B54C-01
Manned submersible observations at cold seeps in the North Anatolian Fault zone, Sea of Marmara
Cold seeps in the Sea of Marmara are associated with active deformation within the North Anatolian Fault system, a transcurrent plate boundary. The Marnaut cruise of Ifremer RV L'Atalante took place in May-June 2007 with objectives (1) to locate gas outflow sites through acoustic means; (2) to better define the relations between active faults and fluid outlets using the Nautile manned submersible; (3) to sample these fluids to determine their nature and origin; (4) to install instruments to monitor the activity of three fluid outflow sites as well as the microseismicity during several months; (4) to sample carbonate crusts, testimonies of the passed activity of fluid outflow; (5) to evaluate the impact of fluid outflow on the present biological and microbiological activity on the seafloor and within the water column; (6) to sample through coring the sediment deposited during the previous earthquakes. Several new zones of fluid emission were found including unexpected locations in areas previously explored with ROV and deep towed cameras. In the Cinarcik Basin, seeps were found along outcrops of Paleozoic sedimentary rocks at the base of the fault-controlled northern cliff, and on en-echelon normal faults extending over the southern slope. In the Tekirdag Basin, bubble emissions were found at the base of the Ganos cliff along NW-SE tension gashes affecting Eocene turbidites of the Kesan formation. On the topographic highs the most active fluid emission sites were found on the top of NE-SW anticlinal ridges at some distance (100-2000 m) from the main fault trace. Active fault scarps were explored at several locations. Very little cold seep activity and no evidence for seafloor rupture from 1999 or 1912 earthquakes was found at the entrance of the Ganos and Izmit Gulf. Observations suggest basement structures along the edges of the subsiding basins and compressive structures on the topographic highs contribute to fluid channeling and expulsion. Cold seeps are found along the main fault scarps in Cinarcik and Tekirdag basins, but also along less prominent fault zones along the opposite side of the basins. The strike-slip fault segment cutting the Central High (west of Istanbul) has comparatively little cold seep activity and we wonder whether this relates to the seismic gap there.
B54C-02
Pore fluid chemistry of cold seeps in the Sea of Marmara
During the Marnaut cruise in the Sea of Marmara, south of Istanbul, fluid and gas seeps were identified and explored along the Main Marmara Fault, the submerged western extension of the North Anatolian Fault Zone that cuts across the entire length of the sea. Our objective was to study the relationship between fluid expulsion sites and active faults at a transform plate boundary. Utilizing the Nautile submersible, ten cold seep sites spanning the fault zone were explored. The tectonic environments of the seeps included strike-slip faults in transtensional and transpressive contexts, normal faults, folds and landslides. Most seeps were extensive, patchy, and diffuse, displaying patches of black sulfidic sediment with typically white to yellow/orange microbial mat on the surface. One endmember type was highly focused, emitted ambient temperature shimmering fluids of low salinity that precipitated chimney structures. At other sites gas bubbles were seen coming from both the sediment cover and from open fractures. Another type had the appearance of a mud volcano with bacteria covered sediment and a fan morphology extending downslope from the seep. This latter site was associated with very high salinity fluids and significant traces of hydrocarbons and included shallow gas hydrate well outside its normal stability field. Hypothetically, the brackish water seeps (already known from Marmarascarps ROV cruise) can be explained by a local fluid source of lake water trapped in the first 100 m of sediment during the last glaciation. However, deeper sources are required at the hydrocarbon emission site and, probably, contribute to steady gas bubble flow at other sites. Push cores were collected, where possible, at the seeps for chemical and biological analysis. As all seeps had carbonate crusts or outcrops to a varying degree this was not always successful. Kullenberg piston cores of up to 10 m were also collected in the seep areas during night operations. Fluids were extracted at intervals along the cores using vacuum extraction (Rhizon). Pore fluids from these cores exhibit chlorinities from 100 to over 1000 mM and a comparably wide range of other major and trace ion compositions. We will report the preliminary results of these core pore fluid analyses. http://tryonlab.ucsd.edu/Marmara/marnaut_objectives.html
B54C-03
Complex Plumbing Systems in the Near Subsurface: Geometries of Authigenic Carbonates From Dolgovskoy Mound (Black Sea) Constrained by Analogue Experiments
Targeted sampling on the Dolgovskoy Mound (northern Shatsky Ridge), revealed the presence of stunning laterally extensive and differently shaped authigenic carbonates. The sampling stations were selected based on sidescan sonar and profiler images that show patchy backscatter and irregular and discontinuous reflectors in the near subsurface. The interpretation of acoustic data from the top part of the Mound supports the seafloor observations and the sampling that revealed the presence of a complex subsurface plumbing system characterized by carbonates and gas. The crusts sampled consist of carbonate cemented layered hemipelagic sedimentary Unit 1 associated with several cm thick microbial mats. Three different carbonate morphologies were observed: a) tabular slabs, b) subsurface cavernous carbonates consisting of void chambers up to 20 cm3 in size, and c) chimney and tubular conduits vertically oriented or forming a subhorizontal network in the subsurface. The methanogenic origin of the carbonates is established based on visual observations of fluids seepage structures, 13C depletion of the carbonates (d13C varying between -36.7 and -27.4 permil), by carbonates within the thick microbial mats. Laboratory experiments with a Hele-Shaw cell were conducted in order to simulate the gas seepage through contrasting grainsize media present of the seafloor. Combined petrography, visual observations, and sand box simulations allowed a characterization of the dynamics and the structures of the plumbing system in the near subsurface. The shapes of the authigenic carbonates are interpreted respectively as a) Darcian porous flow through the finely laminated clayey/coccolith-rich layers, b) gas accumulation chambers at sites where significant fluid escape was impeded by thicker clayey layers forming the laminated Unit1, and c) focussed vertical fluid venting and subhorizontal migration of overpressured fluids released from b). The Hele-Shaw cell experiments represent a promising tool for investigating shallow fluid flow pathways in marine systems.
B54C-04 INVITED
Paleozoic Hydrocarbon-Seep Limestones
To date, five Paleozoic hydrocarbon-seep limestones have been recognized based on carbonate fabrics, associated fauna, and stable carbon isotopes. These are the Middle Devonian Hollard Mound from the Antiatlas of Morocco [1], Late Devonian limestone lenses with the dimerelloid brachiopod Dzieduszyckia from the Western Meseta of Morocco [2], Middle Mississippian limestones with the dimerelloid brachiopod Ibergirhynchia from the Harz Mountains of Germany [3], Early Pennsylvanian limestones from the Tantes Mound in the High Pyrenees of France [4], and Late Pennsylvanian limestone lenses from the Ganigobis Shale Member of southern Namibia [5]. Among these examples, the composition of seepage fluids varied substantially as inferred from delta C-13 values of early diagenetic carbonate phases. Delta C-13 values as low as -50 per mil from the Tantes Mound and -51 per mil from the Ganigobis limestones reveal seepage of biogenic methane, whereas values of -12 per mil from limestones with Dzieduszyckia associated with abundant pyrobitumen agree with oil seepage. Intermediate delta C-13 values of carbonate cements from the Hollard Mound and Ibergirhynchia deposits probably reflect seepage of thermogenic methane. It is presently very difficult to assess the faunal evolution at seeps in the Paleozoic based on the limited number of examples. Two of the known seeps were typified by extremely abundant rhynchonellide brachiopods of the superfamily Dimerelloidea. Bivalve mollusks and tubeworms were abundant at two of the known Paleozoic seep sites; one was dominated by bivalve mollusks (Hollard Mound, Middle Devonian), another was dominated by tubeworms (Ganigobis Shale Member, Late Pennsylvanian). The tubeworms from these two deposits are interpreted to represent vestimentiferan worms, based on studies of the taphonomy of modern vestimentiferans. However, this interpretation is in conflict with the estimated evolutionary age of vestimentiferans based on molecular clock methods, which suggest a maximal age of 126 million years for this group. 1. Peckmann et al. (1999) Facies 40, 281. 2. Peckmann et al. (2007) Palaios 22, 114. 3. Peckmann et al. (2001) Geology 29, 271. 4. Buggisch and Krumm (2005) Facies 51, 566. 5. Himmler et al. (submitted) Palaeogeogr., Palaeoclimatol., Palaeoecol.
