OS53B-01
COOPERATIVE MANAGEMENT OF TRANSBOUNDRY OIL AND GAS RESOURCES IN THE MARITIME BOUNDARY REGION OF THE GULF OF MEXICO
Finding and exploiting oil and gas resources in the ultra-deepwater areas of the Gulf of Mexico is occurring at an accelerated pace. Huge new discoveries have recently been made in a large geological structure known as the Lower Tertiary Wilcox Trend that is located in the U.S.-Mexico Maritime Boundary Region. These discoveries have been projected to boost current U.S. oil reserves by as much as fifty percent. Technological advancements and market conditions have finally reached a point where production of hydrocarbons in these ultra-deepwaters is commercially feasible. However, due to the transboundary characteristics of many of these hydrocarbons, some form of bi-national cooperation is necessary to effectively manage the shared resources, protect the oceanic environment and comply with evolving norms of international law before commercial production can begin. Well established international customary norms prohibit unilateral exploitation of transboundary oil and gas resources. Consequently, it is important for the two nations to address these issues today rather than putting them off until they become a critical political problem in their bilateral relations. The United States and Mexico have already agreed to temporarily cooperate in the exploration of potential oil and gas resources in one portion of the Gulf of Mexico known as the Western Gap. This is an area in the center of the Gulf of Mexico that falls outside of the 200 mile exclusive economic zones of the two nations. After scientific studies provided evidence that the Western Gap qualifies as part of each nation's extended continental shelf, a Delimitation Treaty was negotiated and ratified in 2000. This Treaty gave Mexico access to about 62 percent of the Gap, while the U.S. retained about 38 percent. The Treaty also established a 2.8 nautical mile buffer zone along the new boundary to account for the possibility that straddling oil and gas reservoirs may be located there. The nations agreed to a ten year drilling moratorium and to share information on the geological and geophysical characteristics of any reservoirs in the buffer zone. In 2010, the moratorium expires and either side may exploit the resources in the zone. Similar transboundary reservoirs of immense size exist along significant portions of the U.S.-Mexico maritime boundary. Yet, proper management and production of these resources will be severely hampered by a variety of legal and policy impediments that await resolution. Resolving many of these impediments will only be possible through the collaborative efforts of both nations. It is time for the U.S. and Mexican Governments to take a more proactive role in managing the transboundary hydrocarbon resources in the deep waters of the Gulf of Mexico. If successful, rather than an arena for competition and legal strife, the U.S.-Mexico Maritime Boundary Region can serve as a model of cooperative management. Such a model would benefit both nations as well as serve as a useful guide for the rest of the international community.
OS53B-02
Depth Profiles of Stable Nitrogen and Carbon Isotopes and C:N Ratios in Surficial Sediments From the NW Insular Slope of Cuba.
