OS51B-01
Monitoring Hydrocarbons Transfer in the Gulf of Mexico with Synthetic Aperture Radar; Radarsat-1, Envisat, ERS-1, ERS-2 and JERS-1.
The Gulf of Mexico contains hundreds of active hydrocarbons seeps. Seepage in the Northern and Southern Gulf of Mexico tend to ‘leak' through conduits generated by salt techtonism and related processes. Hydrocarbons are then transported to the sea surface in oily bubbles, once on the surface the oil spreads into layers approximately .1µm thick, detectable by Synthetic Aperture Radar (SAR) instruments onboard spacecrafts like Radarsat, Envisat, Ers-1, Ers-2 and Jers-1. Since 1992, SAR coverage have been collected over the Gulf of Mexico. In total more than 4000 orbital passes over the Gulf of Mexico generated more than 30,000 images that can be used to examine natural hydrocarbon seepage in this region. Much of these data have been archived by the Alaska Satellite Facility through data sharing agreements with NASA and with satellite operators. We have completed a spatial, temporal and SAR Mode classification of the entire SAR coverage. This catalog can be used to plan studies of natural variation in hydrocarbon seepage and to delineate coverage in areas of special interest. The complete catalog is being made available online to facilitate collaborative research in this field.
OS51B-02
A conceptual model for hydrocarbon accumulation and seepage processes inside Chapopote asphalt volcanism site, Southern Gulf of Mexico: from high resolution seismic point of view
As part of the German R/V Meteor M67/2 expedition in 2006 to the southern Gulf of Mexico, a set of 2D high resolution seismic profiles was acquired across the Chapopote knoll to study sea floor asphalt occurrences and their origin. Based on regional seismic stratigraphy studies, correlated to DSDP sites, a higher reflective coarse grained sediment unit of Late Miocene age is identified as a potential shallow gas reservoir, overlain by a low permeability fine grained Pliocene and Pleistocene cover. As a result of salt diapirism, local uplift has caused reduced accumulation rates above the diaper since the late Pliocene, while the rates had been uniform throughout the area before. This has further improved the seal properties, since more fine grained material deposited in elevated locations. Nevertheless, on the crest of Chapopote, sediments above the coarse sediment unit are only around 150-75 m thick. Since oil and gas production can well be expected at depth in Jurassic and Tertiary source rocks, the presence of high amplitude reflector packages within the reservoir unit is interpreted as a result of the presence of hydrocarbons. This interpretation is further supported by the observation that some reflectors are cross-cutting and/or reveal a drop in instantaneous frequency. But, the thin seal above the reservoir unit, located directly underneath a widespread occurrence of asphalts at the sea floor, probably facilitates the leakage of hydrocarbons trapped inside the reservoir through a ~ 750 m wide acoustically chaotic zone partly aided by faulting. Since the top of Chapopote shows a high structural complexity, more seepage sites may exist beyond where seafloor asphalts have been found so far. Evolution and structure of the migration and reservoir system, which may be deep rooted, will be discussed both with respect to shallow gas and asphalt occurrences.
OS51B-03
Effect on the macrobenthic community structure of an abyssal salt diapir in the central Gulf of Mexico
A salt diapir was mapped in the Sigsbee abyssal plain in the central Gulf of Mexico and studied to evaluate the effect on the variability of the macrobenthic community structure. Samples were collected along a linear transect in 6 stations associated with the salt diapir, including abyssal stations. The diapir affects sediment factors such as percentage of sand and silt and the elemental C/N ratio and percentage of organic carbon and nitrogen determining the distribution of the macrobenthic taxonomic richness and biomass. The densities of taxa such as ostracods, cumaceans, nematodes, polychaetes and turbelarids respond specifically to the availability of organic carbon and nitrogen in sediment. This salt diapir acts as a topography feature that modifies the superficial sediment conditions and food availability affecting the composition and taxonomic richness, density and biomass of the abyssal benthos.
OS51B-04
Geologic Characteristics of Hydrocarbon Seeps in the Gulf of Mexico That Potentially Contribute to the Development of Sea Surface Slicks
Numerous Gulf of Mexico hydrocarbon seeps are well documented through studies of chemosynthetic communities, seafloor deposits, and gas hydrates associated with seeps. Furthermore, sea surface oil slicks can be identified on satellite images, and subsurface geology can be determined through geophysical recordings. Initial reviews of satellite images indicate that certain areas with recurring surface slicks are associated with known seafloor chemosynthetic communities. However some areas with documented seafloor chemosynthetic communities produce no sea surface hydrocarbon slicks. The relationship of sea surface slicks to underlying seafloor seeps is influenced by several geologic characteristics and these characteristics may determine which seeps develop into surface slicks. Site specific studies have investigated the relationship of hydrocarbon seeps to seismic character, carbonate character, geothermal patterns, source and pathway, chemosynthetic communities, mineralogy and geochemistry, gas hydrates, and remote sensing of slicks. An improved understanding of factors influencing Gulf of Mexico hydrocarbon seep and surface slick relationships will contribute to a more accurate assessment of natural seep contributions to the global carbon cycle.
OS51B-05
Variability of Total Organic Carbon and Nitrogen in Abyssal Sediments in the Gulf of Mexico
Marine sediments represent the largest reservoir of carbon on Earth and an important factor in climate change and allow us to understand the role of the deep sea sediments as a reservoir for the organic carbon and the chemical balance of nitrogen in the basin. This study determined the precision (CV < 3%) and accuracy (CV < 3%) of the elemental analysis carried out that described the variability of total organic carbon (TOC) and nitrogen from samples collected at 115 locations during several cruises in a depth interval of 1025 and 3725 m. The average values recorded for abyssal sediment samples in the SW Gulf of Mexico were 0.95% +/- 0.135% for TOC and 0.12% +/- 0.025% for N with a C/N ratio of 7.96 +/- 1.406. Only samples from chemosynthetic locations (i.e. Chapopote asphalt volcano) recorded S in 3.35% +/- 1.503%. The elemental composition of TOC and N is controlled by depth and follows a polynomial pattern; the C/N followed a sigmoid pattern and together with TOC varies with the distance to the coast and depth. The presence of topographic features in the seafloor influence the distribution of TOC and N.
OS51B-06
Reconnaissance Strategy for Seep Chemosynthetic Communities in the Gulf of Mexico
The Continental Slope of the Gulf of Mexico hosts diverse chemosynthetic communities at oil and gas seeps. Exploration is needed to extend knowledge of the Gulf of Mexico chemosynthetic ecosystem in the zones anticipated to receive energy exploration and production activities over the coming decades. A nested survey approach can be used to identify representative sampling sites within this vast offshore area. Potential sites where chemosynthetic community could occur are selected on the basis geophysical, geochemical, and satellite remote-sensing indicators. Photo-reconnaissance using cost-effective camera systems is then used to confirm the presences or absence of chemosynthetic communities at high-probability sites. Follow-up sampling can then proceed with submersibles or ROVs to acquire tissue and or geochemical samples. However, because access is limited, submersible dives may not be possible at all sites. Two examples of this approach have recently been applied in the northern and southern Gulf of Mexico, respectively. We compared community characterizations obtained from the initial reconnaissance with more detailed characterizations forthcoming from submersible sampling. Our results show that major differences in community type and geochemical substrata are evident from preliminary reconnaissance, while details of animal densities and species compositions require targeted sampling with submersibles. However, given the limited access to submersibles, cost-effective surveys with deep-sea camera systems would greatly expand understanding of the zoogeography of chemosynthetic fauna in the Gulf of Mexico and Caribbean Sea.