B54C-05
Lipid Biomarkers Indicating Aerobic Methanotrophy at Ancient Marine Methane- Seeps
The inventory of lipid biomarkers of a number of ancient methane-seep limestones has been studied over the last decade. The molecular fingerprints of the chemosynthesis-based microbial communities tend to be extremely well-preserved in these limestones. The key process at seeps is the anaerobic oxidation of methane, performed by consortia of sulfate-reducing bacteria and methanotrophic archaea. Compounds preserved within modern and ancient seep settings comprise C-13-depleted lipid biomarkers. Besides the occurrence of C-13- depleted isoprenoids (archaea) and n-alkyl-chains (bacteria), C-13-depleted hopanoids have been reported in seep limestones. Here, lipid biomarker data are presented from three ancient methane-seep limestones embedded in Miocene and Campanian strata. These examples provide strong evidence that methane was not solely oxidized by an anaerobic process. In a Miocene limestone, 3-beta-methylated hopanoids were found (delta C-13: -100 per mil). Most likely, 3-beta-methylated hopanepolyols, prevailing in aerobic methanotrophs were the precursor lipids. In another Miocene limestone, a series of C-13-depleted 4-methylated steranes (lanostanes; -80 to -70 per mil) is derived from aerobic methanotrophs. Lanosterol is the most likely precursor of lanostanes, known to be produced by aerobic methanotrophs, some of which are outstanding among bacteria in having the capacity to produce steroids. In a Campanian seep limestone a suite of conspicuous secohexahydrobenzohopanes (-110 to -107 per mil) is found. These hopanoids probably represent early degradation products of seep-endemic aerobic methanotrophs. This interpretation is supported by the presence of "regular" hopanoids that can be discriminated from the unusual secohexahydrobenzohopanes by only moderately low delta C-13 values (-49 to -42 per mil). Structural and carbon isotope data reveal that aerobic methanotrophy is more common at ancient methane- seeps than previously noticed. Our data indicate that anaerobic and aerobic oxidation of methane at ancient seeps occurred in the same setting, probably in close proximity to each other.
B54C-06
A big old cold seep revisited: the Paleocene Panoche-Tumey Hills system
One of the largest ancient cold seeps known is exposed in the Panoche Hills and adjacent Tumey Hills, in central California. In outcrop the most striking components of the Panoche-Tumey Hills paleoseep (PTHP) are authigenic carbonate bodies representing methane-derived cementation at or just below the Paleocene seafloor, and sandstone injectites thought to delineate the subseafloor plumbing of the system. The carbonates and underlying injectites are largely contained and best displayed within the Moreno Formation, a dominantly shaley unit that was deposited at outer shelf depths on the western margin of the San Joaquin forearc basin in the Maastrichtian-Thanetian. Since our first (2002) publications about the PTHP we have learned a great deal about its scale and development. Field investigation has extended the length of the seep zone from an initial estimate of 5km to a minimum of 20km, beyond which the main seep horizon (the Cima Sandstone Lentil) is truncated by an unconformity. However, injectites lower in the Moreno Fm. persist for at least 100km along strike to the south, suggesting that the PTHP originally spanned much of the western margin of the Paleocene forearc. The stratigraphic range of the seep carbonates has also been revised upwards from 45m to at least 250m (representing approximately 3 my). The carbonates themselves have δ13C values between +3 and - 54‰ and δ18O values between -7 and +7‰(VPDB) and a range of habits, with irregular mounds and stratiform bodies volumetrically dominant. Mounds contain multiple fluid conduits, breccias, distinctive cement phases, and a zoned, low diversity chemotroph-rich paleofauna (including microbial mats, lucinid bivalves and vestimentiferan tubeworms). They represent focused, prolonged fluid flow, methane expulsion and ecosystem development, and are the best archives of individual seepage events. Stratiform bodies are uniformly micritic, laterally extensive and more biologically diverse. They represent pervasive seepage and locally preserve seafloor lag deposits. Vertical distribution of carbonates shows that the PTHP fluid system began, ended and was locally dominated by focused seafloor flow. Hydrate Ridge off of Oregon is an interesting modern analog for the PTHP. It is comparable in terms of size, tectonic setting, fauna and carbonate structures, though it formed at greater depth and overlies a different type of plumbing system.
