Characteristics and possible origin of native aluminum in cold seep sediments from the northeastern South China Sea

Journal of Asian Earth Sciences 40 (2011) 363–370

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Characteristics and possible origin of native aluminum in cold seep sediments from the northeastern South China Sea Zhong Chen a, Chi-Yue Huang b,*, Meixun Zhao c, Wen Yan a, Chih-Wei Chien b, Muhong Chen a, Huaping Yang a, Hideaki Machiyama d, Saulwood Lin e a

Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, ROC Key Laboratory of Marine Chemistry Theory and Technology of the Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, China d Kochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi, Japan e Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC b c

a r t i c l e

i n f o

Article history: Received 26 June 2008 Received in revised form 1 June 2010 Accepted 14 June 2010

Keywords: Native aluminum (Al°) Cold seep Formation mechanism South China Sea Microbial processes

a b s t r a c t Although native aluminum (Al°) has been reported to occur in various geological settings for more than 20 locations but its mechanism of formation still remains to be elucidated. We report the occurrence and characterization of Al° particles recovered from the surface sediment (CF4) and a short core sediment (ROV-G, 37 cm length) obtained at cold seeps in the northeastern continental slope (NCS) of the South China Sea (SCS). X-ray diffraction analysis shows that the collected particles are metallic aluminum with the unit cell edge a of 4.059 ± 0.005 Å (CF4#2) and 4.029 ± 0.004 Å (T26–28#2, ROV-G). The Al° particles occur as spherules, irregular plates and elongated forms with typical lamellar structures. Their chemical compositions are 95.07–99.84% Al (the average values is 98.42%) with very small amounts of Si, Fe, Ti, S, Zn, Mg, Ca, K, Na, Cu, Co and P, and are similar to Al° particles from the East Pacific Rise and the Central Indian Basin but differ markedly to those from other locations. After ruling out several possibilities of Al° sources and mechanism of formation, an alternative cold seep mechanism is proposed for the origin of Al° in the SCS. During the processes of anaerobic oxidation of methane (AOM) and pyrite formation, high H2S and H2 partial pressures result in strong reducing micro-environments, under which AlðOHÞ 4 is reduced to its metallic state by the microbial-bacterial processes. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Aluminum, making up about 8% by weight of the Earth’s solid surface, is the third most abundant element in the Earth’s crust, second only to oxygen and silicon. With its reactive chemical properties, aluminum is usually combined with other elements such as oxygen, silicon or fluorine to form different compounds. However, it can occur in nature in its elemental form, native aluminum (Al°). The first discovery of Al° was reported in trap rock on the Siberian Platform (Oleynikov et al., 1978). To date, more than 20 discoveries of Al° have been reported in different geological settings and rocks, e.g., in olivine kimberlitic tube (Kovalski and Oleynikov, 1985), volcanoes (Novgorodova and Mamedov, 1996; Novgorodova et al., 1997), ore deposits (Novgorodova et al., 1981; Stolyarov et al., 1988; Kozlov and Skachkova, 1989; He et al., 1990), beresitization zone (Deng et al., 1983), jarositized quartzitic rock (Jiang et al., 1985), ophiolitic chromitites (Bai et al., 2004), and in potash rhyolites (Filimonova and Trubkin, 1996). In addition, Al° is also present * Corresponding author. Tel./fax: +886 6 276 1692. E-mail address: [email protected] (C.-Y. Huang). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.06.006

in sediments of the Pacific Ocean (Shterenberg and Vassileva, 1979; Shterenberg et al., 1986; Dekov et al., 1995; Davydov and Aleksandrov, 2001), the Atlantic Ocean (Shiyukov et al., 1987), the Indian Ocean (Iyer et al., 2007) as well as from the Red Sea (Butuzova et al., 1987) and the Lake Baikal (Dombrovskaya et al., 1985). Evidently, the wide occurrence of Al° in various geological settings also indicates its considerable genetic diversity. Many mechanisms have been proposed for Al° formation, and most require reducing conditions. However, there are no reports of Al° associated with cold seeps even though cold seep environment is normally one of reducing and could be conducive for Al° formation. Cold seeps occur in a variety of geotectonic settings, such as canyon walls, active or passive continental margins, limestone escarpments, hydrocarbon deposits, even inland lakes and seas (e.g., Kvenvolden and Lorenson, 2001; Campbell, 2006). In subsurface sediments, anaerobic oxidation of methane (AOM) takes place (Ritger et al., 1987; Boroski et al., 1996) when upward advecting and methane-charged fluids come in contact with seawater sulfate. Recently, molecular, isotopic and phylogenetic evidence revealed that AOM is performed by a consortium of CH4oxidizing archaea and sulfate-reducing bacteria (e.g., Valentine,

