Exploration Geology and Geoinformatics

Editors

S. ANBAZHAGAN R. VENKATACHALAPATHY

R. NEELAKANTAN

MACMILLAN

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Benthic Foraminifera as Effective Tools for Exploration of Gas Hydrate Rich Zones at Blake Ridge, Northwest Atlantic Ocean

               M. S U N D A R R A J, S O M A D E  A N D  A N I L K. G U P T A 

 

ABSTRACT Gas hydrates, also known as methane hydrates, are solid ice like crystals composed of water and methane molecules (with small amounts of carbon dioxide, propane and ethane), which are stable under high pressure, low temperatures and adequate concentration of gas (Sloan, 1990; Kvenvolden, 1993). They are trapped in marine sediments and permafrost regions. For the comprehensive study of methane rich zones, researchers have been using deep sea benthic foraminifera and their carbon isotopic signatures, Total Organic Carbon; Dissolved Inorganic Carbon, etc. as key indicators.

Blake Ridge is one of the earliest documented marine gas hydrate province in the northwestern Atlantic Ocean (Katz et al., 1999; Holbrook et al., 2002; Robinson et al., 2004). Blake Ridge consists of a pile of Tertiary to Quaternary drift deposits dominated by fine grained nanno fossil bearing hemipelagic sediments (Markl et al., 1970). The organic carbon content in the sediment often closely relates to the surface water productivity (Pedersen and Calvert, 1990). Thus, variations of organic carbon in marine sediments can be used as a proxy for productivity. While consistent abundance of intermediate to high organic carbon associated biofacies and high TOC along with low carbon isotopic values indicate increased marine biological productivity, lower TOC values indicate decreased terrigenous flux. Presence of dysoxic species combined with geochemical data and physical properties of sediments evidently indicates in-situ gas hydrates were formed at Blake Ridge using biogenic methane (Bhaumik and Gupta, 2005). Some benthic foraminiferal groups like Bolivina, Cassidulina, Chilostomella, Epistominella,

32 EXPLORATION GEOLOGY AND GEOINFORMATICS

Gavelinopsis, Globobulimina, Nonionella, Trifarina, Uvigerina, etc. are known to colonize hydrocarbon-seeped bacterial mats and may be attracted from methane gas or hydrogen sulphide gas emissions (Torres et al., 2003; Hill et al., 2003, 2004; Robinson et al., 2004; Gupta, 2004; Panieri, 2005). Also, highly depleted δ13C excursions of marine carbonates are important indicators of gas hydrate rich environment (Hill et al., 2003; Hill et al., 2004). Uvigerinids, Bolivinids, elongated benthics along with some other intermediate to high organic carbon taxa (Cibicides kullenbergi, C. bradyi, Eggerella bradyi, Globocassidulina subglobosa, Gyroidinoides cibaoensis, Robulus gibbus) are abundant in the methane and hydrate rich zones of Blake Ridge indicating its adaptability to such highly reducing organic carbon rich environment (Rathburn et al., 2000; Hill et al., 2003; Robinson et al., 2004; Panieri, 2005; Bhaumik and Gupta, 2005). Thus, benthic foraminiferal analyses combined with geochemical data are effective tools in exploring methane hydrate rich zones. Keywords Benthic Foraminifera, Gas Hydrate and Blake Ridge

