International Conference on Gas Hydrates (ICGH) (6th : 2008)


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DEEP SEA BENTHIC FORAMINIFERA AS A PROXY OF METHANEHYDRATES FROM IODP SITE 890B CASCADIA MARGINAmit Kumar *Trainee office (Geologist)National Gas Hydrates ProgramDirectorate General of HydrocarbonMinistry of Petroleum, Sector-63, Noida, 201301INDIAProf Anil Kumar GuptaGeology and GeophysicsIndian Institute of Technology, Kharagpur, 721302INDIA.ABSTRACTRelease of methane from large marine reservoirs has been linked to climate change, as a causalmechanism and a consequence of temperature changes, during the Holocene to Late Quaternary.These inferred linkages are based primary on variation in benthic foraminifer?s singnatures. Thisstudy examines and illustrates deep sea benthic foraminifera from Holocene to Late Quaternarysample from North Pacific Ocean IODP site 890B,Cascadia Margin. Deep sea benthic foraminiferahas been quantatively analyzed in samples>125 ?m size fractions. Factor and Cluster analysis of the29 highest ranked species made it possible to identify six biofacies, characterizing distinct deep seaenvironmental setting. The environmental interpretation of each biofacies is based on the ecology ofrecent deep sea benthic foraminifera. The benthic faunal record indicates fluctuating deep secondition in environmental parameter including oxygenation, surface productivity and organic foodsupply. The benthic assemblage show a major shift at 2 to3 kyrs BP and 6 to10.5 BP marked bymajor turnover in the relative abundance of species coinciding with in increasing amplitude of inter-stadial cycles. There are strong possibilities of methane flux in this site. Dissociation of gas hydratesand release of methane to the atmosphere could be a cause of increase in the population abundance ofhighly reducing environmental species, which we interpreted in our data.Keywords: gas hydrates, benthic foraminifera, reducing environment___________*Corresponding author: Phone: +91 9910270670 Fax +91, the world faces challenges to meet itsrequirements of conventional source of energy likecoal, petroleum and natural gas. Economically thiscritical situation is the result of increase inconsumption rate than that of the production;hence an alternative source is highly demanded.Here comes the role of gas hydrates. The Gashydrates are naturally occurring solid ice likecrystalline structure composed of water moleculeand short chain hydrocarbon like methane [1], [2].The formation of natural gas hydrates require lowtemperature (?? 12 ?C), high pressure (2.6 Mpa) andhigh organic carbon content of sediments (2.0% to3.5%) and water depths between 300 to 1000meters below sea floor [1], [3], [4], [5], [6]. Themethane formed in gas hydrates may be biogenic[1] or thermogenic [7] in origin.Apart from other proxies like geophysical seismicsurveys that produces Bottom StimulatingReflector (BSR) due to change in acoustic velocityat the interface of base of gas hydrate and free gas,geochemical studies (Chlorine and iodine contentof pore water) and benthic foraminifera also helpin gas hydrate exploration Climate is the endproduct of a multitude of interactions betweenseveral subsystems- the atmosphere, oceans,biosphere, land surface and cryosphere, whichcollectively make up the climate system. Oceansare a sluggish component of the climate system.Surface layers of the ocean respond to externalforces on a timescale of months to years, whereaschanges in the deep oceans are much slower.Paleoceanography and paleoclimatology studypast changes in ocean and climate, which requirean understanding of the proxy data available sothat it can be analyzed methodically.Cascadia Margin is a well-established gas hydratefield and provides an ample opportunity tounderstand methane formation and eruptions usingvarious proxies during the Quaternary. Benthicforaminifera are an important component of themarine community and sensitive to environmentalchanges. The potential of benthic foraminifera haslong been recognized in marine paleo-environmental studiesOver the last three decades, scientists haveincreased their interest to understand differentaspects of benthic foraminifera forpaleoenvironmental reconstructions. The widegeographic and bathymetric distribution, highsensitivity to various ecological factors, extensivemorphological diversity and well-preserved fossilrecord make them an important tool inpaleoceanography and paleoclimatology. Somespecies of benthic foraminifera have been foundassociated with rich organic carbon contentwhereas others with varying oxygen levels of themarine sediments.Numerous species of benthic foraminifera havebeen found in different methane rich marinesettings and have proved to be good indicator ofmethane releases [8], [9], [10], [11]. Some speciesprefer to feed