6th International Conference on Gas Hydrates

REDOX VARIATIONS AT COLD SEEPS RECORDED BY RARE EARTH ELEMENTS IN SEEP CARBONATES Feng, Dong; Chen, Duofu; Lin, Zhijia; Peckmann, Jörn; Bohrmann, Gerhard; Roberts, Harry H. 2008

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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  REDOX VARIATIONS AT COLD SEEPS RECORDED BY RARE EARTH ELEMENTS IN SEEP CARBONATES  Dong Feng, Duofu Chen, Zhijia Lin Key Laboratory of Marginal Sea Geology Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Guangzhou, 510640 Graduate university of Chinese Academy of Sciences Beijing 100049 CHINA Jörn Peckmann, Gerhard Bohrmann MARUM, University of Bremen Post Box 330 440, D-28334 Bremen, GERMANY Harry H. Roberts Coastal Studies Institute, Louisiana State University Baton Rouge, LA 70803, USA  ABSTRACT Understanding the formation conditions of seep carbonate is crucial to better constrain the dynamic fluid flow and chemical fluxes associate with cold seeps on the seafloor. Rare earth element (REE) in seep carbonates collected from modern cold seeps of Gulf of Mexico, Black Sea, Congo Fan, ancient seeps of Beauvoisin (Oxfordian, J 3, Southeastern France) and Marmorito (Miocene, Northern Italy) were studied. Our focus has been on 5% HNO3-treated solution (authigenic carbonate minerals) of carbonates. Several crystalline forms of carbonate minerals have been selected for analysis. Total REE (∑REE) contents in seep carbonates varies widely, from 0.068 to 43.655 ppm, but the common trend is that the ∑REE in microcrystalline phases is highest and lowest of in sparite, suggesting that the ∑REE of seep carbonates may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates show varied Ce anomalies across several seep sites and even within one site, suggesting that the formation condition of seep carbonate is variable and complex. Overall, our results show that apart from anoxic, oxic formation condition is also common at hydrocarbon seep environments. Keywords: cold seep, rare earth element, redox variation, seep carbonate INTRODUCTION Seep carbonate precipitation is a widely observed phenomenon in the modern and ancient marine   seep environments of the world [1-3]. Carbonate precipitation at cold seep sites is a result of microbial oxidation of methane, as well as higher  Corresponding author: Phone: +86 20 8529 0286 Fax +86 20 8529 0130 E-mail: cdf@gig.ac.cn  molecular weight hydrocarbons, through the combined metabolism of methane oxidizing archaea (MOA) and sulfate reducing bacteria (SRB) [4-6]. Seafloor observations of cold seep sites show that the chemical and physical characters of cold seep sites are complex and variable [7] and [8]. However, it is difficult to trace this dynamic characterization, especially the variation of redox condition induced by bacterial processes at modern and ancient cold seeps. The Ce anomaly of authigenic sediment deposited at the seafloor is an effective indicator of the sedimentary redox variation [9-11]. Here we report rare earth element (REE) in 5% HNO3-treated solution (authigenic carbonate mineral) of seep carbonates collected from modern seeps of Congo Fan, Black Sea, Gulf of Mexico, ancient seep from Beauvoisin (Oxfordian, J3, Southeastern France) and Marmorito (Miocene, Northern Italy). Our aim has been to trace the variation in redox conditions during seep carbonate precipitation. MATERIALS AND METHODS Seep carbonates analyzed in this paper were recovered from three modern seep areas: Bush Hill (540 m water