1. INTRODUCTION Presently, the world faces challenges to meet its requirements of conventional sources of energy like coal, petroleum and natural gas whose continuous depletion brings attention on alternative sources of energy. Researchers like MacDonald, (1990) and Gupta, (2004) have mentioned that the energy potential of methane hydrates is significantly larger than that of the other unconventional sources of gas, such as coal beds, tight sands, black shales, deep aquifers and conventional natural gas. Gas hydrates, solid ice like crystals composed of water and methane molecules, are found in many regions of the world (Table 1). Current geophysical surveys such as seismoprofiling, Well log methods and Bottom Simulating Reflectors (BSRs) give indirect information about hydrate content of sediments. But, they are not always reliable. For example BSRs have failed to locate gas hydrate horizons at Ocean Drilling Program Site 994C located on the Blake Ridge, North Atlantic, where much data comes from the geochemical and sediment parameters (Paull, 1996). Thus the need arises to develop new methods for exploring gas hydrates (Table 2). Key indicators like deep sea benthic foraminifera and their carbon isotopic signatures, Total Organic Carbon; Dissolve Inorganic Carbon, etc. have been used for the study of methane fluxes and seep zones. Benthic foraminifera are an important component of the marine community and sensitive to environmental changes. Benthic foraminifera has a capacity to adapt and are able to survive and proliferate in a wide range of marine environments, including extreme ecosystems, such as oligotrophic abyssal plains (Coull et al., 1977) or hydrothermal vents (Sen Gupta and Aharon, 1994) as well as deep-sea trenches.

 

BENTHIC FORAMINIFERA AS EFFECTIVE TOOLS FOR EXPLORATION OF… 33

Table 1. Some Major Gas ‐ Hydrate (Methane seepage) Zones of the World. 

Area  Water depth (m)  References 

Continental margin off Peru  252  Wefer et al., 1994 

Gulf of Mexico  150 ‐ 700 Sen Gupta and Aharon, 1994; Sen 

Gupta et al., 1997 

Eel River, Northern California 

Margin 500 ‐ 525  Rathburn et al., 2000 

Hydrate Ridge, Oregon  600 ‐ 900 Torres et al., 2003; Hill et al., 

2004a; Cannariato and Stott, 2004 

Santa Barbara Channel  120 ‐ 580 Kennett et al., 2000; Hinrichs et 

al., 2003; Hill et al., 2003, 2004b 

Blake Ridge, northwest Atlantic  1981 ‐ 2158 

Katz et al., 1999; Dillon et al., 

2001; Holbrook et al., 2002; 

Robinson et al., 2004 

Miocene limestone of Italy  600 ‐ 100  Barbieri and Panieri, 2004 

Rockall Trough  800 ‐ 1000  Panieri, 2005 

Studies of dead and living benthic foraminifera have shown that benthic foraminiferal distribution patterns are closely tied to the organic carbon flux and the organic carbon content of the sediment (Fariduddin and Loubere, 1997; Schmiedl et al., 1997; De Stigter et al., 1998; Gupta and Thomas, 1999; 2003; Gupta et al., 2004; Singh and Gupta, 2004). Other studies have demonstrated the sensitivity of the biofacies composition to changes in oxygen levels of the bottom water and pore water oxygenation (Loubere, 1996; Jannink et al., 1998). Over the last three decades, scientists have increased their interest to understand different aspects of benthic foraminifera for paleoenvironmental reconstructions. Numerous species of benthic foraminifera have been found in different methane rich marine settings and have proved to be good indicator of methane releases (e.g. Wefer et al., 1994; Sen Gupta et al., 1997; Rathburn et al., 2000; Hill et al., 2003).

Table 2.  Methane Fluxes Identified Using Different Methods.

Method  References 

Highly negative carbon isotopic 

excursions of benthic and planktic 

foraminifera, total organic carbon 

Wefer et al., 1994; Dickens et al., 1995; Katz et al., 1999; 

