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


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DEEP SEA BENTHIC FORAMINIFERA AS A PROXY OF METHANE HYDRATES FROM IODP SITE 890B CASCADIA MARGIN Amit Kumar * Trainee office (Geologist) National Gas Hydrates Program Directorate General of Hydrocarbon Ministry of Petroleum, Sector-63, Noida, 201301 INDIA Prof Anil Kumar Gupta Geology and Geophysics Indian Institute of Technology, Kharagpur, 721302 INDIA. ABSTRACT Release of methane from large marine reservoirs has been linked to climate change, as a causal mechanism 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. This study examines and illustrates deep sea benthic foraminifera from Holocene to Late Quaternary sample from North Pacific Ocean IODP site 890B,Cascadia Margin. Deep sea benthic foraminifera has been quantatively analyzed in samples>125 µm size fractions. Factor and Cluster analysis of the 29 highest ranked species made it possible to identify six biofacies, characterizing distinct deep sea environmental setting. The environmental interpretation of each biofacies is based on the ecology of recent deep sea benthic foraminifera. The benthic faunal record indicates fluctuating deep se condition in environmental parameter including oxygenation, surface productivity and organic food supply. The benthic assemblage show a major shift at 2 to3 kyrs BP and 6 to10.5 BP marked by major 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 hydrates and release of methane to the atmosphere could be a cause of increase in the population abundance of highly reducing environmental species, which we interpreted in our data. Keywords: gas hydrates, benthic foraminifera, reducing environment ___________ *Corresponding author: Phone: +91 9910270670 Fax +91 1204029410 INTRODUCTION Presently, the world faces challenges to meet its requirements of conventional source of energy like coal, petroleum and natural gas. Economically this critical situation is the result of increase in consumption rate than that of the production; hence an alternative source is highly demanded. Here comes the role of gas hydrates. The Gas hydrates are naturally occurring solid ice like crystalline structure composed of water molecule and short chain hydrocarbon like methane [1], [2]. The formation of natural gas hydrates require low temperature (?12 ºC), high pressure (2.6 Mpa) and high organic carbon content of sediments (2.0% to 3.5%) and water depths between 300 to 1000 meters below sea floor [1], [3], [4], [5], [6]. The methane formed in gas hydrates may be biogenic [1] or thermogenic [7] in origin. Apart from other proxies like geophysical seismic surveys that produces Bottom Stimulating Reflector (BSR) due to change in acoustic velocity at the interface of base of gas hydrate and free gas, geochemical studies (Chlorine and iodine content of pore water) and benthic foraminifera also help in gas hydrate exploration Climate is the end product of a multitude of interactions between several subsystems- the atmosphere, oceans, biosphere, land surface and cryosphere, which collectively make up the climate system. Oceans are a sluggish component of the climate system. Surface layers of the ocean respond to external forces on a timescale of months to years, whereas changes in the deep oceans are much slower. Paleoceanography and paleoclimatology study past changes in ocean and climate, which require an understanding of the proxy data available so that it can be analyzed methodically. Cascadia Margin is a well-established gas hydrate field and provides an ample opportunity to understand methane formation and eruptions using various proxies during the Quaternary. Benthic foraminifera are an important component of the marine community and sensitive to environmental changes. The potential of benthic foraminifera has long been recognized in marine paleo- environmental studies Over the last three decades, scientists have increased their interest to understand different aspects of benthic foraminifera for paleoenvironmental reconstructions. The wide geographic and bathymetric distribution, high sensitivity to various ecological factors, extensive morphological diversity and well-preserved fossil record make them an important tool in paleoceanography and paleoclimatology. Some species of benthic foraminifera have been found associated with rich organic carbon content whereas others with varying oxygen levels of the marine sediments. Numerous species of benthic foraminifera have been found in different methane rich marine settings and have proved to be good indicator of methane releases [8], [9], [10], [11]. Some species prefer to feed on rich bacterial food sources at methane seeps showing their potential as indicators of methane release in the geological record. Some methane loving taxa include species of Uvigerina, Bolivina, Bulimina, Chilostomella, Globobulimina and Nonionella, which can withstand such stressful conditions. We expect the results of this study to be helpful in understanding more on the benthic foraminiferal distribution in the methane rich environment and paleoceanography of Cascadia Margin. LOCATION IODP Leg 146, Hole 890 B is located on the Continental slope of the Cascadia Margin (48°39.750¢N; 126°52.890¢W; water depth 1326.3 meters), northeast Pacific, off the north west coast of Northern America (Fig.1). The Cascadia Margin is a positive topographic sedimentary feature on the continental slope. This is a bathymetric high located on the Pacific continental slope approximately 11 km SSW from off Vancouver Island. It is approximately 15-20 km wide region with gentle undulating topography and lies in a tectonically active setting close to the active margin. Fig. 1: Location map is showing site 890B, Cascadia Margin [55] METHODOLOGY Total 174 core samples (5cc) were procured from IODP Leg 146, Site 890B, Cascadia Margin (as per sample request no.20844A). Samples of Core 1H were sliced at 1 cm interval where as samples from Core 2H were sliced at 5 cm interval. The numerical age was calculated on the basis of Blake event with the help of Magnetostatigrapy [53] All the samples have been catalogued and processed using standard procedures [12]. Samples were soaked in water with few drops of diluted Hydrogen Peroxide [H2O2 (15%)] for 8-10 hours in clean and labeled beakers. Hydrogen Peroxide helps disintegrate the sample matrix faster. The soaked samples were washed over a 63mm size sieve using a jet of water. The methylene blue dye is spread on the sieve after each washing which prevents contaminations from the previous sample and dried in an electric oven at ~50°C. The dried samples were transferred to labeled glass vials. The samples were split using Otto splitter into suitable aliquots to obtain 250-300 individuals from sample under microscopic examination. The splitting process homogenizes the samples and each aliquot represent the total sample. Coarser than 125?m size fraction was studied under the microscope for better comparison with the recent studies on benthic foraminifera. So far, 59 species were identified and the benthic foraminiferal census data were generated (census data are made available in (Table: 1). For attaining a perfect paleooceanographic reconstruction by reducing the noises from the data set, factor and cluster analyses were performed on the relative abundance of the highest ranked benthic species using SAS/STAT package [54]. 29 species were selected having dominance of >3% or more in one sample and present in at least two samples for R-mode factor analysis and Q-mode cluster analysis. This procedure involved a Principal Component Analysis (PCA) followed by a Varimax rotation. On the basis of the screen plot of Eigen values and screening of factors helped to obtain six factors that account for 50.89% of the total variance. Fig. 2: Dendogram based on Q- mode cluster analysis of 174 samples from IODP site 890 B using Ward's minimum variance method. Q-mode cluster analysis was performed using Ward’s Minimum Variance method to identify sample groups (Fig. 2). To standardize the data, a PCA was performed on a covariance matrix of 29 species prior to cluster analysis. Six major clusters representing six biofacies were identified on the basis of a plot of semi-partial R-squared values versus the number of clusters. The relative abundances of the primary species of each assemblage of the biofacies versus age plots are given in (Fig 3). Ecological preferences of recent benthic foraminifera were used to interpret the environments of these biofacies. [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. RESULTS AND DISCUSSION Microscopic Analysis observed that the samples show low diversity of fauna compared to Hole 888B (water depth 2516m). Cause of such low diversity may be due to the high sedimentation rate since the latest Pleistocene (Shipboard Scientific Party 1994) and leads to high faunal density (greater abundance of few particular species). Interpretation of characteristic species comparising different biofacies at site 890 B. Bulimina exilis An endemic species of upper oxygen minimum zone, having very low oxygen contain and high Organic carbon [24]. Bolivina alata An epibenthic, oxygen-poor abyssal water species in the upwelling off northeastern Africa. Where organic flux is high throughout the year [25], [16]. Cassidulina minuta Cosmopolitan taxon [25], [16], Mesocosm [26], high organic carbon supply [27], [13], [15], [16]. Bolivina ordinaria An indicative of high organic carbon productivity and they are not hydrocarbon seep indicator; it is generally present where the concentration of H2S is high [9]. Uvigerina crassicostata Hydrocarbon seeps environment [9], low oxygen conditions shallow infaunal microhabitat in organic carbon rich sediments, productivity is high year round and food supply is low or absent [19], [22], [28], [29], [25], [30], [20], [31], [21], [32]. Gobocassidulina pacifica Cosmopolitan, indicating variable flux of organic matter (oligotrophic), high seasonality, Low- oxygenated and cold deep water environment. Hence this biofacies is an indicative of high productivity deep water with seasonal organic fluxes. Elphidium incertum Low oxygen conditions and high food supply, high productivity and continuous flux of organic matter [33], [34], [16]. Melonis berleeanum Opportunistic, largely epifaunal taxa. phytodetrital carbon flux [35],[14]. Bolivina spathulata High continuous flux of organic matter to the sea floor [21],Oxygen deficient and high organic carbon bottom water environment with deep Oxygen Minimum Zone during intense upwelling [20]. Uvigerina proboscidea Low oxygen conditions shallow infaunal microhabitat in organic carbon rich sediments, productivity is high year round and food supply is low or absent [19], [22], [28], [29], [25], [30], [20], [31], [21], [32]. Hydrocarbon seeps environment [9]. Table 1: Benthic foraminiferal biofacies with preferred paleoceanographic environment. Studies indicate that the benthic faunal composition is strongly correlated with productivity of the surface waters and the delivery of organic matter to the seafloor [33], [37], [38], [39]. Thus benthic foraminifera are good indicators of paleoproductivity especially in areas where carbon flux is high [39]. In some cases it was suggested that lateral advection and bottom water ventilation are important factors to control the distribution of the group [16]. Others related the seasonal pulses of organic matter delivered to the deep sea with the distribution of certain opportunistic species [40], [41]. The degree of degradation of the organic matter that reaches the sediment influences the distribution of certain species [42], [31]. At Hole 890B benthic faunal biofacies show a major change near 5 Kyrs BP. From 12-5 Kyrs BP an interval dominated by biofacies Be-Ba, Cm-Bo, Uc-Gp, Ei-Us, Ba-Bs, Uc-Up, characteristic of intermediate to high rates of organic flux. The dominance of biofacies Be-Ba, Uc-Gp, Ei-Us, Ba- Bs, Uc-Up during 2 to 3 and 6.5 to10.5 Kyrs suggests warmer conditions marked by sustained and high flux of organic matter with low oxygenation when the seasonality was low. Primary productivity in surface waters (e.g. CaCO3, SiO2) was accumulated at very high rates throughout the Pacific oceans and biogenic bloom relates to an elevated supply of nutrients to the ocean [43], [44], [45], [46], [47]. Interest in the study of deep-sea benthic foraminifera from different gas hydrate settings from different oceans has increased over the last one decade. Taxa like Bolivina, Bulimina, Cassidulina, Chilostomella, Globobulimina, Nonionella and Uvigerina have been reported from different seep related environments and can be used as proxy for methane rich environments. Little is known about the hydrographic setting of the study area. It is suggested that sluggish deep thermohaline currents in transport suspended sediment toward south [48]. These sediments may have influenced the population of benthic foraminifera in the past. Biofacies Factor  Environment Be-Ba Bulimina exilis Bolivina alata Nonionella bradyi Bolivina subaenaiensis 0.41777 0.40713 0.32414 0.28792 Highly reducing environment with increased flux of organic matter Cm-Bo Cassidulina minuta Bolivina ordinaria Brizalina subaen. Melonis berleeanum Globocassidulina paci. Hoeglundina elegans Bulimina striata Karierrella bradyi -0.51506 -0.37111 -0.32459 -0.29982 -0.22093 -0.19795 -0.11477 -0.11367 High organic carbon productivity and they are not hydrocarbon seep indicator Uc-Gp Uvigerina crassicostata Globocassidulina paci. Uvigerina costata Epistominella exigua Nonionella bradyi 0.55638 0.37635 0.35165 0.33396 0.30911 High productivity deep water with seasonal organic fluxes Ei-Us Elphidium incertum Uvigerina schwageri Melonis berleeanum Cassidulina carinata Uvigerina crassicostata Epistominella exigua 0.56094 0.45742 0.36685 0.33829 0.33166 0.30827 High Continuous flux of organic matter and high productivity Ba-Bs Bolivina alata Bolivina spathulata Bolivina ordinaria Brizalina subaen. Hoeglundina elegans Bulimina marginata Allomorphina sp Bolivina subaenaiensis 0.70614 0.64225 0.6144 0.59071 0.46032 0.35311 0.33692 0.31246 Indicative of methane seepage and or highly redox environment Uc-Up Uvigerina crassicostata Uvigerina proboscidea Bolivina semicoastata Nonionella bradyi Karierrella bradyi Cassidulina laevigata 0.67701 0.60864 0.59805 0.45852 0.45445 0.31286 High productivity and seasonality of the food supply is low Fig. 3: Benthic biofacies plotted against interpolated ages, combined with relative abundances of two major benthic foraminifers in each biofacies assemblage, and lowermost panel shows percentage abundances of benthic foraminifera Uvigerina species. In the present study, we found six biofacies dominating over the studied section. Benthic foraminiferal biofacies assemblage suggests that during the early to Mid-Holocene (7.4 to 4.5 Kyr) time, productivity was high with higher production of algal matter in the surface. Dominance of Elphidium incertum and Uvigerina schwageri in this interval indicates presence of degraded organic carbon, since these species have a preference for labile organic matter. The second important biofacies Ba-Bs dominates the Early Holocene interval (3.5 to 2 Kyr). Major components of this biofacies are Bolivina and Bulimina. These are showing increasing trend toward 10 to 2 Kyrs. These taxa are reported from seep zones and indicate high reducing environment. They prefer dysoxic environment irrespective of organic carbon. High productivity may be due to presence of organic carbon or sulphate or methane. So dominance of these fauna of methane loving taxa in this interval could be linked to dissociation of gas hydrate. Biofacies Cm-Bo shows organic carbon rich dysoxic to anoxic environment during the time interval 3.5 to 7 Kyr. But they are not hydrocarbon seep indicator; it is due to present of high concentration of H2S. Biofacies Uc-Up associated with Uvigerina proboscidea is showing high surface productivity interval from 0 to 0.5 Kyr and may be due to terrestrial organic carbon. CONCLUSIONS The high resolution studies of deep sea benthic foraminifera using 174 samples from the methane rich environment of Cascadia margin (IODP, Leg 164, Site 890 B) for the past 13.49 Kyrs found the major distribution of 29 dominant species (ie, >3% or more in at least one sample and present in at least two samples) .The R mode factor and Q mode cluster analysis performed on the 29 dominant species found six clusters and their respective six biofacies. Based on the available and known paleoceanographic environment of the benthic foraminifera paleoceanic conditions for the past 13.49 kyrs. The particular taxa of foraminifera like Uvigerina, Bolivina, Bulimina, Epistominella are adapted to high organic, low oxygen, reducing environments of modern methane 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 found flourishing in hydrocarbon seeps environment of Green Canyon, Gulf of Mexico [9] and is considered to be facultative anaerobes and Bulimina spathulata association and relatively high abundance are also reported from methane seep environments of Green Canyon, Gulf of Mexico [51]. The biofacies Ba-Bs suggests a highly reducing methane seep environments with sustained flux of organic matter to the sea floor and high organic carbon productivity and the assemblage of biofacies El-Us indicates high productivity and low oxygenated environment with high food supply. Close study of our data of U.peregrina and oxygen minimum zone species abundance is sudden decrease at ~12 Kyrs (fig.3). It shows the Younger Dryas event. It is interval of cold spells in northern hemisphere [52]. In (fig 3) increasing trend of U. peregrina during the Early Holocene from 10.5 to 6 Kyr BP and decreasing trend of Ei- Us showing the transition period of Mid Holocene to Early Holocene. On the basis of these results and availability of methane loving taxa, like Uvigerina,Epistominella, Melonis, Bolivina, Bulimina, Chilostomella and Cassidulina, there are strong possibilities of methane flux in this site. Dissociation of gas hydrates and release of methane to the atmosphere could be a cause of increase in the population abundance of highly reducing environmental species, which we interpreted in our data. We can thus, conclude the site is a probable zone for the methane fluxes and may be fruitful for future study on methane seepage in the deep sea location. Acknowledgements: I am thankful to my supervisor Prof Anil Kumar Gupta, HOD, Department Geology and Geophysics, IIT, Kharagpur for providing me the samples, his indispensable guidance, valuable suggestions and providing all the necessary facilities during the course of this project work. I am highly grateful to integrated ocean drilling programme (IODP) for providing the sample from leg 164, site 890B, obtained through my supervisor References: [1]Claypool, G. and Kaplan, I., (1974). The origin and 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 natural gas. Marcel Dikker, New York. [3]Kvenvolden, K. A. (1993), Gas hydrate– geological perspective and global change. Reviews of Geophysics,  31, 173-187. [4]Malone, R. D. (1994), Gas hydrate geology and geography. International Conference on Natural Gas hydrates. 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