Marine and Petroleum Geology 26 (2009) 1190–1198 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo Petrographic and geochemical characterization of seep carbonate from Bush Hill (GC 185) gas vent and hydrate site of the Gulf of Mexico Dong Feng a, c, Duofu Chen a, *, Harry H. Roberts b a CAS Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Wushan, Guangzhou 510640, China Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA c Graduate University of Chinese Academy of Sciences, Beijing 100049, China b a r t i c l e i n f o a b s t r a c t Article history: Received 23 January 2008 Received in revised form 19 June 2008 Accepted 1 July 2008 Available online 6 July 2008 Authigenic carbonates are common at cold seep sites as a result of microbial oxidation of hydrocarbons. Seep carbonate samples were collected from the surface of the Bush Hill (Green Canyon Block 185, Gulf of Mexico), a mound containing gas hydrate. The carbonates consisted of oily, porous limestone slabs and blocks containing bioclasts and matrix. Analysis by X-ray diffraction shows that aragonite is the dominant mineral (89–99 wt% with an average of 94 wt%) in the matrix of seep carbonate. This cement occurs in microcrystalline, microspar, and sparite forms. The moderate 13C depletion of the seep carbonate (the most depleted one has d13C value of 29.4&, and 26 of 38 subsamples have d13C values >20.0&) indicates that the non-methane hydrocarbons was incorporated during seep carbonate precipitation. Relative enrichment of 18O may be related to localized destabilization of gas hydrate or derived from 18Oenriched pore water originated from smectite–illite transition in the deep sediments. The total content of rare earth elements (REE) of the 5% HNO3-treated solution of the carbonates is from 0.40 ppm to 30.9 ppm. The shale-normalized REE patterns show varied Ce anomalies from significantly negative, slightly negative, and no to positive Ce anomalies. Variable content of trace elements, total REE, and Ce anomalies in different samples and even in the different carbonate mineral forms (microcrystalline, microspar and sparite) of the same sample suggest that the formation condition of the Bush Hill seep carbonate is variable and complex, which is possibly controlled by the rate of fluid flux. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Seep carbonate Carbon and oxygen stable isotopes Rare earth elements Redox variation Bush Hill Gulf of Mexico 1. Introduction Seep carbonate precipitation is a widely observed phenomenon in the modern and ancient marine seep environments of the world (Roberts and Aharon, 1994; Peckmann et al., 2001; Peckmann and Thiel, 2004; Campbell, 2006; Naehr et al., 2007). Carbonate precipitation at hydrocarbon seep sites is a result of microbial oxidation of methane, as well as higher molecular weight hydrocarbons, through the combined metabolism of methane oxidizing archaea (MOA) and sulfate reducing bacteria (SRB) (Hinrichs et al., 1999; Boetius et al., 2000; Valentine and Reeburgh, 2000; Michaelis et al., 2002). The minerals in seep carbonate are mainly Mg-calcite, aragonite, and dolomite (Hovland et al., 1987; Roberts and Aharon, 1994; Peckmann et al., 2001; Naehr et al., 2007). The chemophysical factors controlling the mineralogy in seep carbonates are still not well understood (Peckmann et al., 2001). It is obvious that at seepage sites, the precipitation of aragonite is promoted over calcite when high sulfate concentration occurs (Aloisi et al., 2000). * Corresponding author. Tel.: þ86 20 8529 0286; fax: þ86 20 8529 0130. E-mail addresses: fd@gig.ac.cn (D. Feng), cdf@gig.ac.cn (D. Chen), hrober3@ lsu.edu (H.H. Roberts). 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.07.001 The carbon isotopic composition of seep carbonate serves as an indicator of the carbon sources incorporated during carbonate precipitation because the carbonates inherit the stable carbon isotopic signature from the carbon sources present in the seeps (Roberts and Aharon, 1994; Peckmann et al., 2001; Peckmann and Thiel, 2004). The d18O values, on the other hand, provide information pertaining to the temperature and fluid source from which seep carbonate is precipitated (e.g. Naehr et al., 2000). Additionally, due to the enrichment of 18O in gas hydrates, anomalously positive d18O of seep carbonate could argue in favor of gas hydrate destabilization at seep sites (Bohrmann et al., 1998; Aloisi et al., 2000, 2002; Greinert et al., 2001; Peckmann and Thiel, 2004; Chen et al., 2005). However, the smectite-illite transition