Kennett et al., 2000; Rathburn et al., 2000; Torres et al., 

2003; Hill et al., 2003, 2004a,b 

Presence of chemosynthetic bacteria 

and biota Hinrichs et al., 2003; Van Dover et al., 2003 

Reflection seismic profiles  Dillon et al., 2001; Holbrook et al., 2002 

Pore water chemistry  Luff and Wallmann, 2003 

34 EXPLORATION GEOLOGY AND GEOINFORMATICS

Some species are attracted to bacterial mats and feed on bacterial rich food near methane seeps or hydrogen sulphide gas emissions showing their potential as indicators of methane release in the geological record. Some methane loving benthic foraminiferal groups include species of Bolivina, Cassidulina, Chilostomella, Epistominella, Gavelinopsis, Globobulimina, Nonionella, Trifarina, Uvigerina etc. (Sen Gupta and Aharon, 1994; Wefer et al., 1994; Sen Gupta et al., 1997; Rathburn et al., 2000; Bernhard et al., 2001; Torres et al., 2003; Hill et al., 2003, 2004; Robinson et al., 2004; Gupta, 2004; Panieri, 2005) which can withstand such stressful conditions. A detailed table of environment inferred from each species is given in Appendix1. 1.1. Origin of Gas Hydrates Gas hydrates occur mainly in two geologic settings viz. permafrost regions on land or oceanic sediments of continental margins. These are also found in deep lakes, inland seas, arctic localities associated with petroleum accumulations etc. (Shipley et al, 1979; Kvenvolden, 1990, 1993a, 1998). The methane formed in gas hydrates may be biogenic (Claypool and Kaplan, 1974) or thermogenic (Hyndman and Davis, 1992) in origin. Biogenic methane is formed from bacterial decomposition of sedimentary organic matter (SOM) in low temperature and anaerobic condition at shallow depths (Paul et al, 1994) which produce food for benthic foraminifera. On the contrary if the SOM breaks in high temperature (80°C-150°C) to produce primary and secondary thermogenic gases containing less methane and more short chain hydrocarbons like ethane, propane, butane etc., accounts for their thermogenic origin. The gas hydrate formed from biogenic hydrocarbon is mainly 99% pure methane. 2. LOCATION AND OCEANOGRAPHIC SETTINGS Blake Ridge, in the northwestern Atlantic Ocean (Fig.1) (Katz et al., 1999; Holbrook et al., 2002; Robinson et al., 2004) contains nearly 15 Gt (Gt = 1015 gm) (Dickens et al, 1997) to 40 Gt (Holbrook et al., 1996) of stored carbon in the form of gas hydrates. Presently the area underlies the periphery of the subtropical central gyre and is influenced by the northerly flowing, warm, saline Gulf Stream surface current as well as the southerly flowing Western Boundary Under Current (WBUC). While bottom water temperature of the Blake Ridge Diaper (water depth 2155m) is of 3.2 ºC (Van Dover et al., 2003), the modern lysocline lies in between the 4000 to 4350 m water depth, which is linked to the mixing zone of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) in the subtropical northwest Atlantic (Balsam, 1983). The disseminated gas hydrate rich sediments lies approximately 185 to 450 meter below sea floor sandwiched between methane rich sediments below and methane free sediments above. Blake Ridge is a well established gas hydrate field and provides an ample opportunity to understand methane genesis and eruptions using various proxies during the Quaternary.

BENTHIC FORAMINIFERA AS EFFECTIVE TOOLS FOR EXPLORATION OF… 35

Fig. 1. Location map of Gas Hydrate rich zones (ODP Holes 991 to 997), Blake Ridge, Northwest Atlantic. 2.1. Lithology Blake Ridge consists of a pile of Tertiary to Quaternary drift deposits dominated by fine grained nannofossil bearing hemipelagic mud and silty clay (Markl et al., 1970; Shipboard Scientific Party, 1996). The thickness of the methane-hydrate stability zone in this region ranges from zero along the northwestern edge of the continental shelf to a maximum thickness of about 700 m along the eastern edge of the Blake Ridge (Collett, 1993). The gas thus produced from deep beneath oceanic sediments enters into Gas Hydrate Stability Zone (GSHZ) and forms gas hydrates while the free gas persists beneath it. Favorable factors for the formation of gas hydrate in this region include high pressure (~2.6 Mpa), low temperature (0-10oC), high organic carbon (2.0%-3.5%), high porosity, adequate amount of methane and pore water, water depths of 300-1000 m and rapid sedimentation rate (Claypool and Kaplan 1974; Kvenvolden, 1993, 1998; Malone, 1994; Ginsberg and Soloviev, 1997; Sloan, 1990; Fehn et al., 2000). Figure 2 shows a cross section along the Blake Ridge depicting the bathymetry and temperature variance in the area. Shipboard examinations of smear slides indicate that clays, calcite, and quartz are the dominant mineral components; feldspars, dolomite, and pyrite are minor components. Siliceous microfossils are present primarily as diatoms, although there are some sponge spicules and radiolarians. The presence of strong BSR is found in Blake Ridge, with other proxies it is also evident that disseminated methane hydrates occurs through out sedimentary section between ~180 and ~450 m below seafloor, which may extend about ~30 mbsf (Paull et al, 1996; Lorenson, T. D. and Shipboard Scientific Party, 2000).