depth) and Alaminos Canyon (2200 m water depth) of Gulf of Mexico, Romania shelf (120 m water depth) and Ukraine slope (190 m water depth) of Black Sea, Hydrate Hole (3113 m water depth) and Diapir Field (2417 m water depth) of Congo Fan, two ancient seeps of Beauvoisin (Oxfordian, J3, Southeastern France) and Marmorito (Miocene, Northern Italy) (Table 1). The 0.1 to 0.5 g sample powder was treated with 50 ml of 5% HNO3 in a centrifuge tube for 2-3 hours 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 analysis using Finnigan MAT ELEMENT high resolution ICP-MS. Precision of the REE analysis was checked by multiple analyses of standard samples. The average standard deviations are less than 10%, and average relative standard deviations are better than 5%. For detail of the analyses see Qi et al. [15]. In this paper, Ce/Ce* denotes 3CeN/(2LaN+NdN), where N refers to normalization of concentration against the standard Post Archean Australian Shale  (PAAS) [9]. Ceanom denotes Log(Ce/Ce*), Ceanom >-0.1 is positive anomaly, which means anoxic formation condition, while Ceanom <-0.1 is negative anomaly, which means oxic formation condition [10]. RESULTS AND DISCUSSION Seep carbonate of Gulf of Mexico Bush Hill seep carbonates: The total REE content (∑REE) is 7.070~26.565 ppm for microcrystalline and 0.402~3.096 ppm for sparite. The ∑REE in microcrystalline is higher than that of in sparite for the same sample (Table 2). The shale-normalized REE patterns show varied Ce anomalies, from negative (Ceanom=-0.615~-0.100) to positive (Ceanom= -0.062~-0.006) (Table 2 and Figure 1) between different samples and even in one sample (BH-A). The varied Ce anomalies between samples and even in the different carbonate phases of the same sample (microcrystalline and sparite, e.g. BH-A) strongly indicated the spatial and temporal change of the precipitation conditions of seep carbonate [12]. We suggest that the rate of fluid flow at Bush Hill seep site may be the primary factor that controls redox variations in seep environment. During conditions of relatively slow seepage, the carbonate precipitation occurs deep below the water/sediment interface, where the formation conditions are anoxic, yielding carbonates with positive Ce anomalies. On the other hand, higher seepage rates cause methane to be transported into the shallow sediments or even in the water, where conditions are relatively oxic, and the carbonates show negative Ce anomalies. Alaminos Canyon seep carbonates: The ∑REE is 12.725 ppm for microcrystalline and 2.227 ppm for sparite. Similar to seep carbonate at Bush Hill, the ∑REE in microcrystalline is higher than that of in sparite (Table 2). The shale-normalized REE patterns show distinct negative Ce anomalies (Ceanom=-0.349~-0.294) (Table 2 and Figure 1), indicating that the seep carbonates formed under oxic conditions, which may be related to the dynamic characterization of the cold seeps [13]. Seep carbonate of Black Sea Romania shelf seep carbonates: The ∑REE of seep carbonates is very low, 2.026 ppm for microcrystalline, 0.896 ppm for microspar and 0.552 ppm for sparite (Table 2). The shalenormalized REE patterns show negative Ce anomalies (Ceanom=-0.218~-0.100) (Table 2 and  Figure 1), indicates that the Romania shelf seep carbonates formed under oxic conditions. Ukraine slope seep carbonates: Similar to the Romania shelf seep carbonates, the ∑REE of Ukraine slope seep carbonates is very low, 2.817 ppm for microcrystalline, 0.695 ppm for microspar and 0.068 ppm for sparite (Table 