free hydrocarbon gas gas hydrate and authigenic minerals in chemosynthetic communities of the northern gulf of mexico continental slope relation to microbial processes research submersibles and piston cores were used to sample two chemosynthetic communities in the gulf of mexico continental slope at 540 m water depth vent gas from the deep subsurface is the starting material from which other carbon pools are derived including gas hydrate free hydrocarbon gas in sediment and authigenic carbonate rock gas crystallizes as exposed mounds of structure ii gas hydrate and as massive vein fillings in hemipelagic mud venting rates from gas hydrate mounds periodically increase after the temperature of the bottom water increases gas hydrate decomposition is largely restricted to exposed hydrate and at shallow depth in sediment overall gas hydrate is accumulating not decomposing at study sites most free gas in sediment appears to be destroyed in situ by anaerobic microbial oxidation in chemosynthetic communities leading to sequestration of carbon as abundant authigenic carbonate rock depleted in 13c free methane is rapidly oxidized leaving residual methane enriched in 13c some in situ microbial co2 reduction occurs and methane depleted in 13c mixes with vent methane enriched in 13c the c2 c5 hydrocarbons of vent gas initially most depleted in 13c ethane isobutane isopentane are least affected by microbial oxidation whereas hydrocarbons initially enriched in 13c propane normal butane normal pentane are most affected anaerobic microbial oxidation of all c1 c5 hydrocarbon gases not only methane may be significant in chemosynthetic communities microbial processes contribute to the development and stability of chemosynthetic communities by providing required h2s anaerobic microbial processes lead to deposition of diagnostic authigenic minerals related to the carbon carbonate minerals and sulfur cycles pyrite elemental sulfur altering the seafloor seafloor cementation favors fauna such as chemosynthetic tubeworms and seep mussels by forming carbonate hardgrounds on an otherwise unfavorable mud dominated seafloor

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

pyritization processes and greigite formation in the advancing sulfidization front in the upper pleistocene sediments of the black sea pyritization in late pleistocene sediments of the black sea is driven by sulfide formed during anaerobic methane oxidation a sulfidization front is formed by the opposing gradients of sulfide and dissolved iron the sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content two processes of diffusion limited pyrite formation were identified the first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of fesn n 1 10 1 29 including greigite elemental sulfur and polysulfides formed from h2s by a reductive dissolution of fe iii containing minerals serve as intermediates to convert iron sulfides into pyrite in the second process a direct pyrite precipitation occurs through prolonged exposure of iron containing minerals to dissolved sulfide methane driven sulfate reduction at depth causes a progressive formation of pyrite with a 34s of up to 15 0 the s isotopic composition of fes2 evolves due to contributions of different sulfur pools formed at different times steady state model calculations for the advancement of the sulfidization front showed that the process started at the pleistocene holocene transition between 6360 and 11 600 yr bp our study highlights the importance of anaerobic methane oxidation in generating and maintaining s enriched layers in marine sediments and has paleoenvironmental implications

anaerobic methane oxidation and a deep h2s sink generate isotopically heavy sulfides in black sea sediments the main terminal processes of organic matter mineralization in anoxic black sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2 4 m and methanogenesis below the sulfate zone the modern marine deposits comprise a ca 1 m deep layer of coccolith ooze and underlying sapropel below which sea water ions penetrate deep down into the limnic pleistocene deposits from 9000 years bp sulfate reduction rates have a subsurface maximum at the so42 ch4 transition where h2s reaches maximum concentration because of an excess of reactive iron in the deep limnic deposits most of the methane derived h2s is drawn downward to a sulfidization front where it reacts with fe iii and with fe2 diffusing up from below the h2s fe2 transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur the pore water gradients respond dynamically to environmental changes in the black sea with relatively short time constants of ca 500 yr for so42 and 10 yr for h2s whereas the fes in the black band has taken ca 3000 yr to accumulate the dual diffusion interfaces of so42 ch4 and h2s fe2 cause the trapping of isotopically heavy iron sulfide with 34s 15 to 33 at the sulfidization front a diffusion model for sulfur isotopes shows that the so42 diffusing downward into the so42 ch4 transition has an isotopic composition of 19 close to the 23 of h2s diffusing upward these isotopic compositions are however very different from the porewater so42 43 and h2s 15 at the same depth the model explains how methane driven sulfate reduction combined with a deep h2s sink leads to isotopically heavy pyrite in a sediment open to diffusion these results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth s history