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2002; Levin, 2005). In AOM process, microorganisms consume methane and sulfate to produce the energy that sustains dense chemosymbiotic communities including bacterial mats, vestimentiferan tubeworms, bathymodiolid mussels, mytilid mussels and vesicomyid clams (Sibuet and Olu, 1998; Valentine, 2002; Levin, 2005). On the other hand, AOM results in an increase of sulfide and alkalinity in the ambient environments, promoting the seep precipitations of authigenic minerals, composed mainly of calcite, aragonite, dolomite, pyrite, and less commonly minerals, siderite and barite (e.g., Peckmann et al., 2001; Greinert et al., 2002; Peckmann and Thiel, 2004). Presently, compelling evidence reveals that seep minerals are fingerprints of fluid expulsion and microbialmediated activity on modern or ancient sea floor (Peckmann and Thiel, 2004; Chen et al., 2005; Huang et al., 2006; Han et al., 2008). Therefore, it is surprising that so far there have been no reports on the occurrence of Al° in modern or ancient seep sediments. Recently, several active and fossil cold seeps were discovered in the northeastern continental slope (NCS) of the South China Sea (SCS) and Kaoping Slope off Taiwan (Yang et al., 2003; Wu et al., 2005; Huang et al., 2006; Liu et al., 2006; Lin et al., 2007) (Fig. 1). In this paper we report the occurrence of Al° particles in cold seep sediments of the NCS of the SCS. We present morphological and geochemical data to suggest that they are formed in situ, but their formation mechanism needs to be further investigated.

2. Background The geotectonic evolutions of the NCS of the SCS and the Kaoping Slope off Taiwan (Fig. 1) were controlled by the Eurasian and Philippine Sea Plates. Since the latest Miocene, the SCS oceanic crust of the Eurasian Plate has been subducting under the Luzon volcanic arc, which brought about a large, thick accretionary

wedge in the Kaoping Slope off Taiwan (active continental margins; Huang et al., 2006; Liu et al., 2006). Simultaneously, this subduction created a large number of structures such as diapirs, seabed valleys, fan channels and slope slumps in the passive continental margins of NCS (Wu et al., 2005; Wang et al., 2006). Recently, a great deal of surveys about gas hydrates have been carried out in both passive and active continental margins (Fig. 1), including seismic surveys specially of bottom simulating reflectors (BSRs). Furthermore, sediment sampling, TV-controlled grab system and remotely operated vehicle (ROV) surveys were also conducted. BSRs are widely distributed beneath the sea floor in the NCS of the SCS and the Kaoping Slope off Taiwan, possibly indicating the presence of gas hydrates beneath the sea floor (McDonnell et al., 2000; Wu et al., 2005; Liu et al., 2006; Wang et al., 2006). These results also showed that formations of authigenic carbonate and pyrite were associated with AOM (Chen et al., 2005; Lu et al., 2005; Huang et al., 2006; Han et al., 2008). Combined with other robust evidence such as geochemical anomalies (Oung et al., 2006; Chuang et al., 2006), and mud or volcanic structures (Wang et al., 2006; Chiu et al., 2006), several potential cold seeps were discovered and outlined (Yang, 2003; Huang et al., 2006) (Fig. 1). On the sea floor of the NCS, buildups of authigenic carbonates (Jiulong Methane Reef) (Fig. 1) relating to microbial-mediated activity were discovered during the R/V Sonne cruise 177 in 2004 (Han et al., 2008). In this area shell debris of chemoauthotrophic bivalves such as Calyptogena sp. and Acharax sp. are distributed patchily (Han et al., 2008). Of special interest is the active cold seep at location ROV-F (Figs. 1 and 2c and d), discovered using ROV Hyper Dolphin of the JAMSTEC during the cruise of chemosynthetic community investigation in March, 2007, by Taiwanese and Japanese scientists. At location ROV-F bacterial mats (Fig. 1c), mussels and white crabs (Fig. 1d) are prolific (Lin et al., 2007). This cruise also abundant chimney-like carbonates (Fig. 2a and b) and shell

