mineralogical and stable isotopic characterization of authigenic carbonate from a hydrocarbon seep site gulf of mexico slope possible relation to crude oil degradation abstract one of the characteristic features on the cold deep seafloor on atwater valley at block 425 1930 m water depth in the gulf of mexico is the occurrence of seepage of crude oil and hydrocarbon gases associated with thermogenic gas hydrate and authigenic carbonate deposits mineralogical petrographic and isotopic analyses revealed that the precipitation of these carbonates was related to the microbial oxidation of hydrocarbons carbonate deposits are composed of heavily oil stained small nodules to large cobbles of dense limestones of fine grained textures other carbonate fragments are composed of small intraclasts pellets and some bivalve shell fragments cemented together by aragonite cement xrd analysis and petrographic investigations revealed that the authigenic carbonate is predominantly composed of aragonite that occurs as microcrystalline matrix and cement microcrystalline sparitic and botryoidal acicular aragonite crystals are observed as void filling cement the 18o values of the authigenic carbonate 3 80 to 4 16 v pdb suggest that the precipitating fluid has equilibrated with the cold bottom seawater the calculated 18o values 0 18 to 0 54 smow of the precipitating fluid show subtle deviation from the values reported for the present bottom seawater this is attributed possibly to the precipitation from pore water with temperature ranged from 3 3 to 1 7 c which is less than the measured bottom seawater 4 c however the mixing of the marine pore water with some deep hydrocarbon rich brines or hydrate water would provide a source of 18o enrichment fluid for the precipitation of the authigenic carbonates based on the predominance of aragonite with botryoidal and clotted fabric the occurrence of pyrite and from the oxygen isotopic data it is plausible that the authigenic carbonates may have formed near the sediment water interface in an oxygen depleted environment with low temperature the narrow range of the 13c values of the authigenic carbonate 23 88 to 28 62 v pdb suggests that a single source of carbon is predominant based on the moderately low 13c values and the occurrence of spots of crude oil coatings of the carbonate samples in addition to the application of a carbon isotope mass balance model the carbon involving in the formation of the carbonates of the at 425 seep site could be mainly derived from microbial degradation of crude oil microbial oxidation of non methane thermogenic hydrocarbon gases such as ethane propane and isobutane would play some role as a source of carbonate carbon

polar water column stability abstract an expression is derived for the surface salt input needed to induce complete convective overturning of a polar water column consisting of 1 a layer of sea ice 2 a freezing temperature mixed layer 3 a pycnocline with linearly varying temperature and salinity and 4 deep water with fixed temperature and salinity this quantity has been termed the bulk stability by martinson the bulk stability is found to consist of three components the first two make up martinson s salt deficit and are the salt input needed to increase the density of the mixed layer and the pycnocline layer to that of the deep water the mixed layer stability and pycnocline layer stability respectively the third component is martinson s thermal barrier the potential for pycnocline heat to melt ice reducing the surface salinity it is found that when the pycnocline density gradient due to temperature offsets more than one half of that due to salinity the pycnocline layer stability is negative consequently it is poss

on the linkage between antarctic surface water stratification and global deep water temperature the suggestion is advanced that the remarkably low static stability of antarctic surface waters may arise from a feedback loop involving global deep water temperatures if deep water temperatures are too warm this promotes antarctic convection thereby strengthening the inflow of antarctic bottom water into the ocean interior and cooling the deep ocean if deep waters are too cold this promotes antarctic stratification allowing the deep ocean to warm because of the input of north atlantic deep water a steady state deep water temperature is achieved such that the antarctic surface can barely undergo convection a two box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts such as the bounds on the operation of this loop the model illustrates the possible dominating influence of antarctic upwelling rate and antarctic freshwater balance on global deep water temperatures

volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern taiwan abstract the accretionary wedge of offshore southwestern taiwan contains abundant deposits of gas hydrate beneath the sea floor high concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available to understand temporal variation of methane venting in sediments over geologic time a 23 m long calypso piston core md05 2911 was collected on the flank of the yung an ridge pore water sulfate dissolved sulfide dissolved iron methane sedimentary pyrite acid volatile sulfide reactive iron organic carbon and nitrogen as well as carbonate 13 c were analyzed three zones with markedly different pyrite concentration were found at the study site unit i sediments 20 mbsf were characterized with a high amount of pyrite 251 380 mol g and a 13 c depleted carbonate unit ii sediments 15 20 mbsf with a low pyrite 15 43 mol g and a high content of iron oxide mineral and unit iii sediments the oscillation records of pyrite concentrations are controlled by temporal variations of methane flux with an abundant supply of methane to unit i and iii anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation no aom signal was found in unit ii characterized by typical organically limited normal marine sediments with little pyrite formation the aom induced pyrite formation near the smi generates a marked pyrite signature rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment

u th dating of carbonate nodules from methane seeps off joetsu eastern margin of japan sea abstract we performed u th radioactive disequilibrium analyses of carbonate nodules and sediment samples recovered from methane seep sites off joetsu of the eastern margin of japan sea to decipher the active period of the methane seep the carbonates contain 230th part of which is located in detritus such as silicate and organics at the time of precipitation the initial 230th renders accurate dating with u th radioactive disequilibrium method difficult we assessed the feasibility of correction using radioactive disequilibrium data of ambient sediment to overcome this difficulty a 230th 232th 234u 232th isochron drawn by three chips divided from a carbonate nodule pc05 04 50 passed through data points of local sediments we conclude that the problem of initial 230th can be resolved by measurements of local sediments results show that carbonate nodules include local sediment as impurities furthermore the results of trace element analyses such as rb zr nb ree pb and th also support the idea in all 18 carbonate samples were dated with correction of initial 230th using the mean value of local sediment in this study the u th correction ages show 12 35ka with an isochron age of 26 3ka results indicate that during the time interval of u th ages from 12ka to 35ka environmental conditions must have been favorable for enhanced methane flux through sediment the extensive methane flow period at 20ka accords with the lowest stand sea level during the last glacial age results of this study also suggest that u th ages of carbonate are useful as a reliable chronometer with regard to methane seep activation in order to acquire u th ages of carbonate at methane seep sites however it is important to evaluate the amount of initial 230th accurately using the value of sediment

sr isotopic compositions of cold seep carbonates from the south china sea and the panoche hills california usa and their significance in palaeooceanography 87sr 86sr ratios 13c and 18o values and concentrations of sr and mn in cold seep carbonates collected from the south china sea and the panoche hills california usa are determined in this study the carbon isotopic values of carbonates reflect different carbon sources carbonate samples with high sr concentrations and low mn sr ratios are considered to have very little or no influence by diagenetic alteration sr isotopes of seep carbonates from both the south china sea and the panoche hills record the information on the original seawater or pore water together with the published sr isotopic data of cold seep carbonates in italy we applied the sr isotope stratigraphy sis and sis lowess version 3 curve to calibrate the ages for the cold seep carbonates these ages are consistent with their sedimentary strata indicating that the cold seep carbonates with high sr concentration and low mn sr ratio are good candidates for dating by their sr isotopic compositions

evidence of intense methane seepages from molybdenum enrichments in gas hydrate bearing sediments of the northern south china sea abstract the application of molybdenum mo enrichment to sediments can potentially provide unique constraints on the methane seepage dynamics at continental margins we report herein elemental and isotopic measurements for authigenic carbonates and sediment samples in drill cores 95 m long site gmgs2 08 at 800 m water depth from gas hydrate bearing sediments in the northern south china sea five intervals that were impacted by methane seepages in the sediment profile were identified by the presence of strongly 13 c depleted carbonates 13 c values as low as 56 8 the carbonates often cement chemosynthetic bivalve shell fragments and have anomalous 18 o positive values these observations suggest that methane seepage was intense during those time intervals and that carbonate precipitation occurred in the uppermost sediments probably due to gas hydrate dissociation interestingly the five intervals were all characterized by strong mo enrichments we thus hypothesize that the presence of strong mo enrichments is a good indicator of past episodes of methane seepage such an environment results in sulfidic conditions that occur in a narrow zone in proximity to seawater close to the seafloor leading to resultant mo enrichments in associated sediments in contrast environments with low methane flux would lead to the distribution of mo throughout a wide sulfidic zone and consequently an absence of mo enrichment in the sediments the insights into the environment for mo enrichments at seeps have significant implications for tracing past methane seepages their intensities and even possible occurrences of gas hydrate dissociation