36 EXPLORATION GEOLOGY AND GEOINFORMATICS

Fig 2. Depth vs Temperature Plot at Blake Ridge (Courtesy: Ocean Data View).

3. EXPLORATION OF GAS HYDRATE RICH ZONES As Gas hydrates are not preserved in cores or in exposed outcrops, it is necessary to find digenetic “finger prints” (or proxies) to identify sediments that contained gas hydrate (Rodriguez et.al, 2000). Also, in the absence of free methane gas emission, BSR’s are unable to detect gas hydrate deposits, particularly in Blake Ridge as free methane is believed to have already escaped to the atmosphere, so here micropaleontological fingerprints can be regarded as more suitable tools in studying gas hydrate deposits. Uvigerinids, Bolivinids, elongated benthics along with some other intermediate to high organic carbon taxa (e.g. Cibicides kullenbergi, C. bradyi, Eggerella bradyi, Globocassidulina subglobosa, Gyroidinoides cibaoensis, Robulus gibbus) are abundant in the methane and hydrate rich zones of Blake Ridge which indicates their adaptability to such highly reducing organic carbon rich environment (Rathburn et al., 2000; Hill et al., 2003; Robinson et al., 2004; Panieri, 2005; Bhaumik and Gupta, 2005). Often surface water productivity is closely related to the organic carbon content in the sediment (Muller and Suess, 1979; Pederson, 1983; Sarnthein et al., 1987; Pedersen and Calvert, 1990) and thus, variations of organic carbon in marine sediments can be used as a proxy for productivity. The Total Organic Carbon (TOC) concentrations transformed into mass accumulation rates of TOC can be used for the interpretation of changes in preservation conditions or supply of OM (Jia, et al., 2002). For example: the Arabian Sea and the Bay of Bengal with thick pile of sediments (3 - 4 km) and high organic carbon content (in the Arabian Sea, total organic carbon (TOC) ranges from 0.48 to 4% and in the Bay of Bengal from 0.26 to 2%), are potential areas for gas hydrate rich zones (Gupta, et al., 1998, 2003; Kuldeep et al., 1998; Veerayya et al., 1998; Subrahmanium et al., 1999).

BENTHIC FORAMINIFERA AS EFFECTIVE TOOLS FOR EXPLORATION OF… 37

While high TOC and low carbon isotopic values along with consistent abundance of intermediate to high organic carbon associated biofacies indicate increased marine biological productivity, lower TOC values indicate decreased terrigenous flux. At Blake Ridge, the occurrence of dysoxic species along with geochemical data and physical properties of sediments evidently indicates in-situ gas hydrates were formed using biogenic methane (Bhaumik and Gupta, 2005).

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APPENDIX Appendix 1. List of Benthic Foraminiferal Species and their Inferred Environments.

Genus  Environment  References Bolivina  d’Orbigny, 1839  Opportunists, cosmopolitan, 

infaunal taxon, associated with the OMZ, found in phytodetritus rich dysaerobic environments. 