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

on the isotope composition of reactive iron in marine sediments redox shuttle versus early diagenesis the isotope composition of reactive iron fe in marine sediments and sedimentary rocks is a promising tool for identifying fe sources and sinks across ocean basins in addition to cross basinal fe redistribution which can modify fe isotope signatures fe minerals also undergo diagenetic redistribution during burial the isotope fractionation associated with this redistribution does not affect the bulk isotope composition but complicates the identification of mineral specific isotope signatures here we present new fe isotope data for peru margin sediments and revisit previously published data for sediments from the california margin to unravel the impact of early diagenesis on fe isotope compositions of individual fe pools sediments from oxic california margin sites are dominated by terrigenous fe supply with fe release from sediments having a negligible influence on the solid phase fe isotope composition the highly reactive fe pool sum of fe bound to oxyhydr oxide carbonate monosulfide and pyrite of these sediments has a light isotope composition relative to the bulk crust which is consistent with earlier studies showing that continental weathering shifts the isotope composition of fe oxyhydr oxides to lighter values ferruginous sedimentswithin the peruvian oxygen minimumzone are depleted in fe relative to the lithogenic background which we attribute to extensive fe release to the water column the remaining highly reactive fe pool has a heavier isotope composition compared to california margin sediments this observation is in agreement with the general notion of an isotopically light benthic fe efflux most of the reactive fe delivered and retained in the sediment is transferred into authigenic mineral phases within the topmost 10 to 20 cm of the sediments we observe a first order relationship between the extent of pyritization of fe monosulfide and the isotope composition of authigenic pyrite with increasing pyritization the isotope composition of authigenic pyrite approaches the isotope composition of the highly reactive fe pool we argue that the isotope composition of authigenic pyrite or other fe minerals that may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is separately evaluated and either close to 100 or 0 alternatively one may calculate the isotope composition of the highly reactive fe pool thereby avoiding isotope effects due to internal diagenetic redistribution in depositional settings with high fe but lowsulfide concentrations source and sink signatures in the isotope composition of the highly reactive fe pool may be compromised by sequestration of fe within authigenic silicate minerals authigenic silicate minerals appear to be an important burial phase for reactive fe below the peruvian oxygen minimum zone

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

origin of two distinct multiple sulfur isotope compositions of pyrite in the 2 5ga klein naute formation griqualand west basin south africa abstract quadruple sulfur isotope ratios 32s 33s 34s 36s were measured by a high precision sf6 method for early diagenetic pyrite in the 2 5 ga upper prieska facies of the griqualand west basin from the agouron drill core gkp01 measured 33s 33s 0 515 34s values range between 2 3 and 8 5 with no apparent meter scale stratigraphic trend for 70 m section it was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools nodular and layered pyrite yield mostly negative down to 2 3 and relatively constant 33s values over a few tens of cm scale disseminated pyrites in contrast yield positive 0 0 8 5 33s values that are heterogeneous in sub cm scales the former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction and the latter preferentially sampled aqueous h2s s0 reservoirs in late archean oceans relatively constant 33s values for layered pyrite suggest archean sulfate reservoirs had residence times of a few million years most data yield a well defined 36s 33s ratios of 0 9 suggesting predominance of a single s mif source reaction four analyses of a two cm length sample 306 5 m yield 36s 33s slope of 1 6 this slope is consistent with the relationship expected for a symmetry breaking isotope effect during the s s2 reaction but deviations from the main archean array i e 36s 33s 0 9 may also result from various combinations of so photolysis and so2 oxidation high spatial resolution high precision sf6 measurements of archean pyrite provide significant new insights into the origin of archean sulfur mass independent fractionation s mif and the late archean sulfur cycle

observed relationship between the turkana low level jet and boreal summer convection the turkana low level jet is a circulation feature over northern kenya that is not spatially well resolved in many observational datasets here high resolution trmm imerg and cmorph precipitation estimates along with the era5 and merra2 reanalyses are analyzed to better understand the controls on jet strength and the relationship between the jet and boreal summer rainfall variability in the jet exit region over south sudan the height of the south sudan wet season august is analyzed for 2000 2017 climatologically rainfall peaks over the jet exit region during the early morning hours coinciding with the jet maximum the strong jet enhances low level wind convergence and briefly shifts the low level zonal moisture gradient westward over the western south sudan rainfall peaks in the late afternoon evening in association with daytime heating the correlation between the daily turkana jet strength and precipitation indicates a significant negative relationship over eastern south sudan and over the ethiopian and east african highlands composite analysis of strong and weak jet periods reveal that the atmospheric conditions associated with a strong jet include enhanced low level ridging along the east african coast south of the equator resulting in a stronger height gradient in the turkana channel as well drier conditions over the channel and adjacent highlands these conditions support a stronger jet that is further enhanced by katabatic flow due to the stronger nighttime cooling over the highlands as well as enhanced dry air advection over the jet exit region thus while low level convergence may increase convection weakens over eastern south sudan due to the reduced atmospheric moisture content and weaker instability the opposite occurs for the weak jet case

pathways of organic carbon oxidation in three continental margin sediments we have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of denmark and norway rates of nh4 and sigma co2 liberation sediment incubations were used with o2 penetration depths to conclude that o2 respiration accounted for only between 3 6 17 4 of the total organic carbon oxidation dentrification was limited to a narrow zone just below the depth of o2 penetration and was not a major carbon oxidation pathway the processes of fe reduction mn reduction and sulfate reduction dominated organic carbon mineralization but their relative significance varied depending on the sediment where high concentrations of mn oxide were found 3 4 wt mn only mn reduction occurred with lower mn oxide concentrations more typical of coastal sediments fe reduction and sulfate reduction were most important and of a similar magnitude overall most of the measured o2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon we suspect that the importance of o2 respiration in many coastal sediments has been overestimated whereas metal oxide reduction both fe and mn reduction has probably been well underestimated

the enrichment of heavy iron isotopes in authigenic pyrite as a possible indicator of sulfate driven anaerobic oxidation of methane insights from the south china sea abstract sulfate driven anaerobic oxidation of methane so4 aom and organiclastic sulfate reduction osr in marine sediments commonly lead to the precipitation of authigenic pyrite with characteristic sulfur isotopic compositions yet no reports on the iron isotopic composition of so4 aom generated pyrite exist to the best of our knowledge and the processes controlling the distribution of iron isotopes during this biogeochemical process are not understood to investigate the mechanisms involved in iron isotope fractionation during so4 aom we analyzed the isotopic composition of authigenic pyrite from two seafloor sites hs148 and hs217 in the shenhu gas hydrate area south china sea the 56fe values of pyrite were found to vary from 0 349 to 0 267 in core hs148 and from 0 793 to 0 178 in core hs217 relative to average igneous rock with distinct trends towards higher values with increasing burial depth in both cores the 56fe values correlate positively with the extent of pyritization fepy fehr derived from a sequential iron extraction procedure osr was found to drive early diagenetic pyrite formation at shallow depth resulting in relatively 56fe depleted pyrite peaks in the extent of pyritization of 0 45 and 0 47 at ca 6 and 7 m below the sediment surface respectively correlate with high 56fe and high 34s values and are interpreted to reflect periods of enhanced so4 aom in paleo sulfate methane transition zones smtzs the great variability in iron isotopic composition and the observed trends are best explained as a function of the availability of dissolved sulfide reflecting sulfide formation by osr and so4 aom the successive conversion of reactive iron to pyrite via sulfidization apparently resulted in the enrichment of 56fe in the iron reservoir upon the preferential consumption of isotopically light iron leading to the formation of successively 56fe enriched pyrite especially at or in close proximity to the smtz therefore we hypothesize that the combined enrichment of heavy iron isotopes and 34s in authigenic pyrite is a possible tracer of so4 aom in modern and ancient sedimentary sequences

a newly found jet in north kenya turkana channel abstract on the basis of recent pilot balloon observations which have become available in northern kenya it is shown that there exists a strong southeasterly low level jet in the turkana channel which separates the ethiopian highlands the east african highlands the jet exists throughout the year with speeds exceeding 30 m s 1 60 kt on a number of occasions and sometimes exceeding 50 m s 1 100 kt during february and march the mean monthly winds based on the morning observations exceed 25 m s 1 50 kt the morning winds are stronger than afternoon winds presumably due to stronger vertical mixing and dilution of the jet maximum in the afternoon the hodograph turns in a counterclockwise direction from the lowest levels up to 0 75 km 2500 ft above ground level and sometimes even aloft up to 0 45 km 1500 ft above ground level the hodograph manifests some of the characteristics of the southern hemisphere ekman layer this jet is different from the now well known east african low level jet find

characterization of natural gas hydrate recovered from pearl river mouth basin in south china sea abstract china carried out a drilling campaign in the eastern part of the pearl river mouth prm basin and successfully obtained natural gas hydrate samples in 2013 laboratory based studies for the characteristics of gas hydrate were carried out on one of the hydrate bearing sediment sample the gas hydrate sample shows nodular occurrence with host matrix predominantly composed of silty sand and clay on the basis of the raman spectroscopic and x ray diffraction results the gas hydrate demonstrates a typical structure i si with cage occupancy of methane is more than 99 5 in large cage and 91 4 in small cage respectively corresponding to a hydration numbers of 5 90 by thermodynamic calculation the molecular compositions of hydrate bound gas suggest that the guest molecules are predominantly methane 99 9 with trace amount of ethane 0 04 and propane 0 01 the isotopic analysis indicates that the methane of hydrate bound gas of the studied drilling well in prm basin is a typical microbial origin

sulphur isotope geochemistry of pyrite from the upper cretaceous marshybank formation western interior basin abstract variations in the texture and sulphur isotopic composition of pyrite in the upper cretaceous marshybank formation are linked to depositional environment abundant framboidal and lesser euhedral pyrite precipitated in offshore marine rocks moderate quantities of pyrite also crystallized in brackish coastal plain rocks however in contrast to marine pyrite coastal plain pyrite is dominantly euhedral in texture reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration in the marine rocks pyrite 34s values range from 35 7 to 27 4 avg 4 8 canyon diablo troilite cdt pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition 49 8 to 10 6 cdt however isotopic values are often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments pyrite in coastal plain rocks is characterized by relatively high 34s values 4 2 to 35 5 avg 13 2 cdt while the overlying sideritized conglomerates have the lowest 34s values reported for cretaceous rocks from the western interior basin of north america 49 8 to 41 7 cdt very low 34s values which are only observed in the marine rocks are indicative of microbial sulphate reduction and pyrite formation in a sulphate replete i e open system higher 34s values up to 18 which were obtained for both the marine and coastal plain rocks are indicative of progressive pyrite crystallization in a sulphate limited i e closed system such conditions are expected in marine sediments as burial occurs and in brackish i e low sulphate sediments pyrite with very high 34s values 18 is common in the coastal plain rocks these high values are the result of influx of 34s enriched residual sulphide derived from overlying marine units a minor amount of 34s enriched pyrite is also present within and proximal to marine sedimentary unit h which was deposited at the same time as the coastal plain rocks we hypothesize that porewater containing 34s enriched residual sulphide flowed from the coastal plain and along unit h into the marine sediments thus producing anomalously high 34s pyrite

convectively coupled kelvin waves over tropical africa during the boreal summer structure and variability the structure and variability of convectively coupled kelvin waves during the boreal summer are explored using satellite observed brightness temperature data and ecmwf reanalyses kelvin wave activity is most prominent between the central and eastern pacific across africa and the indian ocean composite analysis shows that over sub saharan africa kelvin wave convection is peaked north of the equator while the dynamical fields tend to be symmetric with respect to the equator convectively coupled kelvin waves propagate faster over the pacific and western atlantic 24 m s 1 and slow down over tropical africa 14 m s 1 consistent with stronger coupling between the dynamics and convection over tropical africa the kelvin waves observed over africa generally propagate into the region from anywhere between the eastern pacific and the atlantic and decay over the eastern indian ocean basin results show marked interannual variability of kelvin wave activity over africa anomalously high kelvin wave variance tends to occur during dry years while low variance occurs during wet years african kelvin wave activity is positively correlated with sst anomalies in the equatorial east pacific the same warm sst anomalies that are favorable for enhanced kelvin wave activity suppress the mean rainfall over tropical africa via a more slowly varying teleconnection and associated subsidence a brief analysis of an intense kelvin wave in august 1987 a dry year shows a clear impact of the wave on convective development and daily rainfall over tropical africa this kelvin wave was also associated with a series of easterly wave initiations over tropical africa

abrupt rainfall transitions over the greater horn of africa observations and regional model simulations 1 we examine the yearly occurrence of a monsoon jump of approximately 20 latitude during the boreal spring and summer rainy seasons over the greater horn of africa gha this jump is in contrast with a simple model of a smoothly varying itcz over the region the rainfall jump is observed annually during april and may in three precipitation data sets and in regional climate model simulations using the psu ncar mesoscale model mm5 the mm5 simulations show the rainfall jump to be roughly coincident with abrupt circulation changes that occur as the somali jet develops during april may and june in particular the cross equatorial meridional branch of the somali jet forms along the east african coast in april bringing moisture and rainfall northward to the southern slopes of the ethiopian plateau this meridional branch forms well before the northern zonal branch of the jet which diverts moisture eastward from southern ethiopia and feeds the indian monsoon these results establish a framework for understanding the precipitation cycle over the gha and provide a foundation for improving subseasonal forecasts over drought prone regions of eastern africa

pyritic event beds and sulfidized fe oxyhydr oxide aggregates in metalliferous black mudstones of the paleoproterozoic talvivaara formation finland the paleoproterozoic 2 0 1 9 ga talvivaara formation of finland was deposited during the shunga event a worldwide episode of enhanced accumulation of organic rich sediments in the aftermath of the lomagundi jatuli carbon isotope excursion sulfidic carbonaceous mudstones in the talvivaara formation contain one of the largest known shale hosted nickel deposits in order to gain new insight into this shungian sedimentary environment sedimentological petrographical and in situ s and fe isotopic microanalyses were carried out on samples representing depositional and early diagenetic conditions the event bedded lithology with tidal signatures in the organic rich mudstones strongly indicates deposition from predominantly river delivered mud on a highly productive coastal area below storm wave base the riverine supply of phosphorus sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water the stratigraphic upper part of the talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds the pyrite beds were deposited by seaward excursions of the concentrated acidic fe rich river plume subsequent to droughts or dry seasons which led to intense pyrite precipitation upon mixing with euxinic waters s34 and fe56 values of the bedded pyrite median s34 10 3 and fe56 0 79 are consistent with the reaction of dissolved fe ii with h2s from bacterial sulfate reduction organic rich clayey fe monosulfide bearing granules were transported from the muddy estuary and enclosed in fe oxyhydr oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river delivered iron the isotopic composition of these presently pyrrhotitic inclusions median s34 3 3 and fe56 1 6 indicates microbial iron reduction the fe oxyhydr oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor their fe oxyhydr oxide matrix was replaced by pyrite median s34 5 8 and fe56 0 81 at shallow sediment depths with 34s and 56fe enriched porewater wavy crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation these results indicate episodic deposition at seasonal to multiannual time scales s34 and fe56 values in the studied fe sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early diagenetic origin finding no support for the previously proposed local hydrothermal activity in the talvivaara mudstones

