carbon and nitrogen loss rates during aging of lake sediment changes over 27 years studied in varved lake sediment we used a collection of ten freeze cores of annually laminated varved lake sediment from nylandssjon in northern sweden collected from 1979 to 2007 to follow the long term loss of carbon c and

organic chemistry in a co 2 rich early earth atmosphere the emergence of life on the earth has required a prior organic chemistry leading to the formation of prebiotic molecules the origin and the evolution of the organic matter on the early earth is not yet firmly understood several hypothesis possibly complementary are considered they can be divided in two categories endogenous and exogenous sources in this work we investigate the contribution of a specific endogenous source the organic chemistry occurring in the ionosphere of the early earth where the significant vuv contribution of the young sun involved an efficient formation of reactive species we address the issue whether this chemistry can lead to the formation of complex organic compounds with co2 as only source of carbon in an early atmosphere made of n2 co2 and h2 by mimicking experimentally this type of chemistry using a low pressure plasma reactor by analyzing the gaseous phase composition we strictly identified the formation of h2o nh3 n2o and c2n2 the formation of a solid organic phase is also observed confirming the possibility to trigger organic chemistry in the upper atmosphere of the early earth the identification of nitrogen bearing chemical functions in the solid highlights the possibility for an efficient ionospheric chemistry to provide prebiotic material on the early earth

δ2h of water from fluid inclusions in proterozoic halite evidence for a deuterium depleted hydrosphere abstract 2h were measured in water present in tiny primary fluid inclusions trapped in mesoproterozoic 1 4 ga and neoproterozoic 0 8 ga halite crystals while the hydrogen concentrations range from 300 to 1500 ppm for the whole sample collection 2h values range from 74 to 54 vsmow for the mesoproterozoic halite sampled from the sibley group ontario canada and from 89 to 38 for the neoproterozoic halite from the browne formation officer basin australia the amount of evaporation required to precipitate halite is accompanied by a deuterium enrichment of 30 to 90 it means that the original aqueous solutions had 2h values comprised between 140 and 90 considering the estimated tropical paleolatitudes for both depositional environments surface waters were most likely significantly deuterium depleted relative to modern ones if such interpretation is partly at variance with the geological record it could be reconciliated with some modelling outputs of the long term water cycle indeed several mechanisms produce a progressive deuterium enrichment of the oceans those identified so far are the addition of deuterium rich extraterrestrial water cometary dust the loss of h during water photodissociation by ultraviolet radiation in the upper atmosphere and the isotopic fractionation that takes place during the hydroxylation of nominally anhydrous mantle rock forming minerals we also propose that the major process able to shift through time the 2 of the hydrosphere is the reduction of h2o into h2 during the serpentinization of mantle rocks at mid ocean ridges

high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

mauna loa volcano is not a methane source implications for mars 1 thirteen years of continuous atmospheric carbon dioxide and methane measurements at the mauna loa observatory in hawaii are used to determine the methane emission rate from the summit of mauna loa volcano we find no measurable methane emissions coming from the summit area with a 95 confidence upper limit of 9 t ch4 yr 1 recent studies have detected 10 ppb ch4 in the martian atmosphere requiring emissions of about 300 t ch4 yr 1 volcanic activity has been suggested as a source of abiogenic ch4 on mars either by magmatic degassing or reactions in hydrothermal fluids heated by a magma intrusion the most recent lava flows on mars 2 my ago are on the tharsis shield volcanoes which may still be active if mauna loa is a valid terrestrial analog our findings suggest that volcanic activity is not a significant source of methane to the martian atmosphere

geochemistry of sedimentary organic matter and trace elements in modern lake sediments from transitional karstic land sea environment of the neretva river delta kuti lake croatia this paper investigates recent sedimentation processes and geochemistry of sedimentary organic matter som and trace elements in a unique semi enclosed lacustrine sedimentation system in a transitional land sea environment of the neretva river delta the results have shown that the influence of the neretva river on the lake sedimentation processes is limited to short term flood events carrying terrigenous loads while the authigenic formation of nanostructured calcite is the primary source of sediments in the recent past the enhanced release of biogenic co2 into the pore water deriving from the decomposition of som acts as a major contributor to this process the occurrence of early formed syngenetic and diagenetic pyrite indicates the existence of oxygen depleted conditions in the investigated system in the recent past the input of terrigenous material was identified as the main source of trace elements pb cu zn sn in the lake sediments the distribution of redox sensitive elements mo tl v particularly depends on the oxygen deficiency and the formation of sedimentary pyrite the human impact in the study area is negligible although somewhat elevated concentrations of sn were found in the surface sediment layers finally the results obtained indicate that the kuti lake represents a unique lacustrine environment which can be viewed as an isolated sedimentological and biogeochemical system in the neretva river delta

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

mud volcanoes at the shelf margin of the east china sea abstract roughly circular sediment mounds varying from tens to hundreds of meters in diameter and from a few meters up to 40 m in height have been mapped for the first time at the shelf edge of the east china sea ecs high resolution seismic profiles together with swath bathymetric and acoustic data indicate that these features are common on the ecs outer shelf and on the upper western slope of the okinawa trough they are associated with unusually large pockmarks up to 40 m in depth and 600 m in diameter on seismic records bright spots phase inversions and other acoustic anomalies indicate that gas and or fluid escape plays an important role in the formation of these mounds they are often associated with normal faults that occur along the western slope of the okinawa trough these faults acting as conduits for migrating fluid based on regional stratigraphic correlation we propose that the source of mud and fluid is deeper older than 235 ka the seepage process is suspected to be very recent and probably still active based on the facts that 1 despite high sedimentation rate the mounds are not buried by recent sediments and 2 last glacial maximum deposits are reworked at the emplacement of the mounds carbonate cemented sediments and deep water reefs possibly associated with fluid seepage could be the origin of local patches of very high backscattering acoustic facies mapped with the swath bathymetric system

enhanced marine ch4 emissions to the atmosphere off oregon caused by coastal upwelling methane in surface waters and marine air off oregon 44 24 n 44 54 n 124 36 w 125 24 w was continuously surveyed in july 1999 during a high resolution survey after a period of steady winds from the north ch4 concentrations were high in the northeastern region near the shelf edge the highest ch4 concentrations were 2 5 times higher than equilibrium with the atmospheric partial pressure in contrast concentrations were near equilibrium in the western part of the survey area the hydrate ridge the increase in ch4 from southwest to northeast correlates with a drop in sea surface temperature sst from 16 5 c to 13 5 c toward the shelf edge the observed sst pattern was caused by summer upwelling off oregon the results suggest that ch4 derived from bottom sources near the shelf slope break and methane found in connection with shallow 100 300 m turbidity layers is transported to the surface by coastal upwelling which causes an enhanced net flux of ch4 to the atmosphere vertical profiles of the methane distribution on the shelf in october demonstrate the accumulation of methane introduced by shelf sources surface concentrations at these stations in october during nonupwelling conditions were lower than in july during upwelling and were only slightly oversaturated with respect to the atmosphere an acoustic doppler current profiler survey indicates that the observed trend cannot be attributed to a surface flow reversal in the area the low salinity waters in the core of the columbia river plume s 31 showed no enhanced ch4 concentrations the trend of higher ch4 concentrations at lower temperatures existed over the whole 17 day survey but large spatial and temporal variations existed the presence of methane sources in regions of coastal upwelling worldwide such as shallow seeps gas hydrates and intermediate nepheloid layers suggests that the enhancement of ch4 fluxes to the atmosphere by coastal upwelling occurs on a global scale

fluid migration and origin of a mud volcano in the northern apennines italy the role of deeply rooted normal faults introductionsurface seeps of gas oil and salinewaters areconsideredtobeindicatorsof deep seated hydrocarbon accumu lationandhavebeenused forover60years as a guide to hydrocarbonprospectivity a wide spectrum ofseepages belongs to di erent geologi cal settings e g camerlenghi etal 1992 1995 thrasher etal 1996 schulz etal 1997 kopf etal 1998 graue 2000 spectacular seepagescan be found in basins with highsedimentation rates that generateoverpressureandmuddiapirism gu liev and feyzullayev 1996 up dipcarrier beds and active tectonic fea tures act as channels for subsurface uidandlocalizesurfaceemissionsuptotens orevenahundred kilometresfrom the reservoir thrasher etal 1996 during migration the domi nant mechanism that changeshydrocarbon composition is phasepartitioning migration of hydrocar bons is then largely the result ofbuoyancy of the free phase formedduringthisprocess matthews 1996 eventually thegeologyofthebasinisthe key to interpreting complicated uidmigrationpathways manystud ies simply focus on the oil and gascomponents of the migrating uidrather than on the characteristics ofthewaterandmudthatareassociatedwithhydrocarbonmigrationandalsoextruded by spontaneous seeps eventhough water is invoked as beingresponsibleforatleastlighthydrocar bon removal klusman and saaed 1996 watersofdi erentprovenancecontributetotheascending uidsandtheir geochemical characteristics canhelp in understanding their evolutionduring diagenesis and migration e g white 1965 hanor 1987 holyshandto th 1996 thispaperpresentssomeanalytical data from a spontaneoussurface seep bearing saline water mud gas and oil at the foothills ofthe northern apennines the aimsare i to provide some insight intothedepthandoriginofthe uidsandthe nature of the seepage pathway and ii tode nethelossofsubsurface uids accumulation related to thegeologicalpattern geological setting and structuralcontrol of the regnano mudvolcanoin the external portion of the nor thern apennines spontaneous uidemissions are distributed along twobelts borgia etal 1986 nanni andvivalda 1999 minissaleetal 2000 the rst belt stretches along the poplainfoothills whilethesecondoneisinternal and runs nearly parallel tothe main apenninic divide fig 1 theregnanomudvolcanoconsistsofmud breccias and mud ows spread ing over an area of 0 5km

abiogenic methanogenesis during experimental komatiite serpentinization implications for the evolution of the early precambrian atmosphere abstract hydrothermal alteration of oceanic ultramafic rocks can produce significant abiotic methane as a byproduct however the abundance of peridotites is limited in the modern basaltic oceanic crust abiogenic methanogenesis was likely more productive in the early precambrian when komatiitic volcanism delivered a much larger proportion of ultramafic material to the crust it is therefore possible that hydrothermal alteration in the hadean and archean could have led to a higher methane flux to the atmosphere than today potentially influencing the average surface temperature of the early earth this hypothesis was tested in a series of experiments in which natural and synthetic komatiite powders were reacted with an aqueous solution containing isotopically labeled formic acid at 300 c and 350 bars over 2 months experiments containing the natural powder generated labeled methane 13 ch 4 at a rate 5 8 faster than concurrent experiments containing the synthetic komatiite a similar difference was also observed between the natural komatiite and previous experiments on olivine serpentinization at the same conditions faster rates of methanogenesis in the natural komatiite experiments could be due to catalysis by magnesiochromite which was present only in the natural komatiites however this was ruled out because parallel experiments containing additional magnesiochromite failed to increase methane yields the most likely catalysts of the remaining candidate minerals were deduced to be ni sulfides common accessory minerals in komatiites that have previously been shown to accelerate organic reactions because ni sulfides were particularly abundant in archean komatiites the experiments imply that methane yields during seafloor alteration were higher on the early earth than today this inference in combination with the assumptions of faster crustal production rates and greater volumetric proportions of ultramafic lithologies leads to the conclusion that methane flux from serpentinization of an archean komatiite bearing oceanic crust was likely higher than that from metamorphism of the modern oceanic crust incorporation of this result into existing geodynamic and atmospheric models suggests that alteration of the archean seafloor may have contributed to the regulation of temperature of the paleoatmosphere however additional sources of greenhouse gases are required to solve the faint young sun paradox for the early archean

constraining methane release due to serpentinization by the observed d h ratio on mars it has been suggested that mars atmospheric ch4 could be produced by crustal hydrothermal systems the two most plausible mechanisms proposed so far not exclusive from each other are homogeneous formation by fluid rock interaction during magmatic events and serpentinization of ultramafic rocks the first goal of the present paper is to provide an upper limit on the release rate of serpentinization derived ch4 due to the release of numerous h2 molecules together with one ch4 molecule followed by thermal escape of all released h atoms to space and subsequent h isotopic fractionation even a relatively modest serpentinization derived ch4 release acting over geological time scales may result in a significant enrichment of d wrt h in mars cryo hydrosphere including atmosphere polar caps and subsurface reservoirs by assuming that the ch4 release rate has been proportional to the volcanic extrusion rate during the last 4 billion years we calculate the present d h ratio resulting from the crustal oxidation due to serpentinization including the additional effect of sulfur oxidation we show that this rate doesn t exceed 20 within a factor of 2 of the estimated present value of the ch4 release rate if not the present d h ratio on mars would be larger than observed 5 smow this result suggests that either the production of ch4 is sporadic with a present release rate larger than the average rate or there are other significant sources of ch4 like homogeneous formation from mantle carbon degassing or bacterial activity second assuming further that most of the h isotopic fractionation observed today is due to serpentinization we show that a 400 m thick global equivalent layer of water may have been stored in serpentine since the late noachian this result doesn t depend on the chemical form of the released hydrogen h2 or ch4 such a quantity is generally considered as the amount required for explaining the formation of valley networks on mars serpentinization therefore appears as a potentially efficient sink of water on mars much more efficient than o escape for having removed large amounts of water from the hydrosphere

pretoria saltpan crater impact origin confirmed the origin of the well preserved 1 13 km diameter pretoria saltpan crater in south africa has been debated throughout this century the structure of this pleistocene crater resembles that of other simple bowl shaped impact craters but the presence of volcanic intrusive rocks along the crater rim has suggested a cryptovolcanic origin in 1989 a drill core from the crater became available the core was studied in detail to establish a paleoenvironmental record for the mid latitudes of the southern hemisphere by analyzing the undisturbed crater sediments that had accumulated since formation of the crater and to determine the origin of the crater the discovery of shock metamorphosed quartz and feldspar fragments melt breccia and siderophile element enriched glasses and sulfide spherules in crater breccia deposits provides clear evidence for the impact origin of the pretoria saltpan crater

a review of nitrogen isotopic alteration in marine sediments key points use of sedimentary nitrogen isotopes is examined on average sediment 15n 14n increases approx 2 per mil during early burial isotopic alteration scales with water depth abstract nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record however bulk sedimentary nitrogen isotope ratios which can be determined routinely and at minimal cost may be altered during burial and early sedimentary diagenesis particularly outside of continental margin settings the causes and detailed mechanisms of isotopic alteration are still under investigation case studies of the mediterranean and south china seas underscore the complexities of investigating isotopic alteration in an effort to evaluate the evidence for alteration of the sedimentary n isotopic signal and try to quantify the net effect we have compiled and compared data demonstrating alteration from the published literature a 100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that at sites located off of the continental margins an increase in sediment 15n 14n occurs during early burial likely at the seafloor the extent of isotopic alteration appears to be a function of water depth depth related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of n isotopic alteration moreover the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global n cycle

separating the influences of diagenesis productivity and anthropogenic nitrogen deposition on sedimentary δ15n variations abstract the stable isotopic composition of nitrogen 15n in organic lake sediments is frequently used to infer changes in the source or cycling of n prior to sedimentation however diagenetic processes that occur after sedimentation can systematically alter the primary isotopic signal recorded in sedimentary organic matter and must be accounted for in order to detect changes in the 15n of nitrogen inputs to the sediment surface here we present a null model that estimates the diagenetic effect on sedimentary lake 15n records the model was tested using lake sediment cores from relatively pristine alpine lakes in the wind river and teton ranges of wyoming usa model inferred increases in productivity were tested against independent productivity proxies diatom accumulation rates and the planktonic to benthic ratio and inferred changes in anthropogenic nitrogen contribution were validated with records of atmospheric nitrogen deposition from the nearby fremont glacier wyoming diagenetic overprinting significantly altered sediment 15n profiles and the degree of alteration was not constant through time of the cores analyzed 30 70 of the variability can be explained by diagenesis alone with the remainder explained by either a change in productivity or a change in the isotopic composition of the source material our null model of isotopic fractionation proved to be successful at separating the diagenetic overprinting from other causes of isotopic shifts thereby providing environmental scientists with an analytical tool to partition the effects of diagenesis and environmental change on sedimentary 15n values

geologic evidence for archean atmospheric and climatic evolution fluctuating levels of co2 ch4 and o2 with an overriding tectonic control the 3 5 3 2 ga sedimentary record shows evidence of surface temperatures of 70 15 c nahcolite nahco 3 as a primary evaporitic mineral and an aggressive weathering regime even in the absence of land vegetation these features are best explained by a mixed ch 4 and co 2 atmospheric greenhouse in which ch 4 co 2 ratios were 1 and pco 2 was at least 100 1000 times the present value and perhaps as high as several bars the formation of large areas of continental crust at 3 2 3 0 ga including the kaapvaal and pilbara cratons resulted in the gradual depletion of atmospheric co 2 through weathering by 2 9 2 7 ga declining pco 2 was associated with climatic cooling and siderite free soils transitory ch 4 co 2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric ch 4 reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 c depleted organic c surface temperatures of 60 c after 2 9 ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes eventual lowering of new continental blocks by erosion reduced loss of atmospheric co 2 due to weathering and continued long term tectonic recycling of co 2 resulted in rising pco 2 and decreasing ch 4 co 2 ratios in the later archean and eventual reestablishment of a mainly co 2 greenhouse similar events may have been repeated in the latest archean and earliest proterozoic but gradually rising production of o 2 effectively kept ch 4 co 2 ratios to 1 at this time

methane emission from the mud volcanoes of sicily italy 1 mud volcanoes represent the largest expression of natural methane release into the atmosphere however the gas flux has never been investigated in detail methane output from vents and diffuse soil degassing is herewith reported for the first time measurements were carried out at 5 mud volcano fields around sicily italy each mud volcano is characterized by tens of vents and bubbling pools in the quiescent phase methane emission from single vents ranges between 0 01 and 6 8 kg day diffuse soil leakage around the vents is in the order of 102 104 mg m 2 d 1 an exceptional flux of 106 mg m 2 d 1 was recorded close to an everlasting fire soil ch4 flux is positive even at large distances from the mud volcano fields suggesting a diffuse microseepage over wider areas a total of at least 400 tons ch4 per year can be estimated over the area investigated alone 1 5 km2

geochemical and mineralogical characterization of sediments from lake futalaufquen 42 8 s andean patagonia to evaluate their potential as paleoclimatic proxies lake sediments are key archives for paleoenvironmental investigation as they provide continuous records of the depositional history of the lake and its watershed lake futalaufquen 42 8 s is an oligotrophic waterbody located in los alerces national park in the andes of northern patagonia south america a sedimentary sequence covering 1600 years was recovered to analyze the potential for paleoenvironmental reconstructions of the last millennia integration of different geochemical and mineralogical parameters and comparison with climatic reconstructions from other patagonian records give clues for the identification of a warm period around ad 800 1000 associated with the medieval climatic anomaly the high frequency of tephra layers beginning in the mid sixteenth century precludes identification of the little ice age recorded in northern patagonia as a cold period from the fourteenth to the eighteenth century furthermore the parameters analysed do not provide evidence of late twentieth century global warming however zn deposition a long distance atmospheric transport process of anthropogenic origin was identified during the last century

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

mud volcanoes in deepwater nigeria abstract detailed study of 3d seismic data from deepwater nigeria has revealed the presence of features interpreted to be mud volcanoes they occur in an upper slope environment seen as 1 2 km circular features at the seabed seabed cores from the mud volcanoes contain oil gas and sand shale clast content richer than the seabed background pliocene fossils have been identified in the cores demonstrating transport of material from depth the features show a high seabed seismic amplitude above a chimney of chaotic seismic reflections and data wipe out the mud volcanoes in the study area shows two distinct clusters located over deeper structural culminations four active mud volcanoes area 1 are located above a rollover anticline in the central part of the area cuspate listric faults reach the seabed on the up slope side of these mud volcanoes four abandoned mud volcanoes have also been identified with progressively older ages towards the crest of the underlying structure these abandoned features are associated with an extremely chaotic seismic signature a combination of over pressured carrier beds and low integrity top seals are believed to be responsible for the formation of these mud volcanoes it is further believed that gas expansion subsequent to seal failure was the main driving force for what must have been violent eruptions the long lived mud volcano activity over the deep structural closure suggests a plumbing system that focuses on escaping compaction water and hydrocarbons through time in the south west of the study area another cluster of five mud volcanoes is located above a shale diapir the seabed expression of the members of this cluster is more varied some show a positive relief at the seabed while others show circular depressions the largest feature represents an assemblage of many smaller mud volcanoes with a common root system their seismic expression is also different from that of area 1 with a well defined sediment wedge at surface and a shallower root system these features are believed to represent a less violent type of mud volcano characterised by ductile flow tectonic stress due to growth of the underlying diapir is thought to have played an important role during eruption in addition to focused methane escape and the low mechanical strength of the overburden

evidence for extreme floods in arid subtropical northwest australia during the little ice age chronozone ce 1400 1850 abstract here we report a 2000 year sediment sequence from the fortescue marsh martuyitha in the eastern pilbara region which we have used to investigate changing hydroclimatic conditions in the arid subtropics of northwest australia the pilbara is located at the intersection of the tropical indian and pacific oceans and its modern rainfall regime is strongly influenced by tropical cyclones the intertropical convergence zone itcz and the indo pacific warm pool we identified four distinct periods within the record the most recent period p1 ce 1990 present reveals hydroclimatic conditions over recent decades that are the most persistently wet of potentially the last 2000 years during the previous centuries p2 ce 1600 1990 the fortescue marsh was overall drier but likely punctuated by a number of extreme floods which are defined here as extraordinary strongly episodic floods in drylands generated by rainfall events of high volume and intensity the occurrence of extreme floods during this period which encompasses the little ice age lia ce 1400 1850 is coherent with other southern tropical datasets along the itcz over the last 2000 years suggesting synchronous hydroclimatic changes across the region this extreme flood period was preceded by several hundred years p3 ce 700 1600 of less vigorous but more regular flows the earliest period of the sediment record p4 ce 100 700 was the most arid with sedimentary and preservation processes driven by prolonged drought our results highlight the importance of developing paleoclimate records from the tropical and sub tropical arid zone providing a long term baseline of hydrological conditions in areas with limited historical observations

saltpan impact crater south africa geochemistry of target rocks breccias and impact glasses and osmium isotope systematics abstract the pretoria saltpan crater is a well preserved 220 000 year old 1 13 km diameter simple impact crater the crater was formed in nebo granites of the bushveld complex some minor intrusions thought to be younger than the nebo granite are present at the crater and have earlier been believed to support a volcanic origin of the structure but recent geological studies showed them to be part of the regional geology and of proterozoic age we studied the petrology and geochemistry of fourteen target granite samples three suevitic breccias nine intrusive rocks as well as melt agglutinates handpicked impact glass fragments and sulfide spherules from the saltpan impact crater unconsolidated suevitic breccias recovered from different depths in the crater were found to contain abundant evidence of shock metamorphism the target rock granites show only limited compositional variability the major and trace element composition of the bulk breccia is very similar to that of average basement granite impact glass fragments recovered from the unconsolidated suevitic breccia have a cipw normative composition similar to that of the basement granites no evidence for admixture from any of the minor intrusions was found the similarity of trace element abundances and ratios and ree patterns between impact glasses and granites favors derivation of the glasses from the granites the impact glass fragments show considerable enrichments of mg cr fe co ni and ir compared to the basement granites the abundances of these elements in the glasses after correction for indigenous concentrations can be explained by admixture of about 10 of a chondritic component high ir concentrations 100 ppb have been found in sulfide spherule samples which may complement the lower ir abundances in the glasses and could indicate some fractionation during impact re os isotopic studies were applied to further investigate the presence of a meteoritic component in the suevitic breccia the target granite shows very low osmium abundances of about 7 ppt and high 187 os 188 os ratios of about 0 72 that would be expected for old continental crust in contrast the breccia samples were found to have much higher osmium abundances 80 ppt and lower 187 os 188 os ratios of about 0 205 these values can be explained by mixing of target rocks with a chondritic component

microbial utilization of abiogenic carbon and hydrogen in a serpentinite hosted system abstract mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling global geochemical cycles and microbial activity serpentinite hosted hydrothermal activity is exemplified by the lost city hydrothermal field 30 n mid atlantic ridge where fluid rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen methane c 2 alkanes and formate such systems have been proposed as possible analogs to the early earth environments that gave rise to the first biochemical pathways thus characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important up to 50 of the microbial biomass is synthesized from mantle carbon conversely mantle carbon contributes only 10 of the biomass in areas with minimal sulfate reduction we attribute this difference to greater incorporation of formate or methane by the dominant microbial species the lost city methanosarcinales in locations where sulfate reducers are able to facilitate this assimilation the ability of autotrophic communities at lost city to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments

a review of the geochemistry of methane in natural gas hydrate the largest accumulations on earth of natural gas are in the form of gas hydrate found mainly offshore in outer continental margin sediment and to a lesser extent in polar regions commonly associated with permafrost measurements of hydrocarbon gas compositions and of carbon isotopic compositions of methane from natural gas hydrate samples collected in subaquatic settings from around the world suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of co2 from sedimentary organic matter typically these hydrocarbon gases are composed of 99 methane with carbon isotopic compositions 13cpdb ranging from 57 to 73 in only two regions the gulf of mexico and the caspian sea has mainly thermogenic methane been found in gas hydrate there hydrocarbon gases have methane contents ranging from 21 to 97 with 13c values ranging from 29 to 57 at a few locations where the gas hydrate contains a mixture of microbial and thermal methane microbial methane is always dominant continental gas hydrate identified in alaska and russia also has hydrocarbon gases composed of 99 methane with carbon isotopic compositions ranging from 41 to 49 these gas hydrate deposits also contain a mixture of microbial and thermal methane with thermal methane likely to be dominant published by elsevier science ltd

the origin and cycling of particulate and sedimentary organic matter and nitrate in lake superior abstract the elemental and isotopic composition of water column particulate and sedimentary organic matter and nitrate in lake superior was determined to assess the origin and cycling of these materials the 15 n and 13 c of sedimentary organic matter and suspended particles at three stations were consistent with an origin primarily from autochthonous production the 15 n of seston was controlled by a balance between the isotope effects associated with nitrate uptake and microbial degradation the ratio of chlorophyll fluorescence to light hindrance 100 transmittance was used in this study as an indication of the relative composition of recently produced photosynthetic vs refractory and non photosynthetic materials such as bacteria or microzooplankton chlorophyll fluorescence to light hindrance cf lh ratios were greatest within the region of the thermocline at the shallow station and lowest in the near bottom waters of the unstratified deep station these changes in cf lh indicated a predominance of recently produced photosynthetic material in surface stratified waters and an increase in refractory or non photosynthetic material at the deepest station relative to the shallow station seston at the deepest station was characterized by the highest 15 n value of the three stations suggesting that degradation bacterial growth and or an enrichment in microzooplankton resulted in an increase in the 15 n content of seston seston at the shallowest station was characterized by high cf lh ratios and low 15 n values suggesting a greater relative contribution of labile material and an influence of an isotope effect during nutrient assimilation suspended particles in the benthic nepheloid layer were characterized by marked depletions in 13 c and 15 n relative to seston and sedimentary organic matter and indicated a unique origin for this material possibly from recent primary production the 15 n and 13 c of particles within the sediment boundary layer were intermediate those of sediments and nepheloid layer particles and were suggestive of an origin from the mixing of these two materials nitrate in lake superior was characterized by the lowest 15 n reported for an aquatic environment average of 4 1 these low 15 n values and large input of water from precipitation directly to the lake surface suggests that much of the nitrate in lake superior is derived from atmospheric deposition

global rates of mantle serpentinization and h2 production at oceanic transform faults in 3 d geodynamic models previous studies have estimated that mantle serpentinization reactions generate h2 at a rate of 1010 1012mol per year along the global mid ocean ridge mor system here we present results of 3 d geodynamic simulations that predict rates of additional mantleserpentinization and h2 production at oceanic transform faults otf we find that the extent and rate of mantle serpentinization increases with otf length and is maximum at intermediate slip rates of 5 to 10 cm per year the additional global otf relatedproduction of h2 is found to be between 6 1 and 10 7 x 1011mol per year which is comparable to the predicted background mor rateof 4 1 15 0 x 1011mol h2 yr this points tooceanic transform faults as potential sites of intense fluid rock interaction where chemosynthetic life could be sustained by serpentinization reactions

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

gas hydrates in the ginsburg and yuma mud volcano sediments moroccan margin results of chemical and isotopic studies of pore water abstract two active mud volcanoes on the atlantic continental margin of morocco were sampled during a 1999 expedition on the r v professor logachev the active mud volcanoes named yuma and ginsburg are each about 4 km in diameter rise 150 250 m above the seafloor and are the first active methane related mud volcanoes identified in this region gravity cores from the crest of the active mud volcanoes recovered methane hydrates clathrates thirty five pore water samples were extracted and analyzed from seven gravity cores taken in and around the active mud volcanoes estimated gas hydrate content in mud volcano sediments is 2 11 by volume and 4 19 by pore space chloride ion concentrations of the original mud volcano water are similar to those of surrounding seawater but are isotopically different the water is very high in oxygen to 35 and light in hydrogen to 70 pore water with these features has not been previously described in the published literature

a note on the fluxes of abiogenic methane and hydrogen from mid ocean ridges the concentrations of methane and hydrogen in hydrothermal vent fluids issuing from mid ocean ridges tend to fall into two groups one with high concentrations of these gases in ultramafic hosted vent fields and a second group with relatively lower concentrations in basalt hosted vent fluids ultramafic hosted systems however appear to be restricted to slow spreading ridges and constitute only a fraction of the hydrothermal systems found there in this note the hydrothermal fluxes of methane and hydrogen have been calculated by estimating the percentages of the total subsurface hydrothermal circulation that circulate through each type of host rock even though the percentage of the total subsurface flow that is affected by serpentinization appears to be rather small 8 it still appears that this process produces about 70 of the total mid ocean flux of these gases the total production of methane and hydrogen is calculated to be about 20 x 10 9 mol yr 1 and 190 x 10 9 mol yr 1 respectively the hydrogen flux is comparable to that most recently calculated on the basis of the rate of hydration of mantle rock in newly formed crust and the stoichiometry of the serpentinization reaction this suggests that except for the production of methane a major portion of the hydrogen produced in the subsurface is not consumed before venting

gas hydrate and mud volcanoes on the southwest african continental margin off south africa widespread occurrence of bottom simulating reflectors bsrs has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the orange river delta probably indicating the presence of large quantities of gas hydrate in this area this report is the first to show the presence of bsrs on seismic records on the southwest african continental margin south of the walvis ridge another remarkable feature in the area is the occurrence of a large number of mud volcanoes the distribution of the bsrs and the location of the mud volcanoes are controlled by the locations of active faults the gas hydrate in this region may consist of a mixture of microbial and thermogenic gas whereas much of the gas flowing through the mud volcanoes probably originated from deep seated aptian source shales

isotope fractionation during nitrogen remineralization ammonification implications for nitrogen isotope biogeochemistry abstract the 15 n 14 n isotopic ratio of marine particulate organic matter usually increases as a consequence of preferential 14 n remineralization ammonification during decay in the oxic water column and at the oxic seafloor however a fractionation factor 15 e of sediment ammonification in natural environments has not been determined yet here the 15 e of sedimentary ammonification is calculated in a set of seven sapropel containing sediment cores two subrecent sediment cores and four sediment trap deployments from the eastern mediterranean sea data clearly reveal rayleigh type isotope fractionation and a fractionation factor 15 e between 1 43 and 2 3 hence sediments and probably sinking particulate organic matter as well can be important sources of isotopically depleted reactive nitrogen the knowledge of 15 e further enables to reconstruct paleo export fluxes from sediment cores if original and actual 15 n as well as n accumulation rates are known

millennial scale decline in coho salmon abundance since the middle holocene in a coastal oregon watershed usa the population size of anadromous salmon in the pacific northwest is strongly influenced by decadal variation in watershed and oceanographic conditions and therefore should also be influenced by larger magnitude millennial scale variations in these conditions we studied 15 n of bulk organic matter in lake sediment from woahink lake oregon as a proxy of marine derived nutrients mdn from spawning coho salmon we compared this site to a control lake with a natural barrier to salmon migration from 7 5 to 5 4 ka a dune was emplaced breached and emplaced again forming woahink lake in a former estuary 15 n decreased steadily since 5 4 ka at woahink but not at the control lake 15 n reached a minimum just prior to anthropogenic nutrient loading which caused an increase in 15 n thus precluding a comparison with the historical decline in salmon abundance a mixing model of lake water nitrate developed to explore alternate scenarios for the observed range of 15 n could not explain these results without invoking mdn input from at least several hundred salmon annually our results show a previously unreported pattern of a millennial scale decline in salmon that has plausible linkages to parallel changes in ocean circulation and productivity

methane and climate during the precambrian era abstract the sun was substantially less bright in the distant past yet earth s surface temperature remained above freezing higher concentrations of the greenhouse gases co 2 and ch 4 were likely responsible for keeping the early climate warm ch 4 concentrations of 1000 ppm or higher are predicted for the late archean paleoproterozoic atmosphere prior to the rise of o 2 photolysis of this ch 4 may have created an optically thin organic haze during much of this time the rise of o 2 at 2 3 ga eliminated most of the methane and probably triggered the paleoproterozoic glaciations ch 4 concentrations could have remained elevated throughout much of the proterozoic however as a consequence of low concentrations of dissolved o 2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis

decadal diagenetic effects on δ13c and δ15n studied in varved lake sediment to assess the long term 27 yr effects of sediment aging on stable carbon and nitrogen isotope values 13c and 15n we used a collection of eight freeze cores of annually laminated varved lak

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

measurements of the fate of gas hydrates during transit through the ocean water column 1 we report on controlled experiments to document the fate of naturally occurring methane hydrate released from the sea floor 780 m 4 3 c by remotely operated vehicle rov disturbance images of buoyant sediment coated solids rising 0 24 m s from the debris cloud soon revealed clear crystals of methane hydrate as surficial material sloughed off decomposition and visible degassing began close to the predicted phase boundary yet pieces initially of 0 10 m size easily survived transit to the surface ocean smaller pieces dissolved or dissociated before reaching the surface ocean yet effectively transferred gas to depths where atmospheric ventilation times are short relative to methane oxidation rates

abiotic hydrogen h2 sources and sinks near the mid ocean ridge mor with implications for the subseafloor biosphere free hydrogen h2 is a basal energy source underlying chemosynthetic activity within igneous ocean crust in an attempt to systematically account for all h2 within young oceanic lithosphere

preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis abstract the carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical processes in marine and lacustrine environments in order to reconstruct past environmental changes from sedimentary organic matter it is crucial to consider potential alteration of the primary isotopic signal by bacterial degradation in the water column and during early diagenesis in the sediments in a series of oxic and anoxic incubation experiments we examined the fate of organic matter and the alteration of its carbon and nitrogen isotopic composition during microbial degradation the decomposition rates determined with a double exponential decay model show that the more reactive fraction of organic matter degrades at similar rates under oxic and anoxic conditions however under oxic conditions the proportion of organic matter resistent to degradation is much lower than under anoxic conditions within three months of incubation the 13c of bulk organic matter decreased by 1 6 with respect to the initial value the depletion can be attributed to the selective preservation of 13c depleted organic compounds during anoxic decay the 15n values continuously decreased to about 3 below the initial value the decrease probably results from bacterial growth adding 15n depleted biomass to the residual material in the oxic experiment 15n values increased by more then 3 before decreasing to a value indistinguishable from the initial isotopic composition the dissimilarity between oxic and anoxic conditions may be attributed to differences in the type timing and degree of microbial activity and preferential degradation in agreement with the anoxic incubation experiments sediments from eutrophic lake lugano are on average depleted in 13c 1 5 and 15n 1 2 with respect to sinking particulate organic matter collected during a long term sediment trap study

hydrocarbon gas seeps of the convergent hikurangi margin north island new zealand abstract two hydrocarbon gas seeps located about 13 km apart have distinctive molecular and isotopic compositions these seeps occur within separate tectonic melange units of narrow parallel trending and structurally complex zones with incorporated upper cretaceous and palaeogene passive continental margin deposits which are now compressively deformed and imbricated along the convergent hikurangi margin of north island new zealand at brookby station within the coastal high the seeping hydrocarbon gas has a methane ethane ratio of 48 and 13 c and d values of methane of 45 7 and 188 respectively relative to the pdb and smow standards within the complex core of the elsthorpe anticline at campbell station seep gas has a methane ethane ratio of about 12000 and the methane has 13 c and d values of 37 4 and 170 respectively the source of the gases cannot be positively identified but the gases probably originate from the thermal decomposition of organic matter in tectonically disturbed upper cretaceous and or lower tertiary sedimentary rocks of passive margin affinity and reach the surface by migration along thrust faults associated with tectonic melange the geochemical differences between the two gases may result from differences in burial depths of similar source sediment

gaia s breath global methane exhalations abstract methane ch 4 is the most abundant organic compound in the earth s atmosphere where it acts as a greenhouse gas and thus has implications for global climate change the current atmospheric ch 4 budget however does not take into account geologically sourced ch 4 seepage geological sources of ch 4 include natural macro and micro seeps mud volcanoes and other miscellaneous sources such as gas hydrates magmatic volcanoes geothermal regions and mid ocean ridges macro seeps contribute 25 tg teragrams ch 4 yr to the atmosphere whereas micro seepage contributes perhaps 7 tg ch 4 yr mud volcanoes emit 5 tg ch 4 yr and miscellaneous sources emit 8 tg ch 4 yr to the atmosphere thus the total contribution to the atmosphere from geological sources is estimated to be 45 tg ch 4 yr which is significant to the atmospheric organic carbon cycle and should be included in any global inventory of atmospheric ch 4 we argue that the atmospheric ch 4 global inventory of the interplanetary panel on climate change must be adjusted in order to incorporate geologically sourced ch 4 from naturally occurring seepage

implications of present day abiogenic methane fluxes for the early archean atmosphere 1 during earth s early history greenhouse warming by atmospheric methane helped to maintain elevated surface temperatures here we estimate the present day abiogenic ch4 flux generated by mineral alteration serpentinization at mid ocean ridges volcanic emissions and geothermal sources in addition we assess the impact that abiogenic methane may have had on greenhouse warming during the early prebiotic archean based on estimates of the rate of seafloor spreading and the degree of serpentinization within the oceanic crust the flux of methane generated by serpentinized lithosphere is calculated to be 1 35 mt ch4 y 1 while volcanic and geothermal sources are estimated to contribute 0 1 and 0 9 mt ch4 y 1 respectively furthermore it is shown that if atmospheric co2 partial pressures were above 0 01 bar the present day level of abiogenic methane production could have been sufficient to maintain above freezing surface temperatures during the archean the very high temperatures 70 c that have been suggested for the early archean however would have required extremely high methane fluxes or more likely greatly elevated atmospheric co2 levels