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

carbon and oxygen isotope analysis of hydrate associated oligocene authigenic carbonates using nanosims and irms abstract petrography and stable c and o isotope compositions of methane derived carbonates from the polish outer carpathians were analyzed two methods were employed in isotopic determinations conventional irms measurements on powdered bulk samples were performed with high precision errors 0 06 for 13c and 0 08 for 18o 1 whilst in situ measurements were made using nanosims with a high spatial resolution 5 m but with low precision errors 3 0 for 13c and 2 3 for 18o 2 the nanosims study proved very useful in identifying the sources of the fluids precipitating the carbonates they were formed predominantly by the anaerobic oxidation of biogenic methane aom the very fine spatial resolution of the analyses revealed highly variable 13c values even in individual crystals from extremely negative related to aom to positive related to methanogenesis me other methods including laser ablation and standard sims would fail to detect this variability and would not indicate the true sources of the fluids because they would show average values therefore any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the 13c values are extreme i e resulting either from aom or me unusual clast like druses from the carbonates were analyzed in detail with nanosims they are filled with strongly 13c depleted down to 54 and 18o enriched up to 6 fringe calcite which precipitated on the walls of the druses from heavy oxygen containing water released from the dissociation of gas hydrates the rocks can thus be termed clathrites the bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks precipitation of the carbonate cements started when hydrates were still present so they are a type of melt and seal fabric the recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane derived carbonates the isotopic results have demonstrated that nanosims can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements this provides a basis for the application of nanosims to tracing diagenetic pathways from various even complexly formed authigenic precipitates

repeated pulses of vertical methane flux recorded in glacial sediments from the southeast bering sea 1 there is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period in this study of two sediment cores from the southeast bering sea 700 m and 1467 m water depth we identify multiple episodes during the last glacial period of intense methane flux reaching the seafloor within the uncertainty of the radiocarbon age model the episodes are contemporaneous in the two cores and have similar timing and duration as dansgaard oeschger events the episodes are marked by horizons of sediment containing 13c depleted authigenic carbonate minerals 13c depleted archaeal and bacterial lipids which resemble those found in anme 1 type anaerobic methane oxidizing microbial consortia and changes in the abundance and species distribution of benthic foraminifera the similar timing and isotopic composition of the authigenic carbonates in the two cores is consistent with a region wide increase in the upward flux of methane bearing fluids this study is the first observation outside santa barbara basin of pervasive repeated methane flux in glacial sediments however contrary to the clathrate gun hypothesis kennett et al 2003 these coring sites are too deep for methane hydrate destabilization to be the cause implying that a much larger part of the ocean s sedimentary methane may participate in climate or carbon cycle feedback at millennial timescales we speculate that pulses of methane in these opal rich sediments could be caused by the sudden release of overpressure in pore fluids that builds up gradually with silica diagenesis the release could be triggered by seismic shaking on the aleutian subduction zone caused by hydrostatic pressure increase associated with sea level rise at the start of interstadials

description of a contourite depositional system on the demerara plateau results from geophysical data and sediment cores the demerara plateau belonging to the passive transform margin of french guiana was investigated during the iguanes cruise in 2013 the objectives of the cruise were to explore the poorly known surficial sedimentary column overlying thick mass transport deposits as well as to understand the factors controlling recent sedimentation the presence of numerous bedforms at the seafloor was observed thanks to newly acquired iguanes bathymetric high resolution seismic and chirp data while sediment cores allowed the characterization of the deposits covering the mass transport deposits modern oceanographic conditions were determined in situ using mooring monitoring over a 10 month period our data indicate the presence of a contourite depositional system along the demerara plateau most likely related to bottom current activity of the north atlantic deep water nadw however at the regional scale large longitudinal waves parallel the nadw flow their shape and orientation seem to be inherited from interactions between bottom currents and paleomorphologies expressed at the top of mass transport deposits their evolution is possibly driven by the intensity of bottom current activity that might have changed over time overall this work presents an integrated approach combining seismic and sedimentological evidence to study the processes at the origin of contourite formation in the demerara plateau other regional factors such as local slope values and slope instability also control sedimentation

stable isotopes and rare earth element compositions of ancient cold seep carbonates from enza river northern apennines italy implications for fluids sources and carbonate chimney growth abstract cold seep carbonates are remarkable fossil records of hydrocarbon seepage and related conduits in the present research we analysed the carbon and oxygen isotopes rare earth element ree and trace element content of carbonate chimneys and concretions in the pleistocene argille azzurre formation northern apennines italy strong 13 c depletions values as low as 40 3 observed in both carbonate concretions and chimneys indicate that anaerobic oxidation of biogenic methane played a major role in the formation of these carbonates slightly higher 13 c values mostly found in the outer layers of the carbonate chimneys reflect a contribution of inorganic carbon from seawater ree patterns reveal the carbonates formed under mostly anoxic pore water conditions combined ree and trace element data reveal the growth of chimneys occurred from the outside in with the outer chimney walls experiencing rapid growth excellent correlations between ce ce ree contents and la n sm n and y ho ratios observed in the chimney walls provide additional evidence for the influence of seawater in the formation of the chimneys meanwhile o isotopic compositions of the samples suggest an 18 o depleted fluid was involved in carbonate precipitation this study has major implications for tracking fluid sources of seep carbonates and patterns of carbonate chimney growth

horizontal circulation and volume transports in the weddell gyre derived from argo float data argo floats provide both hydrographic and trajectory data affording the opportunity to investigate surface to mid depth ocean dynamics here argo float data are used to determine the absolute geostrophic velocity field of the upper 50 2000 m of the weddell gyre from which the overall circulation pattern is investigated the weddell gyre plays a pivotal role in the modification of climate by advecting heat towards the antarctic ice shelves and by modifying the water masses that feed into the lowest limb of the global ocean overturning circulation warm deep water the source water mass that delivers heat to the weddell gyre is conveniently located within the upper 2000 m domain covered by the floats we investigate its volume transport as it circulates the gyre full depth volume transports are estimated by applying a quadratic function to extrapolate the relative dynamic height field component to the full ocean depth using ctd profiles from ship based surveys to determine an extrapolation error major new insights are provided by this study there is an established double gyre structure to the circulation with a strong eastern cell and a weaker western cell regional variation of the baroclinic component of the flow field is revealed indicating a northeast to southwest reduction in the baroclinic flow along with a strong meridional gradient of baroclinic flow along the northern limb of the gyre especially east of 25 w the zonal mean gyre strength away from the shelf edge is 32 5 sv 1 sv 1 106 m3 s 1 of which 13 3 sv are associated with the advection of warm deep water there is a considerable amount of recirculation within the gyre interior where water does not traverse the full zonal extent of the gyre the recirculation is stronger in the eastern cell of the observed double gyre structure the interior circulation cells partly explain the large variations in previous gyre strength estimates we provide an extensive review of previous estimates in context of the new results obtained

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

lipid biomarkers and their specific carbon isotopic compositions of cold seep carbonates from the south china sea abstract cold seep carbonates were studied from two locations dongsha and chiasian in the northern south china sea and on taiwan island lipid biomarkers and their stable carbon isotopic compositions extracted from these seep carbonate samples were analyzed in this study and the results pointed at the anaerobic oxidation of methane aom as responsible for carbonate formation distinct aom communities were indicated by isotopically highly depleted archaeal biomarkers with 13c values as low as 140 8 vpdb and bacterial biomarkers with 13c values as low as 128 6 vpdb which confirmed that biogenic methane was the dominant carbon source lipid biomarker distributions indicated that anme 2 archaea were prevalent in sample tvg3 c2g from dongsha and sample jx8 from chiasian in addition differences of the aom communities between the carbonates appeared to be caused by the respective biogeochemical environments anme 1 archaea tended to be abundant in high mg calcite carbonates and their formation might be linked to low methane fluxes in contrast anme 2 consortia were prevalent in aragonite carbonates and appeared to have prospered at sites with high methane supply our data imply that the mineralogy of the seep carbonates precipitated in the northern south china sea appears to be influenced by the distinct anme consortia which are induced by methane flux

massive vein filling gas hydrate relation to ongoing gas migration from the deep subsurface in the gulf of mexico abstract a gas hydrate mound that contains massive vein filling structure ii gas hydrate occurs on the upper continental slope 540 m water depth of the gulf of mexico southwest of the mississippi delta the mound is located in the green canyon gc block 185 adjacent to jolliet field in gc 184 jolliet field contains oil and gas that filled fault traps caused by salt deformation during late pleistocene holocene time in contrast to reservoir oil in jolliet field which shows bacterial oxidation effects the c 1 c 5 reservoir gas is unaltered by bacterial oxidation disassociated gas is assumed to have recently entered from the subsurface hydrocarbon system vertical migration of gas along faults is ongoing manifested on the sea floor by gas vents gas hydrate complex chemosynthetic communities and by a large gas plume that extends from the vents to the sea surface the isotopic properties of c 1 c 5 hydrocarbons from reservoirs gas vents and gas hydrate correlate closely although outcropping gas hydrate is transiently stable because of variations in seawater temperature the bulk of buried gas hydrate at gc 185 is stable and perhaps increasing in volume because of the copious gas flux the massive accumulation of gas hydrate at the gc 185 site is attributed to the gas that has recently entered the vents largely from jolliet field and to the synchronous activation of fault conduits allowing gas migration to the sea floor synchronous late gas charge and faulting could also explain the wide distribution of gas hydrate across the upper gulf slope

pseudofossils in relict methane seep carbonates resemble endemic microbial consortia pleistocene age methane seep carbonates from the eel river basin california contain aggregate like structures composed of tightly packed hollow spheres that morphologically resemble syntrophic archaeal bacterial consortia known to catalyze the anaerobic oxidation of methane aom tetragonal microstructures also present in the carbonates resemble seep endemic methanosarcinales cell clusters despite morphological similarities to the seep endemic microbes that likely mediated the authigenesis of eel river basin carbonates and sulfides detailed petrographic sem and magnetic microscopic imaging remanence rock magnetism laser raman and energy dispersive x ray spectroscopy suggest that these microstructures are not microfossils but rather mineral structures that result from the diagenetic alteration of euhedral fe sulfide framboids electron microscopy shows that during diagenesis reaction rims composed of fe oxide form around framboid microcrystalites subsequent dissolution of greigite or pyrite crystals leaves behind hollow cell like casings external molds a transformation that occurs on timescales of 100 kyr or less despite their superficial resemblance to morphologically distinctive extant microbes in local sediments the presence of acellular precursor grains as well as of partially altered transitional forms complicate the interpretation of these and other framboidal microstructures that have been reported from the rock record

rapid sea level rise along the antarctic margins in response to increased glacial discharge melting of the antarctic ice sheet and ice shelves reduces the salinity of the coastal seas satellite and model data suggest that the freshwater discharge has also caused coastal antarctic sea level to rise by about 2 mm yr 1 more than the regional mean

antarctic sea ice control on ocean circulation in present and glacial climates in the modern climate the ocean below 2 km is mainly filled by waters sinking into the abyss around antarctica and in the north atlantic paleoproxies indicate that waters of north atlantic origin were instead absent below 2 km at the last glacial maximum resulting in an expansion of the volume occupied by antarctic origin waters in this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around antarctica a simple theory further suggests that these deep waters only came to the surface under sea ice which insulated them from atmospheric forcing and were weakly mixed with overlying waters thus being able to store carbon for long times this unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean s role in regulating atmospheric carbon dioxide on glacial interglacial timescales previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide here it is shown that many of these changes are dynamically linked and therefore must co occur