authigenic carbonates from seeps on the northern continental slope of the south china sea new insights into fluid sources and geochronology seep carbonates represent excellent archives of seepage activities near seafloors in order to provide insights on sources of seeping fluids stable carbon and oxygen isotopes and sr 87 sr 86 ratios were applied to seep carbonates from shenhu sw dongsha and ne dongsha on the northern continental slope of the south china sea u th dating was selected to constrain the time involved in carbonate formation and the potential driving force the delta c 13 values of the shenhu and ne dongsha carbonates vary from 52 3 parts per thousand to 32 6 parts per thousand v pdb indicating that biogenic methane is the primary carbon source at both locations the delta c 13 values of the sw dongsha carbonates are much higher ranging from 18 8 parts per thousand to 11 parts per thousand v pdb suggesting that thermogenic methane is the predominant carbon source in this region a relatively deep origin of thermogenic methane for the sw dongsha seeps is also supported by somewhat lower sr 87 sr 86 ratios of the samples from 0 709025 to 0 709097 the delta o 18 values of the shenhu and ne dongsha carbonates vary from 1 4 to 5 1 v pdb while the delta o 18 values of the sw dongsha carbonates range from 2 5 to 3 4 v pdb many samples from shenhu and ne dongsha demonstrate an o 18 enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate u th ages of the shenhu carbonates show a large range from 330 ka to 152 ka bp and the ne dongsha carbonates have ages from 77 ka to 63 ka bp interestingly u th ages of most samples are either within times of sea level lowstands or correspond to periods when the sea level was falling taken together our data suggest that in the south china sea gas hydrate dissociation during sea level lowstands or falling stages resulted from reduced hydrastastic pressures which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor c 2013 elsevier ltd all rights reserved

fossil plants from the eocene london clay the use of pyrite textures to determine the mechanism of pyritization pyritized twigs and roots from the eocene london clay of se england were studied to gain a better understanding of the process of pyritization by investigating pyrite textures in relation to cell type and quality of preservation highly polished sections and fractured surfaces taken from 124 specimens were examined using optical microscope and sem the latter equipped to map pyrite and carbon pyrite textures include microcrystalline framboidal massive polycrystalline and subhedral or euhedral forms the highest fidelity of preservation is always associated with microcrystalline pyrite precipitation on wall surfaces with subsequent infilling of cells with framboids or polyhedra preventing compression during burial but contributing nothing to actual ultrastructural preservation ultrastructurally parenchymatous cell walls are coalified whereas microcrystalline pyrite plus coalified material were observed within lignified cell walls in all four stages of pyritization are documented observations are interpreted in the light of recent experiments on pyritization of living material and the chemistry of pyrite formation in anoxic environments involving an aqueous and hence mobile fes cluster complex as a precursor the complexity of the fossilization process is reflected in the presence of different textures in adjacent cells of the same tissue this demonstrates the development of isolated chemical microenvironments as ph and eh vary in response to decay and mineralization and pyrite overgrowths within a cell indicate local microenvironmental changes through time

the characteristics of gas hydrates recovered from shenhu area in the south china sea abstract for the first time gas hydrate samples were recovered in the south china sea during a drilling campaign implemented in shenhu area in june 2007 laboratory based studies for the characteristics of gas hydrate were carried out on two hydrate bearing sediment samples observations found that gas hydrates occurred in the pores of fine grained sediments the hydrates were structure i based on the raman spectroscopic results the guest gas molecules in hydrate cages were predominantly methane 99 3 and trace amount of ethane 0 4 and propane 0 04 as determined with the method of gas chromatography as estimated from the intensities of raman peaks the cage occupancy rates were more than 99 in large cage and 86 in small cage respectively corresponding to a hydration number of 5 9

cold seep status archived in authigenic carbonates mineralogical and isotopic evidence from northern south china sea abstract cold seep carbonates are precipitated under high alkalinity conditions created by the anaerobic oxidation of methane in cold seep sites multiple ca mg carbonate phases are identified including aragonite low mg calcite lmc high mg calcite hmc protodolomite and dolomite these phases result from different conditions that are related with cold seep activities here we report on the relationship between the ca mg carbonate phases and the cold seep status authigenic carbonates were sampled from northern slope of south china sea carbon isotopic compositions of samples from shenhu area are lower than 40 indicating methane derived carbon the 13c values of samples from southwest sw taiwan area range from 30 to 20 which is the result of the mixture of methane carbon and seawater carbon carbonate phases were identified according to the composition and structure results samples from shenhu area are composed of protodolomite and hmc three zones were discovered from the center to the rim of the cross section of the tube like sample from sw taiwan area from the external to the internal zones the carbonate phases are hmc lmc and protodolomite hmc respectively the intensity of superstructure reflections of the protodolomite from shenhu area is stronger than that from sw taiwan area indicating higher mgco3 content based on the formation conditions of ca mg carbonates from lmc to dolomite those with higher mgco3 content are formed in more active cold seep environment according to the distribution of carbonate phases in each sample the cold seep flux was high in shenhu area and was sustained for a long time by contrast the flux in sw taiwan area was relatively low and not stable it once became higher but finally returned to low

fe isotope fractionation on fes formation in ambient aqueous solution iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments the sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient earth environments and it is likely that fe isotopes will too once fractionations for key processes are known we report the results of an experimental study of fe isotope fractionation on precipitation of fes synthetic mackinawite from excess aqueous fe ii solutions by addition of sodium sulfide solution at 2 40 c the results show a significant kinetic isotope effect in the absence of a redox process no detectable effect of temperature on the fractionation factor was observed the fe isotope fractionation for zero age fes is fe ii fes 0 85 0 30 across the temperature range studied giving a kinetic isotope fractionation factor of fe ii fes 1 0009 0 0003 on ageing the fes in contact with aqueous fe ii becomes progressively isotopically heavier indicating that the initial fractionations are kinetic rather than equilibrium from published reaction mechanisms the opportunity for fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua fe ii and aqueous sulfide complexes fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive fe source is available since fes aq is a key reactive component in natural pyrite formation kinetic fe isotope fractionations will contribute to the fe isotope signatures sequestered by pyrite subject to the relative rate of fes2 formation versus fes fe ii aq isotopic equilibration

coupled fe and s isotope evidence for archean microbial fe iii and sulfate reduction direct fossil evidence for early microbial life on earth is rare microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in earth9s history we present fe isotopic data for sedimentary pyrite from the 2 7 ga belingwe sedimentary basin in zimbabwe isotopically light fe and a remarkable covariation between fe and s isotopes provide strong evidence for coexisting fe and s reduction our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial fe reduction the covariation of fe and s isotopes may provide a useful new tracer of microbial evolution early in earth history

the impact of ocean deoxygenation on iron release from continental margin sediments in the oceans high nitrate low chlorophyll regions such as the peru humboldt current system and the adjacent eastern equatorial pacific1 primary productivity is limited by the micronutrient iron within the peruvian upwelling area bioavailable iron is released from the reducing continental margin sediments2 the magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones3 4 here we show that measurements of molybdenum uranium and iron concentrations can be used as a proxy for sedimentary iron release and use this proxy to assess iron release from the sea floor beneath the peru upwelling system during the past 140 000 years we observe a coupling between levels of denitrification as indicated by nitrogen isotopes trace metal proxies for oxygenation and sedimentary iron concentrations specifically periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column we attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides we argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present we therefore suggest that future deoxygenation in the peru upwelling area would be unlikely to result in increased iron availability whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply

mechanistic model simulations of the east african climate using ncar regional climate model influence of large scale orography on the turkana low level jet abstract the national center for atmospheric research regional climate model regcm is employed to study the dynamics of the turkana low level jet that lies between the ethiopian and the east african highlands and also investigate the mechanisms responsible for the observed dry conditions over the lake turkana basin that lies in the wider section of the turkana channel the role of the large scale orography and two other forcing factors namely the large scale monsoonal flow and the turkana channel depth are investigated in order to understand the kinematics of the jet the simulated patterns of the turkana easterly low level jet compares well with its observed characteristics strong winds are indicated in the channel throughout the study period of october to december with the wind speed decreasing in the middle and wider region of the channel a split in the jet core is also shown in the middle of the channel the level of maximum winds 11 m s 1 occurs in the layers 930 hpa and 650 hpa levels the

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

mineralogy and organic petrology of oil shales in the sangkarewang formation ombilin basin west sumatra indonesia abstract the ombilin basin is filled by late eocene to early oligocene marginal fan deposits brani formation and lacustrine shales sangkarewang formation unconformably overlain by a late oligocene to early miocene fluvial sequence sawahlunto and sawahtambang formations and capped by an early to mid miocene marine sequence ombilin formation significant oil shale deposits occur in the sangkarewang formation intercalated with thin laminated greenish grey calcareous sandstones x ray diffraction shows that the sediments consist mainly of quartz feldspar carbonates and a range of clay minerals together in some cases with minor proportions of sulphides evaporites and zeolites feldspar and non kaolinite clay minerals decrease up the sequence relative to kaolinite suggesting a changing sediment source as the basin was filled calcite thought to be mainly of authigenic origin is also more abundant in the middle and upper parts of the sequence the organic matter in the oil shales of the sequence is dominated by liptinite macerals particularly alginite mainly lamalginite and sporinite cutinite also occurs in some samples along with resinite and traces of bituminite the dominance of lamalginite in the liptinite components suggests that the material can be described as a lamosite samples from the sangkarewang formation have vitrinite reflectance values ranging between 0 37 and 0 55 these are markedly lower than the vitrinite reflectance for coal from the overlying sawahlunto formation 0 68 possibly due to suppression associated with the abundant liptinite in the oil shales fischer assay data on outcrop samples indicate that the oil yield is related to the organic carbon content correlations with xrd data show that with one exception the oil yield and organic carbon can also be correlated directly to the abundance of carbonate calcite and inversely to the abundance of quartz plus feldspar this suggests that the abundance of algal material in the lake sediments was preferentially associated with carbonate deposition high yields of oil are noted in some samples as a percentage of the organic carbon content this may indicate that partial generation of hydrocarbons from the material has already taken place in association with thermal maturation of the sangkarewang succession

tracing the paleo sulfate methane transition zones and h2s seepage events in marine sediments an application of c s mo systematics 1 microbially mediated anaebic oxidation of methane aom coupled with sulfate consumption within the sulfate methane transition zone smtz in marine sediments is a widely recorded biogeochemical reaction and has profound influence on the atmospheric ch4budget marine carbon cycle and composition of sediment pore fluids recognizing the paleo smtzs in the marine sediments rock records can throw light on the variation of paleo methane fluxes and occurrences of cold seep h2s ch4 events through geologic time here we present results from carbonate carbon pyrite sulfur and molybdenum analyses for two sediment cores overlying the methane hydrate deposits in the bay of bengal the results show intimate association of isotopically depleted carbonate carbon and enriched pyrite sulfur constraining the paleo smtz within the sediment column in addition anomalous enrichments of mo concentrations indicate hydrogen sulfide seepage events here we propose a geochemical tool using c s mo sytematics to decipher the paleo smtzs in marine sediments and rocks