The deep sea floor in the studied area remained unexplored for several decades. Recent searching for fossil fuels and gas hydrates in the seabed has renewed interest in studying deep sea processes in the region. Near- surface sediments were recovered with a Reineick box-corer at 3 preselected quadrants located at the channel axis of the Florida Straits and the slope rise off NW Cuba at depths ranging from 1468 to 2094 m. A total of 12- 30 cm long- subcores were sampled for isotopic (15N/14N and 13C/12C) and C:N ratio analyses. Surficial sediment samples exhibited mostly enriched δ15N values ranging from +3.6 to +6.4‰ with an average of +5.4 ± 0.7. δ15N values in the deeper quadrants (I and II) near the channel axis were fairly homogeneous in contrast to the shallower one (III) located at the slope rise, which showed a higher variability and significantly depleted values (+3.6‰). Testing of equality of δ15N values among quadrants was rejected (Friedman's test p<0.368. From the estimated δ15N average value here recorded a significant input of organic matter from a pelagic source is inferred. The δ13C values had a narrow range in all quadrants (-18.5 to -19.13‰) with an average of - 18.71±0.17. A gradient slightly enriched is noted on the seabed from the westernmost quadrants(I and II)towards the slope rise (quadrant III). The average δ13C signal in surficial sediments from the Southern Straits approaches that known for the continental shelf of South Florida (-18.5±0.7). Vertical profiles of TOC and TN are highly heterogeneous among quadrants displaying a diminishing trend with depth (0- 18 cm). TOC values are mostly impoverished ranging from 0.16 to 0.67 mmol/g. Slope rise sites concentrated less TOC than locations near the channel axis. The opposite occurred with TN values. Sites near the slope rise attained 0.90 mmol/g whereas in the channel axis, nitrogen was reduced to 0.46mmol/g. C:N ratios ranged from 1.9 to 10.2. An increasing gradient was noted from quadrants located at the channel axis towards the slope rise. The mean molar C:N ratio (5.4) is indicative of marine hemipelagic deposition. δ13C vertical profiles indicate episodic OC inputs at 3, 10, and 20 cm. Assuming a conservative sedimentary rate of 2.8 cm/k, such a OC input may have been buried 1000, 3600, and 6200 yr between carbonate layers. Sedimentary organic matter impoverishment in the three quadrants is further supported by the low C and N percentages (0.03-0.15% C; 0.0004-0.005% N). It is postulated that the hydrodynamic conditions may exert an important influence on the vertical and advective fluxes of particulate organic material (POC) which are trapped at the mix layer (700 m) in the Straits, reducing the input of nutrients to bottom dweller communities.
OS53B-03
Rubidium, Strontium, Bromide, and Total Iodine Concentrations Consolidate Evidence for Seawater Dissolution of the Jurassic Louann Salt as the Source of the Orca Basin Brine
A profile of filtered seawater and brine samples was collected in the summer of 2003 from a depth of 1500 m down to the bottom of the anoxic, hypersaline Orca Basin (northern Gulf of Mexico). Using ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS), these samples were analyzed for alkali cations (Na+, K+, Rb+), alkaline earth cations (Mg2+, Ca2+, Sr2+, Ba2+), and the major anions chloride (Cl-) and sulfate (SO42-). Major ion concentrations in the brine are consistent with previous studies, confirming that Na plus Cl make up more than 95% of its composition, hence governing its density and hydrodynamic stability. Binary mixing plots across the interface between deep Gulf of Mexico seawater and the anoxic brine are generally linear, but display substantial deviations from conservative behavior at the steepest part of the pycnocline for all analytes except Na, Cl, and Ca. Negative deviations signify localized cation removal by an adsorption or ion-exchange process, probably associated with the dense layers of particles that are trapped there. Especially strong Mg removal may be indicative of dolomitization, whereby the concomitant release of Ca counters its adsorption, resulting in zero net Ca removal. A positive deviation for sulfate is attributed to bacterial sulfide oxidation. Concentrations of Rb, Sr, and Ba in the homogeneous brine, reported here for the first time, are enriched by factors of 1.5, 1.4, and ~9, respectively, with respect to the overlying seawater. Unlike Ca and Sr, Ba concentrations in the brine are clearly controlled by the solubility of its sulfate salt (barite), causing a maximum of 670 nmol/kg at the interface. Several independent lines of evidence, for example downward decreasing salinity gradients in the sediment pore waters, seismic surveys revealing salt exposure on the upper slope, and the discovery of a brine river flowing into the Orca Basin, suggest that the brine is formed outside the basin, most likely by the interaction of seawater with regionally extensive evaporite deposits. A simple mass balance shows that the dissolution of about 280 g of halite per kg of seawater can account for the extreme concentrations of Na and Cl in the Orca Basin brine. The same mass balance was applied to a number of minor constituents (K, Rb, Mg, Ca, Sr, SO4, Br, IT) in order to calculate what abundances in the halite are required to reproduce their concentrations in the brine as measured in the present work and by others. The results are entirely compatible with the composition of the Jurassic Louann Salt, specifically with the average of compositions published for a transect spanning early to late stage halites. Elevated abundances of K and Rb point to contributions from bittern facies modified by prior diagenetic contact with seawater.