on rich bacterial food sources atmethane seeps showing their potential asindicators of methane release in the geologicalrecord. Some methane loving taxa include speciesof Uvigerina, Bolivina, Bulimina, Chilostomella,Globobulimina and Nonionella, which canwithstand such stressful conditions.We expect the results of this study to be helpful inunderstanding more on the benthic foraminiferaldistribution in the methane rich environment andpaleoceanography of Cascadia Margin.LOCATIONIODP Leg 146, Hole 890 B is located on theContinental slope of the Cascadia Margin(48?39.750minuteN; 126?52.890minuteW; water depth 1326.3meters), northeast Pacific, off the north west coastof Northern America (Fig.1). The CascadiaMargin is a positive topographic sedimentaryfeature on the continental slope. This is abathymetric high located on the Pacific continentalslope approximately 11 km SSW from offVancouver Island. It is approximately 15-20 kmwide region with gentle undulating topographyand lies in a tectonically active setting close to theactive margin.Fig. 1: Location map is showing site 890B,Cascadia Margin [55]METHODOLOGYTotal 174 core samples (5cc) were procured fromIODP Leg 146, Site 890B, Cascadia Margin (asper sample request no.20844A). Samples of Core1H were sliced at 1 cm interval where as samplesfrom Core 2H were sliced at 5 cm interval. Thenumerical age was calculated on the basis of Blakeevent with the help of Magnetostatigrapy [53]All the samples have been catalogued andprocessed using standard procedures [12]. Sampleswere soaked in water with few drops of dilutedHydrogen Peroxide [H2O2 (15%)] for 8-10 hoursin clean and labeled beakers. Hydrogen Peroxidehelps disintegrate the sample matrix faster. Thesoaked samples were washed over a 63?m sizesieve using a jet of water. The methylene blue dyeis spread on the sieve after each washing whichprevents contaminations from the previous sampleand dried in an electric oven at ~50?C. The driedsamples were transferred to labeled glass vials.The samples were split using Otto splitter intosuitable aliquots to obtain 250-300 individualsfrom sample under microscopic examination. Thesplitting process homogenizes the samples andeach aliquot represent the total sample. Coarserthan 125 m size fraction was studied under themicroscope for better comparison with the recentstudies on benthic foraminifera. So far, 59 specieswere identified and the benthic foraminiferalcensus data were generated (census data are madeavailable in (Table: 1).For attaining a perfect paleooceanographicreconstruction by reducing the noises from thedata set, factor and cluster analyses wereperformed on the relative abundance of the highestranked benthic species using SAS/STAT package[54]. 29 species were selected having dominanceof >3% or more in one sample and present in atleast two samples for R-mode factor analysis andQ-mode cluster analysis. This procedure involveda Principal Component Analysis (PCA) followedby a Varimax rotation. On the basis of the screenplot of Eigen values and screening of factorshelped to obtain six factors that account for50.89% of the total variance.Fig. 2: Dendogram based on Q- mode clusteranalysis of 174 samples from IODP site 890 Busing Ward's minimum variance method.Q-mode cluster analysis was performed usingWard?s Minimum Variance method to identifysample groups (Fig. 2). To standardize the data, aPCA was performed on a covariance matrix of 29species prior to cluster analysis. Six major clustersrepresenting six biofacies were identified on thebasis of a plot of semi-partial R-squared valuesversus the number of clusters.The relative abundances of the primary species ofeach assemblage of the biofacies versus age plotsare given in (Fig 3). Ecological preferences ofrecent benthic foraminifera were used to interpretthe environments of these biofacies. [13], [14],[15], [16], [17], [18], [19], [20], [21], [22], [23].RESULTS AND DISCUSSIONMicroscopic Analysis observed that the samplesshow low diversity of fauna compared to Hole888B (water depth 2516m). Cause of such lowdiversity may be due to the high sedimentationrate since the latest Pleistocene (ShipboardScientific Party 1994) and leads to high faunaldensity (greater abundance of few particularspecies).Interpretation of characteristic species comparisingdifferent biofacies at site 890 B.Bulimina exilisAn endemic species of upper oxygen minimumzone, having very low oxygen contain and highOrganic carbon [24].Bolivina alataAn epibenthic, oxygen-poor abyssal water speciesin the upwelling off northeastern Africa. Whereorganic flux is high throughout the year [25], [16].Cassidulina minutaCosmopolitan taxon [25], [16], Mesocosm [26],high organic carbon supply [27], [13], [15], [16].Bolivina ordinariaAn indicative of high organic carbon productivityand they are not hydrocarbon