2). But, unlike to the Romania shelf seep carbonates, the shalenormalized REE patterns of Ukraine slope seep carbonates show positive Ce anomalies (Ceanom=0.053~0.014 (Table 2 and Figure 1), indicates that the Ukraine slope seep carbonates formed under anoxic conditions. It is suggested from the REE patterns that Romania shelf and Ukraine slope seep carbonates formed under oxic and anoxic conditions, respectively. Which is consistent with the obvious oxygen value (11 mol/L) in the bottom water of Romania shelf, and no oxygen was detected in bottom water of Ukraine slope [3]. Thus, there should be an oxic–anoxic interface between the depth from Romania shelf to Ukraine slope during the formation of those seep carbonates. Seep carbonate of Congo Fan Hydrate Hole seep carbonate: The ∑REE is 23.173~42.539 ppm for microcrystalline, 23.461~30.986 ppm for microspar and 1.59 ppm for sparite (Table 2). The shale-normalized REE patterns show distinct positive Ce anomalies (Ceanom=0.188~0.266) (Table 2 and Figure 1), indicates that the Hydrate Hole seep carbonates formed under anoxic conditions. Diapir Field seep carbonate: The ∑REE is 41.636 ppm for microcrystalline (Table 2). Similar to Hydrate Hole, the shale-normalized REE pattern shows positive Ce anomalies (Ceanom=0.105) (Table 2 and Figure 1), indicates that the Diapir Field seep carbonates also formed under anoxic conditions. It is obvious that Hydrate Hole and Diapir Field seep carbonates formed under anoxic conditions. This is confirmed by the depleted δ13CPDB values from -60.2 to -48.2 of those seep carbonates, which clearly indicates that the carbon is derived from a methane source and carbonate precipitation was due to anaerobic methane oxidation (Table 1), further evidence is also from biomarker analyses [14]. Seep carbonate of Marmorito The ∑REE of seep carbonates is 5.220~43.655 ppm for microcrystalline, 3.220~7.177 ppm for microspar, 0.992~1.031 ppm for sparite, and 2.416 ppm for blocky cement. The shale-normalized  REE patterns show varied Ce anomalies, from negative (Ceanom=-0.435~-0.134) to positive (Ceanom=-0.099~-0.029) Ce anomalies (Table 2 and Figure 1) between different samples. The varied Ce anomalies between samples indicates that the spatial or/and temporal change of the precipitation conditions of Marmorito seep carbonates. Remarkably, apart from lipid biomarkers of MOA and SRB, biomarkers of aerobic methanotrophic bacteria have been found in this rock [2] and [16]. Seep carbonate of Beauvoisin The ∑REE is 11.430~21.005 ppm for microcrystalline, 16.847~24.773 ppm dolomite, and 3.32 ppm for sparite (Table 2). The shalenormalized REE patterns show distinct positive Ce anomalies (Ceanom=-0.066~0.037) (Table 2 and Figure 1), indicates that the Beauvoisin seep carbonates formed under anoxic conditions. SUMMARY The ∑REE of carbonates from modern seeps of Gulf of Mexico, Black Sea, Congo Fan, ancient seeps from Beauvoisin (Oxfordian, J3, Southeastern France) and Marmorito (Miocene, Northern Italy) has a wide range, from 0.068 to 43.655 ppm, but usually less than 10 ppm. The common trend is that the ∑REE in microcrystalline phases is higher than in sparite, suggesting that the REE content of seep carbonate may be a function of diagenesis. The shalenormalized REE patterns of the seep carbonates show varied Ce anomalies, indicating the different formation conditions. Congo Fan and Beauvoisin seep carbonates formed under anoxic condition, Black Sea seep carbonates formed under oxic (120 m water depth) and anoxic (190 m water depth) condition, while Gulf of Mexico and Marmorito seep carbonates formed under both anoxic and oxic condition, suggesting that the formation condition of the seep carbonate is variable and complex. Overall, our results show that apart from anoxic, oxic formation condition is also common in hydrocarbon seep environments. The carbonate precipitation under oxic condition may be related to a decrease of seepage rates. When the seep rate decreases, the interface of oxic and anoxic conditions migrates downwards, thus the HCO3 - in former anoxic zone will continue to precipitate carbonate. Furthermore, the intermittent variation of the seepage rate may also result in the carbonate precipitation under aerobic conditions. The HCO3 produced in deep anaerobic zone could be brought  Location  Sampling information  Age  Carbonate microfabrics  δ13C PDB ‰  References  Gulf of Mexico  Bush Hill (GC185, 27°46'N; 91°30'W): Johnson-Sea-Link I Submersible dive 2904 (1997), 4061 (1998) and 4063 (1998), 540 m water depth  Modern (0.8~10 ka)  Microcrystalline aragonite, clotted micrite, banded/botryoidal aragonite cement  -29.4~-15.1  [12]  Alaminos Canyon 645 (26°21'N/94°31'W): DSV Alvin Submersible dive 2209 (1990), 2200 m water depth Romanian shelf: R/V Poseidon cruise (1994), 120 m water depth  Modern (10.6~11.6 ka)  Microcrystalline aragonite, clotted micrite, banded/botryoidal aragonite cement Microcrystalline high-Mg-calcite aragonite, and aragonite cement  -31.3~-23.4  [13]  or  -37.3~-28.9  [3]  Ukraine slope: R/V Professor cruise (1993, 1994), 190 m water depth Hydrate Hole (04°48′56′′S/09°54′50′′E): R/V Meteor M56-B Cruise (2002), 3113 m water depth Diapir field (06°11′04′′S/10°25′53′′E): R/V Meteor M56-B Cruise (2002), 2417 m water depth Marmorito: Cold seep deposits occur near the village of Marmorito in the Monferrato hills, east of Torino in northern Italy Beauvoisin: Located in southeastern France near Buisles-Baronnies  Modern  microcrystalline high-Mg-calcite or aragonite, and aragonite cement Microcrystalline aragonite and high-Mg calcite, pure aragonite Microcrystalline aragonite and high-Mg calcite Microcrystalline dolomite, calcitic veins, botryoidal aragonite, in situ brecciation  -41.0~-14.8  [3]  -60.2~-48.2  [14]  -40.2~-17.3  [2]  Microcrystalline high-Mg-calcite and botryoidal aragonite  26.5~+15.1  [2]  Black Sea  Congo Fan  Northern Italy  Southeastern France  Modern  Modern Modern Miocene  Oxfordian (J3)  Table 1. Background information of the studied samples.  Number++  Type+  La  Ce  Pr  Nd  Sm  Eu  Gd  Tb  Dy  Ho  Er  Tm  Yb  Lu  ∑REE  Ce/Ce*  Ceanom  BH-A-1  mi  2.935  5.406  0.695  2.748  0.627  0.135  0.530  0.087  0.471  0.090  0.253  0.037  0.209  0.031  14.253  0.868  -0.062  BH-A-2  sp  0.521  0.863  0.122  0.465  0.097  0.023  0.092  0.013  0.079  0.015  0.042  0.005  0.035  0.005  2.377  0.794  -0.100  BH-B-1  mi  3.726  7.309  0.922  3.585  0.748  0.164  0.579  0.093  0.513  0.091  0.249  0.035  0.209  0.030  18.254  0.916  -0.038  BH-C-1  mi  6.086  5.734  1.275  5.213  0.859  0.164  0.541  0.086  0.409  0.075  0.204  0.025  0.159  0.022  20.852  0.457  -0.340  BH-C-2  sp  1.004  0.782  0.190  0.731  0.116  0.023  0.096  0.012  0.062  0.011  0.034  0.004  0.026  0.004  3.096  0.397  -0.401  BH-D-1  mi  4.928  5.421  1.033  4.170  0.711  0.149  0.508  0.075  0.369  0.070  0.190  0.024  0.170  0.021  17.840  0.536  -0.271  BH-D-2  sp  0.113  0.055  0.024  0.092  0.021  0.005  0.023  0.003  0.025  0.005  0.016  0.002  0.014  0.003  0.402  