natural gas hydrates on the southeast u s margin constraints from full waveform and travel time inversions of wide angle seismic data strong bottom simulating reflectors bsr have been mapped over a region of approximately 50 000 km2 on the southeastern u s margin and have been associated with possible abundance of natural gas hydrates in june 1992 coincident single channel seismic and wide angle ocean bottom seismic data were acquired in the region focusing on the blake ridge and the carolina rise wide angle reflections from the bsrs were clearly observed at offsets up to 6 km joint travel time inversion was conducted with wide angle and vertical incidence data in order to explore possible regional variation and the resultant two dimensional average velocity models imply higher background velocities on the carolina rise full waveform inversion was then performed to determine the seismic origin of the bsrs the best fit model shows a similar low velocity 1 4 km s beneath the bsr at both sites indicating trapped free gas with low saturation 10 the inversion results also indicate that a thin high velocity wedge with a maximum velocity of 2 3 km s is present just above the blake ridge bsr sediment reflectivities were also calculated and higher reflectivities are observed on the carolina rise an increase in reflectivity below the bsr seems to correspond to the gas bearing zone at both sites concentration of hydrates were estimated based on these velocity models whereas average hydrate concentration of 3 of the total sediment volume is suggested for the lower half of the hydrate stability zone at the blake ridge only a very low average concentration of hydrate can be expected at the carolina rise the hydrates seem to be concentrated near the base of the hydrate stability zone and the maximum hydrate concentration is estimated as 20 at the blake ridge and 7 at the carolina rise both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism it is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites additional enhancement by upward fluid expulsion may also be viable for the blake ridge

changing atmospheric δ14c and the record of deep water paleoventilation ages we propose a new calculation method to better estimate the deep water ventilation age from benthic planktonic foraminifera 14 c ages our study is motivated by the fact that changes in atmospheric 14 c through time can cause contemporary benthic and planktonic foraminifera to have different initial 14 c values this effect can cause spurious ventilation age changes to be interpreted from the geologic data using a new calculation method 14 c projection ages we recalculate the data from the pacific ocean contrary to previous results we find that the pacific intermediate and deep waters were about 600 years older than today at the last glacial maximum in addition there are possible signals of ventilation age change prior to ice sheet melting and at the younger dryas however the data are still too sparse to constrain these ventilation transients

monazite dating of granitic gneisses and leucogranites from the kerala khondalite belt southern india implications for late proterozoic crustal evolution in east gondwana two stages of granitic magmatism occurred during the pan african evolution of the kerala khondalite belt kkb in southern india granitic gneisses were derived from porphyritic granites which intruded prior to the main stage of deformation and peak metamorphism subsequently leucogranites and leucotonalites formed during fluid absent melting and intruded the gneiss sequences monazites from granitic gneisses leucogranites and a leucotonalite were investigated by conventional u pb and electron microprobe dating in order to distinguish the different stages of magma emplacement u pb monazite dating yielded a wide range of ages between 590 520 ma which are interpreted to date high grade metamorphism rather than magma emplacement the results of this study indicate that the kkb experienced protracted heating 50 ma at temperatures above 750 800 c during the pan african orogeny the tectonometamorphic evolution of the study area is comparable to southern madagascar which underwent a similar sequence of events earlier than the kkb the results of this study further substantiate previous assertions that the timing of high grade metamorphism in east gondwana shifted from west to east during the late proterozoic

