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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:
CO2 + 8H+ + 8e- «—» CH4 + 2 H2O
A small amount of amount of thermogenic methane from depth may account for some methane near carbonate crust site # 2 (6/27/94).
3000
20001000
500
100
1500
2500
36o-121o30'
o-121o30'
36o-123o10'
38o-123 10'
Mo
nterey Bay N
ational Marine S
anctuary
o
o '
38o'
o '
o '
t
Santa Cruz
Monterey
SanFrancisco
Monterey Bay
Farallon Shelf
Smooth RidgeSeep Site
Soquel Canyon
MontereyCanyon
n
SanFrancisco 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 km2 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. Rosenbauer
U. S. Geological Survey, Menlo Park, CA
J.B. Martin
University of Florida
D.L. Orange
Monterey Bay Aquarium Research Institute
And the ROVVentana
Califo
rnia
Califo
rn
ia
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-C15 to n-C36 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
2423
25T
TsTm
BNab
29ab
30bb
29
O
D
m/z 191
ab32
ab33
ab31
2423
25T
TsTm
BNab
29ab
30bb
29
m/z 191
O Dab
32ab
33ab31
n-C
17n-
C18
n-C
19n-
C20
n-C
21n-
C22 n-C
23n-
C24
n-C
25n-
C26
n-C
27n-
C28
n-C
29n-
C31
n-C
33
PrPh
AlkanesPr: pristanePh: phytane
n-C
17n-
C18
n-C
19n-
C20
n-C
21n-
C22
n-C
23n-
C24
n-C
25n-
C26
n-C
27n-
C28
n-C
29n-
C31
n-C
33PrPh
Alkanes
Pr: pristanePh: phytane
Darker blue and red barsindicate that the sampleis from 10 to 14 cm belowthe seafloor rather than withinthe upper 10 cm.
Fault
Chemosynthic clams host sulfide-metabolizingbacteria in their gills. The bacteria producefood and enzymes for clam to live and grow.
Other bacteria metabolize methane to S CO2 which
can combine with Ca2+ from sea water to make CaCO3. Carbonate crusts arefound in many places, commonly associated with faults. Areas with carbonate crusts are likely fossil vent sites.
Ca2+
S CO2
CaCO3
Fault related fluid flow ?
Fluid expulsion brings gases to the sediment - water interface
Carbonate crust
Control Room on the Pt. Loboswhere 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
JF Near Clam Field
H Near Flat Carbonate Crust
B Edge of Clam Field
Bacterial
Fermentation
BacterialCarbonate
Reduction
Geothermal
MixandTransition
Thermogenic
d13C
met
hane
(‰
)
JJJJJJJJ
F
F
F
B
HIn Clam Field
Production
Oxidation
a c=1.055
a c=1.09
a c=1.03
a c=1.04
a c=1.02
a c=1.005
CarbonateReduction
MethylFermentation
MethaneOxidationd1
3C c
arbo
n di
oxid
e (‰
)
d 13C methane (‰)
J
J
JJ
J
F H
B-50
-40
-30
-20
-10
0
10
20
-100 -90 -80 -70 -60 -50 -40 -30 -20
o ' "
36o44'40"
36o44'45"
36o44'47"-122o16'32"-122o16'37"
o
-122o16'42"
-122o16'42"
0 50
Meters
100
#2, 6/27/94
#6 & 7, 6/27/94
Low mound w/carbonate crusts
Area of thin verticalcarbonate ridges
NClam Growth 1003.1m waypoint
X
Approximate location of Clam Growth1003.1 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
UpDown
Met
hane
and
Eth
ane
(mM
)
1
10
100
1000
10000Methane
Ethane
d 13C -79
d 13C -54
d 13C-74
Thermogenic methane?
d 13C-31
d 13C-71d 13C -85
d 13C -78
d 13C -75
d 13C-78
d 13C-87
d 13C -74d 13C -79
d 13C -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