in the deep sediments can also generates 18O-enriched pore water which could provide a source of 18O-enriched pore water for carbonate precipitation (Hesse, 2003). The behavior of rare earth elements (REE) has been widely reported in hydrothermal carbonate deposits (Barrat et al., 2000) and in carbonate of the normal marine environments, like coral (Sholkovitz and Shen, 1995). However, there has been relatively little investigation of the behavior of trace elements, especially REE, from hydrocarbon seep carbonates. D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 Bush Hill is one of the most studied areas for gas hydrate and hydrocarbon seeps in the world. Seep communities, authigenic carbonate, gas and oil seeps, and gas hydrates are common on the surface of seafloor (MacDonald et al., 1989, 1994; Roberts and Aharon, 1994; Sassen et al., 2004; Tryon and Brown, 2004). Previous research has shown that vent gas and hydrate gas derives from thermogenic gas that migrated to seafloor from the subsurface petroleum system (Jolliet reservoir) (e.g. Chen and Cathles, 2003; Chen et al., 2004; Sassen et al., 2003, 2004). Some seep carbonate samples collected from the Bush Hill site display extremely negative d13C values (55& to 40&), and thereby have been considered to favor a methane carbon source (Roberts and Aharon, 1994). However, additional data from recent investigations show that the majority of sulfate reduction at the Bush Hill sites is likely fueled by the oxidation of organic matter, possibly heavier hydrocarbons (C2þ) like oil, rather than methane (Joye et al., 2004). Moreover, the measurement of gas and water flux and chemical analyses show large degrees of spatial and temporal variability of vent rate and fluid sources in this region (Roberts et al., 1999; Roberts, 2001; Leifer and MacDonald, 2003; Chen et al., 2004; Tryon and Brown, 2004). In this study, we have investigated sedimentary structure and mineralogy, stable carbon and oxygen isotope, REE, and trace elements of the seep carbonate collected at Bush Hill. We provide evidence that the formation conditions of the seep carbonates are highly variable and complex, and the fluid flow rate at this hydrocarbon seep site may be the primary factor controlling the formation conditions of the authigenic carbonates and their variable geochemical characteristics. 2. Sampling and analytical methods Bush Hill (27460 N; 91300 W) is a fault-related seep and hydrate site located near the boundary of GC 184 and GC 185, Gulf of Mexico, where water depth is w540 m (Fig. 1), and the average bottom water temperature is w7  C. The seep carbonates described 1191 here were collected in 1997 and 1998 during the Johnson-Sea-Link I (JSL I) manned submersible dives 2904, 4061 and 4063 using the robot arm of the submersible (Table 1). Petrographic observation of thin sections of the samples was made using a LEICA-DMRX optical microscope with Leica Qwin Program. The microstructure of the seep carbonate on the fresh surfaces of fractured samples was examined with a scanning electron microscope (SEM). The samples were prepared by gold coating to a thickness of w200 Angstroms for the SEM observations. Photographs were taken using a Sirion 200 FE-SEM equipped with EDAX GENESIS. For X-ray diffraction (XRD), the samples were crushed into powder less than 200 mesh using an agate mortar and pestle. The XRD analyses were performed using a Rigaku DXR 3000 computer-automated diffractometer utilizing Bragg–Brentano geometry. The X-ray source was a Cu anode operated at 40 kV and 40 mA using CuKa radiation equipped with a diffracted beam graphite monochromator. The orientated samples were scanned at an interval of 5–65 (2q) with a step size of 0.02 and count time of 5 s per step. Divergence, scattering, and receiving slits were 0.5 , 0.5 and 0.15 mm, respectively. Relative abundance of the minerals was semi-quantified by Rietveld analysis of the diffractograms with the program SIROQUANT (Taylor, 1991). The powdered samples were processed with 100% phosphoric acid to release CO2 for stable carbon and oxygen isotope analysis. Carbonate carbon and oxygen isotopic compositions in permil (&) relative to PeeDee Belemnite (PDB) standard were measured by using the GV Isoprime II stable isotopic mass spectrometry with deviations less than 0.01& (2s) for both d18O and d13C values. The seep carbonate powder (0.1–0.5 g) was treated with 50 ml of 5% HNO3 in a centrifuge tube for 2–3 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 10 times to be used for the REE and trace elements analysis by using Finnigan MAT ELEMENT high Fig. 1. Bathymetric map of the Bush Hill study site, showing the area of carbonate outcrops, where the samples were collected, and the area of tube worm communities, mussel beds, and gas hydrate outcrops. Small black solid-square in the middle of the insert is the location of the Bush Hill mound. 