Norman L. Guinasso

Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico

We report observations that concern formation and dissociation of gas hydrate near the sea floor at depths of ∼540 m in the northern Gulf of Mexico. In August 1992, three lobes of gas hydrate were partly exposed beneath a thin layer of sediment. By May 1993, the most prominent lobe had evidently broken free and floated away, leaving a patch of disturbed sediment and exposed hydrate. The underside of the gas hydrate was about 0.2 °C warmer than ambient sea water and had trapped a large volume of oil and free gas. An in situ monitoring device, deployed on a nearby bed of mussels, recorded sustained releases of gas during a 44 day monitoring period. Gas venting coincided with a temporary rise in water temperature of 1 °C, which is consistent with thermally induced dissociation of hydrate composed mainly of methane and water. We conclude that the effects of accumulating buoyant force and fluctuating water temperature cause shallow gas hydrate alternately to check and release gas venting.

Natural oil slicks in the Gulf of Mexico visible from space

Natural oil seepage in the Gulf of Mexico causes persistent surface slicks that are visible from space in predictable locations. A photograph of the sun glint pattern offshore from Louisiana taken from the space shuttle Atlantis on May 5, 1989, shows at least 124 slicks in an area of about 15,000 km2; a thematic mapper (TM) image collected by the Landsat orbiter on July 31, 1991, shows at least 66 slicks in a cloud-free area of 8200 km2 that overlaps the area of the photograph. Samples and descriptions made from a surface ship, from aircraft, and from a submarine confirmed the presence of crude oil in floating slicks. The imagery data show surface slicks near eight locations where chemosynthetic communities dependent upon seeping hydrocarbons are known to occur on the seafloor. Additionally, a large surface slick above the location of an active mud volcano was evident in the TM image. In one location the combined set of observations confirmed the presence of a flourishing chemosynthetic community, active seafloor oil and gas seepage, crude oil on the sea surface, and slick features that were visible in both images. We derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate, downstream movement on the surface, half-life of floating oil, and threshold thickness for detection. Applying this equation to the lengths of observed slicks suggested that the slicks in the Atlantis photograph and in the TM image represent seepage rates of 2.2–30 m3 1000 km−2 d−1 and 1.4–18 m3 1000 km−2 d−1, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 m3 yr−1 (120,000 barrels yr−1).

Chemosynthetic Mussels at a Brine-Filled Pockmark in the Northern Gulf of Mexico

A large (540 square meters) bed of Bathymodiolus n. sp. (Mytilidae: Bivalvia) rings a pool of hypersaline (121.35 practical salinity units) brine at a water depth of 650 meters on the continental slope south of Louisiana. The anoxic brine (dissolved oxygen ≤0.17 milliliters per liter) contains high concentrations of methane, which nourishes methanotrophic symbionts in the mussels. The brine, which originates from a salt-cored diapir that penetrates to within 500 meters ofthe sea floor, fills a depression that was evidently excavated by escaping gas. The spatial continuity of the mussel bed indicates that the brine level has remained fairly constant; however, demographic differences between the inner and outer parts of the bed record small fluctuations.

Gulf of Mexico Hydrocarbon Seep Communities: VI. Patterns in Community Structure and Habitat

Communities of chemosynthetic fauna that depend on seeping oil and gas have been found in the Gulf of Mexico at approximately 45 sites between 88°W and 95°W and between the 350 and 2,200 m isobaths. Investigations suggest that the number of sites and the range of occurrence will increase with additional exploration. The dominant fauna consist of species within four groups: tube worms, seep mussels, epibenthic clams, and infaunal clams. These species co-occur to some degree, but tend to form assemblages dominated by a single group. Community development is closely coupled to the geological and geochemical processes of seepage.

Bacterial methane oxidation in sea-floor gas hydrate: Significance to life in extreme environments

Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. The study area is characterized by low water temperature (mean = 7 C), high pressure (about 5,400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH{sub 4}) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in {sup 13}C by as much as 3.8% PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37% SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO{sub 2}) is depleted in {sup 13}C by as much as 22.4% PDB. Hydrate-associated authigenic carbonate rock is also depleted in {sup 13}C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.

Pulsed oil discharge from a mud volcano

In this paper we document instances where change in the magnitude of natural oil seepage coincided with fluctuations of fluid temperature in a seafloor mud volcano. Oil slicks were detected floating near commercial oil fields in the northern Gulf of Mexico in a time series of six satellite synthetic aperture radar (SAR) images collected over a 10 month interval. The oil escaped naturally from a complex of fluid expulsion features at seafloor depths of about 600 m. One of these features was a 50-m-wide, mud- and brine-filled crater. Temperature in the crater fluctuated rapidly during an interval of ~1 yr (minimum 6.1 °C, maximum 48.3 °C, mean 26.1 °C, standard deviation 9.07). The areas of the oil slicks in the SAR images fluctuated repeatedly between 1000 ha. The largest oil slicks detected by SAR occurred along with the fastest increase in fluid temperature.

Emanation of radon-222 from marine sediments

Abstract The behavior of radon in the sea-floor region provides a useful test of theories which describe mass transport in sediments. We have made measurements of Rn-222 and Ra-226 in near-bottom waters and near-surface sediments at the same location. The distribution of radon in sediments can be described by a simplified diagenetic equation when advection, adsorption, and bioturbation are ignored. Sediment measurements show a radon deficit relative to radium emanation. A reasonable balance is found between integrated radon deficit in sediment and radon surplus in the overlying water. In most cores radium increased with depth in the top 10 cm of sediment. This implies that bioturbation and other mixing processes do not homogenize the radium concentration in the zone of diffusive radon loss, and that radium is diffusing out of the sediments. Radon leakage is less than that predicted by previous authors. Radon leakage depends upon the physical distribution of radium in marine sediments. We present a model that predicts leakage of 30–40% for normal deep-sea sediments, in agreement with measured values. Radon surplus in near-bottom waters depends upon the radium distribution, radon leakage, and effective diffusion coefficients. These in turn depend on the properties of the sediment, such as composition, accumulation rate, and porosity. As we learn how these factors interact we may be able to infer sedimentary features from measurements of radon in overlying waters.

COMBINING NEW TECHNOLOGIES FOR DETERMINATION OF PHYTOPLANKTON COMMUNITY STRUCTURE IN THE NORTHERN GULF OF MEXICO 1

In situ analysis of phytoplankton community structure was determined at five stations along the Texas Gulf coast using two instruments, the Fluoroprobe and FlowCAM. Results were compared with traditional methods to determine community structure (pigment analysis and microscopy). Diatoms and small nanoplankton (most likely haptophytes) dominated the phytoplankton community at all stations. Estimated chl concentrations for diatoms+dinoflagellates obtained via the Fluoroprobe were not significantly different for three of the five stations sampled when compared with HPLC‐chemical taxonomy analysis, whereas the concentrations of green algal and cryptophyte chl were overestimated. The FlowCAM estimates of overall nanoplankton and microplankton cell abundance were not significantly different when compared with epifluorescence microscopy, and recorded images of phytoplankton cells provided a representative population of the phytoplankton community at each station. The Fluoroprobe and FlowCAM, when used in tandem, are potentially capable of determining the general characteristics of phytoplankton community structure in situ and could be an important addition to biological observing systems in the coastal ocean.

Some improved techniques for measuring radon and radium in marine sediments and in seawater

Abstract By careful sampling and sample processing, radon and radium may be determined in close-spaced sediment segments. These determinations permit estimation of the radon deficit in sea-floor sediments. Collecting radium on manganese-treated fiber greatly simplifies analysis of near-bottom waters. Some improved radon separation and counting techniques are described. With these methods a single operator can process four samples for radon in 140 min. Overall efficiency (extraction-transfer-counting) is 88%. Blanks are negligible. Reproducibilities of 2.6% and 3.8% are achieved for radium and radon, respectively.

Quantitative evaluation of bioturbation rates in deep ocean sediments. II. Comparison of rates determined by 210Pb and 239,240Pu

Abstract Excess 210 Pb was determined in six abyssal cores from the Eastern Atlantic, Hatteras Abyssal Plain, and Puerto Rico Trench. A bioturbation rate for each core was evaluated using the mixing model of Guinasso and Schink. The rates were compared to mixing rates evaluated for the same cores using 239,240 Pu. In all cases, the excess 210 Pb-derived mixing rates were lower than the plutonium-derived mixing rates. 239,240 Pu was also present at greater depths than excess 210 Pb. One possible explanation for these observations is the association of plutonium with particles of greater food value which are bioturbated at faster rates.