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

authigenic carbonates from an active cold seep of the northern south china sea new insights into fluid sources and past seepage activity abstract site f also named formosa ridge represents the most vigorous cold seep on the northern south china sea continental slope in order to constrain the fluid sources and intensities of seepage we investigated the petrography mineralogy stable carbon and oxygen isotopic compositions element geochemistry and radiocarbon dating of authigenic carbonate rocks retrieved from the seafloor carbonate rocks mainly occurred as crusts nodules and nodular masses incorporated in carbonate breccias the carbonates were comprised mainly of high mg calcite and aragonite the 13 c of authigenic carbonate varied from 55 3 to 34 3 mean 48 5 n 47 vs v pdb suggesting biogenic methane is the dominant carbon source fuelling the system the 18 o carbonate values were from 3 6 to 4 8 mean 3 9 n 47 the observed 18 o enrichement in relation to calculated equilibrium values in the carbonates probably reflects dissolution of gas hydrates combination of seafloor observations and the obtained ams 14 c ages suggest that 1 initiation of methane seepage from at least 10 6 ka ago 2 environmental conditions may have been favorable for enhanced fluid seepage around 6 ka bp and 3 relatively low intensity of seepage from 2 ka bp till today

distinct activity phases during the recent geologic history of a gulf of mexico mud volcano abstract laser line scan imaging and chirp sub bottom profiling were used to detail the morphology of a submarine mud volcano and brine filled crater at 652 m water depth in the northern gulf of mexico the mud volcano has a relief of 6 m and a basal diameter of about 80 m the feature comprises a central brine filled crater 253 m 2 surrounded by a continuous bed of methanotrophic mussels bathymodiolus childressi covering 434 m 2 and a patchy bed covering an additional 214 m 2 of the periphery the brine pool was mostly 28 m and 12 m deep respectively at the northern end of the pool which emitted continual streams of small clear bubbles sub bottom profiles indicated three distinct strata beneath the present surface of the mud volcano integration of 17 profiles shows that the mud volcano has been built in at least three successive stages the lowest stage deposited 35 400 m 3 while the middle and upper stages deposited 7700 and 20 400 m 3 respectively piston cores were taken at the northern edge of the mussel bed and a site 100 m southwest of the pool mussel and lucinid shells were recovered from the closer core lucinid shells from the distant core a mussel shell from 3 4 m sub bottom had a 14 c age of 16 2 ka mixture of modern carbon with carbon dead reservoir material would produce actual ages 2 ka less than the radiocarbon ages

last glacial emplacement of methane derived authigenic carbonates in the sea of japan constrained by diatom assemblage carbon 14 and carbonate content abstract we studied diatom assemblages and caco 3 contents of methane derived authigenic carbonates from the eastern margin of the sea of japan and assessed the formation time of these samples radioactive 14 c date was determined in selected samples to obtain the maximum age of the time the results of our study suggest mass formation of carbonate nodules in a glacial period within 40 ky consistent with a published u th dating result of carbonate nodules in the study area diatom assemblages and contents in the carbonate nodules abundance of 10 6 g dominance of neritic littoral species warm cold water species ratio lower than 25 differ from the near seafloor sediments in the study area which have characteristics of holocene sediments in the sea of japan and suggest cementation of glacial sediments laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi enclosed ocean similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules glacial sediments with oceanic cold water species as low as holocene sediments restricts the sediment age to before 20 cal ky bp carbonate contents higher than 78 wt suggest the cementation of poorly compacted sediments near the seafloor and the date of carbonate cementation is therefore close to that of the cemented sediments most carbonate nodule samples in this study were formed in a glacial period and detection of 14 c restricts this period to within 40 ky

the chimney forest of the deep montenegrin margin south eastern adriatic sea a spectacular field of columnar carbonates has been discovered on the montenegrin margin in the southern adriatic sea at a depth of about 450 m the site exposes many columnar carbonates protruding from the substrate or abated on the bottom such carbonates attain maximum visible lengths of ca 60 cm with diameters up to 20 cm display an annular growth and are either hollow or plugged by indurated sediment petrographic and geochemical analyses document the pervasive presence of dolomite and 13c values as low as 30 vpdb these chimneys are therefore interpreted as former conduits related to hydrocarbon expulsion in this sector of the adriatic basin however available data suggest that hydrocarbon flows at this site have ceased our results show that chimneys formed inside the local depositional units glacial pleistocene shelf shelly rich muddy sediment and were successively exhumed from the host sediment today the chimneys offer substrate to benthic life including cold water corals and sponges the u series dating of these carbonate concretions is complicated by the presence of a significant fraction of detrital sediment which represents a major source of initial 230th ams 14c and 87sr 86sr dating of shells embedded in one of the chimney provided ages beyond the range of radiocarbon dating and 600 kyrs indicating a pleistocene age of the host sediment uncorrected u series dating of the carbonate chimneys yielded and age of ca 250 270 kyrs providing a temporal upper limit for conduit formation driven by lithification caused by the degradation of seeping hydrocarbons in addition u th dating of cold water coral bases settled on chimneys indicates a holocene age for their first exposure after exhumation and subsequent function as hard substrate for benthic organisms the montenegrin chimney forest is a rare case where many such carbonate columnar concretions are still in their original vertical position

past methane release events and environmental conditions at the upper continental slope of the south china sea constraints by seep carbonates authigenic carbonates and seep biota are archives of seepage history and record paleo environmental conditions at seep sites we obtained the timing of past methane release events at the northeastern slope of the south china sea based on u th dating of seep carbonates and seep bivalve fragments from three sites located at 22 02 22 09 n 118 43 118 52 e water depths from 473 to 785 m also we were able to reconstruct the paleo bottom water temperatures by calculating the equilibrium temperature using the ages the corresponding past 18o of seawater 18osw and the 18o of the selected samples formed in contact with bottom seawater with negligible deep fluid influence a criterion consists of mineralogy redox sensitive trace elements and u th isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid our results show that all methane release events occurred between 11 5 0 2 and 144 5 12 7 ka when sea level was about 62 104 m lower than today enhanced methane release during low sea level stands seems to be modulated by reduced hydrostatic pressure increased incision of canyons and increased sediment loads the calculated past bottom water temperature at one site site 3 water depth 767 771 m during low sea level stands 11 5 and 65 ka ago ranges from 3 3 to 4 0 c i e 1 3 to 2 2 c colder than at present the reliability of 18o of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18o enriched fluids that might be emitted from gas hydrate and or clay dehydration our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the south china sea

cold seeps and authigenic carbonate formation in monterey bay california abstract authigenic carbonate associated with modern cold seep biological communities and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of monterey bay although such variations in ancient samples have been used to infer differing tectonic settings these carbonates all formed within a faulted continental margin environment and chemical variations reflect local differences in the sources and flux of carbon to sediment pore fluids the slow seepage of fluid and with it dissolved carbon along the transform faulted continental margin results in discrete areas of enhanced microbial sulfate reduction oxidation of methane from both biogenic and thermogenic origins as well as the active precipitation of both high mg calcite hmc and dolomite the authigenic carbonates include semicontinuous pavements of shallow cemented sediments surrounding benthic communities circular or pipe like chimneys interpreted as cemented conduits formed as a result of methane gas expulsion centimeter to meter scale rings doughnuts or slabs winnowed from variable depths within sediments and carbonate veins ankerite or calcite or cements in faulted basement rocks draped with bacterial mat abundant pyrite framboids preferentially filling the tests of the benthic foraminifer uvigerina peregrina and characteristic of the hmc bearing samples are products of a zone of shallow microbial sulfate reduction a process fundamental to the nourishment of the chemosynthetic cold seep communities sites on a sedimented ridge west of the san gregorio fault zone have carbon isotopic values between 35 and 56 that are strongly influenced by carbon derived from methane the higher values of 18 o more enriched in 18 o are found closer to the active fault zone carbonates from sites within the san gregorio fault zone or from monterey canyon floor exposures of miocene sediments have carbon isotope values between 7 and 26 that are mixtures of sedimentary organic carbon and microbially oxidized hydrocarbons from organic rich petroleum source rocks dolomite bearing samples are more enriched in both 18 o and 13 c bulk carbonate samples with heavier 18 o 18 o 3 5 result from the abundance of authigenic dolomite and low temperatures of formation local destabilization of clathrates might also create pore fluids that are more enriched in both 13 c and 18 o