1192 D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 Table 1 Samples collection information and mineral composition of Bush Hill seep carbonates based on X-ray diffraction Sample Year-Dive number Relative percentages (wt%) number Aragonite Calcite Muscovite Kaolinite Dolomite BH-A BH-B BH-C BH-D BH-F BH-G BH-H 1997-JSL I-2904 1997-JSL I-2904 1997-JSL I-2904 1997-JSL I-2904 1998-JSL I-4061 1998-JSL I-4063 1998-JSL I-4063 94.9 88.5 99.0 99.1 97.1 92.5 88.7 5.10 3.80 0.90 0.90 2.90 7.50 3.30 <0.1 4.8 0.1 – – – – – 1.7 – – – – – – – – – – – 7.8 resolution ICP-MS. 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 less than 10%, and average relative standard deviations are better than 5%. For the detail of ICP-MS analysis see Qi and Gregoire (2000) and Qi et al. (2005). 3. Petrography The authigenic carbonate samples occur as solidified to semisolidified crusts from grey, white to grey, yellow in color with variable porosity. Tube worm casts, serpulid wormtubes, and microbial degraded crude oil are present (Fig. 2). Lucinid-vesycomyid clam shells, up to 9.5 cm in length and 4.5 cm in width, are well preserved in the carbonates. This single type of bivalve shell occurs as imbricate structure, and is the volumetrically dominant component (>40% in volume) in some samples (Fig. 2). The XRD analysis of carbonate matrix shows that aragonite is the most abundant mineral (89–99 wt%, average 94 wt%), with minor amounts of calcite and dolomite (Table 1). Samples BH-C and BH-D contain the most aragonite. However, samples BH-A, BH-B, BH-F, BH-G, and BH-H seem to contain much more calcite, and BHB has some portion of clay minerals (muscovite and kaolinite) whereas other samples are pure in aragonite and calcite. Pyrite was also observed under optical microscope and SEM (Figs. 3 and 4). Aragonite occurs in microcrystalline, microspar, and sparite forms in the carbonate sample (Fig. 3A and B). The microcrystalline aragonite is black to grey in color under binocular, <5 mm and mostly w1 mm in diameter. Some microcrystalline crystals were recrystallized or partially recrystallized to form microspar from 5 mm to 20 mm in diameter. The sparite crystals usually, up to 0.6 mm in length, form multiple layers in the botryoidal cements (Fig. 3B). Pyrite framboids, aragonite clotted microfabrics, and botryoidal cements were observed in the seep carbonate (Figs. 3 and 4). Pyrite framboids, w5–10 mm in diameter (average w7 mm) are dispersed within seep carbonates, and they also occurs within the foraminifer chambers. The framboids are composed of numerous smaller particles, w0.5 mm in diameter, and mostly occurring as cubic or pentagonal dodecahedron crystals (Fig. 4). Clotted microfabrics with misty borderline <100 mm in diameter are the aggregates of microcrystalline aragonite (Fig. 3C). These aggregates generally show an intense fluorescence. Botryoidal cements consist of fibrous aragonite crystals, usually arising from a dark nuclear mass. This type of cement commonly occurs as an isopachous layer showing multiple stages of growth (Fig. 3B), or it fills voids up to 0.5 mm in diameter. 4. Geochemistry 4.1. Isotope geochemistry Stable isotopes of carbon and oxygen were measured on the different components of seep deposits, including microcrystalline aragonite, microspar aragonite, sparite aragonite, bivalve shells, and serpulid worm tubes (Table 2 and Fig. 5). The results show that the carbon and oxygen isotopic values of the Bush Hill seep carbonates can be divided into three groups (Fig. 5). Group I has relatively depleted 13C (d13C: 29.4& to 15.1&) and enriched 18O (d18O: 2.4& to 5.0&) as shown by all microcrystalline aragonite samples, eight microspar aragonite samples, and five sparite aragonite samples. Group II has relatively enriched 13C (d13C: 10.6& to 1.4&) and 18O (d18O: 3.1& to 4.5&) shown by all bivalve shell and serpulid worm tubes samples, one sparite aragonite sample and two microspar aragonite samples. Group III has relatively enriched 13C (d13C: 8.0& to 2.7&) and depleted 18 O (d18O: 0.6& to 0.3&) as represented by eight microspar aragonite samples (Table 2 and Fig. 5). 