Fig. 1. A map showing the study area in the South China Sea (a), and site locations of the Al° particles and potential areas of cold seeps (b). (1) Analyzed sites, (2) Al° sites, (3) seep carbonates (this paper), (4) and (5) seep carbonates from Chen et al. (2005) and Huang et al. (2006), (6) BSRs, A from Liu et al. (2006) and B from Wang et al. (2006), respectively, (7) subduction zone, (8) and (9) promising area of cold seeps, from Yang (2003) and Huang et al. (2006), and (10) Jiulong Methane Reef (Han et al., 2008).

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Fig. 2. Underwater photographs of typical seep carbonates and chemoautosynthesis-based community at cold seeps in NCS of the SCS. (a) Seep carbonates (black arrows) and mussels (white arrows), (b) chimney-like seep carbonates (black arrows), (c) bacterial mats (black arrows), and (d) mussels (white arrows) and white crabs (black arrows). Photographs (a) and (b) were taken at location ROV-G where Al° particles were collected, and (c) and (d) were taken at location ROV-F. All of seep carbonates, mussels and bacterial mats indicate that AOM occurs ubiquitously on the sea floor where native aluminum was discovered. Mussels are 12–15 cm long and chimney carbonates in (b) are 15–20 cm long,

debris of mussels (Fig. 2a) on the sea floor at the core site ROV-G, indicating that the cold seeps were inactive. Though there is no sea-floor visualization data at site CF4, many seep carbonate debris and pyrites associated with AOM were observed in the sediments. Therefore, it is believed that AOM occurs ubiquitously on the sea floor, especially at CF4, ROV-F and ROV-G (Figs. 1 and 2). 3. Sampling and analytical methods Of the more than 150 sediment samples we have analyzed (Fig. 1), Al° grains were found in only a few samples. In this paper, we focus on the CF4 surface sample and samples from the short core sediment ROV-G (Fig. 1). Core ROV-G (118°52.3490 E, 22°8.8900 N, 490 m water depth, 37 cm length) was collected during the investigation of chemosynthetic community in March, 2007. The sample at CF4 (118°57.0060 E, 22°0.5740 N, 1632 m water depth) was recovered by a grab sampler during the Northern South China Sea Opening Research Cruise by R/V Shiyan 3 in August, 2007, carried out by the South China Sea Institute of Oceanology, Chinese Academy of Sciences. Sediment samples were hermetically sealed in polythene bags after sub-sampling on board. The samples were washed with distilled water and passed through 63 lm bronze sieves at National Cheng Kung University (core ROV-G) and South China Sea Institute of Oceanology, Chinese Academy of Sciences (site CF4), respectively. All particles of Al° in the >63 lm size fractions were carefully hand-picked under a stereo microscope. Two particles of Al° were picked out from both the 24–26 cm and the 26–28 cm samples in core ROV-G, and five particles were picked out from the surface sample of site CF4. The morphology and microstructure of Al° particles were measured by Quanta 400 SEM connected to the energy dispersive X-ray microanalyzer (EDS), operating at 20 kV and 25 mA with a 10– 12 mm sample working distance. Later, the samples were mounted in organic resin and polished for chemical analysis. Chemical com-