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

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

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

tectonics of the neoproterozoic southern granulite terrain south india geological correlations between south india and adjacent gondwana fragments are hampered by the lack of a coherent structural framework for india s southern granulite terrain sgt this paper is an attempt at delineating its large scale geometry and neoproterozoic kinematics and tectonics the basic regional pattern is that of a high grade gneissic layering produced during several deformation stages grouped as d1 which became refolded during two regionally separated phases of upright folding d2a nnw trending d2b ne trending and reworked in e to ese trending zones of increased strain d3 of these the palghat cauvery shear zone pcsz which marks the boundary between archaean terrains in the north and proterozoic belts in the south may represent a major crustal shear zone whereas the achankovil zone az is better considered as a high strain zone characterized by flattening with a reverse component d1 structures developed close to the peak of pan african high grade metamorphism d2a and d2b occurred on the retrograde path partly at supersolidus and partly at subsolidus conditions d3 high strain zones show subsolidus textures in view of the general clockwise nature with a clear decompressional component of all published p t paths we propose that d2 d3 occurred during partial exhumation of the terrain correlations with madagascar are weakened as the az is no longer considered a major crustal shear zone whereas correlations with sri lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor

neoproterozoic alkaline magmatism at sivamalai southern india abstract the sivamalai alkaline complex lies at the southern margin of the cauvery shear system that separates the archaean and proterozoic domains of the southern granulite terrain in india u pb tims dating of zircon from a pegmatitic syenite sample in the complex yields a concordant age of 590 2 1 3 2 ma which is interpreted to date the intrusion of the alkaline rocks a lower concordia intercept at 168 210 ma defined by two grains with high common lead may indicate post magmatic disturbances due to recrystallisation which is also evident in the cl images of the zircons epma dating of monazite from a post kinematic pegmatite which intrudes the crystalline basement hosting the alkaline rocks yields an age of 478 29 2 ma and provides a lower bracket for the main phase of tectonism in this part of the southern granulite terrain the pan african high grade metamorphism and ductile deformation has thus most likely affected the alkaline rocks this is supported by the presence of a metamorphic foliation and extensive recrystallisation textures seen in the rocks the major and trace element concentrations measured on selected samples reveals the presence of both enriched and depleted rock types the enriched group includes ferrosyenite and nepheline syenite while the depleted group has only nepheline syenites the trace element depletion of some nepheline syenites is interpreted to be a result of fractional crystallization involving the removal of accessory phases like zircon titanite apatite and allanite

acquisition footprints and seafloor coupling in multicomponent obc seismic data in four component 4 c towed ocean bottom cable obc data sets acquisition footprints are often observed sometimes these exhibit a spatial period equal to the length of the receiver cable i have analyzed a 2d 4 c obc data set looking at common offset gathers cog spectral analyses and hodogram analyses of the direct p wave first breaks the acquisition footprint is seen to be directly related to the following effects observed on a few of the multicomponent receivers namely those nearest to the towing vessel significant delays on the inline component though not on the downgoing direct p first breaks depletion of higher frequencies narrower bandwidth on the inline component and oscillatory motion closer to the vertical on the direct p first breaks equivalent to decreased amplitude on the in line component this is interpreted to be a result of the towing procedure wherein the leading end of the cable with the first few receiver modules is raised from the seafloor and laid down again relat

depth registration of p wave and c wave seismic data for shallow marine sediment characterization gulf of mexico multicomponent seismic data combining p wave and converted p to sv wave c wave wavefields provide independent measurements of rock and fluid properties unlike p waves c waves are minimally affected by changes in pore fluids and in cases of azimuthal anisotropy will be split into two modes fast and slow with differing polarization the 4c 3d ocean bottom cable obc multicomponent seismic data discussed here were acquired in shallow water 300 ft offshore louisiana over approximately 455 miles2 figure 1 because these data are still being marketed to interested oil and gas operators only data above targeted oil and gas reservoirs 5000 ft were used the p wave migrated data extend to only 1 s and the c wave migrated data volume to only 2 s

