The interpretation of 3D seismic data from Ship Shoal South Addition Blocks 349-358, Gulf of Mexico shows a complex interaction between salt, faults, and sedimentary strata. Reconstruction of the geometry of early Pliestocene (about 3.65 Ma) through recent salt and associated sediments reveals the evolution of a supralobal basin in the study area. The basin depocenter shifted from the northeastern part to the center of the study area through time. A small, bulb-shaped, salt-stock structure occurs in the northwest, and a salt sheet structure is present in the southeastern part of the study area. Those structures are part of a pennant-shaped structure bounded by counter regional faults trending northeastward. Salt movements created instability and triggered extensive faulting of the overlying strata. Three-dimensional reconstruction suggests that salt blocked the sediment during the early Pleistocene. The sediment was diverted around the salt high on both east and west sides of the salt body to the southwest and southeast. Stratigraphic interpretation of the interval between 1.35 Ma and 1.95 Ma led to the identification of a highstand systems tract (HST), a transgressive systems tract(TST), and two lowstand systems tracts (LST). The strata are developed normally in the depocenter area, whereas the strata at the basin margin were deformed by salt movement and faulting. Each systems tract is uniquely associated with a certain seismic facies. Three seismic facies were identified associated with LST, TST, and HST. Additionally, seismic sections reveal channel geometries in the LST. Seismic attribute analysis elucidates facies distribution in the systems tracts. Because of its ability to move, to divert sediment, to create instability, and to block sediment transport pathways, salt exercises the main control on the sedimentary processes in the study area.

The bioavailability of metabolizable organic matter within marine sediments is one of the more important driving mechanisms controlling benthic pelagic communities. Interactions between organic material and mineral surfaces within the sediment, such as adsorption, can cause organic matter to be unavailable for degradation by organisms; therefore for this study we have used the relationship of organic carbon-to-sediment surface area as an indicator of available organic carbon in northern Gulf of Mexico sediments. We have determined that these sediment interactions demonstrate a significant association with benthic fauna abundances; however they are not the most dominant environmental variables. It may be the combination of biogeochemical parameters, such as organic carbon content, sediment surface area, grain size, water depth and other geophysical variables, that is the ultimate control on the bioavailability of metabolizable organic matter in the northern Gulf of Mexico.

Satellite images of chlorophyll concentration in the surface waters of the Gulf of Mexico suggest a high degree of heterogeneity in the phytoplankton biomass. The causes of this variability and the amount of variability in the phytoplankton community structure are not well understood. The physical and chemical conditions of a specific environment can influence phytoplankton community structure by selecting for those phytoplankton species able to survive within that environment. Varying salinity and temperature characteristics give water masses distinct surface water density signatures. This study examined the relationship between phytoplankton biomass, community structure, and different water mass properties by measuring chlorophyll a and algal group concentration across frontal zones. Continuous salinity and temperature measurements were used to calculate continuous density along transects during four cruises on the R/V Gyre between summer 2002 and spring 2004. Frontal zones were identified as areas of sharp density change where σt changed by 1.5 points over a distance of 1 km. Density fronts that coincided with visible temperature fronts (satellite AVHRR images) were selected for biomass and community structure analysis. Discrete water samples were analyzed using fluorometric analysis (total chlorophyll a concentration) and HPLC analysis (photosynthetic pigments). Community composition for discrete samples was determined using CHEMTAX and these values were used to interpolate community composition. Phytoplankton biomass and community structure were examined at a total of 21 density fronts. Unlike previous studies of frontal zones, phytoplankton biomass (measured as chl a concentration) was not significantly higher within frontal zones than in adjacent waters at any of the 21 fronts. Community composition (measured as algal group abundance and diversity) was significantly different between the front and at least one adjacent water mass at front 2 during summer 2002, at front 6 during summer 2003, at front 3 during fall 2003, and at front 3 during spring 2004. Both biomass and community composition were significantly different between fronts at all front pairs during summer 2002. The results of this study suggest that density fronts are not biologically important features in the northern Gulf of Mexico. Lack of high phytoplankton biomass at fronts in the Gulf of Mexico could indicate that unique physical, chemical, or biological processes are occurring.

The distribution of benthic polychaetes in the Mississippi Canyon was examined to evaluate impacts of environmental variables on species assemblages. Environmental variables considered included depth, bathymetric slope, hydrographic features, sediment grain size, food availability and sediment contamination. Samples were collected using GOMEX boxcorer. Density decreased with increasing depth exponentially. Diversity exhibited a unimodal pattern with depth with a maximum value in the intermediate depth range (about 1269 m). Deposit feeders were the most abundant feeding guild. Both the feeding guilds and faunal composition could be divided into three groups along the depth gradient: shallow (300 – 800 m), intermediate (800 – 1500 m) and deep (> 1500 m). Results of statistical analyses revealed that depth was the most important determinant in organizing polychaete assemblages in the study area. The Mississippi Canyon and the Central Transect (a non-canyon area) were found not contaminated by trace metals or Polynuclear Aromatic Hydrocarbons (PAHs) in sediments, although the highest PAHs concentration occurred at the head of the Canyon, MT1. The mean density was higher in the Mississippi Canyon (1668 N/m2) than in the Central Transect (979 N/m2), while the mean diversity in the Canyon (ES(100) = 26.9 ) was lower than the Central Transect (ES(100) = 33.1). Large amounts of terrigenous input from the Mississippi River to the Canyon could enhance polychaete density and accelerate competitive exclusion, and thus lead to lower diversity. The faunal composition was significantly different between the two transects, with higher species richness in the Mississippi Canyon (301 species). This could be attributed to structure complexity in the Mississippi Canyon. The distribution of feeding guilds was similar between two transects. The differences observed in polychaete assemblages between two transects may be largely due to high terrigenous sediment and organic matter input to the Mississippi Canyon by the Mississippi River.