4.2. Trace element geochemistry The results of trace element analysis are listed in Table 3. The REE content is 7.07–26.57 ppm for microcrystalline aragonite, 0.40–3.10 ppm for microspar aragonite, 0.85–2.38 ppm for sparite aragonite, and 1.65–30.86 ppm for bivalve shells. The REE content of microcrystalline aragonite is higher than that of sparite aragonite and microspar aragonite in the same sample (Table 3). The shale-normalized REE patterns of seep deposits exhibit negative Ce anomalies (Ce/Ce* < 0.95, e.g. samples BH-C and BHD), no Ce anomalies (0.95 < Ce/Ce* < 1.05, e.g. sample BH-H), and positive Ce anomalies (Ce/Ce* > 1.05, bivalve shell in sample BHG) (Table 3). The Ce anomaly of microcrystalline aragonite is more negative than that in sparite and microspar aragonite in the same sample (Fig. 6 and Table 3). Strontium contents in microcrystalline aragonite are from 5168 ppm to 9147 ppm, except for sample BH-H which has low Sr content (94 ppm). The Sr concentrations vary from 2490 ppm to 9054 ppm in microspar aragonite, 8787 ppm to 9206 ppm in sparite aragonite, 113 ppm to 2624 ppm in bivalve shells. Barium contents in microcrystalline aragonite samples range from 17 ppm to 93 ppm although sample BH-H has a very high content (902 ppm). Barium contents are 10 and 19 ppm in the two microspar aragonite samples, 15 ppm and 26 ppm in the two sparite aragonite samples, 12 ppm and 55 ppm in the two bivalve shell samples (Table 3). Elements V, Mo, U, and Cd contents are highly variable, however, it appears that the higher contents are more likely in the microcrystalline aragonite than in the microspar and sparite aragonite. 5. Discussion 5.1. Hydrocarbon-derived carbonate 5.1.1. Carbon and oxygen stable isotopes The d13C value of methane in the Gulf of Mexico is from 120& to 30& (Sackett, 1978; Whiticar et al., 1986), crude oil is averaging 25& (Roberts and Aharon, 1994), and seawater CO2 3 is 0  3& (Anderson and Arthur, 1983). The carbon and oxygen isotopes of the Bush Hill seep carbonates can be divided into three groups (Fig. 5). Group I has moderate depletion d13C values from 29.4& to 15.1& in the microcrystalline, sparite, and microspar aragonite (Table 2 and Fig. 5). These moderately depleted d13C values could be derived from non-methane hydrocarbons, probably the microbial degradation of crude oil. These values are also supported by the frequent occurrences of biodegraded crude oil in the pores of carbonate samples (Fig. 2). Based on a carbon isotope mass balance model (Formolo et al., 2004), the seep carbonate carbon source at the Bush Hill is mainly from non-methane hydrocarbons and only minor carbon has been contributed from methane, which is consistent with the previous results (Joye et al., 2004). D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 1193 Fig. 2. Morphology of seep carbonate from Bush Hill mound. BH-A, asphalt (upper right part) within carbonate; BH-B, large amount of warm tubes preserved in seep carbonate; BHC and BH-D, porous carbonate; BH-F: lucinid-vesycomyid clam shells up to 9.5 cm length and 4.5 cm width in seep carbonate; BH-G, single type of bivalve shell occurs as imbricate structure; BH-H, occurrence of the microbial degradation of crude oil in seep carbonate, Serpulid worm tubes (arrows) attached on its surface. Allscale bars ¼ 1 cm. In contrast, the d13C values of Group II shown by the shells and small worm tubes are from 10.6& to 1.4& (Fig. 5 and Table 2), much less 13C-depleted than that of Group I, which is similar to the d13C values of tube worms and chemosymbiotic shells reported by Roberts and Aharon (1994) and Aharon et al. (1997). Two samples of microspar and one sample of sparite have similar values (Fig. 5). These higher d13C values of Group II are possibly the mixed source of seawater (d13C ¼ 0  3&, Anderson and Arthur, 1983) and nonmethane hydrocarbons, such as oil (d13C ¼ 28& to 25&, Aharon et al., 1997). The microspar aragonite in Group III has similar d13C values (8.0& to 2.7&) with the Group II (Fig. 5). The microspar aragonite is partially recrystallized from microcrystalline aragonite. Thus, the carbon source of microspar aragonite may be inherited from the carbon of microcrystalline aragonite but modified by recrystallization in later diagenesis. The oxygen isotopic composition of authigenic carbonate is controlled by a combination of factors including: (1) sample mineralogy and chemistry, (2) temperature of carbonate precipitation, and (3) pore fluid isotopic composition (Anderson and Arthur, 1983). Aragonite at Bush Hill precipitated in equilibrium with bottom water (d18O is about 0.7& V-SMOW) at 7  C has a d18OPDB value of approximately 3.67&, according to Hudson and Anderson (1989). The d18O ratios of Groups I and II carbonates are from 2.44& to 4.96&. Most of them fall into the range of 3–4& (Table 2), indicating that they are likely precipitated from fluids at ambient bottom water temperature. However, Group III has the 1194 D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 Fig. 3. The microscopic images of seep carbonate from Bush Hill. (A) Matrix has peloid (labeled ‘‘pel’’), foraminifer (labeled ‘‘for’’), and bivalve shell (labeled ‘‘she’’), plane-polarized light; (B) sparite aragonite occurs as isopachous layer, plane-polarized light; (C) clotted microfabric has irregular shape and unclear margin, plane-polarized light; (D) framboidal pyrite consists of numerous small particle of pyrite (white) and aragonite (black) grains, reflected light. d18O ratios that range from 0.58& to 0.35&, indicating an equilibrium temperature from 21.4  C to 25.5  C, higher than w7  C of normal bottom water temperature. Measured bottom water temperature at Bush Hill gas vent site could be up to 10  C (Roberts et al., 1999; MacDonald et al., 2005) and even over 20  C (Roberts, 2001) at active vents, suggesting that these lower d18O ratios result from the effect of higher temperature which is most probably related to the expulsion of warm fluids at seafloor because of the rapid flux. These warm fluids may result in the recrystallization of microcrystalline aragonite. In addition, some samples in the Group I have even higher d18O ratios (up to 4.96&). The smectite–illite transition generates 18Oenriched pore water, and the advection of such pore water along migration pathways to seafloor vent sites would provide a source of 18O-enriched water for carbonate precipitation (Hesse, 2003). On the other hand, gas hydrate preferentially incorporates heavier oxygen isotopic water during crystallization (Davidson et al., 1983; Matsumoto, 2000). Therefore gas hydrate decomposition liberates water enriched in 18O about 1–2.9& (Hesse and Harrison, 1981). The ongoing dissolution/decomposition of gas hydrates was Fig. 4. SEM images of pyrite framboids. (A) Pyrite framboids w5–10 mm in diameter dispersed within seep carbonates. The framboids are composed of numerous smaller particles, w0.5 mm in diameter, and mostly occurring as pentagonal dodecahedron (B), spheric (C), and cubic (D) crystals. Very small filaments <0.1 mm in diameter on the pyrite surface are unknown, and may be clay mineral or bacteria. D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 Table 2 Carbon and oxygen stable isotopes of seep deposits from Bush Hill (&, PDB) Number Typea d13C d18O Number Typea d13C d18O BH-A BH-A BH-A BH-A BH-B BH-C BH-C BH-C BH-C BH-C BH-D BH-D BH-D BH-D BH-D BH-D BH-D BH-D BH-F mi ms ar ms mi mi ms ar ms ms ms ms mi ms ms ar ms ms bi 21.37 24.52 29.41 19.65 24.17 20.05 19.56 19.87 5.01 7.99 5.64 5.33 15.13 4.30 5.04 16.51 2.70 4.18 6.00 4.96 3.38 3.56 3.76 3.10 3.54 3.41 4.34 0.35 0.35 0.58 0.35 2.44 0.03 0.21 4.04 0.31 0.53 3.62 BH-F BH-F BH-F BH-F BH-F BH-F BH-F BH-G BH-G BH-G BH-G BH-G BH-G BH-H BH-H BH-H BH-H BH-H BH-H bi bi swt mi ms ar ms bi bi swt mi ms ms mi ms ar ar ms ms 2.96 1.68 1.39 22.79 25.28 27.59 27.94 10.56 5.58 1.55 26.95 24.18 27.12 17.93 15.35 4.05 15.19 5.05 6.41 3.33 3.62 3.30 3.30 3.35 3.17 3.22 3.15 3.17 3.26 3.59 3.24 3.32 3.84 3.60 3.34 3.72 3.39 3.45 a mi, microcrystalline aragonite; ar, sparite aragonite; ms, microspar aragonite; bi, bivalve shell; swt, serpulid worm tubes. observed at Bush Hill site (MacDonald et al., 1994; Sassen et al., 1998; Roberts et al., 1999). Thus, the 18O-enriched seep carbonate may be related to the destabilization of gas hydrate. This hypothesis of the destabilization of gas hydrate providing 18Oenriched water has been suggested by other studies at this site (Formolo et al., 2004), and at other cold seep sites worldwide (Bohrmann et al., 1998; Aloisi et al., 2000, 2002; Greinert et al., 2001; Chen et al., 2005). 5.1.2. Carbonate mineralogy Authigenic carbonate precipitation at seep sites will only take place when pore fluids become sufficiently supersaturated with respect to a carbonate phase, and crystallization is not inhibited by kinetic factors (Burton, 1993). Factors that influence carbonate mineral precipitation at hydrocarbon seep sites, including the degree of carbonate supersaturation, the concentrations of Ca and Fig. 5. Plot of d13C and d18O values of seep carbonates from Bush Hill. The dashed line represents d18OPDB value (3.67&) of aragonite precipitated in equilibrium with bottom water at 7  C (d18Owater is about 0.7& V-SMOW). 1195 Mg, the presence of complex forming anions such as SO2 4 and PO3 4 , temperature, pCO2, the degree of microbial activity, and the phylogeny of the microbes involved (Naehr et al., 2007 and references therein). Aragonite seems to be favored in more oxic environments (high SO2 4 ) with higher total alkalinity concentrations (Burton, 1993; Savard et al., 1996). The crystallization of Mg-calcite preferentially occurs under slightly more anoxic conditions with and total alkalinity concentrations (Greinert et al., lower SO2 4 2001). Elevated temperatures generally seem to favor aragonite precipitation, while changes in saturation state seem to have little effect (Naehr et al., 2000). Aragonite can also be formed at a very fast venting site where not all of the hydrocarbon is consumed in the sulfate reducing zone, and is oxidized aerobically in shallow sediment or even in the water column directly over the seep (Hovland et al., 1987; Terzi et al., 1994). Based on the samples analyzed in this study, the dominant mineral of the matrix of Bush Hill seep carbonate is aragonite (89– 99 wt% with an average of 94 wt%, Table 1). Samples BH-C and BH-D have the highest aragonite content (up to 99 wt%), and have more pores than other samples (Fig. 2). These two samples also have the highest formation temperature based on the oxygen isotopic equilibrium calculation, suggesting that elevated temperatures seem to favor aragonite precipitation (Naehr et al., 2000). The in situ observation of temperature and gas venting flux at Bush Hill shows that higher venting rate is consistent with higher temperature (Roberts et al., 1999), suggesting that fast venting may more generally result in aragonite precipitation (Hovland et al., 1987; Terzi et al., 1994). 5.1.3. Biogenic fabrics Evidence for an involvement of microbes in the formation of cold seep carbonates comes from stable isotopes, biomarkers, and biogenic fabrics. Pyrite occurs as framboids are often associated with authigenic carbonates, thereby indicating that sulfate reduction was active during carbonate precipitation. The association between authigenic carbonates and pyrite has already been observed in fossil seeps (Peckmann and Thiel, 2004; Campbell, 2006), and modern seeps (Aloisi et al., 2000; Sassen et al., 2004; Chen et al., 2005, 2006, 2007). Size distribution in framboids can yield insight into their genesis since rapidly-growing framboids are smaller and more homogeneous (Wilkin et al., 1996). The framboids from Bush Hill seep carbonates have an average diameter about 7 mm and are not significantly different in size. Additional evidence for the origin of the framboidal pyrite results from its size distribution of pyrite microcrystallites within framboids. Popa et al. (2004) have shown that the size distribution of biogenic framboids have a remarkably low number of microcrystallites (10–100), which is similar to the framboids of Bush Hill seep carbonates (the number of microcrystallines could not be larger than several hundred), while most abiotic framboids contain a large number of microcrystallites (up to 109; Goldhaber and Kaplan, 1974). Botryoidal aragonite is one of the typical carbonate fabrics in both modern and ancient seep carbonates (Roberts et al., 1993; Peckmann et al., 2001; Peckmann and Thiel, 2004). The botryoidal aragonite in seep carbonate usually arises from a nuclear mass of dark organic material, and was suggested to be induced by microbial activity (Roberts et al., 1993; Goedert et al., 2000). This hypothesis is in agreement with the negative stable carbon isotope values (as low as 29&). These biologically controlled fabrics, like clotted microfabric, botryoidal cement, and pyrite framboids preserved in seep carbonates suggest that they are being produced during methane oxidation and sulfate reduction by MOA and SRB, and those microbes are directly involved in the precipitation of authigenic carbonate at the Bush Hill gas vent site. Thus, the distinctive 1196 D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 Table 3 Elemental content (ppm) of 5% HNO3-treated solution in seep carbonates samples from Bush Hill Number BH-A BH-A BH-B BH-C BH-C BH-D BH-D BH-F BH-F BH-F BH-G BH-G BH-H Typea La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE Ce/Ce* Eu/Eu* mi ar mi mi ms mi ms ar bi mi bi 2.93 5.41 0.69 2.75 0.63 0.13 0.53 0.09 0.47 0.09 0.25 0.04 0.21 0.03 14.25 0.87 1.10 0.52 0.86 0.12 0.47 0.10 0.02 0.09 0.01 0.08 0.01 0.04 0.01 0.03 0.01 2.38 0.79 1.14 3.73 7.31 0.92 3.58 0.75 0.16 0.58 0.09 0.51 0.09 0.25 0.04 0.21 0.03 18.25 0.92 1.18 6.09 5.73 1.28 5.21 0.86 0.16 0.54 0.09 0.41 0.07 0.20 0.02 0.16 0.02 20.85 0.46 1.13 1.00 0.78 0.19 0.73 0.12 0.02 0.10 0.01 0.06 0.01 0.03 <0.01 0.03 <0.01 3.10 0.40 1.03 4.93 5.42 1.03 4.17 0.71 0.15 0.51 0.08 0.37 0.07 0.19 0.02 0.17 0.02 17.84 0.54 1.17 0.11 0.06 0.02 0.09 0.02 <0.01 0.02 <0.01 0.02 <0.01 0.02 <0.01 0.01 <0.01 0.40 0.24 0.99 mi 1.28 2.79 0.37 1.40 0.31 0.07 0.28 0.04 0.24 0.04 0.12 0.01 0.09 0.01 7.07 0.97 1.05 0.17 0.33 0.04 0.16 0.04 0.01 0.04 <0.01 0.03 <0.01 0.01 <0.01 0.01 <0.01 0.85 0.89 1.00 0.34 0.60 0.08 0.31 0.07 0.02 0.07 0.01 0.06 0.01 0.03 <0.01 0.03 <0.01 1.65 0.83 1.09 5.31 10.59 1.33 5.15 1.08 0.24 0.94 0.14 0.80 0.15 0.39 0.06 0.34 0.05 26.57 0.93 1.12 3.37 18.67 1.10 5.11 0.92 0.17 0.68 0.08 0.38 0.06 0.17 0.02 0.12 0.02 30.86 2.15 1.04 mi 2.30 5.12 0.67 2.55 0.57 0.08 0.51 0.08 0.44 0.08 0.22 0.03 0.19 0.03 12.87 0.99 0.70 Sc V Cr Co Ni Zn As Sr Y Zr Mo Ag Cd Ba Hf Th U 1.66 19.10 9.22 1.96 9.64 12.35 1.95 5169 2.92 1.07 1.49 0.10 0.33 23.99 0.03 0.46 6.25 1.31 5.29 3.51 1.71 9.01 3.22 0.26 8784 0.85 0.06 1.03 0.12 0.18 15.01 <0.01 0.03 5.94 1.99 16.02 7.69 2.70 13.10 14.31 5.29 7614 2.87 1.95 2.96 0.12 1.14 25.89 0.05 0.67 9.93 1.66 13.98 4.76 2.48 13.23 6.60 2.94 9147 2.35 0.75 0.79 0.09 0.11 35.23 0.02 0.22 8.30 1.07 4.62 2.09 1.30 11.72 3.75 0.85 9054 0.64 0.09 0.44 0.51 0.06 19.45 <0.01 0.03 4.20 1.51 14.20 4.91 2.56 17.55 6.41 2.32 8970 2.16 0.79 0.75 0.11 0.10 26.88 0.02 0.24 6.95 1.16 0.44 1.97 1.36 7.01 8.53 0.05 2490 0.25 0.03 0.07 0.09 0.21 10.33 <0.01 <0.01 0.10 0.05 7.59 0.14 0.01 0.15 0.03 0.33 363 0.02 <0.01 0.74 0.14 0.01 17.45 0.01 0.01 22.48 1.17 3.92 2.50 1.68 12.48 15.23 0.18 9206 0.42 0.11 1.67 0.12 0.24 25.89 <0.01 0.01 5.31 1.20 2.93 2.75 1.32 7.63 2.81 0.05 2624 0.41 0.03 1.65 0.06 0.13 12.34 <0.01 0.01 1.01 2.64 28.25 7.70 1.96 11.94 9.79 8.50 8678 4.25 2.61 6.12 0.22 0.34 92.63 0.07 1.05 19.83 0.06 6.91 0.12 0.01 0.14 0.02 0.51 113 0.05 <0.01 0.11 0.15 <0.01 54.62 0.01 0.04 32.04 0.04 5.94 0.14 <0.01 0.09 0.06 0.25 94 0.02 <0.01 0.02 0.03 0.02 902 0.01 0.01 19.42 Ce/Ce* denotes 3CeN/(2LaN þ NdN), Eu/Eu* denotes EuN/(SmN  GdN)0.5, where N refers to normalization of concentration against the standard Post Archean Australian Shale (PAAS) (McLennan, 1989). a mi, microcrystalline aragonite; ar, sparite aragonite; ms, microspar aragonite; bi, bivalve shell. biogenic fabrics might be diagnostic markers of biogeochemical and microbiological processes at the gas vent and hydrate sites. 5.2. Ce anomaly and redox condition The Ce anomaly provides a signal of a redox condition in contemporaneous seawater and modern oceans marked by depletion (negative anomaly) of Ce because of prevalent oxidation of Ce3þ to Ce4þ (Piper, 1974). Unlike seep carbonates from Green Canyon Block 238 and South China Sea which show no Ce anomaly (Chen et al., 2005), the shale-normalized REE patterns of the 5% HNO3-treated solution of carbonate minerals of the Bush Hill seep carbonate show varied Ce anomalies (Table 3 and Fig. 6). The varied Ce anomalies between samples and even in the different carbonate phases in the same sample (the Ce anomaly of microcrystalline aragonite is more negative than that of sparite and microspar aragonite in the same sample, e.g. BH-G) strongly indicate the spatial and temporal change of the precipitation conditions of seep carbonate. Thus, the redox condition of seep carbonate crystallized at the Bush Hill site appears to be highly variable. It is widely accepted that V, Mo, U, and Cd are redox sensitive elements and enriched under anoxic conditions (Morford and Emerson, 1999). Thus, they can be used to illuminate the formation condition of the carbonate. The loosely coupled relationship between the Ce anomaly and redox sensitive elements, on the other hand, indicates that the precipitation condition for Bush Hill carbonates are not simply explained (Table 3 and Fig. 6). A highly variable set of formation conditions is probable. The variable redox conditions as well as small-scale variations in the chemical environment during carbonate precipitation may be related to microbial metabolism at seep sites. Based on the illustrations herein, we suggest that the rate of fluid flow at Bush Hill seep site may be the primary factor that controls variations in seep carbonate characteristics. During conditions of relatively slow seepage, oxidized hydrocarbon became the dominant contributing carbon source which was represented as the 