Gulf Of Mexico Slope Research Articles

Stelis breviloba (Orchidaceae, Pleurothallidinae), a new species from Mexico

Background: Stelis subgen. Physosiphon is basically a Meso-American group, and its best-known species is Stelis emarginata, characterized by its reddish orange flowers and distribution along the Gulf of Mexico slope in Mexico and Guatemala. Morphologically similar specimens from the Sierra Madre del Sur, on the Pacific slope of Guerrero and Oaxaca could represent a different and undescribed taxon.
 Question: Are the Sierra Madre del Sur populations similar to Stelis emarginata a different species?
 Study site: Sierra Madre del Sur in Guerrero and Oaxaca, Mexico.
 Methods: The description of the new species was based on live and pressed specimens; a line drawing and photographs were made from fresh material; its known localities were georeferenced and displayed on a map of Mexico. A morphological comparison of the new taxon with similar species was done based on specialized literature.
 Results: Stelis breviloba is described and illustrated as a new species from southern Guerrero and Oaxaca. It differs from S. emarginata by its shorter plants, less floriferous racemes, smaller flowers, oblong petals, and its allopatric distribution, restricted to the Pacific slope of the Sierra Madre del Sur. A key for the identification of the Mesoamerican species of Stelis subgen. Physosiphon is provided.
 Conclusion: Stelis breviloba is an endemic of the Sierra Madre del Sur and probable vicariant species of S. emarginata. Most species of Stelis subgen. Physosiphon (six out of eight) are present in the Mexican flora.

Impact of Particulate Matter (PM10 and PM2.5) from a Thermoelectric Power Plant on Morpho-Functional Traits of Rhizophora mangle L. Leaves

A conventional thermoelectric plant (TP) in the sandy bar of the Tampamachoco Lagoon (Gulf of Mexico slope) emits particulate matter (PM) transporting trace metals that affect a mangrove forest. Wind transports the emission plume from north to south in the northerly wind season (NWS); the dry season (DS) showed calm periods. We analyzed whether PM2.5 and PM10 emissions from the TP and their trace metals impact Rhizophora mangle leaves. The experimental design included three sampling sites along the main lagoon axis (north to south) during the NWS and DS. Mangrove leaves were collected; PM was obtained with a cascade impactor and trace elements were analyzed by atomic absorption spectrophotometry. Leaves were measured and tested for metal and chlorophyll content, and for metal detection with SEM-EDX. Calm periods in the DS promote high atmospheric PM concentrations. Wet deposition in the NWS caused the highest trace metal deposition on mangrove leaves. A north-to-south gradient was identified on the mangrove forest, being the south site of the lagoon where lower chlorophyll and leaf area, higher stomatal width and density, and higher Cd concentrations were recorded. The morpho-physiological modifications observed on mangrove leaves affect functions such as photosynthesis and gas exchange.

Mitochondrial, morphological and environmental data partially support current subspecies designation inAmazilia yucatanensishummingbirds

AbstractHistorical geological events and Pleistocene climatic fluctuations have played important roles in shaping distribution and population differentiation across taxa. The buff-bellied hummingbird (Amazilia yucatanensis) is widely distributed along the Gulf of Mexico slope and the Yucatan Peninsula. Here, we obtained measurements and sequenced two mitochondrial DNA fragments from currently recognized subspecies: Amazilia yucatanensis yucatanensis (YUC), Amazilia yucatanensis cerviniventris (CER) and Amazilia yucatanensis chalconota (CHA). Additionally, we tested for their genetic and morphological differentiation, demographic expansion, palaeoclimatic distribution and niche overlap. Our results reveal genetic differentiation between two groups of populations: (1) from the Yucatan Peninsula to Veracruz (YUC+CER); and (2) from Veracruz to Tamaulipas (CHA). Neutrality tests and Bayesian skyline plots suggest past demographic expansion without changes in the effective population size over time. The potential distribution was fragmented at the Trans-Mexican Volcanic Belt and expanded northwards during the Last Glacial Maximum and Mid-Holocene to current conditions. Niche overlap was higher between YUC and CER. The environmental space occupied by subspecies was more similar to each other than expected by chance but significantly non-equivalent. Our results provide new insight on the distribution of this widespread hummingbird species and suggest that fragmentation during glaciations and differences in habitat have played a role in the recent diversification.

Variation in patterns of fish assemblage and their environmental correlates in a tropical river basin from the Gulf of Mexico slope

Abstract Understanding patterns of freshwater fish assemblage structure is key to protect them from ongoing human-induced threats to aquatic biodiversity. Yet, studies on associations between fish assemblages and habitat are lacking from many areas of high diversity in Middle America. We assessed fish assemblage structure and environmental associations from a portion of the Lacantún River sub-basin (Usumacinta River, Chiapas, Mexico). Based on environmental data and 17,462 individuals (56 species, 46 genera, and 22 families) captured from 13 sites sampled between 2017–2019, we found that stream order, distance to the Usumacinta, forest cover, temperature, and dissolved oxygen are key to explaining assemblage composition. Four clusters were found via multivariate regression tree analysis, with stream order and dissolved oxygen as defining variables. Our findings suggest that fish communities remain spatially structured even at small scales, in association to environmental gradients among habitats.

A population genetics study of three native Mexican woody bamboo species of Guadua (Poaceae: Bambusoideae: Bambuseae: Guaduinae) using nuclear microsatellite markers

Background: Sporadic flowering contributes significantly to genetic diversity and connectivity among populations. Woody bamboos present sporadic or gregarious flowering patterns with long flowering cycles. In this study, we analyze the genetic diversity of three Guadua species distributed along the Gulf of Mexico slope that have different patterns of flowering. 
 Questions: (1) Are the three Guadua species genetically differentiated? (2) Does the vulnerable species G. inermis have low levels of genetic diversity? (3) What is the relative contribution of geographic and environmental factors to the genetic structure of G. inermis ?
 Species studied: Guadua inermis , G. amplexifolia and G. tuxtlensis 
 Study site and dates: During 2014 and 2015, we collected samples of G. inermis in Puebla and southeastern Mexico, G. amplexifolia in Veracruz and Oaxaca, and G. tuxtlensis in southern Veracruz.
 Methods: We successfully amplified five of nine SSR markers, and genotyped a total of 155 samples.
 Results: The three Guadua species were genetically differentiated. For G. inermis , we found high levels of population genetic diversity, which are relatively higher than those of other monocot species. Genetic differentiation was high and three groups were detected: north, central and south. We found a significant association between genetic distances and the maximum temperature of the warmest month, but not with geographic distance. 
 Conclusions: Our study is the first to analyze levels of genetic diversity in Mexican bamboos and confirms their taxonomic identity. G. inermis has a strong genetic structure, even when populations are geographically close.

Genetic structure and historical and contemporary gene flow of Astyanaxmexicanus in the Gulf of Mexico slope: a microsatellite-based analysis.

Background Astyanax mexicanus from the river basins of the Gulf of Mexico slope are small freshwater fish that usually live in large groups in different freshwater environments. The group is considered successful due to its high capacity for dispersal and adaptation to different habitats, and the species present high morphological variability throughout their distribution in Mexico. This has produced the most extreme morphotype of the group; the hypogeous or troglobite, which has no eyes or coloration, and is probably the cause of taxonomic uncertainty in the recognition of species across the entire range. Most studies of A. mexicanus have mainly focused on cave individuals, as well as their adjacent surface locations, providing an incomplete evolutionary history, particularly in terms of factors related to dispersal and the potential corridors used, barriers to gene flow, and distribution of genetic variability. The aim of the present study is to determine the population structure and the degree and direction of genetic flow in this complex taxonomic group, incorporating geographic locations not previously included in analyses using microsatellite loci. Our aim is to contribute to the knowledge of the intricate evolutionary history of A. mexicanus throughout most of its range.MethodsThe present study included a set of several cave and surface locations of A. mexicanus, which have been widely sampled along the Gulf of Mexico slope, in a genetic population analysis using 10 microsatellite loci.ResultsTen genetic populations or lineages were identified. In these populations, gene flow was recorded at two time periods. Historical gene flow, both inter and intra-basin, was observed among surface populations, from surface to cave populations, and among cave populations, whereas recording of contemporary gene flow was limited to intra-basin exchanges and observed among surface populations, surface to cave populations, and cave populations.

Triple trouble? Invasive poeciliid fishes carry the introduced tilapia pathogen Gyrodactylus cichlidarum in the Mexican highlands

As part of ongoing surveys of the gyrodactylid parasite fauna of freshwater fishes in Mexico, we recorded the infection of three species of poeciliids (Poecilia mexicana, