomers which together with the effects of the molecular structure of arg enhances the initial temperature of condensation and increases the thermal decomposition temperature of the formed macromolecules regardless of whether aas exist in the interlayer sites or on the external surfaces of clay minerals clay om association is able to protect macromolecules postponing their thermal decomposition the present work indicates that macromolecules with peptide bonds could readily form via the condensation of aas under dry high temperature conditions which implies that non aqueous thermal conditions resulting from volcanic activities might be a favorable geological setting for prebiotic chemical evolution on the early earth

potassium sources and illitization in texas gulf coast shale diagenesis abstract the aim of this paper is to identify the controls on the rates of illitization in texas gulf coast shales under diagenetic conditions two different mineral transformations are considered the transformation of smectite to nearly illite and the precipitation of discrete illite i e the formation of pure illite analyses of potassium feldspar and kaolinite abundance in addition to k2o al2o3 and tio2 contents in shales from three wells are used to supplement published data in order to identify the mineral reactions responsible for the smectite to illite transformation in shales involving illite smectite i s mineral series these data indicate that the first stage of the smectite to illite reaction in shales r 0 structures ca 0 50 illite content is controlled by the presence of feldspar and hence also by the dissolution kinetics of feldspar the empirically derived activation energy for the first i s transformation in shales is similar to that for k feldspar dissolution kaolinite is not involved in this transformation in the shales investigated the second stage of the reaction sequence r 1 structures ca 50 100 illite content occurs after potassium feldspar has largely or completely disappeared from the shale system this indicates that the feldspar no longer controls the second stage of the smectite to illite transformation and suggests another external source of potassium for the reaction in the shales this assumption is supported by the relative increase of potassium content of the shales at least in two of the three studied wells kaolinite does not seem to be involved either in this second stage of the i s reaction sequence the formation of discrete illite crystallites in shales is independent of the i s transformation reactions and takes place only after a very significant period of time has elapsed the slow rate of this reaction reflects the low activity of potassium in solutions that are in equilibrium with shale in comparison with sandstones where a rapid precipitation of pure illite can in some cases be observed it is suggested that the different reactions in the smectite to illite transformation necessitate different sources of potassium either from within or outside an argillaceous whole rock system these different sources of potassium give rise to different rates for the i s conversion reaction and the formation of illite

direct influence of gravity waves on surface layer stress during a cold air outbreak as shown by synthetic aperture radar abstract synthetic aperture radar sar images of oceans and the great lakes have provided a highly detailed means of observing atmospheric boundary layer phenomena such as convection land breezes and internal gravity waves this is possible because the backscattered radiation detected by sar can be dominated by scattering from wind driven capillary waves whose spatial variation is visible as patterns in the sar images in this paper we present two case studies in which sar images taken over lake superior demonstrate spatial variability in the surface wind stress created over the lake by coincident gravity waves and boundary layer convection during cold air outbreaks of particular interest is the direct influence of the gravity waves on the lake surface stress despite the intervening highly convective atmosphere as well as the detailed view of the fetch dependence of that stress

organic matter preservation on continental slopes importance of mineralogy and surface area abstract theoretical considerations calculations and data reported by keil et al 1994a were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix linked organic carbon content of continental margin sediments occurs as a monolayer or monolayer equivalent coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains mayer 1994 results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments even in the sand and silt fractions but rather the presence of nonspherical high surface area to volume particles primarily clays but also oxy hydroxides and ultra structured nonorganic bioclasts such as diatom frustules analysis of the mineralogy organic carbon content surface area and in situ microfabric of continental margin sediments off california in the mendocino and san luis obispo areas and off the state of washington indicate a strong correlation between the amount of matrix linked organic matter present and the suite of clay minerals data indicate that organic carbon appears to be preferentially sequestered in smectite rich sediments compared to those whose clay fractions are dominated by chlorite we suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior our data also indicate that organic carbon preservation on the three california transects all of which have a suboxic oxygen minimum zone that impinges on the sea floor is not significantly influenced by differences in bottom water oxygen concentration and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

theoretical expression for an ocean internal soliton synthetic aperture radar image and determination of the soliton characteristic half width this paper deals with the development of techniques for satellite synthetic aperture radar sar ocean image interpretation we derived a theoretical model of a radar image for a korteweg de vries type ocean internal soliton and validated the model using ocean internal wave signals taken from ers 1 sar and radarsat sar images the results indicate that the model perfectly simulates ocean internal soliton signatures with double sign variations of radar backscatter on the basis of the model we developed the curve fitting method and the peak to peak method for determining the internal soliton characteristic half widths which then were used to calculate the internal soliton amplitudes the test results indicate that ocean internal soliton amplitudes derived by the two methods agree with in situ data acquired on the portuguese continental shelf and in the south china sea with reasonable accuracy the role that wind fields play in ocean radar remote sensing was also analyzed finally the modulation ratio of ocean internal waves on radar images was quantitatively estimated

