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Methane carbon and deuterium isotopic composition of sediment gas. Methane in the clam colonies is likely produced by non-clam symbionic bacteria via the carbonate reduction pathway: CO 2 + 8H + + 8e - «—» CH 4 + 2 H 2 O A small amount of amount of thermogenic methane from depth may account for some methane near carbonate crust site # 2 (6/27/94). 3000 2000 1000 500 100 1500 2500 36 o -121 o 30' o -121 o 30' 36 o -123 o 10' 38 o -123 10' M o n t e r e y B a y N a t i o n a l M a r i n e S a n c t u a r y o o ' 38 o ' o ' o ' t Santa Cruz Monterey San Francisco Monterey Bay Farallon Shelf Smooth Ridge Seep Site Soquel Canyon Monterey Canyon n San Francisco Bay San Jose Oakland Hydrocarbons Associated With Fluid Venting Processes in Monterey Bay, California Introduction The Monterey Bay National Marine Sanctuary encompasses about 14,000 km 2 of marine waters along the central coast of California. The centerpiece of this sanctuary is Monterey Bay which is underlain by a network of deep submarine canyons. We have begun a multifaceted study to describe and interpret the hydrocarbons in surface and near-surface sediment of the sanctuary in order to define the hydrocarbon background and to describe the processes responsible for the hydrocarbon occurrences. Of special interest are the presence of chemosynthetic communites nestled in areas of fluid venting. Fluid venting in the deep ocean supports chemosynthetic 'cold seep' ecosystems and may play an important role in world-wide, deep ocean ecology and element cycling. At the base of the food chain are hydrocarbons fueling the process in a world devoid of light. A chemosynthetic 'cold seep' ecosystem occurs at a site 1,000 m deep and is interpreted as the surface expression of a mud volcano on Smooth Ridge near Monterey Canyon. Sediment samples from within and near cold seeps on Smooth Ridge were collected by push cores, using a Remote Operated Vehicle (ROV). T.D. Lorenson, K.A. Kvenvolden, F.D. Hostettler, R.J. Rosenb auer U. S. Geological Survey, Menlo Park, CA J.B. Martin University of Florida D.L. Orange Monterey Bay Aquarium Research Institute And the ROV Ventana C a l i f o r n i a C a l i f o r n i a Cold Seep Hydrocarbon Gases Chemosynthetic 'cold seep' communities occur at a 1,000 m site interpreted as the surface expression of a mud volcano on Smooth Ridge near Monterey Canyon. In order to examine the gas composition of these seeps, sediment samples were collected within and nearby the clam fields using push cores from the Remotely Operated Vehicle (ROV) Ventana . Upon retrieval, the cores (maximum length, 30 cm) were subsampled for measurements of a variety of chemical parameters including compositions of hydrocarbon gases. In a collection of 20 samples, methane concentrations range from 1.4 to 7,000 micromol/L; carbon isotopic compositions of the methane range from -71.0 to -86.6 per mil with two exceptions of -53.8 and -30.6 per mil. These results suggest that most of the methane is microbial in origin; the isotopically heavier methane may represent a thermogenic source or oxidation of the microbial methane. Minor concentrations of other hydrocarbon gases (ethane through butanes) accompany the methane. Colonies of clams mark the locations on the seafloor where gas apparently vents through small conduits. The small amount of putative thermogenic methane which was found adjacent to the clam colonies implies that the expulsion of thermally generated hydrocarbons from depth is not the principal driving mechanism for flow in this region; however, microbial production of methane resulting in reduced fluid density could be a factor in facilitating fluid flow at these cold seeps. Diagram of the Smooth Ridge site. Three separate clam colonies were sampled for sediment gases and pore water chemistry on two separate occasions. Sediment near carbonate crusts and ridges was also sampled once. Methane and ethane concentrations are shown as blue and red bars, respectively, with the carbon isotopic composition of methane. The majority of hydrocarbon gas is methane. Ethane concentrations are relatively high for surficial marine sediment, occurring only within the clam colonies. Trace amounts of propane, butane, and pentane were also noted. These gases commonly appear in thermogenic fluids which may migrate up from depths of 2-3 kilometers along conduits. Sediment Hydrocarbons Heavy hydrocarbons are present in sediment at the seeps as well as in surficial shelf sediment obtained by conventional box coring. Aliphatic and aromatic hydrocarbons have been identified in eight sediment samples from the site of the seeps and in seven samples of surface sediment collected at or near the head of Soquel Canyon, one of the subsidiary canyons of the Monterey Canyon complex. Aliphatic hydrocarbons at the seeps and at Soquel Canyon include acyclic and cyclic compounds. Total concentrations of acyclic hydrocarbons range from about 1 to 3 mg/g. Distributions of these hydrocarbons are similar in samples from both areas and are