Leonardo Macelloni

Hydrocarbon gas hydrates in sediments of the Mississippi Canyon area, Northern Gulf of Mexico

Abstract The Gulf of Mexico Hydrates Research Consortium has begun installing a seafloor observatory to monitor gas hydrate outcrops and the hydrate stability zone in Mississippi Canyon Area Lease Block 118. Relevant background information concerning the Mississippi Canyon Area and gas hydrate occurrences in the northern Gulf of Mexico is presented. Microbial influences and possible scenarios of hydrate accumulation are considered. The design of the observatory was based on field data recorded in the Mississippi Canyon Area, principally lease block 118 (MC118) and the vicinity of lease block 798 (MC798). Swath bathymetry by autonomous underwater vehicle played a large part, as did seismic imaging within the hydrate stability zone and core sampling. These data and the results of their analyses are discussed in detail. Discussion and interim conclusions are presented.

STRUCTURE OF A CARBONATE/HYDRATE MOUND IN THE NORTHERN GULF OF MEXICO

A one-kilometer-diameter carbonate/hydrate mound in Mississippi Canyon Block 118 has been chosen to be the site of a multi-sensor, multi-discipline sea-floor observatory. Several surveys have been carried out in preparation for installing the observatory. The resulting data set permits discussing the mound’s structure in some detail. Samples from the water column and intact hydrate outcrops show gas associated with the mound to be thermogenic. Lithologic and bio-geochemical studies have been done on sediment samples from gravity and box cores. Pore-fluid analyses carried out on these cores reveal that microbial sulfate reduction, anaerobic methane oxidation, and methanogenesis are important processes in the upper sediment. These microbial processes control the diffusive flux of methane into the overlying water column. The activity of microbes is also focused within patches near active vents. This is primarily dependent upon an active flux of hydrocarbon-rich fluids. The geochemical evidence suggests that the fluid flux waxes and wanes over time and that the microbial activity is sensitive to such change. Swath bathymetry by AUV combined with sea-floor video provides sub-meter resolution of features on the surface of the mound. Seismic reflection profiling with source-signature processing resolves layer thicknesses within the upper 200-300m of sediment to about a meter. Exploration-scale 3-D seismic imaging shows that a network of faults connects the mound to a salt diapir a few hundred meters below. Analyses of gases from fluid vents and hydrate outcrops imply that the faults act as migration conduits for hydrocarbons from a deep, hot reservoir. Source-signature-processed seismic traces provide normal-incidence reflection coefficients at 30,000 locations over the mound. Picking reflection horizons at each location allows a 3-D model of the mound’s interior to be constructed. This model provides a basis for understanding the movement of fluids within the mound.

Black coral (Anthozoa, Antipatharia) forest near the western Pontine Islands (Tyrrhenian Sea)

Dense aggregations of antipatharian corals have been discovered by means of remotely operated vehicle (ROV) photo-imaging off the western Pontine Islands (Central-Tyrrhenian Sea, Italy). These black coral assemblages are mainly composed of Parantipathes larix and Leiopathes glaberrima . This discovery constitutes an update of their habitat distribution for the Mediterranean Sea and is relevant for the development of focused protection measures for the study area.

Evidence of a shallow water submarine hydrothermal field off Zannone Island from morphological and geochemical characterization: implications for Tyrrhenian Sea Quaternary volcanism

Discoveries from multibeam bathymetry and geochemical surveys performed off Zannone Island (western Pontine Archipelago, Tyrrhenian Sea) provide evidence of an undocumented hydrothermal field characterized by ongoing fluid emissions and morphologically complex giant depressions located in shallow water ( 250 m) that host pockmarks, mounds, small cones and active fluid vents, and which are interpreted as complex fluid escape features developed both through vigorous-explosive events and steady seepage. Their spatial distribution suggests that the NE-SW trending faults bounding the Ponza-Zannone structural high and the shallow fractured basement are favorable conditions for the upward migration of hydrothermal fluids. Moreover, we performed a detailed geochemical study to investigate the source of the hydrothermal fluids. The geochemical signature of the collected fluids provides information of active CO2-dominated degassing with a significant contribution of mantle volatiles, with measured 3He/4He values > 3.0 Ra that are similar to those recorded at Stromboli and Panarea volcanoes. The hydrothermal system produces volatiles that may originate from residual magma batches, similar to the Pleistocene trachytes cropping out in the SE sector of Ponza Island, that were probably intruded in the shallow crustal levels and never erupted. The discovery of the Zannone hydrothermal field updates the record of active hydrothermal areas of the Mediterranean Sea. Moreover, the recognition of several giant hydrothermal depressions characterized by a complex morphology is peculiar for the Mediterranean Sea.