hydrothermal clay mineral formation in the uraniferous paleoproterozoic fa formation francevillian basin gabon abstract the spatial distribution of neoformed clay minerals was investigated in the paleoproterozoic fa formation of the francevillian basin south east gabon which hosts high grade u ore deposits associated with the only known occurrence of natural nuclear reactors illite appears as the main clay phase in the lower fluvial unit in the mineralized upper fluvio deltaic tidal unit the clay assemblage is more diversified and commonly characterized by illite chamosite berthierine and chamosite berthierine mixed layers usually in association with alteration products of u bearing minerals the clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of u ore deposits polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid rock ratio according to crystalline structure mineral chemistry thermodynamic modeling and geochronology clay phases seem to be mainly hygrometer rather than exclusively a thermometer and their formation as well as associated dissolution precipitation of u bearing minerals took place between ca 2040 and 2010 ma ago at temperatures of about 240 30 c using previous burial estimates this suggests the operation of hydrothermal processes controlled by an external heat source likely associated with volcanism during deposition of the fd and fe formations hydrothermal activity involving oxidized fluids would have driven u remobilization and ultimately formation of high grade u ore deposits the thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich u endowment

the 2 1 ga old injectite network of the franceville basin gabon architecture origin and implications on manganese mineralization abstract detailed sedimentological investigations on numerous outcrops and drill core sections in the 2 1 ga old franceville basin gabon provide evidence for a large scale injectite network the injectites were formed by the injection of sands through a thickness of about 150 m of the fb1 member and now cover a minimum area of 70 km 2 corresponding to the bangombe plateau but are also recognized close to franceville 35 km farther south east the injectite lithology corresponds to a mud or carbonate supported sandstone characterized by a loose and uncompacted fabric contrasting with the host rock because the injectites are often parallel to subparallel to the stratification they were misinterpreted as depositional beds by previous authors at outcrop the injectite bodies exhibit sill dyke wing and protrusive geometries emplaced during early burial within poorly compacted material with partial wall erosion and dissociation the source of the sand material is attributed to a channel levee turbidite depositional system located in the lower part of the fb1 member these deposits were buried by a thick biochemical muddy cap deposited during a starvation phase which increased the seal capacity of the system this type of architecture has a high potential to develop compaction disequilibrium during burial moreover in the case of the francevillian injectites the abundance of microbial organic matter favoured early methane degassing and lateral charging of the sand reservoir the overpressure in the channels was periodically released during early burial through the effect of rapid sedimentary and tectonic loading and possible seismic activity the fluidized sand was injected according to a symmetrical wing pattern thus the injected masses caused a local decrease in the grade of the lateritic manganese ore deposit of the bangombe plateau

carbon isotope record for the onset of the lomagundi carbon isotope excursion in the great lakes area north america carbonate units of the chocolay group in the lake superior area usa and the gordon lake formation of the correlative upper huronian supergroup ontario canada were deposited after the last paleoproterozoic glacial event and an episode of intense chemical weathering the huronian supergroup contains at the base 2 45 ga volcanics and is intruded by the 2 22 ga nipissing diabase dykes and sills while the chocolay group is bracketed in age between 2 29 and 2 20 ga the lomagundi 2 22 2 1 ga carbon isotope excursion started after the paleoproterozoic glacial epoch and before a plume breakout event at 2 22 ga therefore the chocolay and upper huronian carbonates were deposited either before or during the onset of the lomagundi event notably thin carbonates of the basal gordon lake formation and thick carbonate succession of the kona dolomite in the northeastern exposures of the chocolay group record 13 c values as high as 9 5 versus v pdb similar to other successions deposited during the lomagundi event both units contain pseudomorphs and molds after sulfates this observation suggests that seawater sulfate contents rose dramatically in association with the onset of the lomagundi event and the rise of atmospheric oxygen carbonates in the western and southern exposures of the chocolay group randville and bad river dolomites and saunders formation previously assumed to be equivalent to the gordon lake formation and kona dolomite have carbon isotope values close to 0 based on basin analysis we infer that these carbonate units were deposited during a negative carbon isotope excursion after the lomagundi event started and are slightly younger than the kona dolomite and gordon lake formation this interpretation implies that the carbonate platform in the great lakes area transgressed to the west over shallow marine and fluvial deposits the negative carbon isotope excursion in the lake superior area might correspond to similar 13 c values of the mooidraai dolomite in the griqualand west basin south africa supporting correlation between paleoproterozoic successions of north america and south africa and the notion of three global glaciations in the paleoproterozoic era carbonates of the mille lacs group trout lake glen township and denham formations in minnesota have 13 c values ranging from 1 2 to 2 5 combined with geochronologic constraints these data suggest that these units were deposited after the lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized 2 0 ga passive margin in the lake superior area 2006 elsevier b v all rights reserved