positions were determined by an electron probe microanalyzer (GXA-8100) at 15 kV and 20 mA at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The analytical results are expressed in atom%. Detection limits of elements analyzed with the GXA-8100 is 0.01 atom%. Deviations and certified values are within the recommended ranges of standards. The X-ray diffraction patterns of Al° particles were obtained with X-ray diffractometer (XRD), using monochromatic Mo Ka radiation on a Bruker SMART APEX-CCD at the China University of Geosciences (Beijing). Scans were run from 3° to 50° 2h with a scan step of 0.03° 2h at 45 kV/35 mA. The space group of aluminum (synthetic) is Fm3 m, thus its unit cell edge a is calculated by the d2 = a2/(h2 + k2 + l2). 4. Results 4.1. Morphology and type The >63 lm fractions of the analyzed sediment samples are composed of biogenic calcium carbonates, different types of pyrites, a minor amount of seep carbonate debris, and terrigenous minerals such as quartz and mica. Authigenic pyrites occur as four different aggregates such as framboids, granular masses, elongated tubes and foraminiferal infillings which are similar to those from cold fluid sediments (e.g., Kohn et al., 1998; Huang et al., 2006). Typical shapes of Al° particles are presented in Figs. 3 and 4. The particles are grayish or silver white in color with a strongly metallic luster. Some particles, however, have a brownish coating of siliceous sediments (Fig. 3a). The Al° particles vary from 50 to 1100 lm in length and 10–50 lm in width. They exhibit high plasticity, with uneven and sharp edges, indicating they were formed in situ rather than transported from a long distance. The Al° particles appear as three different types: (1) large, irregular plates and sharp edges like specimen CF4#1 (#1 particle in CF4, the same below) (Fig. 3a) and specimen T24-26#2 (#2 particle

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Fig. 3. Photographs of the distinct types of Al° particles in the NCS of the SCS. (a) Large, irregular particle with brownish coating of siliceous sediments (white arrows), (b) elongated, irregular particle with lamellar structure, (c) elongated and platy shapes without obvious lamellar structures, and (d) energy dispersive X-ray spectrum of Al° particle from (a).

in 24–26 cm samples in core ROV-G, the same below). (Fig. 3d); (2) elongated shapes with typical lamellar structures (Fig. 3b and c and Fig. 4a and b). The lamellae are between 50 and 120 lm in length and 5 and 10 lm in thickness. Each lamellae appears as flexural microstructure (Fig. 4d); and (3) spherules with botryoidal structures, which co-occur with lamellar particles (Fig. 4c) or appear as a single spherule. The globular shaped botryoids have smooth edges and vary in diameter from 5 to 60 lm (Fig. 4c). Morphologically the Al° particles are almost identical to those from the Central Indian Basin sediments (Iyer et al., 2007). 4.2. X-ray diffraction analysis The X-ray diffraction patterns of these particles show eight reflections at 111, 200, 220, 311, 222, 400, 331 and 422 corresponding to interplanar spacings of 2.3589, 2.0373, 1.4358, 1.1719, 1.2247, 1.0149, 0.9304 and 0.9056 Å for sample CF4#2, and 2.3239, 2.0122, 1.4223, 1.2153, 1.1636, 1.0082, 0.9427 and 0.9013 Å for sample T26-28#2. The data are close to the characteristic reflections of synthetical Al and in excellent agreement with those from ocean sediments, and basic and ultrabasic igneous rocks (e.g., Oleynikov et al., 1978; Jiang et al., 1985; Dekov et al., 1995; Novgorodova and Mamedov, 1996), confirming the examined particles are native aluminum (Al°). The sharp and clear reflections on the X-ray diffraction patterns show that the particles are well crystallized. The unit cell edge a calculated by the interplanar spacings is 4.069 ± 0.010 Å (CF4#2) and 4.026 ± 0.02 Å (T2628#2, ROV-G), which are similar to those from the East Pacific Rise (Dekov et al., 1995) and other locations (e.g., Oleynikov et al., 1978; Jiang et al., 1985; Arsmakov et al., 1988).

4.3. Chemical composition The studied particles are mainly composed of Al (95.07–99.84%, the average value is 98.42%), with minor amount of Si, Fe, Ti, S, Zn, Mg, Ca, and trace of K, Na, Cu, Co and P (Table 1). Rarely some of elements reach 1%, but Mg concentration in specimens of CF4#1 and CF4#3 is higher than 1%. Compositionally, the constituted elements do not differ between surface and core sediments (Table 1), but the average Al concentration is slightly higher in core ROV-G than that in CF4 (Table 1). The results of electron microprobe analysis show similar chemical compositions in the different lamellae (CF4#2) (Table 1). Al exhibits a significant linear correlations with Fe, S and Zn (r = 0.86, 0.88, 0.97, respectively), implying a close association among these metals. On the contrary, Al exhibits no relationships with other elements such as Si (r = 0.16) and Ti (r = 0.16). As Si and Ti are constituent elements of terrigenous sediments, the non-correlations between Al and Si, Ti indicate that the Al° formation is not closely associated with terrigenous sediments. The chemical compositions of the SCS Al° particles are similar to those of the East Pacific Rise (Dekov et al., 1995), and Central Indian Basin (Iyer et al., 2007), but are markedly different from those of basic and ultrabasic igneous rocks (Novgorodova and Mamedov, 1996; Bai et al., 2004), non-carbonate zeolitic pelagic clays (Arsmakov et al., 1988), kimberlite tube (Kovalski and Oleynikov, 1985), as well as tungsten deposit (He et al., 1990), beresitization zone (Deng et al., 1983) and jarositized quartzitic rock (Jiang et al., 1985). Those published data showed that trace elements such as Mg, Si, Fe, Ca and Ti occur more frequently and in higher concentrations, the contents of Cu, Zn and Mn are lower and rare, whereas Pb,

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Fig. 4. Scanning electron micrographs of the Al° particles from the NCS of the SCS. (a) Typical lamellae of Al° of CF4#2, (b) enlargement of the lamellae indicated by square in (a), (c) bulbous and lamellar structure of Al° of CF4#3, and (d) lamellar structure of Al° of T24-26#1.

Table 1 Chemical composition of Al° particles from the NCS of the SCS. Specimens

Al

Core ROV-G (118°52.3490 E, T24-1 97.62 T24-2 99.73 T26-1 99.71 T26-2 99.84

Fe

Si

22°8.8900 N, 490 m) 0.02 0.43 0.03 0.10 0.09 0.11 0.04 0.18

Site CF4 (118°57.0060 E, 22°0.5740 N, 1632 m) CF4-1 95.15 0.77 – CF4-2 99.77 0.04 0.11 99.78 0.04 0.11 CF4-3 95.07 0.96 0.14 97.27 0.66 0.11 CF4-4 99.62 0.21 – CF4-5 99.09 0.45 0.01

K

Na

Mg

Ca

Co

S

Cu

P

Zn

Ti

Total

Type

– 0.01 – –

0.02 0.02 – 0.02

0.11 – – –

0.01 0.01 0.01 –

0.01 – 0.02 –

0.02 0.01 0.01 –

0.87 0.01 0.02 –

– – – –

0.11 0.02 0.02 0.02

0.08 0.05 0.01 –

99.30 99.99 100.00 100.10

Elongate Irregular Elongate Elongate

0.11 – 0.01 0.19 0.01 – –

0.19 0.02 0.04 0.55 0.02 0.02 –

1.90 – – 1.43 1.19 0.01 –

0.87 0.02 0.02 0.67 0.28 0.02 0.01

– 0.02 0.02 – – 0.02 0.03

1.04 0.02 0.01 1.64 1.24 0.02 0.01

0.06 – – 0.07 0.11 – 0.01

0.08 – – 0.02 0.01 – –

0.32 0.01 0.02 0.22 0.15 0.02 0.02

0.03 0.03 0.04 0.05 0.05 0.04 0.07

100.52 100.05 100.09 101.01 102.00 99.98 99.70

Irregular Lamellae-1 Lamellae-2 Spherule Lamellae Spherule Irregular

Note: (1) All values in atom% and (2) not detectable.

Cr, Sn and Cl are very rarely present (Dekov et al., 1995). It is important to note that chemical compositions of Al° particles in our study differ markedly from those of tungsten deposits, Guangdong Province, China (He et al., 1990), indicating the Al° particles were not from riverine contributions of the Pearl River. 5. Interpretation and discussion 5.1. Possible sources of Al° The particles at cold seeps are identified to be Al° based on the results of X-ray diffraction patterns and chemical compositions. However, their sources are unknown. We first consider the possibility of contamination. We have examined more than 150 sediment samples, and Al° particles were

only found in a few samples, so they were unlikely to be contaminated from sampling devices. Although Al° particles were collected during the different cruises and were picked out at different laboratories using different devices, their chemical compositions were similar. Thus the possibility is ruled out that these Al° particles were of artificial contamination from sampling devices, shipboard operations, or sieving instruments. Following lines of evidence also do not support a terrestrial source for the Al° particles. Freshness and morphology of the specimens (Figs. 3 and 4) suggest that they were likely formed in situ and recently, and this is not consistent with a long distance transport from continental China or from Taiwan Island. Additionally, the significant chemical differences between Al° particles of cold seep sites and tungsten deposits (He et al., 1990) does not support a riverine contribution. Considering, the sedimentation rates of 0.15–0.21 m/ka (Wang

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et al., 2000), the sediments containing Al° particles in core ROV-G (at depths of 24–26 cm and 26–28 cm) were deposited at about 1200 years ago, thus the possibility of airplane crashes as a contributing factor could be ruled out. An extraterrestrial origin is untenable because cosmic spherules found throughout the ocean are enriched in Fe, Ni and Cu compounds, but not Al° (Iyer et al., 2007). Thus, the Al° particles were most likely formed in situ at the SCS. 5.2. Occurrences and formation mechanisms of Al° in nature Though more than 20 discoveries of Al° have been reported to occur in nature, the formation conditions of Al° have not been ascertained. Native metals like Al° can form in reducing oceanic or mud volcanic environments, which are associated with upward migration of basaltic magma (Iyer et al., 2007), hydrothermal activity (Dekov et al., 1995), magmatic or metamorphic process (Howard and Fisk, 1988) or high-temperature hydrocarbon-enriched fluids (Novgorodova and Mamedov, 1996). Alternatively, exsolution of metal-rich fluids in the magma (Yang and Scott, 1996) and degassing of magmatic vapors during submarine eruptions (Rubin, 1997) may lead to reduction of some elements to their metallic state. Several hypotheses have been proposed concerning the mechanism of Al° formation, including: (1) gas-condensate from metalliferous fluids together with fairly other ‘dry’ reducing gases (Eqs. (1a) and (1b)) (Stolyarov et al., 1988; Dekov et al., 1995; Iyer et al., 2007), (2) electrolytic reduction of alumina (Al2O3) dissolved in molten cryolite at 1000 °C (Eq. (2)) (Deng et al., 1983), (3) reduction of carbon compounds (Eqs. (3a) and (3b)) (Deng et al., 1983), (4) reduction by intratelluric fluids (Eq. (4)) (Sobotovich and Ol’khovik, 1987), and (5) reaction of aluminum chloride (AlCl3) with H2 (Eq. (5)) (Stolyarov et al., 1988).

Early stage : 3AlCl ! AlCl3 þ 3Al

ð1aÞ 

Final stage : AlCl3 þ 3Kmetal ðPotassiumÞ ! Almetal ðAl Þ þ 3KCl ð1bÞ 2Al2 O3 ! 4Al þ 3O2 