13C depleted carbonate (Samples BH-A, BH-B, BH-F and BH-G). At the same time, due to the slow seepage rate, carbonate precipitation occurs deep below the water/sediment interface, where being relatively anoxic, the carbonates show no or positive Ce anomalies and relatively high contents of V, Mo, U and Cd (Samples BH-A, BH-B, BH-F and BH-G). On the other hand, during the relatively high seepage, the fast seepage may force methane to be transported up to the subsurface of the seafloor, where hydrocarbons cannot be fully oxidized. Thus, the hydrocarbons become the lesser contributing carbon source which results in less negative 13C signatures of the seep carbonates (Samples BH-C, BH-D and BH-H). The carbonate precipitates at subsurface or close to the water/sediment interface, where the formation condition is relatively aerobic. Thus, the carbonate precipitates here show negative Ce anomalies and relatively lower contents of V, Mo, U, and Cd (Samples BH-C, BH-D and BH-H). 6. Conclusions Seep carbonate samples collected from the surface of the Bush Hill consist of bioclasts and matrix with variable porosity. The D. Feng et al. / Marine and Petroleum Geology 26 (2009) 1190–1198 1197 that there are variable/complex formation conditions for Bush Hill seep carbonate. Through more measurements over a larger area of the GC 185 mound, the variable/complex precipitation conditions of seep carbonates may be further verified. Acknowledgments This study was partially supported by the National Science Foundation of China (Grants: 40725011 and U0733003), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant: KZCX2-YW-108) and the Open Fund of the Key Laboratory of Marine Geology and Environment of CAS (Grant: MGE2007KG05). The seep carbonates were obtained from the field cruises sponsored by the Minerals Management Service through a cooperative agreement (Contract No. 14-35-0001-30660) with the Coastal Marine Institute at Louisiana State University. Dr. Xuanfeng Xu (Stevens Institute of Technology, USA) helped with the quantification of XRD. Dr. Liang Qi (Institute of Geochemistry, CAS, China) helped with the analysis of trace element. Dr. Guofu Xu (Central South University, China) helped with the SEM observations. Editor D.G. Roberts and an anonymous reviewer are thanked for their careful and constructive review. References Fig. 6. Shale-normalized REE patterns of the 5% HNO3-treated solution of seep carbonates from Bush Hill. (A) is microcrystalline aragonite (mi); (B) is sparite aragonite (ar), microspar (ms) and bivalve shell (bi). carbonate is predominantly composed of aragonite (89–99 wt%, average 94 wt%, n ¼ 7) that occurs as microcrystalline, microspar, and sparite. Pyrite framboids, clotted microfabrics, and botryoidal cements were preserved in the seep carbonates are interpreted as indicators of biologically controlled fabrics produced during microbial oxidation of hydrocarbons. The moderately depleted d13C values (>29.4&) combined with frequent occurrences of biodegraded crude oil in the seep carbonate samples suggest that the carbon source of the seep carbonate was mainly derived from microbial degraded crude oil. The oxygen isotopic value suggests that the carbonates were precipitated from fluids at ambient bottom water. Some 18Oenriched samples may be related to the destabilization of gas hydrate locally or the smectite–illite transition in deep sediments, and some 18O-depleted samples may have resulted from elevated temperatures that are most probably the expulsion of warm fluids at the seafloor. Based on the loosely coupled relationship of redox sensitive elements (V, Mo, U and Cd) and REE as well as stable isotopes and morphologies of the samples, it is suggested that the primary factor controlling or influencing the formation characteristics of seep carbonates is the flux rate of fluids and gases at the seep site. Under relatively slow seepage conditions, carbonate precipitated deep below the water/sediment interface, where conditions are anoxic and the carbonates are generally less porous and have depleted 13C values. On the other hand, in relatively fast seepage settings, carbonate precipitated at shallower subsurface and even near the water/sediment interface, where the formation condition is aerobic, and the resulting carbonate occurs as more porous and with relatively less depleted 13C values. This study has examined only limited samples from a specific site. Therefore, our conclusions may be reflective of localized conditions. 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