0.242  -0.615  BH-F-1  mi  1.279  2.793  0.374  1.400  0.312  0.066  0.282  0.040  0.241  0.042  0.122  0.014  0.092  0.013  7.070  0.973  -0.294  BH-F-2  sp  0.173  0.328  0.044  0.164  0.035  0.008  0.036  0.005  0.026  0.005  0.015  0.002  0.011  0.002  0.853  0.889  -0.349  BH-G-1  mi  5.307  10.59  1.326  5.154  1.078  0.239  0.945  0.143  0.798  0.147  0.386  0.059  0.340  0.050  26.565  0.929  -0.650  BH-H-1  mi  2.298  5.123  0.668  2.553  0.567  0.080  0.511  0.077  0.444  0.079  0.220  0.030  0.191  0.028  12.870  0.987  -0.012  AC-1  mi  3.410  3.627  0.709  3.068  0.566  0.123  0.380  0.071  0.338  0.058  0.184  0.025  0.146  0.019  12.725  0.508  -0.051  AC-2  sp  0.595  0.545  0.118  0.499  0.104  0.023  0.101  0.016  0.097  0.018  0.054  0.007  0.044  0.007  2.227  0.448  -0.081  BS-RO-1  mi  0.355  0.595  0.099  0.412  0.110  0.028  0.114  0.018  0.113  0.023  0.072  0.009  0.067  0.010  2.026  0.730  -0.032  BS-RO-2  ms  0.198  0.247  0.040  0.170  0.034  0.009  0.044  0.007  0.051  0.011  0.038  0.005  0.036  0.006  0.896  0.606  0.332  BS-RO-3  sp  0.109  0.172  0.019  0.083  0.017  0.004  0.027  0.004  0.034  0.009  0.032  0.004  0.031  0.005  0.552  0.794  -0.006  BS-UKR-1  mi  0.487  1.072  0.123  0.517  0.128  0.032  0.136  0.021  0.133  0.024  0.070  0.009  0.057  0.008  2.817  0.992  -0.137  BS-UKR-2  ms  0.130  0.271  0.024  0.103  0.023  0.006  0.031  0.005  0.033  0.008  0.026  0.004  0.027  0.004  0.695  1.033  -0.218  BS-UKR-3  sp  0.016  0.025  0.002  0.009  0.002  0.000  0.003  0.000  0.003  0.001  0.003  0.001  0.003  0.000  0.068  0.885  -0.100  CF-HH-2a-1  sp  0.194  0.885  0.073  0.269  0.055  0.030  0.053  0.007  0.035  0.006  0.017  0.002  0.017  0.002  1.586  1.844  0.266  CF-HH-2a-2  ms  3.230  0.885  1.033  4.037  0.839  0.291  0.701  0.109  0.576  0.107  0.279  0.040  0.244  0.033  23.461  1.562  0.194  CF-HH-2a-3  mi  4.497  11.94  1.472  5.951  1.205  0.402  1.079  0.166  0.931  0.169  0.436  0.061  0.366  0.051  33.599  1.542  0.188  CF-HH-2b-1  sp  2.137  16.81  0.612  2.300  0.472  0.103  0.373  0.065  0.350  0.066  0.199  0.030  0.187  0.027  14.360  1.560  0.193  CF-HH-2b-2  ms  4.461  7.439  1.336  4.939  1.041  0.234  0.884  0.137  0.734  0.141  0.381  0.054  0.336  0.047  30.986  1.616  0.208  CF-HH-2b-3  mi  6.126  16.26  1.791  7.096  1.468  0.318  1.244  0.197  1.055  0.205  0.539  0.080  0.430  0.065  42.539  1.559  0.193  CF-DF-2c  mi  6.704  21.93  1.860  7.598  1.537  0.346  1.358  0.209  1.103  0.228  0.609  0.081  0.510  0.078  41.636  1.272  0.105  CF-HH-2d  mi  3.172  19.42  1.040  3.856  0.773  0.329  0.647  0.104  0.610  0.091  0.247  0.034  0.192  0.026  23.173  1.623  0.210  Mar E-1  mi  1.329  2.680  0.329  1.349  0.309  0.067  0.236  0.048  0.249  0.048  0.141  0.020  0.122  0.017  6.945  0.923  -0.035  Mar E-2  ms  1.383  2.818  0.339  1.396  0.302  0.070  0.215  0.052  0.262  0.044  0.137  0.021  0.119  0.017  7.177  0.935  -0.029  Mar E-3  sp  0.258  0.385  0.043  0.154  0.031  0.008  0.041  0.006  0.042  0.008  0.026  0.003  0.022  0.004  1.031  0.804  -0.095  Mar E-4  bc  0.613  0.907  0.099  0.369  0.073  0.019  0.090  0.013  0.090  0.019  0.057  0.007  0.052  0.008  2.416  0.796  -0.099  Mar ES1-1  mi  11.48  12.08  2.259  9.324  1.990  0.451  1.915  0.316  1.676  0.349  0.902  0.121  0.688  0.103  43.655  0.520  -0.284  Mar ES1-2  ms  3.378  2.182  0.418  1.595  0.292  0.075  0.346  0.064  0.357  0.077  0.234  0.033  0.182  0.028  9.262  0.367  -0.435  Mar G-1  mi  1.515  1.599  0.247  1.046  0.209  