B54C-07
Geochemical Variations in Paleoseeps Over Short Time Scales: Stable Isotope Results From the Panoche-Tumey Hills Paleoseep (PTHP), California
Fluctuations in fluids compositions over varying time scales are widely recognized from modern cold seep deposits. We present results from a detailed study of carbonate mineral proxies for fluid flow from the PTHP. This cold seep deposit extends for 20 km along strike and was fed by a system of sandstone injectites during its mid- Paleocene lifespan. We sampled carbonate-bearing portions of the seep horizons both laterally and vertically over an area of approximately 5 km2. We analyzed the elemental and stable isotopic chemistry to examine temporal variations. Carbonate occurrences include dolomite to low-Mg calcite mineralogies in matrix cements, cemented pipe structures, laminated carbonates (fossilized bacterial mats?), and carbonate veins. In all cases the direction of carbonate mineral growth can be determined. Previous work on bulk samples has yielded a range of δ13C of –-54 to +3 ‰ and δ18O of --7 to +7 ‰ (all values VPDB). Our more detailed sampling shows a smaller range, but significant variation of isotopic results over mm's to cm's (δ13C values vary in individual features from as much as –-28.19 to +3.58 ‰; δ18O values from –-8.68 to +4.71 ‰). Sequential sampling by micromilling of transects across carbonate infill show the greatest range in carbon isotopes. Individual transects show ranges in carbon isotopes minimum and maximum of 4.8 and 29.7 ‰, respectively. Oxygen isotopes vary less – from 4.1 to 10.1 ‰ for different transects. The influence of meteoric diagenesis on the oxygen is difficult to constrain well. Coupled with carbonate and sulfate/sulfide mineralogy, the variations in isotopic compositions can be related to fine-scale variations in microchemical environments, perhaps due to repeated changes in the chemistry of fluids fluxing through the features examined. Given the age range of the seep horizon, and the size of individual carbonate features, we estimate that total cementation times of individual features, and hence fluid flow variation time scales, is at the most on the order of a few thousands of years.
B54C-08
Constraining Microbial Cycling of Carbon and Sulfur and Relationships to Macrofaunal Ecology in Cretaceous Cold Seeps
The Pierre Shale in Colorado hosts an extensive fault-controlled network of Late Cretaceous fossil-rich carbonate accumulations expressed topographically as the Teepee Buttes. An ancient methane seep origin for these features is clear from carbonate-carbon isotope values that are as light as -50 per mil and from organic biomakers that point to abundant aerobic and anaerobic methanotrophy. A primary goal is to identify the patterns of sulfate reduction linked to anaerobic oxidation of methane (and other possible hydrocarbons) through a detailed isotopic study of sulfur preserved as carbonate-associated sulfate trapped within authigenic and biogenic phases. We are further constraining these patterns by calibrating them against our studies of modern seeps in the Gulf of Mexico. Delineating micro- and macrofaunal symbiotic linkages driven by chemosynthetic (sulfide-oxidizing) bacterial communities is a related goal. Through ongoing analysis of shell material, we hope to fingerprint thiotrophic activity within specific organisms, including lucinid bivalves. Consistent with the abundant benthic macrofauna, C- S-Fe analysis of the host shales indicates that bottom waters were oxygenated at the time of seep activity. Our work expands on previous studies to include a detailed paragenesis of the carbonate fabrics, which is aided by high-resolution C isotope and trace element analysis. Our results show that early-formed carbonates are characterized by depleted C isotope values, comparatively heavy O, low Fe contents, and high Sr and Mg. Late carbonates show isotope and elemental relationships that are generally opposite those of the early precipitates. Botryoidal cements, pelsparites, and yellow calcite dominate the early diagenetic forms. Blocky white sparry cements formed later, and micrite is mostly a product of later micritization of early fabrics and correspondingly shows broad geochemical properties. The overarching focus of all this work is to understand the timing, location (surface vs. subsurface), mechanisms, and specific microbial factors behind carbonate authigenesis and their relationships to macrofaunal ecology.