seep indicator; it isgenerally present where the concentration of H2Sis high [9].Uvigerina crassicostataHydrocarbon seeps environment [9], low oxygenconditions shallow infaunal microhabitat inorganic carbon rich sediments, productivity is highyear round and food supply is low or absent [19],[22], [28], [29], [25], [30], [20], [31], [21], [32].Gobocassidulina pacificaCosmopolitan, indicating variable flux of organicmatter (oligotrophic), high seasonality, Low-oxygenated and cold deep water environment.Hence this biofacies is an indicative of highproductivity deep water with seasonal organicfluxes.Elphidium incertumLow oxygen conditions and high food supply, highproductivity and continuous flux of organic matter[33], [34], [16].Melonis berleeanumOpportunistic, largely epifaunal taxa. phytodetritalcarbon flux [35],[14].Bolivina spathulataHigh continuous flux of organic matter to the seafloor [21],Oxygen deficient and high organiccarbon bottom water environment with deepOxygen Minimum Zone during intense upwelling[20].Uvigerina proboscideaLow oxygen conditions shallow infaunalmicrohabitat in organic carbon rich sediments,productivity is high year round and food supply islow or absent [19], [22], [28], [29], [25], [30],[20], [31], [21], [32]. Hydrocarbon seepsenvironment [9].Table 1: Benthic foraminiferal biofacies withpreferred paleoceanographic environment.Studies indicate that the benthic faunalcomposition is strongly correlated withproductivity of the surface waters and the deliveryof organic matter to the seafloor [33], [37], [38],[39]. Thus benthic foraminifera are goodindicators of paleoproductivity especially in areaswhere carbon flux is high [39]. In some cases itwas suggested that lateral advection and bottomwater ventilation are important factors to controlthe distribution of the group [16]. Others relatedthe seasonal pulses of organic matter delivered tothe deep sea with the distribution of certainopportunistic species [40], [41]. The degree ofdegradation of the organic matter that reaches thesediment influences the distribution of certainspecies [42], [31].At Hole 890B benthic faunal biofacies show amajor change near 5 Kyrs BP. From 12-5 Kyrs BPan interval dominated by biofacies Be-Ba, Cm-Bo,Uc-Gp, Ei-Us, Ba-Bs, Uc-Up, characteristic ofintermediate to high rates of organic flux. Thedominance of biofacies Be-Ba, Uc-Gp, Ei-Us, Ba-Bs, Uc-Up during 2 to 3 and 6.5 to10.5 Kyrssuggests warmer conditions marked by sustainedand high flux of organic matter with lowoxygenation when the seasonality was low.Primary productivity in surface waters (e.g.CaCO3, SiO2) was accumulated at very high ratesthroughout the Pacific oceans and biogenic bloomrelates to an elevated supply of nutrients to theocean [43], [44], [45], [46], [47]. Interest in thestudy of deep-sea benthic foraminifera fromdifferent gas hydrate settings from different oceanshas increased over the last one decade. Taxa likeBolivina, Bulimina, Cassidulina, Chilostomella,Globobulimina, Nonionella and Uvigerina havebeen reported from different seep relatedenvironments and can be used as proxy formethane rich environments. Little is known aboutthe hydrographic setting of the study area. It issuggested that sluggish deep thermohaline currentsin transport suspended sediment toward south[48]. These sediments may have influenced thepopulation of benthic foraminifera in the past.Biofacies Factor  EnvironmentBe-BaBulimina exilisBolivina alataNonionella bradyiBolivina subaenaiensis0.417770.407130.324140.28792Highlyreducingenvironmentwithincreased fluxof organicmatterCm-BoCassidulina minutaBolivina ordinariaBrizalina subaen.Melonis berleeanumGlobocassidulina paci.Hoeglundina elegansBulimina striataKarierrella bradyi-0.51506-0.37111-0.32459-0.29982-0.22093-0.19795-0.11477-0.11367High organiccarbonproductivityand they arenothydrocarbonseep indicatorUc-GpUvigerina crassicostataGlobocassidulina paci.Uvigerina costataEpistominella exiguaNonionella bradyi0.556380.376350.351650.333960.30911Highproductivitydeep waterwith seasonalorganic fluxesEi-UsElphidium incertumUvigerina schwageriMelonis berleeanumCassidulina carinataUvigerina crassicostataEpistominella exigua0.560940.457420.366850.338290.331660.30827HighContinuousflux oforganicmatter andhighproductivityBa-BsBolivina alataBolivina spathulataBolivina ordinariaBrizalina subaen.Hoeglundina elegansBulimina marginataAllomorphina spBolivina subaenaiensis0.706140.642250.61440.590710.460320.353110.336920.31246Indicative ofmethaneseepage andor highlyredoxenvironmentUc-UpUvigerina crassicostataUvigerina proboscideaBolivina semicoastataNonionella bradyiKarierrella bradyiCassidulina laevigata0.677010.608640.598050.458520.454450.31286Highproductivityandseasonality ofthe foodsupply is lowFig. 3: Benthic biofacies plotted againstinterpolated ages, combined with relativeabundances of two major benthic foraminifers ineach biofacies assemblage, and lowermost panelshows percentage abundances of benthicforaminifera Uvigerina species.In the present study, we found six biofaciesdominating over the studied section. Benthicforaminiferal biofacies assemblage suggests thatduring the early to Mid-Holocene (7.4 to 4.5 Kyr)time, productivity was high with higher productionof algal matter in the surface. Dominance ofElphidium incertum and Uvigerina schwageri inthis interval indicates presence of degradedorganic carbon, since these species have apreference for labile organic matter.The second important biofacies Ba-Bs dominatesthe Early Holocene interval (3.5 to 2 Kyr). Majorcomponents of this biofacies are Bolivina andBulimina. These are showing increasing trendtoward 10 to 2 Kyrs. These taxa are reported fromseep zones and indicate high reducingenvironment. They prefer dysoxic environmentirrespective of organic carbon. High productivitymay be due to presence of organic carbon orsulphate or methane. So dominance of these faunaof methane loving taxa in this interval could belinked to dissociation of gas hydrate.Biofacies Cm-Bo shows organic carbon richdysoxic to anoxic environment during the timeinterval 3.5 to 7 Kyr. But they are not hydrocarbonseep indicator; it is due to present of highconcentration of H2S. Biofacies Uc-Up associatedwith Uvigerina proboscidea is showing highsurface productivity interval from 0 to 0.5 Kyr andmay be due to terrestrial organic carbon.CONCLUSIONSThe high resolution studies of deep sea benthicforaminifera using 174 samples from the methanerich environment of Cascadia margin (IODP, Leg164, Site 890 B) for the past 13.49 Kyrs found themajor distribution of 29 dominant species (ie, >3%or more in at least one sample and present in atleast two samples) .The R mode factor and Qmode cluster analysis performed on the 29dominant species found six clusters and theirrespective six biofacies. Based on the availableand known paleoceanographic environment of thebenthic foraminifera paleoceanic conditions forthe past 13.49 kyrs. The particular taxa offoraminifera like Uvigerina, Bolivina, Bulimina,Epistominella are adapted to high organic, lowoxygen, reducing environments of modernmethane seep environments from Gulf of Mexico[9], Northern California /El River area [10],Monterey, CA, USA [49] and Offshore Japan[50].Uvigerina peregrina species is also foundflourishing in hydrocarbon seeps environment ofGreen Canyon, Gulf of Mexico [9] and isconsidered to be facultative anaerobes andBulimina spathulata association and relativelyhigh abundance are also reported from methaneseep environments of Green Canyon, Gulf ofMexico [51].The biofacies Ba-Bs suggests a highly reducingmethane seep environments with sustained flux oforganic matter to the sea floor and high organiccarbon productivity and the assemblage ofbiofacies El-Us indicates high productivity andlow oxygenated environment with high foodsupply. Close study of our data of U.peregrina andoxygen minimum zone species abundance issudden decrease at ~12 Kyrs (fig.3). It shows theYounger Dryas event. It is interval of cold spellsin northern hemisphere [52]. In (fig 3) increasingtrend of U. peregrina during the Early Holocenefrom 10.5 to 6 Kyr BP and decreasing trend of Ei-Us showing the transition period of Mid Holoceneto Early Holocene. On the basis of these resultsand availability of methane loving taxa, likeUvigerina,Epistominella, Melonis, Bolivina,Bulimina, Chilostomella and Cassidulina, there arestrong possibilities of methane flux in this site.Dissociation of gas hydrates and release ofmethane to the atmosphere could be a cause ofincrease in the population abundance of highlyreducing environmental species, which weinterpreted in our data. We can thus, conclude thesite is a probable zone for the methane fluxes andmay be fruitful for future study on methaneseepage in the deep sea location.Acknowledgements:I am thankful to my supervisor Prof Anil KumarGupta, HOD, Department Geology andGeophysics, IIT, Kharagpur for providing me thesamples, his indispensable guidance, valuablesuggestions and providing all the necessaryfacilities during the course of this project work. Iam highly grateful to integrated ocean drillingprogramme (IODP) for providing the sample fromleg 164, site 890B, obtained through mysupervisorReferences:[1]Claypool, G. and Kaplan, I., (1974). The originand distribution of methane in marine sediments.In:Kaplan, I. (eds) Natural gases in marine sediments.Plenum, New York, 315-340.[2]Sloan, E. D. (1990), Clathrate hydrates of naturalgas. Marcel Dikker, New York.[3]Kvenvolden, K. A. (1993), Gas hydrate?geological perspective and global change. Reviews ofGeophysics,  31, 173-187.[4]Malone, R. D. 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