ð2Þ 

Al2 O3 þ 3C ! 2Al þ 3CO; Al includes Al; Al2 O3 ; Al4 C3

ð3aÞ

2Al4 C3 þ 3SiO2 ! 8Al þ 3Si þ 6CO

ð3bÞ

Al2 O3 þ 3H2ðgasÞ ! 2Al þ 3H2 O 2AlCl3 þ 3H2 ! 2Al þ 6HCl

ð4Þ

5.3. Cold seep environments and native metal formation Bacterial sulfate reduction (Eq. (6)) by the degradation of organic matter and AOM (Eq. (7)) at cold seeps liberate H2S and HCO 3, which cause an increase in alkalinity that promote carbonate precipitation (Eq. (8)) (Coleman and Raiswell, 1995). Ferric-iron-bearing phases (e.g., amorphous FeOOH) can be microbially reduced at cold seeps with high levels of SO2 and organic matter (CH2O) 4 (Peckmann and Thiel, 2004) (Eq. (9)). The process leads to form meta-stable FeS (amorphous FeS, mackinawite (FeS) and greigite (Fe3S4)) and also to cause an increase in alkalinity. If these processes occur in close proximity, seep carbonate formation is even more favored than by AOM alone (Eq. (7)). As meta-stable FeS can not be preserved for a long time under anoxic conditions, it ultimately converts to pyrite (FeS2) through the H2S pathway (e.g., Drobner et al., 1990; Shen and Buick, 2004) (Eq. (10)), in which H2 is liberated.  2CH2 O þ SO2 4 ! 2HCO3 þ H2 S

ð6Þ

  CH4 þ SO2 4 ! HCO3 þ H2 S þ OH

ð7Þ

Ca2þ þ 2HCO3 ! CaCO3 þ H2 O þ CO2

ð8Þ

4CH4 þ CH2 O þ 4FeOOH þ 3H þ þ4SO2 4 ! 5HCO3 þ 4FeS þ 10H2 O FeS þ H2 S ! FeS2 þ H2

Mechanism (1) is the best known and has been used to interpret Al° formation in tin deposit (Stolyarov et al., 1988), East Pacific Rise (Dekov et al., 1995), and Central Indian Basin (Iyer et al., 2007) and other occurrences. This mechanism involves highly reducing conditions resulting from the presence of high CH4 and H2 contents and low fO2 during periods of magma migration or hydrothermal activity, all high temperature environments (e.g., Stolyarov et al., 1988; Dekov et al., 1995; Iyer et al., 2007). Cold seep environments are typically reducing but low temperature, therefore Al° particles may not form by mechanism (1) (Osadchiy et al., 1982; Dombrovskaya et al., 1985; Dekov et al., 1995). Similarly, cold seep Al° particles are not likely formed by mechanism (2), (3) or (4) because these reactions require high temperature conditions. Mechanism (5) could occur under low temperature and reducing environments, but it is not favored under alkaline environments occurring at most cold seeps. As a result, the Al° particles at cold seeps are of special interest from the point of the Al° genesis.

ð10Þ

On the sea floor of the NCS of the SCS, seep carbonates occur as crusts, chimneys (Fig. 2a and b), nodules or chemoherm concretions (Chen et al., 2005, 2006; Lu et al., 2005; Huang et al., 2006; Han et al., 2008), and the chemoautosynthesis-based ecosystem is abundant (Fig. 2), indicating the strong AOM at cold seeps. Additionally, sediments from sites CF4 and ROV-G smelled of H2S when they were separated on shipboard. These lines of evidence demonstrate that strong reducing conditions can form and be maintained in subsurface sediments in the NCS of the SCS, due to the presence of abundant H2, H2S, and CH4. Trivalent aluminum ions (Al3+) needed for reduction to form Al° particles occur in different forms such as clay minerals, gibbsite, nordstrandite and bayerite. But in sedimentary porewater under slightly alkaline environments of cold seeps, dissolved Al3+ most likely exists as the tetrahedral AlðOHÞ 4 (Swaddle et al., 2005). Eq. (11) is one possible mechanism for Al° formation under reducing but alkaline conditions of cold seeps.

2AlðOHÞ4 þ 3H2 ! 2OH þ 6H2 O þ 2Al ð5Þ

ð9Þ



ð11Þ

Aluminum reacts with hydroxyl ions (OH) in the presence of water to form AlðOHÞ 4 plus hydrogen gas depends on suitable Eh and pH conditions (the reverse reaction of Eq. (11)). For reaction (11) to occur, microbial processes likely have to be involved. It is recognized that microbial activities are strong in marine environments (McMullin et al., 2000), and can drive the formation of many authigenic minerals including native metals, oxides, phosphates, carbonates, sulfides, and silicates (e.g., Kawano and Tomita, 2001; Kashefi et al., 2001), although the exact chemical mechanisms still remain to be understood. The investigation of Au3+ reduction to Au0 offers one possible mechanism for Al3+ reduction. Recent evidence suggests that hyperthermophilic and mesophilic dissimilatory Fe(III) – reducing bacteria and archaea can couple oxidation of H2 to the reduction of Au3+, leading to Au0 precipitation (Kashefi et al., 2001). However, these microorganisms could not grow with Au3+ as the sole electron acceptor, but could grow and reduce Au3+ when Fe(III) is also supplied. It appears that the reduction of Au3+ to Au0 can be achieved by a microbial-involved