u th dating and post depositional alteration of a cold seep carbonate chimney from the campos basin offshore brazil abstract dating authigenic cold seep carbonates allows determining the timing of active seepage of hydrocarbon rich fluids at continental margins and ultimately the detection of the geological factors affecting it u th dating has been selected in this study the method having been successfully used before however incorporation of initial thorium in substantial amounts of detrital material due to the precipitation of carbonates within the seabed and post depositional alteration potentially pose a major problem violating the basic assumptions of the method here we report results from several samples from the campos basin off the coast of brazil they are unusual in their high 13 c values mean of 10 0 vpdb high detrital contents and being dominantly dolomite elevated 230 th 238 u activity ratios indicate that most of the carbonates lost uranium during diagenesis isochrones are obtained from the unaffected samples yielding an age of 140 4 3 1 ka vertical growth of the chimney must have been short and rapid since results from the top and bottom parts of the chimney concur within analytical uncertainties furthermore profiles of u th radioisotope specific activity 234 u and 232 th specific activity on two quarter slices of a carbonate chimney reveal the occurrence of three distinct diagenetic processes u loss u exchange with seawater and th gain

u th dating of cold seep carbonates an initial comparison authigenic carbonates from hydrocarbon seeps are unique archives of past seepage and associated environmental parameters in order to constrain the ages of a set of seep carbonates and the time involved in carbonate formation we applied uranium thorium u th dating to samples from the gulf of mexico the congo fan and the black sea the resulting u th ages indicate that environmental conditions must have been favorable for enhanced methane rich fluid seepage during the time intervals of 53 4 to 1 7 ka bp for the gulf of mexico and 45 5 to 3 0 ka bp for the congo fan the seep carbonates from the black sea formed at 1 6 to 1 1 ka bp our results suggest that enhanced fluid flow during these time intervals was closely related to 1 sea level variations associated with glacial interglacial cycles and 2 environmental alterations in the course of late quaternary climate change including variations in bottom water temperatures that affected the stability of gas hydrate reservoirs

radiometric dating of submarine hydrocarbon seeps in the gulf of mexico massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid bathymodiolus sp containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and u series isotopes stable carbon isotopes and 87 sr 86 sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon rich fluids samples from five seepage sites on the northern gulf of mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to 2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives in general paired 14 c and 230 th dates of carbonate buildups consisting of aragonite high mg calcite and dolomite mineralogies are highly discordant the cause of the discordance established on the basis of paired 14 c and 13 c values 14 c 898 to 992 13 c 9 5 to 53 3 for n 27 lies with the impairment of the radiocarbon dates resulting from dilution of the 14 c pool with fossil hydrocarbon derived carbon the validity of the ionium dates based on u rich 2 4 7 6 ppm samples is demonstrated by the concordance between 234 u 238 u and 230 th 234 u evolution in time and by the 234 u 238 u o activity ratios that are generally within the range of sea water value of 1 14 0 04 2 sigma some dolomite rich samples are exceptional because their 234 u 238 u o ratios are significantly higher 1 22 1 38 than sea water suggesting deposition from anoxic pore waters where the soluble u reached anomalous 234 u 238 u ratios the formation of the carbonates from sea water derived fluids rather than from formation fluids advecting from deep aquifers is supported by the 87 sr 86 sr composition of the samples mean 0 709145 19 10 6 n 14 that compares well with modern nonseep marine carbonates and the ambient gulf of mexico sea water 0 709171 8 10 6 calcareous shells the 13 c values of which indicate a carbon source in sea water 13 c 4 3 to 1 1 yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates isotope migration attested by the observed u uptake in the fossil shells is likely to affect the accuracy of their ionium dates radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late pleistocene time 195 13 ka ages derived from nascent seep sites at mid bathyal and abyssal depths 12 3 0 0 ka indicate that currently vigorous seepage was initiated at the end of the last deglaciation these radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor

petrographic and geochemical characterization of seep carbonate from bush hill gc 185 gas vent and hydrate site of the gulf of mexico abstract authigenic carbonates are common at cold seep sites as a result of microbial oxidation of hydrocarbons seep carbonate samples were collected from the surface of the bush hill green canyon block 185 gulf of mexico a mound containing gas hydrate the carbonates consisted of oily porous limestone slabs and blocks containing bioclasts and matrix analysis by x ray diffraction shows that aragonite is the dominant mineral 89 99 wt with an average of 94 wt in the matrix of seep carbonate this cement occurs in microcrystalline microspar and sparite forms the moderate 13 c depletion of the seep carbonate the most depleted one has 13 c value of 29 4 and 26 of 38 subsamples have 13 c values 20 0 indicates that the non methane hydrocarbons was incorporated during seep carbonate precipitation relative enrichment of 18 o may be related to localized destabilization of gas hydrate or derived from 18 o enriched pore water originated from smectite illite transition in the deep sediments the total content of rare earth elements ree of the 5 hno 3 treated solution of the carbonates is from 0 40 ppm to 30 9 ppm the shale normalized ree patterns show varied ce anomalies from significantly negative slightly negative and no to positive ce anomalies variable content of trace elements total ree and ce anomalies in different samples and even in the different carbonate mineral forms microcrystalline microspar and sparite of the same sample suggest that the formation condition of the bush hill seep carbonate is variable and complex which is possibly controlled by the rate of fluid flux

using chemical compositions of sediments to constrain methane seepage dynamics a case study from haima cold seeps of the south china sea abstract cold seeps frequently occur at the seafloor along continental margins the dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction which can significantly impact the global carbon and sulfur cycles the circulation of methane rich fluids at margins is highly variable in time and space and assessing past seepage activity requires the use of specific geochemical markers in this study we report multiple sedimentary proxy records for three piston gravity cores qdn 14a qdn 14b and qdn 31 from the haima seep of the south china sea scs by combining total organic carbon toc total inorganic carbon tic total nitrogen tn total sulfur ts acid insoluble carbon and sulfur isotope 13corganic carbon and 34sacid insoluble and 34s values of chromium reducibility sulfur 34scrs as well as carbon isotopes of tic 13ctic in sediments our aim was to provide constraints on methane seepage dynamics in this area we identified three sediment layers at about 260 300 cm 380 420 cm and 480 520 cm sediment depth characterized by particular anomalies of low 13ctic values and high ts content high ts and crs contents and high 34sacid insoluble and 34scrs values respectively on this basis we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record while the exact mechanisms accounting for the presence or absence of these particular geochemical signals in the sediment are not known we propose that they correspond to variations in methane flux and their duration through time overall our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time

cold seep systems in the south china sea an overview abstract three decades after the discovery of cold seep systems various sites of hydrocarbon seepage have been found in the south china sea scs over the past decade these sites have become model systems for understanding the variability of hydrocarbon seepage and associated biogeochemical processes in this review we describe the cold seep systems of the scs with an emphasis on seafloor manifestations fluid sources biogeochemical processes and macroecology seafloor features associated with seeps include mud volcanoes pockmarks and carbonate deposits a common characteristic of cold seeps is the occurrence of authigenic i e in situ precipitated carbonate minerals these carbonates commonly exhibit low 13c and high 18o values suggesting the incorporation of methane derived carbon and oxygen derived from gas hydrate water biogeochemical processes such as sulfate driven anaerobic oxidation of methane sd aom the key process at seeps have been studied in detail with the aim of establishing geochemical proxies to trace these processes into the geological past we also detail the features characterizing seep ecosystems understanding the impact of decomposing methane hydrate on the marine carbon budget remains challenging and requires additional seafloor observations as well as models predicting how gas hydrate responds to changing conditions such as temperature increase sea level rise and episodic mass wasting

tree ring analysis elucidating palaeo environmental effects captured in an in situ fossil forest the last 80 years within an early permian ecosystem abstract the early permian chemnitz fossil lagerstatte leukersdorf formation chemnitz basin se germany represents a diverse t 0 assemblage of a fossil forest ecosystem around the sakmarian artinskian transition 290 6 1 8 ma which was preserved by pyroclastic deposits of a multi phased volcanic eruption the multi aged plant community consists of predominantly hygrophilous elements which grew on an alluvial plain mineral substrate under sub humid conditions representing a wet spot environment strong seasonality triggered the formation of annual tree rings in arborescent woody plants such as pycnoxylic gymnosperms medullosan seed ferns and calamitaleans from several hundred fossil trees the 53 best preserved specimens were selected and investigated in detail by measuring 2 081 tree rings in individual sequences of up to 77 rings ring sequences were analysed by standard dendrochronological methods to determine both annual growth rates and mean sensitivity morphological and statistical analyses on single tree rings reveal different tree ring types according to the different plant groups pycnoxylic gymnosperms have distinct and regular tree rings whereas medullosan seed ferns and calamitaleans show indistinct and regular tree rings as well as so called event rings results reveal differences between plant groups regarding their physiological reactions or adaptations to seasonal fluctuations in comparison to pycnoxylic gymnosperms both medullosan seed ferns and calamitaleans exhibit reduced growth rates and more sensitive reaction to environmental perturbances as water deficiency pointing to comparably lower adaptation to seasonally dry palaeoclimate in this context event rings are in many cases traced back to plant physiological stress during particularly severe drought periods altogether these fossil trees serve as sensitive environmental archives which shed light on growth conditions several decades back in time from the entombing eruption