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

airborne synthetic aperture radar imagery of atmospheric gravity waves we analyze the oceanic imprints of atmospheric internal gravity waves detected in airborne synthetic aperture radar sar images collected off the west coast of vancouver island british columbia in june 1990 a sequence of sar images taken over a period of 2 5 hours reveals that the waves traveled with group velocities of 3 6 m s 1 toward the northwest counter to the 10 m s 1 surface winds wave crests were oriented normal to the coast and had lateral extents of several tens of kilometers observed wavelengths ranged from 1 2 to 2 3 km while calculated wave periods ranged from 3 to 8 min a comparison of the observed wave properties with values derived from a simple three layer density model of the troposphere suggest that the waves were low mode oscillations trapped within a 0 75 km thick temperature inversion overlying a 1 km thick weakly stratified marine boundary layer wave propagation was counter to the wind shear within the inversion layer generation of the internal wave groups appears to have occurred near the southern boundary of the stable marine air mass where it bordered a convectively active air mass that was intruding northward along the coast

direct observation of subtropical mode water circulation in the western north atlantic ocean abstract eighteen degree water edw is the dominant subtropical mode water of the north atlantic subtropical gyre and is hypothesized as an interannual reservoir of anomalous heat nutrients and co2 although isolated beneath the stratified upper ocean at the end of each winter edw may re emerge in subsequent years to influence mixed layer properties and consequently air sea interaction and primary productivity here we report on recent quasi lagrangian measurements of edw circulation and stratification in the western subtropical gyre using an array of acoustically tracked isotherm following bobbing profiling floats programmed to track and intensively sample the vertically homogenized edw layer and directly measure velocity on the 18 5 c isothermal surface the majority of the clivar mode water dynamics experiment climode bobbers drifted within the subtropical gyre for 2 5 3 5 years many exhibiting complex looping patterns indicative of an energetic eddy field bobber derived lagrangian integral time and length scales 3 days 68 km associated with motion on 18 5 c were consistent with previous measurements in the gulf stream extension region and fall between previous estimates at the ocean surface and thermocline depth several bobbers provided evidence of long lived submesoscale coherent vortices associated with substantial edw thickness while the relative importance of such vortices remains to be determined our observations indicate that these features can have a profound effect on edw distribution edw thickness defined using a vertical temperature gradient criterion exhibits seasonal changes in opposition to a layer bounded by the 17 c and 19 c isotherms in particular edw thickness is generally greatest in winter as a result of buoyancy forced convection while the 17 19 c layer is thickest in summer consistent with seasonal ekman pumping contrary to previous hypotheses the bobber data suggest that a substantial fraction of subducted edw is isolated from the atmosphere for periods of less than 24 months seasonal to biennial re emergence principally within the recirculation region south of the gulf stream appears to be a common scenario which should be considered when assessing the climatic and biogeochemical consequences of edw

nanoscale analysis of preservation of ca 2 1 ga old francevillian microfossils gabon abstract the fc formation of the francevillian of gabon displays the oldest gunflint type assemblage of microfossils that is hosted in shallow water stromatolites the fc formation was deposited between 2 14 and 2 08 billion years ago ga after the great oxygenation event 2 4 2 3 ga and near the end of the lomagundi event 2 3 2 06 ga although they have been used as a benchmark for the search for older microfossils the nature of gunflint type microfossils has remained elusive due to their simple shapes their small sizes and their alteration here we report the first nanoscale study of francevillian gunflint type microfossils we used a combination of raman spectroscopy palynology in situ focused ion beam sectioning and analytical electron microscopy in spite of the relatively high thermal maturity of organic matter inferred peak burial temperature 296 30 c spherical cell walls huroniospora and filamentous sheaths gunflintia minuta 3 m in diameter and broader filaments are preserved organic matter in on cell walls and sheaths is associated with nanocrystalline quartz whereas coarser quartz crystals fill and surround the microfossils this pattern likely inherited from recrystallization of texturally heterogenous opal generations could have allowed the observed preservation of organic structures and limited migrations of organic matter moreover we demonstrate the preservation of thick sheathed broad 3 m filaments for the first time in a stromatolitic gunflint type assemblage such thick sheaths are common in cyanobacteria but not in other filamentous bacteria we distinguished two types of star shaped organic microstructures eoastrion but found no diagnostic evidence for against a microfossil nature at the nanoscale furthermore we show that titanium is commonly associated with organic structures of microfossils likely as a result of diagenetic mineralization in contrast iron rich nanocrystals associated with microfossils occur in quartz not in organic matter and could possibly represent recrystallized biominerals