0.044  0.148  0.031  0.155  0.031  0.092  0.013  0.081  0.011  5.220  0.547  -0.262  Mar G-2  ms  1.010  0.751  0.177  0.674  0.136  0.031  0.138  0.019  0.121  0.024  0.069  0.008  0.053  0.008  3.220  0.389  -0.410  Mar G-3  sp  0.215  0.327  0.048  0.187  0.041  0.010  0.041  0.007  0.048  0.009  0.027  0.003  0.024  0.003  0.992  0.734  -0.134  Mar G-4  sp  0.076  0.111  0.016  0.060  0.013  0.002  0.015  0.002  0.013  0.003  0.009  0.001  0.007  0.001  0.329  0.726  -0.139  Bea A2-1  mi  4.276  8.000  0.997  4.317  0.864  0.186  0.767  0.131  0.690  0.130  0.324  0.044  0.247  0.035  21.005  0.859  -0.066  Bea 97-1  sp  0.544  1.373  0.161  0.646  0.148  0.000  0.137  0.021  0.135  0.023  0.069  0.007  0.049  0.007  3.321  1.088  0.037  Bea 97-2  mi  2.269  4.494  0.528  2.237  0.453  0.117  0.421  0.071  0.382  0.071  0.223  0.023  0.125  0.017  11.430  0.917  -0.038  Bea 97-3-1  do  3.930  9.966  1.291  5.038  1.086  0.223  1.058  0.174  0.940  0.174  0.434  0.056  0.358  0.045  24.773  1.060  0.025  Bea 97-3-2 do 2.498 6.570 0.920 3.641 0.762 0.130 0.779 0.113 0.687 0.122 0.334 0.038 0.222 0.030 16.847 1.040 0.017 ++ BH=Bush Hill of Gulf of Mexico; AC=Alaminos Canyon of Gulf of Mexico; BS-RO=Romania shelf of Black Sea; BS-UKR=Ukraine slope of Black Sea; CFHH=Hydrate Hole of Congo Fan; CF-DF=Diapir Field of Congo Fan; Mar=Marmorito of Northern Italy; Bea=Beauvoisin of Southeastern France. + mi=microcrtstalline, ms=microspar, sp=sparite, bc=blocky cement, do=dolomite.  Table 2. Rare earth elements (REE) content (ppm) of 5% HNO3-soluble parts of seep carbonates samples.  (a) and (b): Bush Hill seep carbonates show varied Ce anomalies, from positive to negative Ce anomalies between different samples and even in the same sample (BH-A); (c): Alaminos Canyon seep carbonates show negative Ce anomalies; (d): Black Sea seep carbonates show negative (BS-RO) and positive (BS-UKR) Ce anomalies; (e): Congo Fan seep carbonates show distinct positive Ce anomalies; (f) and (g): Marmorito seep carbonates show negative (MarES1and Mar G) and positive (Mar E) Ce anomalies; (h): Beauvoisin seep carbonates show positive Ce anomalies.  Figure 1 Shale-normalized REE patterns of modern and ancient cold seep carbonates  up to the subsurface oxic zone to precipitate carbonate during the stage of fast seepage. Additional geochemical data, e.g. high resolution Sr and Ba profiles of fast and slow fluid flow precipitation, is needed to further investigate the current model. ACKNOWLEDGEMENT The Gulf of Mexico seep carbonates were obtained on the cruises sponsored by the Minerals Management Service (MMS). Dr. Qi L. helped the analyses of REE. This study is supported by NSFC (40725011 and U0733003). REFERENCES [1] Campbell K A. Hydrocarbon seep and hydrothermal vent paleoenvironments and paleontology: Past developments and future research directions. Palaeogeography, Palaeoclimatology, Palaeoecology, 2006, 232: 362-407. [2] Peckmann J, Thiel V, Michaelis W, et al. Cold seep deposits of Beauvoisin (Oxfordian; southeastern France) and Marmorito (Miocene; northern Italy): microbially induced authigenic carbonates. International Journal of Earth Sciences, 1999, 88: 60-75. [3] Peckmann J, Reimer A, Luth U, et al. Methane-derived carbonates and authigenic pyrite from the northwestern Black Sea. Marine Geology, 2001, 177: 129-150. [4] Hinrichs K U, Hayes J M, Sylva S P, et al. Methane-consuming archaebacteria in marine sediments. Nature, 1999, 398: 802-805. 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