microbial activity in surficial sediments overlying acoustic wipeout zones at a gulf of mexico cold seep 1 down core concentration gradients of dissolved methane and sulfate isotope gradients of methane dissolved inorganic carbon and authigenic carbonate and organic matter elemental ratios are incorporated into a vent evolution model to describe spatial and temporal variability of sedimentary microbial activity overlying acoustic wipeout zones at mississippi canyon mc 118 gulf of mexico we tested the hypothesis that these zones indicate areas where sediments are exposed to elevated fluid flux and therefore should contain saturated methane concentrations and enhanced microbial activity from sulfate reduction sr anaerobic oxidation of methane aom and methanogenesis mp thirty surficial cores between 22 and 460 cm deep were collected from sediments overlying and outside the wipeout zones and analyzed for pore water and solid phase constituents outside the wipeout zones sulfate and methane concentrations were similar to overlying water values and did not vary with depth indicating low microbial activity above the wipeouts nine cores showed moderate activity with gently sloping sulfate and methane concentration gradients methane concentrations 20 m and isotope depth gradients indicative of organic matter oxidation in stark contrast to this moderate activity four cores showed high microbial activity where sulfate concentrations were depleted by 50 cm below seafloor maximum methane concentrations in the decompressed cores were above 4 mm and down core profiles of 13c ch4 and 13c dissolved inorganic carbon dic indicated distinct depth zones of sr aom and mp bulk organic matter analysis suggested that the high activity was supported by an organic source that was enriched in carbon c n 15 and depleted in 15n and 13c compared to other activity groups possibly due to the influx of petroleum or chemosynthetically fixed carbon within high activity cores the 13c dic values were similar to the 13c caco3 values a result expected for authigenic carbonate recently precipitated however these values were dissimilar in moderate activity cores suggesting that microbial activity was higher in the past this study provides evidence that the fluid flux at mc 118 varies over time and that the microbial activity responds to such variability it also suggests that sediments overlying wipeout zones are not always saturated with respect to methane which has implications for the formation and detection of gas hydrate

using 87sr 86sr ratios to date fossil methane seep deposits methodological requirements and an example from the great valley group california abstractmethane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems for this information to be useful a reliable determination of the carbonates stratigraphic ages is required but this is not always available here we investigate the using strontium isotope stratigraphy to date fossil methane seep carbonates via detailed petrographic and geochemical investigation of the different carbonate phases in biostratigraphically well dated seep carbonates of paleozoic mesozoic and cenozoic age the best results are obtained from banded botryoidal rim cements from carbonate phases showing a weak or no cathodoluminescence signal an oxygen isotope signature close to that of seawater and the lowest mn concentrations we then applied the method to a presumably late jurassic seep carbonate from the great valley group in california strontium isotope ratios of the least diagenetically altered carbonate phases indicate a tithonian late jurassic

circum antarctic shoreward heat transport derived from an eddy and tide resolving simulation almost all heat reaching the bases of antarctica s ice shelves originates from warm circumpolar deep water in the open southern ocean this study quantifies the roles of mean and transient flows in transporting heat across almost the entire antarctic continental slope and shelf using an ocean sea ice model run at eddy and tide resolving 1 48 horizontal resolution heat transfer by transient flows is approximately attributed to eddies and tides via a decomposition into time scales shorter than and longer than 1 day respectively it is shown that eddies transfer heat across the continental slope ocean depths greater than 1 500 m but tides produce a stronger shoreward heat flux across the shelf break ocean depths between 500 m and 1 000 m however the tidal heat fluxes are approximately compensated by mean flows leaving the eddy heat flux to balance the net shoreward heat transport the eddy driven cross slope overturning circulation is too weak to account for the eddy heat flux this suggests that isopycnal eddy stirring is the principal mechanism of shoreward heat transport around antarctica though likely modulated by tides and surface forcing

geochemical paleogradient in pore waters controlled by aom recorded in an oligocene laminated limestone from the outer carpathians abstract a 20 cm thick laminated limestone of oligocene age from the outer carpathians is composed mainly of coccoliths and authigenic calcite 13 c values unequivocally indicate that the authigenic calcite precipitated as a result of methane oxidation the 13 c and 18 o values change gradually from the base to the top of the bed from 38 0 to 17 6 and from 1 1 to 5 7 respectively this trend reflects a geochemical gradient which developed in the zone of anaerobic oxidation of methane aom due to the consumption of sulfate by methane this gradient was related to an inverse relationship between the concentrations of both compounds in the pore waters of the aom zone methane was the source of isotopically light carbon it was accompanied by heavy oxygen containing water liberated from decomposition of gas hydrates the methane bearing fluids migrated upwards to the base of the sulfate reduction sr zone where methane was oxidized ch 4 so 4 2 hco 3 hs h 2 o and the aom zone developed the bicarbonate produced in this reaction was significantly depleted in 13 c and 16 o in the sr zone organic matter was oxidized by sulfate 2ch 2 o so 4 2 h 2 s 2hco 3 the bicarbonate released in that reaction was the source of isotopically heavier c and lighter o the proportion of bicarbonate produced by oxidation of methane to bicarbonate produced by oxidation of organic matter decreased upwards the resultant isotopic composition of bicarbonate dissolved in the pore waters changed upwards towards higher 13 c and lower 18 o values and has been recorded in the limestone s isotopic composition

thickness of the gas hydrate stability zone gulf of mexico continental slope the maximum thickness of the gas hydrate stability zone ghsz at key gas hydrate study sites was estimated and a generalized ghsz profile across part of the central gulf of mexico slope was constructed maximum thickness of the ghsz increases with water depth at the shallowest site 540 m water depth from about 450 m to about 1150 m at the deepest site 1930 m water depth the occurrence of gas hydrate in the subsurface is largely controlled by structural focussing of hydrocarbons with gas hydrate most abundant at the rims of salt withdrawal basins and less abundant within intrasalt basins the postulated geometry of large subsurface gas hydrate accumulations shows strong structural control bottom simulating reflectors bsrs are absent or infrequently observed in the gulf of mexico because of structural focusing of the free hydrocarbon gases that form gas hydrate new insight to the geometry of the ghsz in the gulf of mexico continental slope will contribute to improved application of geophysics to map the distribution of gas hydrates improved understanding of three dimensional geometry of gas hydrate accumulations will contribute to better assessment of gas hydrate volumes as a future energy resource as a geohazard and will constrain theories relating the role of gas hydrate in past climate change

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

paleo environmental controls on cold seep carbonate authigenesis in the sea of marmara abstract the factors controlling fluid emission dynamics at ocean margins are poorly understood in particular there are significant uncertainties on how fluid seepage at cold seeps may have responded to abrupt environmental changes in the geological past this study reports on a detailed geochemical investigation of seafloor carbonate crusts sampled at cold seeps along the submerged part of the north anatolian fault system in the sea of marmara an inland sea which has experienced major paleo environmental changes over the last deglaciation period we also analyzed a series of authigenic carbonate concretions recovered from two sediment cores at the western high ridge an active fluid venting area the ages of seafloor carbonate crusts derived from isochron u th dating cover the last 7 kyr suggesting that fluid activity along the fault system remained continuous over that time interval in the sediment cores carbonate concretions are concentrated at the lacustrine to marine transition which corresponds to the period when mediterranean waters flowed into the marmara basin about 12 14 kyr ago u th isotopic data indicate that most of these concretions formed later during the holocene around 9 10 kyr ago a period coinciding with an important anoxic event that led to the deposition of a sapropel layer in the sea of marmara based upon these results we suggest that the absence of carbonate concretions in the lacustrine sediment unit indicates that dissolved sulfate concentrations in the marmara lake pore waters during glacial time were too low to promote significant anaerobic methane oxidation thereby preventing sedimentary carbonate authigenesis in contrast the progressive inflow of mediterranean waters into the glacial marmara lake after 15 ka provided a source of dissolved sulfate that allowed anaerobic oxidation of methane to proceed within the anoxic sediment importantly the synchronism between the main phase of authigenic carbonate precipitation at the studied sites average 9 4 1 8 ka n 16 and the regional anoxic sapropel event support the idea that the drop in bottom water dissolved oxygen content was probably a key factor to enhance microbial activity and associated carbonate precipitation at that time overall these results provide straightforward evidence that fluid emission dynamics and hydrocarbon oxidation at cold seeps can be directly related to changing environmental conditions through time