analysis of coastal lee waves along the coast of texas observed in advanced very high resolution radiometer images we examine a group of wave like cloud patterns that occurred along the coast of texas on a national oceanic and atmospheric administration satellite advanced very high resolution radiometer ir images taken on january 22 1999 these wave like cloud patterns were interpreted to be signatures of a coastal lee wave packet on the basis of simultaneous field observations and theories developed by zheng et al 1998a the wave packet contains 13 waves with crest lines generally parallel to the coastline the lengths of leading wave crest lines are longer than 500 km the average wavelength is 9 5 km ranging from 6 2 to 14 7 km the width of the horizontal distribution band of the wave packet is as wide as 113 km this case represents the most energetic coastal lee wave packet that has ever been reported

fractionation between inorganic and organic carbon during the lomagundi 2 22 2 1 ga carbon isotope excursion article i nfo the lomagundi 2 22 2 1 ga positive carbon isotope excursion in shallow marine sedimentary carbonates has been associated with the rise in atmospheric oxygen but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen the amount of oxygen released to the exosphere during the lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon which is poorly characterized because dissolved inorganic and organic carbon reservoirs were arguably larger in the paleoproterozoic ocean at a time of lower solar luminosity and lower ocean redox state decoupling between these two variables might be expected we determined carbon isotope values of carbonate and organic matter in carbonates and shales of the silverton formation south africa and in the correlative sengoma argillite formation near the border in botswana these units were deposited between 2 22 and 2 06 ga along the margin of the kaapvaal craton in an open marine deltaic setting and experienced lower greenschist facies metamorphism the prodelta to offshore marine shales are overlain by a subtidal carbonate sequence carbonates exhibit elevated 13 c values ranging from 8 3 to 11 2 vs vpdb consistent with deposition during the lomagundi positive excursion the total organic carbon toc contents range from 0 01 to 0 6 and 13 c values range from 24 8 to 13 9 thus the isotopic fractionation between organic and carbonate carbon was on average 30 3 2 8 n 32 in the shallow marine environment the underlying sengoma shales have highly variable toc contents 0 14 to 21 94 and 13 c values 33 7 to 20 8 with an average of 27 0 3 0 n 50 considering that the shales were also deposited during the lomagundi excursion and taking 13 c values of the overlying carbonates as representative of the 13 c value of dissolved inorganic carbon during shale deposition a carbon isotope fractionation as large as 37 appears to characterize the production of bulk organic matter in the deeper part of the pretoria basin at that time this enhanced fractionation relative to that observed in shallow water environments likely reflects heterotrophic secondary and chemotrophic productivity at and below a pronounced redoxcline consistent with the euxinic conditions inferred from independent evidence for the deeper part of the pretoria basin greater variability in organic carbon vs carbonate carbon isotopic values on the shallow marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the lomagundi excursion our study suggests that in contrast to the late neoproterozoic and phanerozoic when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers in the paleoproterozoic redox stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline this strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the 13 c values of dissolved inorganic carbon in the paleoproterozoic seawater

exceptional preservation of expandable clay minerals in the ca 2 1 ga black shales of the francevillian basin gabon and its implication for atmospheric oxygen accumulation abstract clay minerals are exceptionally well preserved in marine black shale of the ca 2 1 ga francevillian group in southeastern gabon the fb formation of the francevillian group is characterized by smectite rich clay minerals including randomly ordered r0 type and ordered r1 type mixed layer illite smectite i s the preservation of r0 type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis which is unique considering the paleoproterozoic age of the sedimentary rocks r0 and r1 type smectite rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures suggesting formation of clay organic matter complexes our data suggests that clay minerals may have enhanced organic matter preservation providing the oldest example where a link between clay minerals and organic matter sequestration can be established our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time