quantitative detection of methane hydrate through high resolution seismic velocity analysis a laterally extensive high resolution travel time velocity analysis and acoustic wave form inversion were used to quantitatively determine methane hydrate content in deep water sediments of the blake ridge off the southeast u s coast the interval acoustic velocity vp analyses were performed in the p domain by interactively picking the p trajectories of prominent reflections in each of 50 plane wave decomposed common midpoint gathers the reflections correspond to seismic stratigraphic boundaries so that lateral vp changes due to lithology changes are mitigated and vp changes due to changing hydrate content are enhanced two separate interval vp analyses were performed one with thick 0 4 km layers which yielded lower uncertainty but also lower resolution and one with thinner layers 0 1 km yielding higher resolution but slightly larger uncertainties results show no correlation between low sediment reflectivity and vp however in the areas exhibiting a bottom simulating reflector bsr a high vp interval 2 0 km s and 0 15 km thick is seen immediately above the bsr where the bsr is strongest a 256 layer least squares acoustic wave form inversion reveals the bsr to be caused by a vp decrease from 2 0 to 1 5 km s with little or no change in density the inversion also reveals a thin 0 025 km layer of anomalously low vp lying immediately below the bsr two models of methane hydrate distribution are tested each indicating that the volume of methane hydrate in the intervals of elevated vp is up to 25 of the total volume

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate

sea surface temperature estimates from the southwest pacific based on planktonic foraminifera and oxygen isotopes planktonic foraminiferal census data were collected on 34 surficial sediment samples from east of new zealand sea surface temperature estimates were derived from the data using the modern analogue technique mat and compared to data from climap s fa 20 equation they were also computed from oxygen isotope analyses of globigerina bulloides the temperature estimates compared well between the three methods and were found to correlate closely with the appropriate seasonal sea surface temperature as measured by satellite the foraminiferal assemblages and derivative temperatures clearly identify the major watermasses and oceanic fronts in the region and thus can be applied to palaeoceanographic analysis of southwest pacific sediments

high radiogenic heat production in the kerala khondalite block southern granulite province india in situ radioelemental k u and th analysis and heat production estimates have been made at 59 sites in the kerala khondalite block kkb of the southern granulite province sgp of india together with the in situ analyses on granulites and gneisses previously reported from 28 sites and heat production estimated from the published geochemical analyses on granites and syenites of the kkb the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the kkb garnet biotite gneisses are characterized by high levels of th and u with mean values of 60 and 3 ppm respectively khondalites leptynites and charnockites have slightly lower levels of th 23 20 and 22 ppm respectively and u 2 9 2 4 and 0 9 ppm respectively the mean k u th abundances for the granites leucogranites and granitic gneisses ranges from 3 9 to 4 3 2 6 to 4 3 ppm 22 to 50 ppm respectively and for the syenites 4 8 2 ppm and 5 7 ppm mean radiogenic heat production values for garnet biotite gneiss khondalite leptynite and charnockite are 5 5 2 7 2 4 and 2 2 w m 3 respectively for the granites leucogranites granitic gneisses and syenites it is 2 6 3 4 4 6 and 1 4 w m 3 respectively heat production of granulite facies rocks which are the most abundant rocks in kkb correlate well with th but less with u suggesting that variation is caused by th and u bearing accessory minerals such as monazite and zircon the high heat production of the kkb granulites are in contrast to the low heat production of the late archaean granulites of the northern block nb of the sgp which are highly depleted in radioelements and also the granulites of madurai block mb that have higher radioelemental abundances than in the granulites of the nb the high heat production of the kkb granulites could be due to the nature of protoliths and or metasomatism associated with neoproteroic to pan african alkaline magmatic activity represented by alkali granite and syenite carbonatite emplacements and emplacement of pegmatites

glacial and holocene stable isotope distributions in the southeastern indian ocean we have reconstructed glacial interglacial variations in cibicidoides wuellerstorfi 13c and 18o in the southeastern indian ocean using depth transects of cores from off western and southern australia we compare these new results with published stable isotope records to constrain glacial interglacial changes in indian ocean chemistry and circulation the strong last glacial maximum lgm deep water 13c depletions previously observed for the northeastern indian ocean did not extend throughout the low latitude glacial indian ocean we observe only a small lgm 13c decrease off southern australia in marked contrast to the strong lgm 13c depletions observed in many cores from the atlantic and western indian sectors of the southern ocean it is not yet clear whether the very low lgm 13c values in the deep northern and southwestern indian ocean are an artifact of surface ocean productivity or whether they reflect real gradients in deep water chemistry