believed to be mainly autochthonous. The n-alkanes range from n-C 15 to n-C 36 with n-alkanes of lower molecular weight likely derived from aquatic sources (odd- and even-carbon-number molecules equally abundant) and with n-alkanes of higher molecular weight from terrigenous sources (predominance of odd-carbon-number molecules). Cyclic terpanes also have similar distributions in all samples; immature biogenic molecules are present, and concentrations are low. Of particular interest, however, is the ubiquitous presence of 28,30-bisnorhopane and oleanane. These are compounds which have been commonly identified in Monterey Formation petroleum and rock extracts. The presence of these terpanes in surficial sediment of Monterey Bay may signal a connection with the Monterey Formation which is present in the subsurface and in some surface outcrops in parts of Monterey Bay. Soquel Canyon Monterey Canyon Cold Seep Site 24 23 25 T Ts Tm BN ab 29 ab 30 bb 29 O D m/z 191 ab 32 ab 33 ab 31 24 23 25 T Ts Tm BN ab 29 ab 30 bb 29 m/z 191 O D ab 32 ab 33 ab 31 n-C17 n-C18 n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C31 n-C33 Pr Ph Alkanes Pr: pristane Ph: phytane n-C17 n-C18 n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C31 n-C33 Pr Ph Alkanes Pr: pristane Ph: phytane Darker blue and red bars indicate that the sample is from 10 to 14 cm below the seafloor rather than within the upper 10 cm. Fault Chemosynthic clams host sulfide-metabolizing bacteria in their gills. The bacteria produce food and enzymes for clam to live and grow. Other bacteria metabolize methane to S CO 2 which can combine with Ca 2+ from sea water to make CaCO 3 . Carbonate crusts are found in many places, commonly associated with faults. Areas with carbonate crusts are likely fossil vent sites. Ca 2+ S CO 2 CaCO 3 Fault related fluid flow ? Fluid expulsion brings gases to the sediment - water interface Carbonate crust Control Room on the Pt. Lobos where the Ventana is directed Bacterial endosymbionts in chemosynthetic mussels are analogous to those in clams; however hydrogen sulfide rather than methane is oxidized. 0 -20 -40 -60 -80 -100 -120 -450 -350 -250 -150 -50 J F Near Clam Field H Near Flat Carbonate Crust B Edge of Clam Field Bacterial Fermentation Bacterial Carbonate Reduction Geothermal Mix and Transition Thermogenic d 13 C methane (‰) J J JJ J J J J F F F B H In Clam Field Production Oxidatio n a c =1.055 a c =1.09 a c =1.03 a c =1.04 a c =1.02 a c =1.005 Carbonate Reduction Methyl Fermentation Methane Oxidation d 13 C carbon dioxide (‰) d 13 C methane (‰) J J J J J F H B -50 -40 -30 -20 -10 0 10 20 -100 -90 -80 -70 -60 -50 -40 -30 -20 o '" 36 o 44'40" 36 o 44'45" 36 o 44'47" -122 o 16'32" -122 o 16'37" o -122 o 16'42" -122 o 16'42" 0 50 Meters 100 #2, 6/27/94 #6 & 7, 6/27/94 Low mound w/carbonate crusts Area of thin vertical carbonate ridges N Clam Growth 1003.1m waypoint X Approximate location of Clam Growth 1003.1 m site based on average of MOS nav fixes and waypoint location X #9, 11/22/94 #3, 11/22/94 #8, 11/22/94 #10, 11/22/94 2 meters X 6/27/94 #1 #2 #4 #5 Upthrown Downthrown Larval Trap Float marker 11/22/94 2 meters #1 #2 #6 #5 Upthrown Downthrown Larval Trap Float marker 6/2/94 2 meters X #1 #5 Up Down Methane and Ethane (m M) 1 10 100 1000 10000 Methane Ethane d 13 C -79 d 13 C -54 d 13 C-74 Thermogenic methane? d 13 C-31 d 13 C-71 d 13 C -85 d 13 C -78 d 13 C -75 d 13 C-78 d 13 C-87 d 13 C -74 d 13 C -79 d 13 C -71 LEGEND d D methane (‰) Mass chromatograms of terpanes and triterpanes (m/z=191) 23, C23 tricyclic terpane 24, C24 tricyclic terpane 25, C25 tricyclic terpane T, Triplet: C24 tetracyclic terpane, C26 tricyclic terpane (S?), and C26 tricyclic terpane (R?) 28, C28 tricyclic terpanes (S? and R?) 29, C29 tricyclic terpanes (S? and R?) Ts, 18a (H)-22,29,30-trisnorneohopane (C27) Tm, 17a (H)-22,29,30-trisnorhopane (C27) BN, 17a (H),18a (H),21b (H)-28,30-bisnorhopane ab 29, 17a (H),21b (H)-30-norhopane (C29) ab 29, 17b (H),21a (H)-30-normoretane (C29) O, 18a and (or) b (H)-oleanane (C30) ab 30, 17a (H),21b (H)-hopane (C30) M, 17b (H),21a (H)-moretane (C30) ab 31, 17a (H),21b (H)-homohopane (22S and 22R) (C31) D, Diploptene ab 32, 17a (H),21b (H)-bishomohopane (22S and 22R) (C32) ab 33, 17a (H),21b (H)-trishomohopane (22S and 22R) (C33) ab 34, 17a (H),21b (H)-tetrakishomohopane (22S and 22R) (C34) ab 35, 17a (H),21b (H)-pentakishomohopane (22S and 22R) (C35) Acknowledgements We thank the captains, crews, and ROV pilots of the R/V Pt. Lobos whose skill made this work possible. "Mud Volcano" chemosynthetic vesicomyid clam colony Chemosynthetic Clams (Calyptogena) Carbon isotopic compositions of methane and carbon dioxide in sediment gas. Most sample sites fall outside typical boundaries, likely because of carbon recycling of light carbon between methane and carbon dioxide by different bacterial populations. An alternative explanation for the isotopically heavy methane (thermogenic methane?) is the oxidation of a small pool of methane which leaves isotopically heavy methane behind. Carbonate crust with soul