OS53B-04
Modeling Intense Near-Bottom Currents Along the Sigsbee Escarpment
The Sigsbee Escarpment in the northwestern Gulf of Mexico is a steep topographic feature found at depths between 1500m and 3000m depth, depending on location along the escarpment. Observations of intense (50- 100 cm/s) velocities near the ocean bottom over the escarpment have been linked with bottom-intensified Topographic Rossby Waves (TRWs). A modeling methodology has been developed for simulating these energetic bottom-intensified features along the Sigsbee Escarpment. A nested model domain with a horizontal resolution of approximately 800m and up to 80 vertical layers has been configured for the region. A generalized vertical coordinate system implemented in the Navy Coastal Ocean Model has been used to develop a "vanishing sigma" vertical coordinate system that permits accurate representation of bottom topography while reducing the slope of the vertical coordinate surfaces. The high-resolution model is nested within a 1/20° resolution Gulf of Mexico model so that linkages between the intense deep currents and the eddy field of the Gulf can be explored. A set of idealized model experiments demonstrates processes that can be responsible for the generation, propagation, and intensification of the TRWs.
OS53B-05
Properties of Sea Floor Hydrates From the Gulf of Mexico
Near-surface methane hydrates are well known from numerous seep sites on active and passive continental margins. The hydrates exhibit a characteristic macroscopic and microscopic fabric as shown by samples recovered from a large range of water depths. In the Gulf of Mexico gas hydrates have been sampled from shallow northern slope in the Green Canyon area between 600 and 1000 m water depth and in the southern Gulf in 3300 m water depth. The internal fabric of pure gas hydrate has a peculiar structure with pores that result from rising methane gas. The porous gas hydrate structure could also be observed at the surface of gas hydrate outcrops in 3300 m water depth using ROV QUEST during METEOR cruise M67/2. Fabric analyses of hydrate samples indicate that at least parts of the hydrate are formed from free methane gas. Free gas migrates upwards through the sediment column and is also indicated by gas bubbles emanating at the seafloor. These bubbles form plumes in the water column. Gas hydrate decomposition and ice formation is also documented by cryo- stage X-ray diffraction and Rietveld analyses. Preservation of structure I hydrates various between 40-70% and has a average preservation of 60%. Preservation of structure II hydrates from the Gulf of Mexico is much better, which is explained by shallower stability curve of such hydrates.
OS53B-06
Asphalt Flows on Chapopote, a Knoll in the Campeche Bay, Southern Gulf of Mexico - new Results From ROV Investigations
During the German expeditions SO174 in 2003 and M67 in 2006 swath mapping was carried out in the salt diapir province in the Campeche Bay. The seafloor morphology in the north of the area is dominated by elongated hills, called knolls. Asphalts have been discovered at two of the 400 m high knolls during video surveys, but more findings are likely. During M67 dives with the ROV QUEST were carried out at one of the knolls, named "Chapopote", in about 3000 m water depth. Chapopote has a caldera-like central depression with a rim that is depressed in the north and south. The distribution of asphalts is patchy, with a major field south-east of the central depression and several smaller areas some hundred meters apart from each other at the rim. Asphalts cover about 0.5 km2. The main field appears to be the most recent outflow of asphalt. The flow pattern of this asphalt is ropy with little signs for degradation. At the other fields the asphalts are degraded to blocks without visible flow structures and are covered with hemipelagic sediments. Based on detailed observations, we put an earlier model by Hovland et al., EOS, 86, 42, 2006, in question. This model proposes supercritical water transporting hydrocarbons leading to the expulsion of warm or hot asphalts at the seafloor. Alternatively, we favour the view that cold hydrocarbons flew out at several locations at Chapopote. In a subsequent alteration process, the hydrocarbons lose the more volatile components leading to the observed residue of asphalts on top of the sediments. We found evidence of seepage at Chapopote: outflow of gas bubbles, occurrence of gas hydrates and release of oil while sampling. At one site, we observed a package of individual flows stacked on top of each other. This structure suggests that the expelled hydrocarbons, can flow into the water as a viscous fluid, which is positive buoyant. During the alteration the flows get heavier and lay down at the sediments and partly keep on flowing, creating the ropy surface, forming the observed pattern.
OS53B-07
Geophysical Surveys in the Southern Gulf of Mexico - Preliminary Results from R/V Meteor Cruise M67/2
In March/April 2006, R/V Meteor Cruise M67/2 was carried out in the southern Gulf of Mexico to study fluid and gas seepage in shallow sediments and in particular to investigate the occurrences of asphalts on the sea floor. The working program included both geophysical survey work on regional and local scale as well as geologic and geophysical station work. Among the geophysical methods, high resolution multichannel seismics, sediment echosounding, swath bathymetry, deep tow side scan sonar and water column acoustic profiling were used. Main objectives were to study the origin of asphalt ‘volcanism' and the nature and extent of fluid/gas seepage systems on the sea floor and in the water column. Multichannel seismic surveys were ranging in water depth from ~1000 m in the South to more than 3500 m in the North, covering areas of pronounced salt tectonics, rafting and diapirism. The surveys were targeted to investigate the sub-seafloor structures above oil slicks on the sea surface, which were likely locations of pronounced fluid seepage. They reveal a larger number of bright spots at shallow sub-bottom depth, which have probably mostly developed as a result of salt diapirism and associated fault deformation. From North to South, the degree of diapirsm and deformation increases and topography changes from more circular hills (knolls) to diapiric ridges, revealed by a complete swath bathymetric chart. Stratigraphic analyses of sediment echosounder and high resolution seismic data can be further used to identify the onset of deformation, which is imaged in variations of sediment thickness, derived from mostly turbiditic sediment input, as a result of uplift. The new parametric sediment echosounder system Parasound (Atlas Hydrographics) was also used to image the water column in the vicinity of Chapopote Knoll during bathymetric surveys and ROV and station work. A dense grid of survey lines revealed clear evidence for gas bubbles in the water column, originating from a spot near the asphalt occurrences.
OS53B-08
Habitat heterogeneity - biological association relationships in the asphalt volcano, SW Gulf of Mexico
A new class of cold seep, named asphalt volcano, was discovered in the Campeche Knolls region of the southern Gulf of Mexico, supporting chemosynthetic communities alike those lying at similar depth on the Angolan margin and the Barbados Prism suggesting an interesting longitudinal connectivity in the faunal components. The discovery of this novel deep-sea habitat has raised questions about diversity and process dynamics in this novel poorly described milieu. Results from two previous cruises jointly sponsored by German, US and Mexican funding agencies have allowed us to recognize the presence of large densities of background benthic megafauna, mainly represented by sea-cucumbers and galatheid crabs, which occupy diverse habitats in asphalt volcano and feed on microbial assemblages on the asphalt covering extended area. Asphalt displays different degrees of hardness suggesting ongoing activity of asphalt extrusion in the site that is reflected in biological benthic communities in different states succession and complexity. The fresh asphalt and the immediately surrounding soft sediment are colonized by mats of complex microbial assemblages where both background benthic megafauna and chemosynthetic tube worms and mussels aggregate. Our results focus on the diversity of the habitats associated with methane seepage through the example of geological structures in the asphalt volcano considering the small scale with the analysis of the relationships between biological assemblages and habitat heterogeneity assessing the role of the geological structure on biological communities. Bubbling of gas, oil and the content of thermogenic gas and gas hydrate in the asphalt suggests that the asphalt plays an important role as a reservoir of methane in this marginal deep sea.