stratigraphic expression of earth s deepest δ13c excursion in the wonoka formation of south australia the most negative carbon isotope excursion in earth history is found in carbonate rocks of the ediacaran period 635 541 ma known colloquially as the shuram excursion workers have long noted its broad concordance with the rise of abundant macro scale fossils in the rock record collectively known as the ediacaran biota thus the shuram excursion has been interpreted by many in the context of a dramatically changing redox state of the ediacaran oceans for example a result of methane cycling in a low o2 atmosphere the final destruction of a large pool of recalcitrant dissolved organic carbon doc and the step wise oxidation of the ediacaran oceans more recently diagenetic interpretations of the shuram excursion have challenged the various redox models with the very negative 13c values of ediacaran carbonates explained via sedimentary in growth of very 13c depleted authigenic carbonates meteoric alteration or late stage burial diagenesis a stratigraphic and sedimentological context is required to discriminate between these explanatory models and to determine whether the shuram excursion can be used to evaluate oxygenation in terminal neoproterozoic oceans here we present chemo stratigraphic data 13c 18o and trace element abundances from 15 measured sections of the ediacaran aged wonoka formation fm of south australia in some locations the wonoka fm is 700 meters m of mixed shelf limestones and siliciclastics that record a 17 permil 13c excursion 12 to 5 further north in the basin the wonoka fm is host to deep 1 km paleocanyons which are partly filled by tabular clast carbonate breccias canyon filling ceased during ongoing sedimentation on the shelf interior the canyon shoulder as evidenced by upper canyon shoulder units that overlie and cap certain canyon fill sequences the unprecedented size of the chemostratigraphic dataset presented here 2671 13c 18o measurements from the canyon shoulder 1393 13c 18o measurements from canyon clasts and 11 different trace elements measured on 247 wonoka fm carbonate samples when coupled with the unique canyon shoulder to canyon fill depositional system of the wonoka fm allows for new insights into ediacaran carbon isotope systematics the excursion is preserved in a remarkably consistent fashion across 12 000 km2 of basin area fabric altering diagenesis where present occurs at the sub meter vertical scale results in

paleo cold seep activity in the southern south china sea evidence from the geochemical and geophysical records of sediments abstract over the past decade more than 30 cold seep sites have been reported along the continental slope of the south china sea scs these seep sites have all been located in the northern scs and no methane seepage has been discovered in the southern scs here we report on the analysis results of the geochemical and geophysical of the marine core sediments 58s collected from the southern continental slope of the scs we suggest that the sedimentary total sulfur ts total inorganic carbon tic 34scrs values of chromium reducible sulfur crs and carbonate 13ctic values of the bulk sediments coupled with the ratio of nrmaf60mt nrmaf80mt nrm can be used to trace the methane release events in a methane rich environment at least two methane release events were identified at depths of 5 5 6 2 m and 6 8 7 2 m moreover according to the characteristic of the ts toc and nrmaf60mt nrmaf80mt nrm ratio of sediments we suggest the methane flux has changed episodically this inference is supported by the occurrence of the strongly negative carbonate 13ctic values of bulk sediments and the positive 34scrs values which indicate the occurrence of anaerobic oxidation of methane aom and the location of a past or present sulfate methane transition zone smtz respectively ultimately these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment

the thermodynamic balance of the weddell gyre the thermodynamic balance of the weddell gyre is assessed from an inverse estimate of the circulation across the gyre s rim the gyre experiences a weak net buoyancy gain that arises from a leading order cancellation between two opposing contributions linked to two cells of water mass transformation and diapycnal overturning the lower cell involves a cooling driven densification of 8 4 2 0 sv of circumpolar deep water and antarctic bottom water near the gyre s southern and western margins the upper cell entails a freshening driven conversion of 4 9 2 0 sv of circumpolar deep water into lighter upper ocean waters within the gyre interior the distinct role of salinity between the two cells stems from opposing salinity changes induced by sea ice production meteoric sources and admixture of fresh upper ocean waters in the lower cell which contrasts with coherent reductions in salinity associated with sea ice melting and meteoric sources in the upper cell

thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic communities gulf of mexico continental slope abstract where abundant at the sea floor thermogenic gas hydrates impact bacterially mediated processes in chemosynthetic communities dependent on methane and h2s our main gas hydrate sites are at 540 m water depth and relatively low temperature 7 c gas hydrates outcrop as vein fillings in hemipelagic muds near gas vents within chemosynthetic communities molecular and isotopic properties of hydrate forming c1 c5 hydrocarbons and co2 provide insight to bacterially mediated processes hydrate bound methane is altered by bacterial oxidation as indicated by enrichment of 13c and deuterium d and by co2 depleted in 13c the degree of gas hydrate alteration appears related to duration of exposure at the sea floor in hydrate associated sediments bacterial oxidation of a mixed pool of hydrocarbons yields a net production of co2 depleted in 13c bacterial oxidation of hydrate bound methane and free hydrocarbon gases in adjacent sediments could contribute to gas hydrate decomposition some thermogenic carbon in sediments could be recycled via methanogenesis to yield a net production of bacterial methane depleted in 13c our results strengthen the hypothesis that gas hydrates could favor life in other extreme environments at low temperatures

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

geochemical record of methane seepage in authigenic carbonates and surrounding host sediments a case study from the south china sea abstract sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time three gravity cores d 8 d f and d 7 were collected from seep sites on the northern slope of the south china sea where gas hydrates were drilled in the subsurface various carbon and sulfur contents 13c values of total inorganic carbon 13ctic 34s values of chromium reducible sulfur 34scrs trace element contents grain size and ams 14c dating of planktonic foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13c depleted authigenic carbonate nodules 13c values as low as 49 discovered at an interval of 150 200 cm in core d 7 this finding is supported by high s c ratios and molybdenum enrichment in the same interval however low contents of crs and negative 34scrs values are present it is suggested to reflect a transient methane seepage event which continued for about 1 ka based on the 14c ages cores d 8 and d f have 13ctic values close to zero low s c ratios and crs contents negative 34scrs values and no trace element enrichment suggesting a negligible impact of methane seepage on the sediments the negative 34scrs values of the studied seep impacted and background sediments suggest that the application of 34scrs alone as a proxy to identify aom related process may be insufficient sediment carbon sulfur trace element systematics and 14c ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales

the role of tides in ocean ice shelf interactions in the southwestern weddell sea near observed weddell sea gyre continental shelf cavity water masses in interactive ocean sea ice tide ice shelf melt simulations tides amplify basal melt refreeze pattern raising net melt by 50 associated with enhanced time varying kinetic energy at the ice base impacts of additional meltwater production on weddell continental shelf sea ice and newly formed bottom water properties are suggested corresponding author u hausmann uhausc at gmail dot com this article has been accepted for publication and undergone full peer review but has not been through the copyediting typesetting pagination and proofreading process which may lead to differences between this version and the version of record please cite this article as

a multidisciplinary study of middle miocene seep carbonates from the northern apennine foredeep italy several pelitic intervals are intercalated at various levels within the marly arenaceous turbiditic successions of the middle miocene northern apennine foredeep they range in thickness from 30 to 200 m and represent sedimentation on top of ephemeral structural highs related to blind faults sediments are made up of hemipelagites and fine grained turbidites and include 13c depleted carbonates related to the rising of methane rich fluids hydrocarbon seep carbonates large portions of pelitic intervals are involved in chaotic masses by soft sediment deformation slumps slides intraformational breccias revealing an intense sediment instability during middle miocene a stratigraphic mineralogic and geochemical study was conducted on two of these pelitic intervals castagno d andrea mt citerna in order to reconstruct carbonate development the composition of fluids and to document the connections between fluid seepage and syndepositional tectonics this multidisciplinary approach has allowed us to discriminate between the two examined pelitic intervals in terms of age depositional rates and conditions organic carbon and post depositional processes seep carbonates are characterized by chemosymbiotic fossil communities autoclastic fractures and brecciation carbonate bodies show complex facies relationships as indicative of different stages in chemoherm growth the compositional study evidences the peculiar chemistry of chemoherm carbonates calcite low in mg and sr compared with carbonates in associated enclosing pelites and with modern chemoherms in general the non carbonate components within the chemoherms are enriched in detrital minerals and depleted in phyllosilicates with respect to the enclosing pelites the mineralogical changes in the clay component within the brecciated unit of the castagno d andrea chemoherms suggest authigenic precipitation of the mg rich phases isotopic analyses show the distinct carbon signature in the chemoherms from the two examined intervals castagno d andrea chemoherms more depleted from 15 8 up to 41 3 pdb than mt citerna from 5 2 up to 16 7 and the transitional 13c depletion trend observed moving from chemoherms to the enclosing pelites moderately depleted and te bouma sequence turbidites in the range of marine carbonates a slight but significant enrichment in 18o up to 1 4 pdb is observed for all chemoherms when compared to values of carbonate phases present in enclosing pelites geochemical data indicate that the brecciated facies of seep carbonates are related to an explosive release of gaseous fluids probably associated with the rise of deep hypersaline fluids

the contribution of the weddell gyre to the lower limb of the global overturning circulation the horizontal and vertical circulation of the weddell gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports the gyre is found to convey 42 8 sv 1 sv 106 m3 s 1 across the central weddell sea and to intensify to 54 15 sv further offshore this circulation injects 36 13 tw of heat from the antarctic circumpolar current to the gyre and exports 51 23 msv of freshwater including 13 1 msv as sea ice to the mid latitude southern ocean the gyre s overturning circulation has an asymmetric double cell structure in which 13 4 sv of circumpolar deep water cdw and relatively light antarctic bottom water aabw are transformed into upper ocean water masses by mid gyre upwelling at a rate of 2 2 sv and into denser aabw by downwelling focussed at the western boundary 8 2 sv the gyre circulation exhibits a substantial throughflow component by which cdw and aabw enter the gyre from the indian sector undergo ventilation and densification within the gyre and are exported to the south atlantic across the gyre s northern rim the relatively modest net production of aabw in the weddell gyre 6 2 sv suggests that the gyre s prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of indian sourced aabw

genesis and geometry of the meiklejohn peak lime mud mound bare mountain quadrangle nevada usa ordovician limestone with submarine frost heave structures a possible response to gas clathrate hydrate evolution abstract during the early middle ordovician early whiterockian the meiklejohn peak lime mud mound a large whaleback or dolphin back dome grew on a carbonate ramp tens to hundreds of kilometres offshore this ramp extended from the northwest margin of laurentia into the open waters of the ancestral pacific ocean to the north the mound developed in an outer ramp environment in relatively deep and cold water a steep northern margin with a slope that exceeds 55 characterizes the mound this margin is split by a 14 m long vertical fracture that separates a zone of slumped drag folded and brecciated rocks from the main mass of the mound failure along this fracture occurred subcutaneously as highlighted by covering beds that are folded next to the mound brecciated blocks and clasts contain zebra and stromatactis structures indicating that these rocks and structures were lithified early in the history of the mound the southern end of the mound is less steep and is characterized by large echinodermal grainstone cross beds these deposits are part of a large subaqueous dune that grew northwards and preceded the main development of the mound southward dipping and downlapping layers of mud mound mudstone and wackestone overlie the dune these muddy limestone layers are cut in several places by injection dykes and are pierced near the contact with the underlying dune by a 25 m long pipe filled with rotated nodular and brecciated mud mound clasts this long pipe extends to the edge of the mound and appears to have been a conduit where fluidized materials that came from the mound s interior were vented the interior of the mound is typified by light grey limestone with zebra bands and stromatactis structures both structures represent former cavity systems that are filled with fibrous and bladed calcite and pelleted and laminated geopetal mudstone spar bands of zebra limestone often extend for several metres and appear to have been unsupported over these distances zebra banded rocks are also accompanied by snout and socket structures and in some instances are folded and sheared by curving kink bands zebra and stromatactis limestone structures found throughout the mud mound resemble frost heave and cryoturbation structures identified in both holocene and pleistocene cryosols and in laboratory experiments with advancing freezing fronts in clay size sediment significantly modern occurrences of methane clathrate hydrate methane charged ice display parallel and digitate layering similar in depositional appearance to that of zebra and stromatactis limestone from meiklejohn peak early carbonate cements are also commonly associated with these modern clathrate hydrate deposits consequently gas clathrate hydrates may have been the propping agent for zebra and stromatactis structures observed in the mud mound in this scenario carbonate cements would have precipitated and stabilized these structures both with the consolidation and dissociation of gas clathrate hydrates and with the oxidation and reduction of associated gases stable 13 c and 18 o isotope ratios collected from mudstone and spar of zebra and stromatactis structures indicate that they were lithified in equilibrium with ordovician seawater the 13 c isotope ratios recorded at meiklejohn peak are similar to 13 c isotopic ratios obtained from co 2 evolving from modern seafloor these isotopic ratios may indicate that frost heave structures in the meiklejohn peak mud mound are the result of consolidation and dissociation of carbon dioxide clathrate hydrates even though the bulk of gas clathrate hydrates identified to date in modern ocean floors are composed of methane carbon dioxide clathrate hydrates are known from the modern seafloor of the okinawa trough they may also be common in areas of abundant carbonate sediment accumulation as suggested by recent observations from the great australian bight

diagenetic alteration affecting δ18o δ13c and 87sr 86sr signatures of carbonates a case study on cretaceous seep deposits from yarlung zangbo suture zone tibet china abstract seep carbonates provide excellent records of past seepage activities and have been commonly considered to preserve primary unaltered stable carbon isotope signatures however late diagenetic reactions may overprint original isotopic compositions but the mode and effect of such alterations are poorly understood in particular there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis this study reports recently discovered cretaceous hydrocarbon seep deposits from the yarlung zangbo suture zone tibet china that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins unitary linear recursive analysis was applied to 13 c values and 87 sr 86 sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids the 18 o values range from 11 8 to 2 2 13 c values from 34 1 to 12 9 and 87 sr 86 sr ratios from 0 706221 to 0 706808 the heterogeneity in isotopic compositions and the observation that the most negative 18 o values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis the linear correlations between 13 c values and 18 o values for matrix micrites r 2 0 54 and between bulk carbonate 87 sr 86 sr ratios and 18 o values r 2 0 85 are best explained by burial diagenetic overprinting of oxygen strontium and even carbon isotopic compositions rather than by meteoric water hypergenesis extrapolated values of 13 c and ratios of 87 sr 86 sr against a 18 o value of 2 average value of calcite precipitated in isotopic equilibrium with coeval cretaceous seawater that would characterize the primary carbonate give an end member 13 c value of 34 and an end member 87 sr 86 sr ratio of 0 7072 the end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low 13 c values and high 87 sr 86 sr ratios were formed by anaerobic oxidation of methane near the seafloor in contrast the measured 18 o values and 87 sr 86 sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87 sr 86 sr ratio and high temperature our findings reveal that 13 c values can only be moderately and 87 sr 86 sr ratios can be significantly altered during late diagenesis and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates

tracing the evolution of seep fluids from authigenic carbonates green canyon northern gulf of mexico authigenic carbonates from hydrocarbon seeps are unique long term archives of past fluid flow the studied samples were collected from green canyon block 140 at a water depth of 260 m in the gulf of mexico petrography x ray diffraction stable isotopes and c 14 dating were applied to assess the evolution of seep activity and potential driving forces the carbonates are dominated by high mg calcite hmc and aragonite with a minor amount of low mg calcite lmc and dolomite petrographically peloids clotted microfabric acicular aragonite and a variable content of bioclasts were observed three types of carbonates are recognized structure i carbonates with c 14 ages from 46 5 ka to 25 8 ka bp are characterized by delta c 13 values from 23 2 parts per thousand to 5 1 parts per thousand suggesting multiple carbon sources that include thermogenic methane biodegraded crude oil seawater and residual co2 from methanogenesis at greater depth in contrast structure ii carbonates formed between 17 6 ka and 11 7 ka bp and have delta c 13 values varying from 22 2 parts per thousand to 8 8 parts per thousand suggesting carbon sources similar to those of structure i carbonates but with a negligible influence of residual co2 from methanogenesis in addition the presence of lmc in this type of carbonate may be associated with brine seepage structure iii carbonates among the youngest of the samples analyzed with c 14 ages of 1 2 ka bp these carbonates have the most negative delta c 13 values ranging from 36 1 parts per thousand to 26 8 parts per thousand suggesting that thermogenic methane is the primary carbon source the majority carbonates of both structure i and ii are slightly o 18 enriched which is most likely related to the incorporation of water from dehydration of clay minerals the considerable range of mineralogical and isotopic variations of the studied carbonates highlights the local control of the seepage flux it is proposed that factors affecting the activity of hydrocarbon seeps are sea level changes and salt movement the combination of petrography stable isotopes and dating approach used here highlights that these are valuable tools to assess the variability of past fluid flow at hydrocarbon seeps c 2013 elsevier ltd all rights reserved

sr and nd isotopes of cold seep carbonates from the northern south china sea as proxies for fluid sources abstract in this study sr isotopes combined with nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids we presented sr and nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the south china sea scs based on the evidence for negligible detrital contamination and diagenetic alteration carbonate samples are considered dependable archives which record sr and nd isotope signals of the original pore waters from which carbonates have precipitated excellent correlation between 87sr 86sr ratios and 1 sr observed in carbonate samples indicates two end members with distinct sr isotopic signatures including modern seawater as a major source and a lesser contribution from methane rich seep fluid driven from a deep source besides these two end members release of nd from organic matter and fe mn oxyhydroxide is an additional contribution to pore water nd furthermore sr and nd isotopic compositions suggest water rock interactions between upward seep fluid and detrital component in sediments which resulted in the isotopic shift towards pronounced 143nd depleted and slightly 87sr enriched in the fluid phase this study provides insight into the knowledge for tracing seepage fluid circulation by sr nd isotope system at modern seep systems

geophysical signatures of mud mounds at hydrocarbon seeps on the louisiana continental slope northern gulf of mexico abstract mud mounds are common features at cold seeps in many areas of the world they are abundant in the northern gulf of mexico where they occur at the numerous hydrocarbon seeps on the continental margin although the mounds are widely known their geophysical signatures and geology have not been widely studied in detail in this study we examined mounds in two areas of the upper louisiana slope using geophysical data of several types 11 12 khz side scan sonar 3 5 khz echo sounder profiles near bottom 2 12 khz chirp sonar profiles and three dimensional 3d multichannel seismic data in side scan images the mud mounds appear as ovoid high backscatter spots associated with larger irregular high backscatter zones along faults and fault complexes three different sonar responses were noted 1 a more or less uniform appearance 2 a central higher backscatter spot surrounded with a lower return halo bulls eye and 3 a more or less uniform high backscatter area with an acoustic dead spot near its center near bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics implying that the long range acoustic signature of a given mound results from the integration of seafloor characteristics in these records anomalous seafloor echo types include strong seafloor reflectors hardbottom or buried hardbottom acoustic wipe out attenuation and acoustic turbidity prolonged echoes cores from acoustically anomalous seafloor areas indicate that a variety of seep related material has been introduced into the near seafloor sediments the prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath seafloor seismic amplitude maps from these mounds are either generally uniform with a positive or negative anomaly or display a ring of higher reflectivity around the mound periphery positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging the ring feature is thought to result from the preferential concentration of seep debris in the mound periphery relative to the center some mounds which show no seismic reflection anomaly from the seafloor appear inactive altogether observations from this study are consistent with the mud mounds being formed by sedimentary volcanism the data imply they are holocene features that have a life cycle that includes hardbottom formation which progressively crusts over the fluid vent ending its activity

chemical biological and hydrological controls on the 14c content of cold seep carbonate crusts numerical modeling and implications for convection at cold seeps understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations increasing the sediment ocean methane flux evidence for the occurrence of convection at cold seeps however is still limited we use the concentration of 14c d14c in carbonate crusts formed at cold seeps of the eastern mediterranean sea as a tracer for convective recirculation of seawater derived fluids a numerical model is applied to investigate the controls on 14c incorporation in cold seep carbonates our simulations show that increased amounts of ch4 in the expelled fluids result in elevated crust d14c while high ca2 and hco3 concentrations produce the opposite effect convection is the only transport process that can significantly increase crust d14c advection bioirrigation eddy diffusion and bioturbation instead have little effect on or produce a decrease of crust d14c in addition the presence of old or modern carbon mc in host sediments prior to cementation and the 14c decay associated to the time needed to form the crust contribute in defining the d14c of carbonate crusts we then use the model to reproduce the 14c content of the eastern mediterranean sea crusts to constrain the chemical and hydrological conditions that led to their formation some crusts contain relatively low amounts of 14c 945 0 d14c 930 2 which assuming no ageing after crust formation can be reproduced without considering convection other crusts from two sites the amsterdam and napoli mud volcanoes instead have a very high 14c content 899 0 d14c 838 4 which can only be reproduced by the model if convection mixes deep fluids with seawater order of magnitude calculations using the rayleigh criterion for convection suggest that the slow seepage about 10 cm year 1 of low salinity 20 fluids at the amsterdam sites could trigger haline convection there on the napoli mud volcano where high density brines are expelled density driven convection cannot take place and other processes possibly involving the rapid movement of free gas in the sediment could be important

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

u th chronology and formation controls of methane derived authigenic carbonates from the hola trough seep area northern norway abstract we investigated methane derived authigenic carbonate mdac crusts and nodules from a cold seep site on the northern norwegian continental shelf in ca 220 m water depth to determine the timing and mode of their formation gas bubbling observed during remotely operated vehicle rov assisted sampling of mdac crusts revealed ongoing seep activity authigenic carbonates were present as crusts on the seafloor and as centimetre size carbonate cemented nodules at several intervals within an adjacent sediment core aragonite dominated mineralogy of the mdac crusts suggests formation close to the seafloor at higher rates of sulphate dependent anaerobic oxidation of methane aom in contrast dolomite cemented nodules are consistent with the formation at the sulphate methane transition zone deeper within the sediment at lower rates of aom the 13c carbonate values of bulk rock and of micro drilled aragonite samples vary between 22 2 and 34 6 vpdb we interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane with a minor contribution from seawater dissolved inorganic carbon dic aom activity is supported by high concentrations of aom related biomarkers of archaea archaeol and 2 sn hydroxyarchaeol and sulphate reducing bacteria iso and anteiso c15 0 fatty acids in the crusts the dolomite nodules exhibit higher 13c carbonate values 12 vpdb suggesting a smaller amount of methane derived carbon presumably due to the contribution of dic migrating from depth and lower aom rates the latter is supported by orders of magnitude lower concentrations of archaeol and sn 2 hydroxyarchaeol in the sediment interval containing the largest dolomite nodules 18o values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate derived water u th dating of two mdac crusts shows that they formed between 1 61 0 02 and 4 39 1 63 ka bp and between 2 65 0 02 and 4 32 0 08 ka bp we infer both a spatial and temporal change in methane flux and related mdac formation at this seep site these changes might be caused by regional seismic events that can affect pore pressure or re activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed

exhalative mineralization in devonian reef complexes of the canning basin western australia lenticular stromatolite barite sulfide mounds interpreted as sedimentary exhalative deposits are exposed at several localities in devonian reef complexes of the canning basin western australia these mounds hundreds of meters long and tens of meters thick occur along contacts between basinal shales gogo formation and marginal slope limestones sadler limestone and adjacent to synsedimentary faults cutting the gogo formation the mounds are limestone buildups composed of stromatolites intergrown with barite and cut by iron sulfide veins the iron sulfides marcasite and pyrite are weathered to gossans at the surface they fill fissures that probably formed as a result of contemporary seismic activity and the early compaction of underlying and adjoining shales the stromatolite barite sulfide deposits also occur as allochthonous clasts in penecontemporaneous debris flow deposits postdepositional compaction of basinal shales in the gogo formation commonly amounted to about 75 percent this resulted in the expulsion of large volumes of fluids along permeable interbeds shale limestone contacts and synsedimentary faults reaching the sea floor as cool fluid seepages the expelled fluids carried dissolved carbonate barium iron and reduced sulfur they probably nourished chemosynthetic bacteria that mediated the deposition of stromatolites barite and iron sulfides resulting in growth of the stromatolite barite sulfide buildups there is no known economic mineralization in the exhalative buildups but some drill holes encountered minor uneconomic veins of epigenetic zn pb mineralization filling narrow fissures that postdate the exhalative deposits economic epigenetic zn pb deposits that have been mined elsewhere in the same general area of the canning basin were formed during the early carboniferous or latest devonian many millions of years after the exhalative deposits both exhalative and epigenetic phases of mineralization are thought to have been sourced through the compaction driven dewatering of shales the exhalative deposits are interpreted to have formed over seepages of cool fluids having salinities close to that of seawater whereas the epigenetic deposits formed from much hotter fluids probably brines expelled during subsequent shale compaction

phosphogenesis associated with the shuram excursion petrographic and geochemical observations from the ediacaran doushantuo formation of south china abstract the ediacaran period witnessed one of the largest phosphogenic events in earth s history coincidently some phosphorite deposits in south china are associated with the largest known carbon isotope negative excursion i e shuram excursion suggesting an intimate coupling of the biogeochemical carbon and phosphorous cycles however the geomicrobiological linkage between these anomalies remain poorly understood in this study we investigated the phosphorite samples from the uppermost doushantuo formation in south china carbon isotope compositions of authigenic calcite cements and nodules in the phosphorites are as low as 34 vpdb petrographic and geochemical investigations indicate that the 13 c depleted carbonates likely formed as the result of microbial sulfate and iron reduction that released phosphorous from iron oxyhydroxide concentrating phosphorous in pore waters and thereby promoting phosphate mineralization the timing of this event appears to coincide with enhanced sulfate delivery to seawater through continental weathering the basin scale distribution of doushantuo phosphorites suggests a redox control on the availability of iron oxyhydroxide and the recycling of pore water phosphorous both inner and outer shelf regions were likely characterized by an oxic water column and were the main loci for phosphogenesis on the contrary intra shelf and slope regions which are lean in phosphorite were subjected to euxinic or ferruginous water column conditions the intimate coupling between ediacaran phosphogenesis and the shuram excursion suggests strong links among seawater redox conditions c s p fe cycling and fossil phosphatization increased microbial sulfate reduction driven by enhanced sulfate reservoir in the ediacaran ocean may have played an essential role on these biogeochemical events

u th systematics and ages of authigenic carbonates from hydrate ridge cascadia margin recorders of fluid flow variations uranium u concentrations and activity ratios 234u of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon rich fluids cold seeps at hydrate ridge off the coast of oregon usa the low u concentrations mean 1 3 0 4 g g and high 234u values 165 317 of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor their 230th 234u ages span a time interval from 0 8 to 6 4 ka and cluster around 1 2 and 4 7 ka in contrast chemoherm carbonates precipitate from marine bottom water marked by relatively high u concentrations mean 5 2 0 8 g g and a mean 234u ratio of 166 3 their u isotopes reflect the 234u ratios of the bottom water being enriched in 234u relative to normal seawater simple mass balance calculations based on u concentrations and their corresponding 234u ratios reveal a contribution of about 11 of sedimentary pore water to the bottom water from the u pore water flux and the reconstructed u pore water concentration a mean flow rate of about 147 68 cm a can be estimated 230th 234u ages of chemoherm carbonates range from 7 3 to 267 6 ka 230th 234u ages of two chemoherms alvin and se knoll chemoherm correspond to time intervals of low sealevel stands in marine isotope stages mis 2 4 5 6 7 and 8 this observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments the 18o pdb ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times deviations of the chemoherm 18o values from the marine 18o record can be interpreted as to reflect temporally and spatially varying bottom water and or vent fluid temperatures during carbonate precipitation between 2 6 and 8 6 c

chemoherms on hydrate ridge unique microbially mediated carbonate build ups growing into the water column two active chemoherm build ups growing freely up into the oceanic water column the pinnacle and the south east knoll chemoherms have been discovered at hydrate ridge on the cascadia continental margin these microbially mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle shaped morphology with steep flanks the recovered rocks are pure carbonates dominated by aragonite based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates the fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates the strongly 13c depleted carbon isotope values of the samples as low as 48 1 pdb are characteristic of methane as the major carbon source for the carbonate formation the methane rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom simulating reflector bsr and rise through fault systems the 18o values of the aragonitic chemoherm carbonates are substantially higher as high as 5 0 pdb than the expected equilibrium value for an aragonite forming from ambient seawater 3 5 pdb as a first approximation this indicates formation from glacial ocean water but other factors are considered as well a conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates this model explains the function of the consortium of archaea and sulfate reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane aom and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate dependent anaerobic oxidation of methane within an oxic environment the contact between beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro environment where carbonate formation takes place

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr