Offshore Mexico Research Articles

Abundance and mtDNA differentiation of humpback whales (Megaptera novaeangliae) in the Shumagin Islands, Alaska

Despite extensive research on humpback whales (Megaptera novaeangliae (Borowski, 1781)) in parts of the North Pacific, little research has focused on the whales feeding in coastal waters west of Kodiak Island in the Gulf of Alaska. To extend research westward in the North Pacific, small-boat surveys were conducted near the Shumagin Islands during the summers of 1999–2002. Photographs of the natural markings of humpback whales were collected, representing 413 sightings of 171 individual whales. Small samples of skin tissue were collected from 20 individuals, including two mother–calf pairs, for sex identification and comparison of mtDNA haplogroups with previously published results from surveys in other regions of the North Pacific. Individual identification photographs were used in mark–recapture analysis to estimate abundance for the Shumagin Island region. The best estimate was given by a modified Jolly–Seber method: N = 410 (95% CI: 241–683) for 2002. Comparison of photographs with archived photographs from throughout the North Pacific revealed four migratory destinations for 13 of the Shumagin Islands whales: Hawai'i, Japan, offshore Mexico, and coastal Mexico. The frequencies of mtDNA haplogroups differed significantly from those in three other sampled feeding grounds: California, southeastern Alaska, and Prince William Sound. The haplogroup frequencies and migratory destinations of individuals suggested an affinity with the Hawaiian wintering ground but data are insufficient to associate whales off the Shumagin Islands with any surveyed breeding ground.

Classification of Offshore Gulf of Mexico Oils and Gas Condensates (1)

Five groups of petroleums can be recognized in the offshore Gulf of Mexico on the basis of carbon isotope and sulfur isotope ratios, sulfur content, and relative abundances of nickel and vanadium. Of 355 oils analyzed, 341 (96.1%) are evidently closely related, and cannot be subdivided on the basis of carbon or sulfur isotopes. We suggest that these oils represent a single broad superfamily. However, two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium: group 1A, (sigma){13}C = -26.5 +/- 0.4 parts per mil, (sigma){34}S = -5.6 +/- 2.4 parts per mil, %S = 0.25 +/- 0.15%, V/(Ni + V) = 0.27 +/- 0.10; and group 1B, (sigma){13}C = -26.5 +/- 0.3 parts per mil, (sigma){34}S = -5.9 +/- 1.2 parts per mil, %S = 0.73 +/- 0.43, V/(Ni + V) = 0.67 +/- 0.07. Group 1A oils are broadly distributed across the Louisiana and adjacent Texas continental shelf, whereas group 1B oils are found along the Louisiana shelf edge and continental slope. Group 1B oils have substantial genetic affinities to those from the Campeche Shelf, offshore Mexico, of Mesozoic origin. The remaining 14 analyzed petroleums include supermature condensates from offshore Texas, and oils from offshore southern Florida believed to be cogenetic with those from the Jurassic Smackover Formation. A three-dimensional appraisal of geohistory suggests that petroleum generation began in the Eocene in the region of the Rio Grande delta, progressed coastwise to the Mississippi delta area by the Miocene, and then occurred in regions of progres ively deeper water during the Pliocene-Pleistocene.

Methane Hydrate in Slope Sediments on West Coast of Central America: ABSTRACT

Offshore Mexico and Guatemala slope sediments are classic sites of deep-sea gas hydrate occurrence. Gassy, frozen sediment was recovered in cores from this region on Legs 66, 67, and 84 of the Deep Sea Drilling Project. In addition, a massive 3 to 4-m thick layer of nearly pure methane hydrate at a depth of 250 m was cored on Leg 84 and preserved for study. The gas from the hydrate is 99+% methane with a few tenths percent carbon dioxide and traces (10-4v/v) of ethane. Most of the sites with gas hydrate in the sediments have methane with ^dgr13C of -70 to -60 ^pmil, indicating origin from methane-generating bacteria. The massive gas hydrate contained methane with ^dgr13C of -40 ^pmil, and the surrounding sediment had bicarbonate in the por water with ^dgr13C of +35 ^pmil. The 75 ^pmil separation in ^dgr13C between coexisting methane and bicarbonate is consistent with kinetic fractionation during bacterial reduction of carbon dioxide to methane, with continuous replenishment of carbon dioxide by fermentation processes. The areal extent of the massive gas hydrate is not known, but the single point yields a gas-in-place estimate of 5.2 × 108m3/km2 or 48 bcf/mi2. End_of_Article - Last_Page 245------------

Early Diagenesis of Sands and Sandstones from Middle America Trench and Trench Slope, Offshore Mexico and Guatemala: ABSTRACT

Siliciclastic sediments from the northern and southern segments of the Middle American Trench are distinctive both petrologically and diagenetically. Samples of terrigenous sands recovered in piston and DSDP cores from the continental margin of southern Mexico are primarily arkose and lithic arkose of plutonic and metamorphic provenance. Accessory constituents and diagenetic features are useful criteria for distinguishing sands from different tectonic provinces along the Mexican (northern) segment of the trench. Basal, early Miocene sandstones from the upper- and middle-slope regions (continental block) contain abundant skeletal grains and are cemented by calcite or gypsum. Early Pliocene to middle Miocene sands from the accretionary wedge are weakly lithified and contain fractured framework grains. Unlithified trench sands of Quaternary age have undergone significant pore-space reduction at very shallow burial depths. Samples of Holocene terrigenous sands recovered in piston cores offshore from central Guatemala (southern segment) are feldspathic litharenite and litharenite of volcanic provenance. Authigenic pyrite is ubiquitous in these sands, and pore-filling phillipsite occurs locally. Partial dissolution of glass fragments, pyroxene, and plagioclase has occurred in sands from every environment sampled. The differences observed in sands from the two segments of the Middle America Trench may also characterize siliciclastic sediments deposited along other segmented convergent margins. Within a trench segment, changes in sand composition with time could indicate intermittent volcanic activity and changes in relative sea level. End_of_Article - Last_Page 505------------

Tectonic Evolution of the Southern Gulf of Mexico

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth. Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm 3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer. In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba. The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatan and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.