Harmful algal blooms (HABs) in the Gulf of Mexico are primarily caused by dense aggregations of the dinoflagellate species, Karenia brevis. Karenia brevis produces a highly toxic neurotoxin, brevetoxin which has been shown to cause Neurotoxic Shellfish Poisoning (NSP) and respiratory distress in humans in addition to a wide range of negative impacts upon natural ecosystems. Karenia mikimotoi is a co-existing species present during K. brevis blooms. K. mikimotoi has caused major HAB events in other parts of the ocean, but has not been recognized as a major contributor to toxicity of blooms in the Gulf of Mexico. K. brevis and K. mikimotoi have both been associated with the presence of unidentified hemolytic toxins. Production of hemolysins has not previously been investigated for either species to date in the Gulf of Mexico. Presence of hemolysins may affect toxicity and the overall impact of HABs. Therefore, detection of hemolysins is imperative for accurate identification of potential harmful impacts of such blooms. The primary goal of this research is to define whether either species is capable of producing hemolytic activity independent of brevetoxin activity; and to identify if there is significant differentiation between a variety of clonal isolates regarding toxicity and growth rate when subjected to variable experimental conditions.

Over the past five years, gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses of phospholipid fatty acids (PLFAs) has been increasingly used to link organic matter (OM) sources with sedimentary bacteria. This technique has been applied across diverse estuarine and coastal sediments, including lower Laguna Madre, TX, an oligotrophic, coastal lagoon dominated by a single OM source, seagrasses; shelf stations, a eutrophic coastal region receiving multiple sources of OM, hypoxic regions that occur seasonally and deep slope and abyssal plain sediments of the Gulf of Mexico (GOM). Previous reports using the Laguna Madre data as examples, have been used to make comparisons of PLFA 16:0 and PLFA 15:0 isotope ratios and PLFA 16:0 and total organic carbon isotope ratios. Deviations from the 1:1 line in the former indicate living or recently senescent sources of organic matter are not predominantly bacterial. Deviations from the 1:1 line in the latter indicate living or recently senescent sources of organic matter differ isotopically from detrital or older OM in sediments. Prior to the work of Goni et al. (1998), carbon isotope ratios of OM in GOM sediments were interpreted as marine in origin. Based on a series of geochemical measurements, Goni et al. suggested that GOM sediments are largely composed of terrestrial organic carbon (OCterr). Furthermore, They went on to show that shelf and slope sediments were primarily C3 and C4 respectively. I report on the preferential utilization of autochthonous OM by sedimentary bacteria at the sediment surface and the shift to recalcitrant, terrestrially derived OM with depth.

Shallow foundations are used when designing subsea structures on the seafloor of deep water Gulf of Mexico. In order to design these subsea shallow foundations it is important to understand the behavior of the undrained shear strength of the shallow soils (i.e. upper ten to twenty feet of soil). The objective of this research is to analyze a database of soil data from deep water Gulf of Mexico originally produced by Cheon (2011) with a focus on shallow soils. The purpose of this analysis is to gain a better understanding of the soil and how it will be usable with regards to shallow foundation design. The methodology of this analysis involves studying raw data collected from different measurements taken to aid in the creation of design profiles of undrained shear strength versus depth. Within the existing database there are 18 locations with a high resolution of point data from in-situ tests (Halibut Vane) and non in-situ tests (Minivane and Torvane) that provide the clearest picture of undrained shear strength in the shallow region. The data shows that the design profiles originally created for these locations for deep foundations are generally not representative of the strength in the shallow region. They also show that in-situ test data show more variability than non in-situ data. There are also 25 Cone Penetration Tests in the existing database that show very high resolution data in the shallow region. These Cone Penetration Tests also indicate a crust that appears to be about 1 ft thick and exists along the edge of the continental shelf. Recommended future activities to build upon this work include re-evaluating the design profiles at these 43 locations at which high resolution studies have been performed in the shallow region, collecting these design profiles as well as any new design profiles and organizing them into a new database focused on shallow soils, generating a new generic profile base on the data within the new database, and creating a model that uses spatial variability analysis to calculate undrained shear strengths at new locations based on the data in the database. / text

This study defines depositional environments and constructs the sequence stratigraphic framework of the lower Miocene Oakville Formation and the basal part of the middle Miocene Lagarto Formation in the Carancahua Bay area. The Early Miocene of the northwestern Gulf of Mexico represents a tectonically stable period with a high sediment influx. The analysis is based on a data set composed of 45 well logs and 200 mile2 area 3D seismic volume. The study interval was divided into five depositional sequences 1-5 that encompass 0.6-2.5 My. LST, TST, and HST systems tracts were recognized by stacking patterns and bounding surfaces. Sequence thickness increases from sequence 1 to 3 and displays reverse thickness trends from sequence 3 to 5, implying changes in accommodation space relative to sediment supply, beginning with high rates of accommodation and evolving into low accommodation rates relative. Besides type-1 depositional sequence which forms during relative sea-level fall below the shoreline break, regressive units of T-R sequence model were also defined and delineated. The interval contains four regressive units, R-Unit1-4. The R-Unit net sandstone maps exhibit the same characteristic of a dip-oriented source of delta-plain origin and a delta-front depocenter basinward. Integration of well log patterns, sandstone dispersal trends from net sandstone maps and seismic stratal slices led to interpretation of depositional environments in each sequence. LST deposits are represented mainly by incised-valley fill facies. TSTs are composed predominantly of retrogradational barrier/tidal-inlet facies, whereas other TSTs contain lagoonal and reworked deltaic systems. HST1 is composed mainly of fluvial-dominated deltaic systems, whereas deltaic systems in other HSTs exhibit wave-influenced deltaic and strandplain depositional systems. The integrated methodology reveals depositional facies variations in contrast to previous work that interpreted these deposits as shorezone systems. During LSTs coarse-grained sediments bypassed shelf through incised valley systems to a downdip depocenter. More sandy sediments were stored on shelf as deltaic and strandplain deposits during HSTs. In contrast to the others, destructive process occurred in TSTs and reworked sandy sediments, for example from delta fronts to barrier bar and lagoonal facies. Submarine fans form by sediments transported through incised-valley systems and delta fronts are commonly good reservoirs. Hence, presence of such depositional facies in the study area might be genetically linked to exploration targets. / text

The number of subsea production systems placed in deepwater locations in the Gulf of Mexico (GOM) has increased significantly in the last ten to fifteen years. Currently, API-RP2A (2000 a,b) designates the GOM as a low seismic zone, and thus does not require seismic effects to be considered during the design process. However, there have been a number of seismic events with Richter magnitudes between 3.0 and 4.9 that have occurred in this region. As a result, questions have been raised regarding the seismic performance of deepwater subsea systems. This thesis presents an analytical parametric study where a prototype subsea structure was selected based on a survey of subsea systems. The baseline analytical model consisted of a single casing embedded in soft clay soils, which supported a lumped mass at a cantilevered height above the soil. A number of the model characteristics were varied in the parametric study to simulate the structural response of a range of subsea structures. This thesis discusses the impact of API-RP2A Zone 1 and 2 design seismic demands for the performance of subsea structures. The results from the subsequent analyses show that the stresses and deflections produced by the Zone 1 and 2 peak ground accelerations fall within the allowable limits.

Although there has been considerable work on the role of nutrient-derived (mostly nitrate) primary production in fueling hypoxia in northern Gulf of Mexico, very little is known about the relative importance of autochthonous versus allochthonous sources of dissolved organic matter (DOM). Moreover, even less is known about the importance of dissolved organic nitrogen (DON), a critical component of DOM (along with DOC) in supporting hypoxia in this region. Most nitrogen in marine organisms exists in the form of amino acids. Changes in the spatial and temporal distribution of amino acids in the Mississippi River Plume have been shown to be important in the dynamic microbial cycling in the plume. In this study, concentrations of amino acids, DON and DOC were linked with hydrography data (e.g., DO, salinity, temperature, fluorescence) to determine how these sources of DOM are related to seasonal and diurnal changes in hypoxia on the inner Louisiana shelf. The general working hypothesis of this work was that allochthonous and autochthonous sources of DOM on the Louisiana shelf have been largely underestimated in their role in fueling hypoxia in northern Gulf of Mexico. A positive correlation between DOC, DON and fluorescence demonstrated that the main source of both DOC and DON was likely to be in situ phytoplankton production. Surface waters in the near-field showed this relationship more than at stations to the west where a sub-surface chlorophyll peak near the pycnocline may also provide a source of DOC and DON in bottom waters. DFAA always had relatively low concentrations at all water depths, which further supports prior work which has shown rapid cycling and high consumption rate of DFAA by heterotrophic bacteria. In addition to biotic controls, selective adsorption of DFAA likely contributed to the dominance of aspartic and glutamic acids at our stations. Hypoxia was generally observed in bottom waters in both spring and summer 2010. Dissolved oxygen generally revealed a negative correlation with nitrate+nitrite concentrations. Based on other work, one possible reason for such linkages may be from NH4+ released from dissimilatory nitrate reduction to ammonium (DNRA). Another possible reason may be the high degradation of labile DOM (such as DFAA) as shown by high respiration in bottom waters in prior work by Amon and Benner (1998).