last glacial maximum paleochemistry and deepwater circulation in the southern ocean evidence from foraminiferal cadmium south atlantic benthic foraminiferal cd ca shows no glacial interglacial variation suggesting that the glacial contribution of north atlantic deep water to the southern ocean was not much different than at present in contrast cd ca in southeast indian ridge cores show lower glacial bottom water cd comparable to levels in intermediate depths of the north atlantic and significantly lower than in the deep south atlantic low glacial cd ca was also recorded in planktonic foraminifera suggesting a substantial decrease in the nutrient concentration of subantarctic surface water during the glacial maximum which most likely was caused by increased biological productivity the cd data are inconsistent with low glacial benthic foraminiferal 13c which suggest higher nutrient concentration we propose that the low cd ca in the southeast indian ridge records reflects a local source of nutrient depleted deepwater formed during the last glacial maximum by open ocean convection near the antarctic polar front downstream of the kerguelene plateau if this source was limited to the southeast indian basin then its impact on the overall chemistry of glacial circumpolar deepwater was rather small however if during glaciations open ocean convection became the dominant mode of bottom water formation it might have had a greater impact on cpdw chemistry

combined vertical incidence and wide angle seismic study of a gas hydrate zone blake ridge wide angle ocean bottom seismic data and single channel seismic reflection data were collected in june 1992 over an area where gas hydrates are thought to be extensive on blake ridge offshore south carolina wide angle reflections were observed on four woods hole oceanographic institution ocean bottom hydrophones at offsets up to 15 km results from traveltime inversion show that the bottom simulating reflector bsr that marks the base of the hydrate stability field lies 400 500 m below the seafloor and is overlain by a 200 to 300 m thick layer of average velocity 1 9 km s 1 there is no evidence for significant lateral velocity variation associated with lateral changes in bsr character ray tracing calculations show that the observation of the bsr out to large offsets 7 km constrains the maximum vertical velocity gradient to about 0 5 s 1 amplitude versus offset avo analysis was performed for ranges up to about 6 km and incidence angles of more than 70 the high amplitude of the bsr in the normal incidence data can be explained by two models both of which include a strong negative impedance contrast the hydrate wedge model and the free gas layer model synthetic seismogram calculation for the wedge model using the reflectivity method shows a strong increase in the bsr amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge this amplitude increase is clearly not observed in the data which shows only a relatively modest increase in amplitude with offset this observation agrees better with the calculated avo for the gas layer model the lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and or that the base of the gas layer is a gradational boundary

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

integrated 2d 4 c obc velocity analysis of near seafloor sediments green canyon gulf of mexico using 2d four component ocean bottom cable 2d 4 c obc seismic data processed in common receiver gathers we developed robust vp and vs interval velocities for the near seafloor strata a vital element of the study was to implement iterative interpretation techniques to correlate near seafloor p p and p sv images initially depth equivalent p p and p sv layers were interpreted by visually matching similar events in both seismic modes complementary 1d ray tracing analyses then determined interval values of subsea floor vp and vs velocities across a series of earth layers extending from the seafloor to below the base of the hydrate stability zone bhsz to further constrain these interpretations iterating interpretation of depth equivalent horizons with velocity analyses allowed us to converge on physically reasonable velocity models simultaneous vp and vs velocity analysis provided additional model constraints in areas where data quality of one reflection mode usually vp in the near seafloor environm

gas venting and subsurface charge in the green canyon area gulf of mexico continental slope evidence of a deep bacterial methane source abstract questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the gulf of mexico continental slope remain unanswered a research submersible was used to sample mixed bacterial and thermal gas 13 c of methane 62 8 d 176 venting to the water column from the gulf slope in green canyon gc 286 the 14 c value of the methane 998 is consistent with fossil carbon another gas vent on gc 185 is 100 methane 13 c 62 9 d 155 and may be from a bacterial source the 14 c 997 of this bacterial methane is also consistent with fossil carbon fossil bacterial methane and thermal hydrocarbons are present in pliocene to pleistocene reservoirs 3509 4184 m of genesis field gc 205 161 160 oil in these reservoirs is biodegraded but gas is not suggesting that gas charge to reservoirs continues presently at 3 4 km depth mixed thermal and bacterial methane may charge the deep reservoirs and fossil methane from depth may ultimately vent on the sea floor at gc 286 and gc 185 results of this study of green canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources

late quaternary caco3 production and preservation in the southern ocean implications for oceanic and atmospheric carbon cycling recent geochemical models invoke ocean alkalinity changes particularly in the surface southern ocean to explain glacial age pco2 reduction in such models alkalinity increases in glacial periods are driven by reductions in north atlantic deep water nadw supply which lead to increases in deep water nutrients and dissolution of carbonate sediments and to increased alkalinity of circumpolar deep water upwelling in the surface southern ocean we use cores from the southeast indian ridge and from the deep cape basin in the south atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by circumpolar deep water this suggests that deep southern ocean carbonate ion concentrations were lower in glacial stages than in interglacials rather than higher as suggested by the polar alkalinity model broecker and peng 1989 our observations show that changes in southern ocean caco3 preservation are coherent with changes in the relative flux of nadw suggesting that southern ocean carbonate chemistry is closely linked to changes in deepwater circulation the pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient depleted water nadw to the southern ocean and with an increase of nutrients in deep water masses carbonate mass accumulation rates on the southeast indian ridge 3200 3800 m and in relatively shallow cores 3000 m from the kerguelen plateau and the south pacific were significantly reduced during glacial stages by about 50 the reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in caco3 corg flux ratios acting as another mechanism which would raise the alkalinity of southern ocean surface waters the polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant even if overall productivity in the southern ocean were held constant a decrease in the caco3 corg ratio would result in increased alkalinity and reduced pco2 in southern ocean surface waters during glacials this ecologically driven surface alkalinity change may enhance deepwater mediated changes in alkalinity and amplify rapid changes in pco2

cooling history of the puttetti alkali syenite pluton southern india abstract the puttetti alkali syenite pluton in southern india belongs to the suite of felsic magmatic intrusives emplaced during the late neoprotrozoic cambrian time during the final phase of amalgamation of the gondwana supercontinent in this study we evaluate the cooling history of this pluton based on various isotopic systems we present whole rock pb pb data on the syenite which yields an isochron age of 508 25ma three phlogopite separates from the syenite pluton give k ar ages of 454 0 9 0 448 5 8 9 and 445 6 8 8 ma indicating cooling age at temperatures of 415 c u pb analyses of zircons from this syenite yielded an age of 572 2 ma in a previous study with u pb closure temperatures 800 o c this age probably indicates the timing of emplacement of the puttetti pluton collectively we estimate from the isotopic age data and respective closure temperatures that the syenite body cooled at about 3 2 o c ma from about 800 o c to about 415 o c the markedly low cooling rate of the syenite pluton absence of chilled margin effects and common occurrence of pyroxene feldspar phlogopite and zircons megacrysts in the rock indicate that the host granulites were at high temperatures during the emplacement of the syenite magma the cooling history of puttetti syenite estimated in this study is closely comparable with the 3 4 o c ma cooling rate estimated for a granite pluton in a previous study from madagascar our study suggests protracted cooling rates for the late pan african intrusives emplaced within the gondwana crust with a long residence history in a hot crust bore they were exhumed to shallower levels

high resolution multicomponent seismic imaging of deepwater gas hydrate systems multicomponent seismic data have unique value for studying near seafloor geology in deepwater environments when properly processed pp compressional and ps converted shear images made from multicomponent seismic data acquired in deepwater with seafloor sensors show near seafloor geology with impressive detail these high resolution images are invaluable for studying deepwater gas hydrate systems