NIUST AUVs - Expanding possibilities

The National Institute of Undersea Science and Technology (NIUST)s Underwater Vehicle Technology Center (UVTC) expanded their operational capabilities by acquiring a SeaBED class AUV in early 2009. This vehicle dubbed, Mola Mola after the Ocean Sunfish, is a superb addition to the UVTC, as it adds photographic capabilities at very slow moving speeds to the centers repertoire. The vehicle is designed to fly at speeds of 0.2 ms-1 about 3m above the seafloor, snapping high resolution digital images of the seafloor at preset intervals of 4 to 5 seconds. Normal mission behavior, programmed prior to launch of the vehicle, is to cover a certain area on the seafloor in a lawn mowing track, with parallel lines covering the entire area. At the end of the mission, geo-referenced photo mosaic maps of the target of interest on the sea floor are computed. The vehicle was deployed for several missions on the NASA vessel Liberty Star in the Bahamas. High resolution imagery of the bottom fauna and flora from depths too deep for deep divers to reach, provided insight into the distribution of Lion Fish in the coral reefs of the Bahamas. At the end of the field season engineering efforts were started to reorganize and replace certain hardware components to allow for improved navigation and data handling within the vehicles software architecture. They vehicle itself has since been modified from its original design, adding guided inertial navigation and improvements in its image acquisition process. Changes further include obstacle avoidance, GPS positioning and addition of a VHF radio beacon. In October 2009 combined efforts of both AUVs, the Mola Mola and the Explorer class Eagle Ray were needed in the Gulf of Mexico aboard the NOAA ship Nancy Foster, to locate and retrieve information about sunken ships of historic interest, some of which may have disappeared below the water surface of the northern Gulf of Mexico, almost 200 years ago. In a collaboration between NIUST, MMS and NOAA Office of Ocean Exploration and Research, targets identified in side scan sonar images, were selected and investigated by the AUVs. Eagle Ray, due to its large size and design features, performed initial multibeam surveys of the target areas, producing high-resolution maps of the seafloor. These maps were used to determine safe working areas for the Mola Mola, which was subsequently launched to take a continuous series of photographs in close proximity to the seafloor, producing a photo-mosaic map of the target area. Eagle Ray served as a platform for a mass spectrometer mapping of the Mississippi Canyon Block 118 Hydrate Mount as a part of the Gas Hydrate Observatory efforts. Results o this dive produced a high resolution spatial map of methane gas distribution 6m above the seafloor, discovering three new methane seeps in the area Continued mapping efforts in the Hudson Canyon together with the National Marine Fisheries Service and Rutgers University. High resolution multibeam data from the canyon revealed interesting never before seen detail and bottom features in this area. Enough data to spark new and diverse interest about sub bottom composition and marine live within the canyon.

New Insights on Failure and Post-failure Dynamics of Submarine Landslides on the Intra-slope Palmarola Ridge (Central Tyrrhenian Sea)

Newly collected multibeam and seismic data on the intra-slope Palmarola ridge show widespread pockmarks and landslide-related morphologies along its flanks. In detail, two main types of slope failures were identified: disintegrative-like and cohesive like landslides. The first type is characterized by a complex of small, nested scars affecting the steep and tectonically-controlled eastern flank of the ridge, suggesting a genesis related to retrogressive processes. The cohesive landslides affect the northern flank of the ridge and are characterized by larger scars, where material was not completely evacuated, and well-defined debris deposits at their base, with the development of pressure ridges. Tectonic activity and slope gradients represent the main controlling factors for the development of instabilities; moreover, we noted a relationship between pockmarks and landslide scars.

Progress toward a sea-floor observatory at a carbonate/hydrate mound in the northern Gulf of Mexico

The Gulf of Mexico hydrates research consortium has been designing a sea-floor observatory to monitor natural gas hydrates in the gulf of Mexico for almost ten years. The observatory will consist of seismo-acoustic receiving arrays, geochemical arrays in the lower water column and upper sediments as well as systems for observing microbial activity. Mississippi Canyon Lease Block 118 (MC118) in the northern part of the Gulf has been selected as the site of the observatory. A carbonate/hydrate mound approximately one kilometer in diameter occurs in the south-central portion of MC118 at a water depth of about 900 m. The surface morphology of the mound has been imaged by multi-beam bathymetric sonar from an autonomous underwater vehicle (AUV) operating 40 m above the sea floor, by video cameras deployed on and a few meters above the sea floor from surface vessels and by visual observations from manned submarines. Gravity and box cores have been collected for lithologic and bio-geochemical studies of the near-surface sediments on the mound. Microbial sulfate reduction, anaerobic methane oxidation, and methanogenesis are all important processes in the upper four meters of sediment. These microbial processes seem to control the diffusive flux of methane from the sediments into the overlying water column. The activity of microbes is also focused within patches or dasiahot spotspsila at the main, active mounds. This activity is primarily dependent upon an active fluid flux of hydrocarbon-rich fluids. The geochemical evidence suggests that the fluid flux waxes and wanes over time and that the microbial activity is sensitive to such change. The subsurface structure of the mound has been investigated by chirp-sonar profiles acquired by the AUV simultaneously with acquisition of the swath bathymetry data and by a psuedo-3D grid of high-resolution seismic profiles obtained using the surface-source/deep-receiver technique. Also, deep seismic 3D volumes obtained by the petroleum industry have been viewed. The surface of the mound is pocked by craters apparently formed during episodic fluid expulsion events. Gases venting from the mound, as well as those contained in outcropping hydrates, have been analyzed and found to be thermogenic, perhaps having migrated up faults from a deep, as yet undiscovered, petroleum reservoir. The deep seismic data show that the faults emanate from a salt diapir located some hundreds of meters below the mound. It was observed during a cruise in November, 2007, that craters and outcrops had changed significantly during the 14-month period since the previous cruise in September, 2006. The mound therefore has been shown to be sufficiently dynamic to warrant continuous monitoring over the five-to-ten years that the observatory is expected to be operational.