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consortium, where the reduction of other electron acceptors such as Fe(III) can supply the energy to promote the reduction of Au3+ to precipitate gold by a microbial-catalyzed reaction. Here, we speculate that this consortium hypothesis could also work in cold seep environments, where abundant electron donors (H2S, H2, and CH4) can be used by microorganisms to reduce Fe(III) to supply the energy, which in turn can be used by other microorganisms to reduce Al3+ by oxidizing one of the above electron donors (such as H2). Besides thermodynamics, another argument against microbial reduction of Al3+ is that this element is toxic. However, a bacterium that requires aluminum to thrive has been reported in Yellowstone National Park (Travis, 1998), although the role of aluminum in the microbe remains mysterious. 6. Conclusion 1. The Al° particles from cold seep marine sediments of the NCS are a new type of occurrence in the natural environment. X-ray diffraction data confirm these particles to be metallic aluminum. Compositionally, the Al° particles are similar to those from the East Pacific Rise and the Central Indian Basin, but differ markedly to those from the other locations. 2. After ruling out the possibility of contamination, and transportation from continental sources, it is concluded that the Al° particles were formed in situ at sites of cold seeps. 3. The mechanism for Al° formation in the SCS cold seep remains unknown, but AlðOHÞ 4 could be reduced to Al° by H2, mediated by a microbial consortium under a reducing and slightly alkaline environment.

Acknowledgements Thoughtful reviews by two anonymous reviewers are highly appreciated. We wish to give special thanks to Prof. Xiong Ming (China University of Geosciences, Beijing), and Drs. Li Jianfeng, Chen Linli (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) for assistance in measurements. The study was partially supported by National Program on Key Basic Research Project (Grant No. 2009CB2195002-2), National Natural Science Foundation of China (Grant Nos. 40676038 and 40730844) to Z. Chen, National Science Council, ROC (Grant Nos. NSC95-2816-M006-003 and NSC95-2116-M-006-002) to CY Huang, and Northern South China Sea Opening Research Cruise by R/V Shiyan 3 in August 2007, South China Sea Institute of Oceanology, Chinese Academy of Sciences. We also appreciate all cruise members of R/V Natsushima NT07-05 cruise conducted under the collaboration between JAMSTEC and NCOR in 2007. References Arsmakov, H.I., Krnglyakov, V.V., Marushkin, A.I., 1988. Native metals and alloys in the pelagic sediments of the Pacific Ocean. Lithology of Ore Deposits 4, 122–126 (in Russian). Bai, W., Yang, J., Fang, Q., Yan, B., Zhang, Z., Ren, Y., Shi, N., Ma, Z., Dai, M., 2004. Some native metals from ophiolitic chromitites in Tibet. Earth Science Frontiers (China University of Geosciences, Beijing) 11 (1), 179–187 (in Chinese with English Abstract). Boroski, W.S., Paull, C.K., Ussler III, W., 1996. Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate. Geology 24, 655– 658. Butuzova, G.Y., Shterenberg, L.E., Voronin, B.I., Korina, E.A., 1987. Native metals in the ore-bearing sediments in the Red Sea. Lithology of Ore Deposits 2, 122–125 (in Russian). Campbell, K.A., 2006. Hydrocarbon seep and hydrothermal vent palaeoenvironments: past developments and future research directions. Palaeogeology, Palaeoclimatology, Palaeoecology 232, 362–407. Chen, D.F., Huang, Y.Y., Yuan, X.L., Cathles III, L.M., 2005. Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils

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