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Exploration Geology and

Geoinformatics

Editors

S. ANBAZHAGANR. VENKATACHALAPATHY

R. NEELAKANTAN

MACMILLAN



c Macmillan Publishers India Ltd., 2009

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Benthic Foraminifera as Effective Tools

for Exploration of Gas Hydrate RichZones 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 δ13

C 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 methanehydrates 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 exploringgas 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 etal., 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




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 3.2ºC (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.




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 ShipboardScientific Party, 2000).




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 heremicropaleontological 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).




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 wereformed 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




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

benthic foraminifera as effective tools for exploration of gas hydrate rich zones at blake ridge,...

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