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

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

cause and timing of the thermal over maturation of hydrocarbon source rocks of the ecca group main karoo basin south africa abstract the source rocks of the main karoo basin south africa are believed to have undergone thermal over maturation close to the jurassic aged karoo large igneous province klip dolerite intrusions yet the cause and timing of this source rock exhaustion event likely to have extensively destroyed their shale gas potential remain controversial based on drill core samples from the central main karoo basin we investigated the clay mineralogy of the 2 1 1 and 2 1 phyllosilicate groups and have carried out 40ar 39ar radiometric dating of illite the x ray diffractograms xrd and mineral chemistry reveal that chlorite and illite phases of samples close to the dolerite intrusions are characterized by higher crystallinity with a chemical composition indicative of formation in a metamorphic environment in contrast chlorite and illite distal to any dolerite intrusion show lower crystal growth rates with chemical compositions suggesting formation in a diagenetic environment 40ar 39ar radiometric dating indicates that illite in samples distal to the dolerite intrusions formed between 292 and 245 ma with some ages clustering around 281 ma the oldest ages are consistent with a detrital fraction probably recycled from the erosion of the ash bed at the top of the underlying dwyka group however ages around 281 ma can be linked with estimated depositional age of the ecca group at 280 ma while the youngest ages may reflect late diagenesis to regional metamorphism illite fractions from samples close to dolerite shows formation age between 204 and 170 ma consistent with the time of the klip dolerite sills and dykes intrusions at 180 ma this illustrates that the ages of these illites were partially or totally re set due to recrystallization in a late thermal anomaly induced by the intrusion of dolerites the correlation between the timing of the youngest mineral transformation processes and the klip represents strong evidence suggesting that source rocks of the ecca group experienced a thermal overprinting event 180 ma ago this late thermal anomaly is consistent with previous data illustrating thermal over maturation and metamorphic devolatilization of ecca group shales situated close to the dolerite intrusions

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

relationships between mineral surfaces and organic carbon concentrations in soils and sediments abstract relationships between mineral specific surface area and organic carbon oc concentration are examined for sediments and soil a horizons from throughout the world i found published elsewhere that continental shelf sediments from many different regions exhibit downcore loss of oc to a refractory background level which shows a consistent relationship with mineral surface area slope 0 86 mg m 2 oc this trend is equivalent to a monolayer of organic matter over all surfaces and is termed the monolayer equivalent me level sediments and soils from other environments are compared to this empirically derived relationship several continental slope areas show extension of this relationship to considerable depth marked excesses of oc above this trend which persist downcore were found in sediments with high carbonate mineral content or slope sediments with low dissolved oxygen concentrations in the overlying water column about half of the soils examined also adhered to this relationship while soils with high carbonate content low ph or poor drainage showed oc concentrations higher than the me level oc concentrations below the me level are found in deltaic regions and areas with low organic matter delivery such as the deep sea or arid soils the nature of mineral surfaces was examined using n2 adsorption desorption isotherms and most surface area was found to be present as pores with

viewing a cold air outbreak using satellite based synthetic aperture radar and advanced very high resolution radiometer imagery in this phenomenological study we compare nearly simultaneous visible wavelength advanced very high resolution radiometer avhrr and synthetic aperture radar sar images of a cold air outbreak that occurred off the ice pack in the bering sea the visible image shows the spatial evolution of the cloud structure from the ice edge to several hundred kilometers offshore near the ice edge the clouds form regular linear features known as cloud streets these streets are spaced about 5 km apart 5 times the depth of the boundary layer by 200 km offshore the visible image shows nascent closed cell mesoscale cellular convection mcc with connected disks and smears of white clouds surrounded by arcs and rings of dark cloud free regions the diameters of the disks range between 10 and 15 km the sar image shows the same broadly defined spatial pattern but with a substantial increase in detail uncorrelated with the avhrr image in the area of the cloud streets shown in the avhrr image are two dimensional elongated regions of enhanced radar backscatter called sar streaks with the same cross wind spacing as the cloud streets this suggests the presence of quasi two dimensional roll vortices in the atmospheric boundary layer downstream in the area of proto mcc are broadly spaced expanses of enhanced backscatter that form two dimensional patterns such as connected arcs that have structure and scales similar to the spaces between the clouds the spatial evolution of the streaks and streets broadly conforms to the patterns of downdrafts and updrafts respectively exhibited in modeling studies of cold air outbreaks however the comparison with modeling studies is more successful for the cloud streets than for the sar streaks we suggest that this difference is because the clouds in the avhrr images give a time integrated signal of large scale quasi two dimensional atmospheric turbulence whereas mesoscale radar patterns of surface roughness give a relatively instantaneous view of fundamentally three dimensional structure within the atmospheric boundary layer