Browsing by Subject "Northern Gulf of Mexico"
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Item Observations and models of venting at deepwater Gulf of Mexico vents(2012-08) Smith, Andrew James; Flemings, Peter Barry, 1960-; Fulton, Patrick M.; Hesse, Marc; Liu, XiaoliNatural vents in the Gulf of Mexico are actively expelling water and hydrocarbons. They are ubiquitous along continental margins, and I characterize a single vent in the Ursa Basin at leaseblocks MC852/853. Seismic data reveal that the vent is elevated ~75 meters above the seafloor and is roughly circular with a ~1.2 km diameter. A transparent zone centered underneath the vent extends to ~1500 meters below seafloor; this zone is commonly interpreted to record the presence of gas. There is a strong negative polarity seismic reflection that rises rapidly at the vent’s boundaries and is horizontal within a few meters of the seafloor beneath the vent edifice. I interpret that this reflection records a negative impedance contrast, marking the boundary between hydrate and water above and free gas and water below: it is the bottom-simulating reflector. Salinities beneath the vent increase from seawater concentrations to >4x seawater salinity one meter below seafloor. Temperature gradients within the vent are ~15x the background geothermal gradient. I model the coexistence of high salinity fluids, elevated temperature gradients, and an uplifted bottom-simulating reflector with two approaches. First, I assume that high salinity fluids are generated by dissolution of salt bodies at depth and that these hot, saline fluids are expelled vertically. Second, I model the solidification of gas hydrate during upward flow of gas and water. In this model, free gas combines with water to form hydrate: salt is excluded and heat is released, resulting in the generation of a warm, saline brine. The two models result in predictable differences of salinity and temperature. A better understanding of the hydrogeological processes at vent zones is important for quantifying the fluxes of heat and mass from submarine vents and is important for understanding the conditions under which deep-sea biological vent communities exist.Item Ocean biogeochemistry in the northern Gulf of Mexico, the East/Japan Sea, and the South Pacific with a focus on denitrification(2012-05) Kim, Il Nam, 1976-; Min, Dong-Ha; Macdonald, Alison M.; McClelland, James W.; Gardner, Wayne S.; Liu, ZhanfeiOcean nitrogen fixation and denitrification are crucial nitrogen source and sink mechanisms for the global ocean environment. While recent studies have reported that oceanic denitrification has increased over the last few decades, others have suggested that global ocean nitrogen fixation rates have been underestimated, and still others that anthropogenic perturbations have altered the global nitrogen cycle. This implies that the current estimates of the oceanic nitrogen inventory are incomplete and they need to be revised with more information. In addition, current denitrification estimates need to be reexamined due to their large associated uncertainties. Thus, I have conducted research estimating denitrification rates in three different locations: the northern Gulf of Mexico (GOM), the East/Japan Sea (EJS), and the South Pacific: from coastal to marginal to open ocean scale in different oceanographic conditions. Denitrification rates in the bottom layer (including bottom waters+sediments) at the shallow and often hypoxic northern GOM ranged from 103-544 [mu]mol N m⁻² d⁻¹ (=1.4 to 7.4 Gg N mon⁻¹ with area=3.24x10¹⁰m²), and were controlled not only by biogeochemical factors (i.e. organic matter supply and remineralization), but also by physical factors (i.e. stratification and relative contributions from different water masses). Despite high dissolved oxygen concentrations, the significant decrease in nitrate concentrations below the expected levels, low N/P ratio (<12.4), and deep nitrite peak in the bottom layer indicate a presence of denitrification in EJS, confined at the Tatar Strait and the Ulleung Basin areas. The estimated denitrification rates range from 0.3 to 33.2 [mu]mol N m⁻² d⁻¹, and was comparable to the directly measured denitrification rates from sediment samples. The high-quality repeat hydrographic datasets observed at 32°S of the South Pacific Ocean offer an opportunity to estimate water column denitrification rates on a basin-scale in the open ocean away from the Eastern Tropical Pacific oxygen minimum zones. The mean water column denitrification rates in the oxygen minimum layer of P06 line (32°S) were estimated to range between 7.1 and 18.5 [mu]mol N m⁻² d⁻¹. The results imply that, although very small at any particular site, once integrated over a basin-scale, the open ocean water column denitrification can be a significant component of the oceanic nitrogen budget. Denitrification is subject to seasonal, decadal and possibly climate scale variations. While it is commonly estimated at the oxygen minimum zones or sediments, denitrification is not merely confined to such regions only, and small amounts of denitrification occur in other oceanic parts. Once integrated, it may be quantitatively significant for the world's oceans. Denitrification is playing a significant role in local, regional, and global ocean scales. In the future, we need to consider variability of denitrification in coastal regions, and to investigate denitrification in unexpected and unexplored regions, in order to improve our knowledge on global oceanic mass balance.Item The Role of Particulate Matter in the Development of Hypoxia on the Texas-Louisiana Shelf(2013-07-31) Cochran, Emma MaryIn the northern Gulf of Mexico, hypoxia occurs annually during the summer on the Texas-Louisiana shelf. This study examines the distribution of particulate and dissolved components relative to hydrography, to better understand the processes controlling the development of hypoxia. Particulate matter on the Texas-Louisiana Shelf has three major sources ? river plumes, primary production, and resuspended sediments. The sources and processes controlling distribution and transport of particles are investigated using optical proxies (backscattering, chlorophyll fluorescence, Colored Dissolved Organic Matter fluorescence (CDOM)), temperature, salinity, dissolved oxygen (DO), and in-situ sampling during June and August 2011 cruises of the Mechanisms Controlling Hypoxia program (hypoxia.tamu.edu). Discrete samples of particulate matter (PM) and particulate organic carbon (POC) concentration were obtained for analysis and calibration of optical instruments interfaced with a profiling CTD, a towed undulating CTD (Acrobat), and the ship?s flow-through system along the shelf from south of Galveston, Texas, to east of the Mississippi delta. The results of this study support a previously hypothesized concept of three primary areas of organic and inorganic particle composition and processes that dominate those areas ? river-dominated water, highly productive surface waters, and clear, nutrient-poor low-productivity surface waters. The distribution and bulk composition of particulate matter in the northern Gulf of Mexico, plus the distribution of chlorophyll fluorescence and CDOM suggest that subpycnocline primary production plays a role in determining oxygen concentration in subpycnocline waters away from the river-dominated water.