Sen  Gupta  and Machain‐Castillo,  1993; Gupta  and  Satapathy, 2000; Gooday, 2003 

Cassidulina  d’Orbigny, 1826  C. laevigata related to cold waters, high seasonality environment and enhanced organic carbon influx. 

Murray,  1991;  Loubere and  Fariduddin,  1999; Schmiedl et al., 1997 

Chilostomella  Reus, 1849  Methane‐loving  taxa  found  in hydrocarbon‐seep bacterial mats and hydrocarbon vents and seep zone. 

Sen Gupta, and Aharon, 1994,  Wefer,  et  al., 1994;  Rathburn,  et  al., 2000,  Hill,  et  al.,  2003, Torres, et al., 2003, Sen Gupta, et al., 1997, Hill, et al., 2004. 

Cibicides  Montfort, 1808  Epifaunal,  well‐aerated  bottom waters and low organic flux. 

Hayward  et  al,  2002; Fariduddin and Loubere, 1997;  Schmiedl,  et  al., 1997 

Eggerella  Cushman, 1935  Eggerella  advena  is  related  to eutrophication  and  increased nutrient  supply;  indicative  of pollution;  found  in  semi‐open inlet  environments  with  silt substrate  and  reflect intermediate  flux  of  relatively degraded organic matter.  

Thomas,  et  al,  2004; Akira  Tsujimoto  et  al., 2006;  Clark  ,1971; Annin,  2001;  Gupta 1997 

Epistominella  Husezima  and Maruhasi, 1944 

Opportunistically exploit phytodetritus (‘phytodetritus species’). 

Gooday ,1993 

Gavelinopsis  Hofker, 1951  Well‐oxygenated bottom water, influenced by lateral input of organic particulate matter transported by bottom current 

Hayward, 2002 

Globobulimina  Cushman, 1927  Infaunal, associated with high food supply, and refractory organic carbon input. 

Gooday, 2003; Fontanier et al., 2002 

 

 

44 EXPLORATION GEOLOGY AND GEOINFORMATICS

Genus  Environment  References Globocassidulina  Voloshinova, 

1960 Infaunal, year round high nutrient supply. 

Rathburn and Corliss, 1994; Mackensen et al., 1995 

Gyroidinoides  Brotzen, 1942  G. cibaoensis reported from low oxygenated deep waters of the northwestern Indian Ocean having moderate flux of organic matter. 

Gupta, and Thomas,  1999 

Noninella  Cushman, 1926 

Infaunal species N. auris prefer anoxic, H2S‐containing sediments, feed on methane oxidizing bacteria and could be an indicator of biogenic methane below the sediment surface. 

Wefer et al. 1994 

Robulus  de Montfort, 1808 

Marked species of upper part of Oxygen Minimum Zone (OMZ) and indicative of high organic carbon flux and low oxygen content. 

Hermelin and Shimmield, 1990 

Trifarina  Cushman, 1923 

T. angulosa is infaunal, free‐living, related to low temperatures, low salinity and high sand content, variable organic flux rates, outer shelf to upper slope, well‐oxygenated environments. 

Hayward et al. 2002; Murray, 1991, Gupta, 1997; Mackensen, et al., 1995; Harloff and Mackensen, 1997 

Uvigerina  d’Orbigny, 1826 

U.  peregrina  is  shallow infaunal,  thriving  underneath OMZ, associated with high and sustained  flux  of  organic matter. In the Cascadia Margin U.  peregrina  was  found attracted to rich bacterial food source  at  methane  seeps.  U. proboscidea  blooms  in  high productivity  regions  of  the Indian,  Atlantic  and  Pacific Oceans  where  productivity  is high  throughout  the  year  and seasonality of  the  food supply is low or absent. 

Sen Gupta and Machain‐Castillo, 1993; Altenbach et al., 1999, Torres et al., 2003, Gupta and Thomas, 1999; Almogi‐Labin et al., 2000, Thomas et al., 1995, Woodruff, 1985