Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

CONCENTRATIONS AND CARBON ISOTOPE COMPOSITIONS OF METHANE IN THE CORED SEDIMENTS FROM OFFSHORE SW TAIWAN

Chuang, P.C. ∗ , Yang, T.F., Hong, W.L. Department of Geosciences, National Taiwan University, Taiwan Lin, S., Chen, J.C. Institute of Oceanography, National Taiwan University, Taiwan Sun, C.H., Exploration & Development Research Institute, CPC, Taiwan Wang, Y. Central Geological Survey, MOEA, Taiwan

ABSTRACT It has been found that Bottom Simulating Reflections (BSRs) widely distribute in offshore southwestern Taiwan which infer the existence of potential gas hydrates underneath the seafloor sediments. We systematically collected cored sediments and analyzed the gas composition of pore-space of sediments through ten cruises from 2003 to 2006. The methane concentrations of cored sediments display an increasing trend with depth. Some abnormally high methane concentrations have been found in offshore southwestern Taiwan. In addition, the profiles of methane and sulfate of cored sediments reveal very shallow depths of sulfate methane interface at some sites in this study. The linear sulfate gradients, low total organic carbon and high methane concentrations imply that sulfate reduction is mainly driven by the process of anaerobic methane oxidation in sediments. Thus, the methane fluxes can be determined through the gradients of sulfate reduction and steady state solutions to diffusion equations and results show that the methane fluxes are very high, especially at sites G23 (4.12×10-2 mmol cm-2yr-1) and N8 (2.11×10-2 mmol cm-2yr-1) in offshore southwestern Taiwan. It indicates that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates, in this area. ∗

Corresponding author: Phone: +886-2-3366-5876 Fax: +886-2-2369-5568 E-mail: [email protected]

Some selected gas samples have also been analyzed for carbon isotopic compositions. The δ13C data of CH4 gases range from -28.29 o/oo ~ -85.17 o/oo. The carbon isotopic compositions of methane show that biogenic gas source is dominated at shallower depth; however, some thermogenic gases might be introduced from deeper source in this region. Key words: Methane, Gas hydrate, Anaerobic methane oxidation, methane flux, carbon isotopic compositions of methane Introduction

We will discuss the distribution of methane

Gas hydrates are a kind of non-stoichiometric

concentrations and fluxes in offshore southwestern

clathrates and metastable crystal products in low

Taiwan. Furthermore, we will compare the

temperature and high pressure conditions. They are

different sources of methane in our study area.

natural occurring solids that contain natural gases, mainly methane, within a rigid lattice of water molecules. Gas hydrates widely distribute in the permafrost of polar region and the strata of continental deep sea area in the world. Since methane is the major gas inside gas hydrates, gas hydrates have been considered as potential energy resources for the future. Methane is also a greenhouse gas that might affect the global climates from the dissociations of gas hydrates (Kvenvolden, 1998; Milkov, 2004; Sloan, 1998). Based on the seismic profiles, scientists have found

Figure 1: Bathymetric map of offshore Taiwan

many Bottom Simulating Reflections (BSRs)

showing sampling sites investigated in this study.

which indicate gas hydrates widely distributed in the depths of 700 m to 3000 m under seafloors in

RESULTS

offshore southwestern Taiwan (Chi et al., 1998;

1. The Distributions of Methane Concentrations

Chow et al., 2000; Schnurle et al., 1999, 2002; Liu

in Bottom Waters and Cored Sediments

et al., 2006). We systematically collected sea

After compiling all the data, we have found the

waters and cored sediments for the dissolved and

distributions of methane concentrations in core top

pore-space gas composition and isotope analysis

waters and pore spaces of cored sediments as

through

ORI-718,

shown in Fig. 2 and Fig. 3. Many samples contain

Marion

methane as the dominant hydrocarbon gas.

ORI-765,

However, most C2+ gases are below detection

eight

ORII-1207, Dufresne

cruises:

ORII-1230, (MD)

cruise,

ORI-697, ORI-732, ORI-758,

ORI-792 and ORI-804 from 2003 to 2006. The

limits,

sampling sites are shown in the Fig. 1.

concentrations are given here. Some extremely 2

so

only

the

results

of

methane

high methane concentrations can be found in offshore southwestern Taiwan (e.g., Chuang et al., 2006; Yang et al., 2006). In addition to those data have been reported previously (Chuang et al., 2006 and Yang et al., 2006), the dissolved methane concentration in the core top water sample from site GS5 of ORI-792 (4,137,250 nL/L) is also higher than the background value of general sea water (20 nL/L). Some sites with high methane concentrations (around 10,000 μL/L) can also be found in pore space of cored sediments, e.g., sites: MD052911, MD052912, MD052913, MD052914

Figure 2: Map of methane concentrations (in nL/L)

of Marion Dufresne (MD) cruise, GH3, GH6,

in bottom seawaters distributed in offshore

GH10, GH16 of ORI-758, A, C, D, H of ORI-765

southern Taiwan

and GS5 of ORI-792. Some duplicate sampling sites of different cruises display

a

continuously

methane

venting

Taiwan

phenomenon in this study area, e.g., sites MD052911, MD052912, MD052913, MD052914 of Marion Dufresne (MD) cruise are located in the same positions of sites G22, G24, G23 of ORI-718, 5 of ORI-735 and sites A, C, D, H of ORI-768 are the same as sites N8, G23, N6, G96 of ORI-718. In addition, sites GH6, GH10, GH16 of ORI-758 are > 10000

close to sites G22, N2, G24 of ORI-718. These

100 ~ 300

observations indicate that methane venting areas of

1000~10000 <100

700~1000

500~700

300 ~ 500

CH4 (μL / L)

G22, G24 of ORI-718 are wide and there is a

Figure 3: Map of methane concentrations (in μL/L)

sustained methane venting source underneath each

of pore spaces distributed in offshore southern

site.

Taiwan. Fig. 2 and Fig. 3 also show that anomalous

high methane concentrations are mainly distributed

2. Methane Fluxes in Offshore Southwestern

in active margins. GS5 of ORI-792 is the only one site

that

contains

unusually

high

Taiwan

methane

Combining the profiles of interstitial sulfate

concentrations in passive margins.

concentration and the methane concentration of pore space in sediments, it usually can be observed that sulfate is depleting and methane is increasing dramatically at one depth, which is called as 3

Sulfate Methane Interface (SMI). Most methane

we use sulfate fluxes to be proxy methane fluxes

profiles show typical concave-upward curvatures

(Fig. 4). The proxy methane fluxes of sites G23

that have been found for many methane profiles in

(4.12×10-2 mmol cm-2yr-1) of ORI-697 and N8

anoxic marine sediments. Those profiles display

(2.11×10-2 mmol cm-2yr-1) of ORI-718 reveal very

steep linear gradient just below the sulfate-methane

high values as compared with other gas hydrate

interface (SMI) indicating the upward diffusive

study areas (Borowski et al., 1996; Dickens, 2001 ).

fluxes of methane (Niewöhner et al., 1998). Linear

The high methane fluxes indicate that there might a

sulfate gradients can imply that sulfate depletion is

methane enriched source below the sea floors in

mainly controlled by anaerobic methane oxidation

the region. The methane enriched source might be

reaction (AMO), rather than by the flux of

from the dissociation of gas hydrates based on

sedimentary organic matter from above. The

other indicators (Lin et al., 2006; Liu et al., 2006)

equation of coupled sulfate-methane reaction shows a one to one stoichiometry. Furthermore, the differences of the depths of SMI can indicate the relative amount of methane fluxes below the sea floors (Borowski et al., 1996, 1999). Dissolved methane concentrations in bottom water and methane concentrations of pore space in deeper sediments are extremely high at sites N8, G23 of ORI-697, G22 of ORI-718. The depths of SMI in these three sites are very shallow, so that the methane fluxes of those sites might be very high (Chuang et al., 2006). Geochemical profiles for cored samples are plotted in Fig. 4 with corresponding sulfate concentrations (Lin et al., 2006) in pore water and methane concentrations in pore space. Since the linear sulfate gradient above SMI can be observed from each site, possibly sulfate is mainly consumed by methane. Therefore, we try to use Fick’s fist law assuming steady-state conditions (Niewöhner et al., 1998) to calculate methane fluxes from linear gradients (Fig. 4). In comparison

with

methane

fluxes,

we

also

calculated sulfate fluxes. The discordant of methane flux and sulfate flux appears at each site. The lower methane concentration might result from delayed sampling procedure after core retrieval, so 4

Figure 4: Selected profiles of methane concentrations of pore space in cored samples and dissolved sulfate concentrations in pore water. Methane concentrations are represented by open blue circles and sulfate concentrations are represented by open black triangles. The shadow in different profiles shows the possibility range of the depths of SMI. Values of methane flux (mmol cm-2yr-1) and proxy methane flux (mmol cm-2yr-1) are showed in each site. The data of sulfate concentrations are from Lin et al. (2006).

Figure 5: Carbon isotopes of methane in pore spaces of cored sediments.

3. Isotopic compositions of methane in pore spaces of cored sediments

CONCLUSIONS

Fig. 5 reveals the isotopic compositions of methane

1. Methane

is

the

main

component

of

of some selected gas samples that have also been

hydrocarbon gases in bottom water and cored

analyzed for carbon isotopic compositions. The

sediments. Ethane is the minor component and

δ13C data of CH4 gases range from -28.29 o/oo ~

most C3+ gases are below detection limits in

o

this study.

-85.17 /oo. The carbon isotopic compositions of methane show that biogenic gas is the main gas

2. Unusually high methane concentrations in the

source. The isotopic signatures of site G96 of

dissolved gases of bottom water and cored

0RI-718 are located in the mixed zone (Fig. 5). Site

samples can be found in SW offshore Taiwan.

G96 of ORI-718 is located in one of the mud

The methane venting areas of G22, G24 of

volcano area. Thermogenic gases might be

ORI-718 are wide and there is a sustained

introduced from deeper source in this region. After

methane venting source underneath each site. 3. Unusually high CH4 concentrations in both

secondary migrations, C1/(C2+C3) ratios become enriched in C1 at site G96.

bottom water and cored sediments are mainly distributed in active margin offshore SW Taiwan. However, the high CH4 concentrations can only be found at site GS5 of ORI-792 in passive margin in this study. 4. Very high methane fluxes and very shallow depth of SMI indicate that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates and/or from mud volcanoes, in this area. 5

5. Both biogenic and thermogenic gases are

Kvenvolden, K.A. (1998) A primer on the

observed from gas samples.

geological occurrence of gas hydrate. In: Henriet, J. P. and Mienert, J. (eds.) Gas

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