Chinese Science Bulletin © 2008
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Petrographic and geochemical characterization of seep carbonate from Alaminos Canyon, Gulf of Mexico FENG Dong1,2, CHEN DuoFu1†, QI Liang3 & Harry H. ROBERTS4 1
Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China; 3 State Key Lab of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China; 4 Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA
Seep carbonates were collected from the Alaminos Canyon lease area, Gulf of Mexico. The carbonates are present as slabs and blocks. Bivalve shell and foraminifer are the dominant bioclasts in carbonate. Pores are common and usually filled with acicular aragonite crystals. XRD investigation shows that aragonite is the dominate mineral (98%). Peloids, clotted microfabirc and botryoidal aragonite are developed in carbonate and suggest a genesis linked with bacterial degradation of the hydrocarbons. The 13C value of bioclasts in carbonate is from 4.9‰ to 0.6‰, indicating that the carbon source is mainly from sea water as well as the small portion incorporation of the seep hydrocarbon. The microcrystalline and sparite aragonite shows the 13C value from 31.3‰ to 23.4‰, suggesting that their carbon is derived mainly from microbial degradation of crude oil. 14C analyses give the radiocarbon age of about 10 ka. Rare earth elements (REE) analyses of the 5% HNO3-treated solution of the carbonates show that the total REE content of the carbonates is low, that is from 0.752 to 12.725 μg·g1. The shale-normalized REE patterns show significantly negative Ce anomalies. This suggests that cold seep carbonate is most likely formed in a relatively aerobic environment. 14
seep carbonate, carbon and oxygen isotope, rare earth element, C dating, redox condition, Gulf of Mexico, Alaminos Canyon
The discovery of modern hydrothermal vents and cold seeps are one of the most important achievements in marine investigations during the last 30 years. Cold seep, due to the close relationship to gas hydrate, global change and origin of life is now the focus of scientific communities. Fluids from deep marine sediments occurs as seep or vent, and migrate to ocean seafloor produce a series of physical, chemical and biological processes[1]. The seep fluids are composed commonly of dissolved and gaseous methane, but heavy hydrocarbons are also observed (e.g. crude oil)[2,3]. Seep carbonate precipitation is a striking phenomenon at modern and ancient marine seep environments, which widely occurs in the marine environments of the ̣ world[4 7]. Carbonate precipitation at seep site is a result of microbial oxidation of methane through the combined www.scichina.com | csb.scichina.com | www.springerlink.com
metabolism of methane oxidizing archaea (MOA) and ̣ sulfate reducing bacteria (SRB)[8 10]. Seep carbonate is diagnostic of sites with gas venting at seafloor, and the deepwater gas vents are commonly the sites of gas hydrate accumulation[11]. The study of present-day gas hydrate-related carbonates could provide a clue to evaluate the role of gas hydrate in geọ logical history[12 14]. As the product of cold seep, seep carbonate fully records the evolution of cold seep. Thus, integrated study of the mineral composition, texture and Received September 23, 2007; accepted January 15, 2008 doi: 10.1007/s11434-008-0157-0 † Corresponding author (email:
[email protected]) Supported by the National Natural Science Foundation of China (Grant Nos. 40725011 and U0733003), the Hundred Talents Program of the Chinese Academy of Sciences (CAS) and the Foundation of Key Laboratory of Marginal Sea Geology of CAS (Grant No. MSGL07-21)
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1 Geological setting and sampling The GOM salt basin is a prolific petroleum province, the opening of the basin commenced in the Late TriassicMiddle Jurassic as a consequence of crustal rifting, thick slat was deposited in the basin during rifting, ongoing slat deformation and active faults provide efficient conduits for fluid migration from the deep subsurface pẹ troleum system in to the shallow sediments[16 18]. The seep carbonates were collected in the Alaminos Canyon (26°21N/94°31W) at the base of the continental slope of GOM during submersible cruise with DSV Alvin in 1990 (Figure 1). Water depth of the site is 2200 m, this
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site displays abundant chemosynthetic communities composed of tubeworm clusters, mussel beds with scattered clams, and limited bacterial mats. The seep carbonate occurs as slabs and blocks and from greyly white or greyly yellow in color, show as different shapes and size (Figure 2(a), (b)).