OS51B-07
Prospects and Techniques for Eddy-Resolving Acoustic Tomography in the Eastern Gulf of Mexico
For several decades monitoring and modeling the dynamics and physical structure of the Gulf of Mexico have been major efforts undertaken by oceanographers of the United States and other American countries. There are very interesting physical oceanographic features in the Gulf, not the least of which are the Gulf Loop Current and the eddies it spawns. Satellite sensing of IR and altimeter imagery has been a major input to modeling those features. Such efforts are very important to the economy and well being of much of the United States and Mexico, including fisheries, mineral economies, hurricane strengths and paths in the summer, and severe snow storms in the eastern US in the winter. A major shortcoming of the present monitoring of the Gulf is the lack of subsurface input to the dynamic models of the Gulf. Acoustic tomography is a viable means of providing that missing input. Several universities have come together to investigate the prospects for establishing a Gulf Eddy Monitoring System (GEMS) for the deep eastern half of the Gulf using acoustic tomography. The group has conducted several acoustics experiments and propagation studies to determine the feasibility of long-range propagation in the eastern Gulf and the mitigation of adverse effects on marine mammal populations in that region under the Office of Naval Research project entitled the Littoral Acoustic Demonstration Center (LADC). The group has also convened an invited session for the 9th World Multiconference on Systemics, Cybernetics and Informatics (WMSCI 2005) Orlando, FL, July 2005. This paper discusses prospects for establishing the GEMS tomographic system, its technical characteristics, and its contributions to advancing the knowledge of the dynamics of the Gulf. This presentation will concentrate on the characteristics of a single-slice tomographic system, called GEMS Phase I, across the approaches to the DeSoto Canyon in the northeastern Gulf and its prospect for monitoring the movements of eddies into the canyon. It will also touch on the prospects for expanding the network to include the entire eastern Gulf with a moored system on the Campeche Bank and several along the US Gulf shelf. This projected tomographic system is called GEMS Phase II. The paper will invite the participation of Mexican organizations to help plan and develop this proposed extensive project.
OS51B-08
The Distribution of Microbial Activity Outside and Within Acoustic Wipe-out Zones at Mississippi Canyon 118, Gulf of Mexico
At cold seep sites, microbial activity is known to be enhanced due to the abundance of reduced compounds entrained in upwardly advecting, hydrocarbon-rich fluids. Yet, it remains to be seen how this microbial activity influences sedimentary gas fluxes and composition on local and regional scales. An integrated biogeochemical and geophysical approach was used to investigate the local and regional role microbial activity plays at hydrate- bearing cold seeps in the northern Gulf of Mexico. Twenty-eight sediment cores were collected both outside and within acoustic wipe-out zones that defined the vent system at Mississippi Canyon Lease Block 118 (MC118), 100 miles offshore of Louisiana. Variations in depth gradients of sulfate and methane concentrations and stable carbon isotopic composition of methane and dissolved inorganic carbon (DIC) between cores were utilized to quantify differences in microbial activity between coring sites. Three distinct zones featuring low, moderate, and high levels of microbial activity including sulfate reduction, methane production and methane oxidation were recognized. Low microbial activity was always found outside the seismic wipe-out zones whereas both moderate and high microbial activities were found within the wipe-outs. Evidence for temporal variability of microbial activity was also found. At sites where microbial activity was high, the carbon isotopic compositions of authigenic carbonates and the DIC pool were similar, suggesting current carbonate precipitation. However, at sites with only moderate microbial activity the isotopic compositions of the carbonates and DIC were different, suggesting their precipitation at some time in the past when microbial activity was higher.
OS51B-09
Stable sulfur and carbon isotope investigations of pore-water and solid-phase compounds in sediments of the Chapopote Asphalt Volcano, southern Gulf of Mexico
During R/V Meteor cruise M67 2a/b (March-April 2006) to the Asphalt Volcanoes of the southern Gulf of Mexico two
gravity cores were retrieved from the central depression of the Chapopote Knoll which contained viscous
oil/asphalt a few meters below the sediment surface. Also several push cores were taken with the remotely
operated vehicle (ROV) QUEST at sites where oil/asphalt reached closely below the sediment surface. From
these cores solid-phase and pore-water samples were taken for on-board and subsequent shore-based
analyses. Together with a core taken from a background site which is not influenced by asphalt/oil seepage these
sediment and pore water samples are currently subject to detailed analyses of (1) the stable sulfur isotopic
composition of both dissolved (sulfate and sulfide) and solid-phase (iron monosulfides, pyrite) sulfur
compounds, and (2) the composition and stable carbon isotopic signatures of hydrocarbon gases.
The major aims of these investigations are to identify whether and to which extent the upward migration of oil,
asphalt and gas (1) stimulates biogeochemical processes and turn-over rates, and (2) influences the stable
sulfur isotopic signatures of both dissolved and solid phase sulfur compounds. Furthermore, we seek to
determine the potential of these - possibly unusual - stable sulfur isotopic signals of solid-phase sulfides to
reconstruct hydrocarbon seepage in older geological records and to elucidate how the composition and the
stable carbon isotopic signatures of the hydrocarbon gases are altered by the action of typical chemosynthetic
communities thriving at these sites.
http:www.rcom.marum.de/Project_E1.html