2 Methods The samples were washed with fresh water and dried in the air. Typical samples were selected to do thin sections that were observed using a LEICA-DMRX optical microscope with Leica Qwin Program. The microstructure of the seep carbonate on the fresh surfaces of crashed samples was examined with a scanning electron microscope (SEM). Photographs were taken at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences using a Quanta 400 SEM and at Central South University using a Sirion 200 FE-SEM equipped with EDAX GENESIS, operating at 10̣20 kV with a 5 to 9 mm working distance. The X-ray diffraction (XRD) was finished at Stevens Institute of Technology. After being washed with distilled water, the samples were crushed into powder less than 200 mesh using an agate mortar and pestle. XRD was performed using a Rigaku DXR 3000 computerautomated diffractometer utilizing Bragg-Brentano geometry. The X-ray source was a Cu anode operated at 40 kV and 40 mA using CuK radiation equipped with a diffracted beam graphite monochromator. The orientated samples were scanned at an interval of 5̣65° (2) with a step size of 0.02° and count time of 5 seconds per step. Divergence, scattering and receiving slits were 0.5°, 0.5° and 0.15 mm, respectively. The powdered samples were processed with 100% phosphoric acid to release CO2 for stable carbon and
Figure 1 Location of Alaminos Canyon study site[19]. FENG Dong et al. Chinese Science Bulletin | June 2008 | vol. 53 | no. 11 | 1716-1724
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geochemical features of seep carbonate could provide the formation condition of seep carbonate. Gulf of Mexico (GOM) is one of the earliest sites well known and the most studied areas in the world for seep carbonate. Submersible dives with Alvin in 1990 confirmed the existence of significant vent related phenomena at the full-depth range of the continental slope[15]. However, few cold seep sites in deep water depths (greater than 1 kilometer) had been visited and sampled in the past surveys due to the difficulties of the work. Like the shallower vent areas (e.g. Bush Hill) of the continental slope, Alaminos Canyon (with a water depth of 2200 m) displayed abundant chemosynthetic communities composed of tube worm clusters and mussel beds, and seep carbonate are well developed and always occur as the habitat of the seep communities[3]. In this paper we investigated the heterogeneity of isotopes and trace elemental concentrations at micro-scales in a seep site of Alaminos Canyon of the GOM, and its importance would be providing an example for studying the cold seeps in other regions and in ancient record.
Figure 2 Typical morphologies of seep carbonate from Alaminos Canyon, GOM. (a), (b), and (c) are the seep carbonate samples in this study. Well preserved bivalve shells are in the seep carbonate (black arrow in (a) and arrows in (b)), indicating its feature of in situ growth and preservation. Forniciform serpulid worm tube attached on the surface of the seep carbonate (white arrow in (a)). Pores are common in seep carbonates, most of them are mm-size, and cm-size chimney pore can also be observed, which might be the conduit of fluid seep. (c) Polished smooth of the carbonate composited of lighter (left) and darker (right) areas, the darker area is composed of microcrystalline aragonite with large amount of bioclasts, and the lighter area is mainly composed of sparite aragonite with little bioclasts. (d) is the enlargement of the darker area of (c), foraminifer and bivalve shells are the dominant bioclasts in seep carbonate, pores are common and usually filled with acicular aragonite. Peloid and botryoidal aragonite are also observed in matrix of the carbonate, plane polarized light.
oxygen isotope analysis at the Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Carbonate carbon and oxygen isotopic compositions are reported relative to PeeDee Belemnite (PDB) standard were measured by using the GV Isoprime II stable isotopic mass spectrometry with deviations less than 0.01‰ (2V) for both 18O and 13C values. The 14C measurements were made on microcrystalline and sparite aragonite (n=2), serpulid worm tube attached on the surface of carbonate (n=1) and shell fragments (n=3) within carbonate cement. The analysis was conducted using EN tandem accelerator at Peking University. To remove contaminants, the serpulid worm tube and shell fragment were carefully stripped off adhering sediments, and the corresponding treatment was also made to authigenic carbonate samples to strip off shell fragments under a microscope. The samples were washed in an ultrasonic water bath for 10 min and then freeze-dried for 24 h under vacuum. The cleaned samples were reacted with phosphoric acid and the evolved 1718
CO2 was reduced to get graphite targets. The analytical precision was 1%̣2%. Trace and rare earth elements analyses were conducted at the University of Hong Kang using VG Plasma-Quad Excell ICP-MS. For detailed parameters of the instrument analyses see Qi et al.[20]. The ICP multielement standard solution from AccuStandard Inc. (USA) was used to prepare a variety of calibration solution. 100ng/ml single standard solution: Ba and Pr were used to correct the interference of BaO+ on Eu and PrO+ on Gd, respectively. The seep carbonate powder (0.5 g) was treated with 50 mL of 5 % HNO3 in a centrifuge tube for 2̣3 h to separate the carbonate mineral phase and residue phase. Then 2500 ng of Rhodium was added as an internal standard for calculating the element concentration of dissolved carbonate mineral phase. Five milliliters of this solution was further diluted 5 times. For reliability of the analysis, see Chen et al.[11]. Precision of the REE and trace element analysis was checked by multiple analyses of international carbonate standard samples CAL-S. The average standard deviations are
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3 Results 3.1 Petrography and mineralogy The seep carbonate shows no obvious stratification and mainly composed of bioclasts and carbonate cement (Figure 2). The carbonate displays lighter and darker areas due to differences in bioclasts content (Figure 2(c)). Pores of different size are very common and some of them are of cm-size (Figure 2(b)). These pores are probably the conduit of cold seep. The XRD of the matrix of seep carbonate shows that the carbonate is composed mainly of aragonite (up to 98%), and only minor calcite (Figure 3). The seep carbonate occurs as microcrystalline, sparite and fibrous or acicular crystals under microscope (Figure 4). Fibrous or acicular aragonite generally develops in open pore space, either between intraclasts or inside the cavities of biogenic components (Figure 2(d); Figure 4(h)).
3.2 Isotope geochemistry The carbon and oxygen stable isotopes of microcrystalline and sparite aragonite show a small range of 13C values from 31.3‰ to 23.4‰, and 18O from 2.8‰ to 5.5‰. The bivalve shells have 13C values from 4.6‰ to 4.9‰, and 18O from 4.3‰ to 4.5‰. The serpulid worm tube has 13C values from 1.9‰ to 0.56‰, and 18O from 3.4 ‰ to 3.3‰ (Table 1; Figure 6). The result of 14C analyses shows that the microcrystalline and sparite aragonite give the ages of 22.53 ka and 21.86 ka, while the shells give the ages from 10.6 ka to 11.6 ka. One serpulid worm tube has large variation (Table 1).
Figure 3 X-ray diffraction patterns of carbonate crust.
Foraminifers and bivalve fragments are the dominant biogenic components of the seep carbonate (Figure 2). Well preserved bivalve shell in the seep carbonate indicating its in situ growth and preservation (Figure 2(a)). The biogenic components are common in some part of the carbonate (Figure 2(c), (d)). Clotted microcrystalline and peloid are developed in micritic aragonite (middle part of Figure 2(b); Figure 4). Peloids are composed of either micirtic aragonite and surrounded by sparite aragonite cement (Figure4 (a), (c)), or fibrous and acicular aragonite aggregation (Figure 4(e)). The peloids, usually 30‰ (Figure 6), showing
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Sample number AC-D-1 AC-D-2 AC-D-3 AC-D-4 AC-D-5 AC-D-6
Table 2 The elemental content in soluble parts of seep deposits from Alaminos Canyon of the Gulf of Mexico (μg·g1) Numbera) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE Ce/Ce*b) AC-mi 3.41 3.63 0.71 3.07 0.57 0.12 0.38 0.07 0.34 0.06 0.18 0.03 0.15 0.02 12.72 0.51 AC-ar 0.59 0.54 0.12 0.50 0.10 0.02 0.10 0.02 0.10 0.02 0.05 0.01 0.04 0.01 2.23 0.45 AC-bi 0.27 0.12 0.04 0.18 0.03 0.01 0.03
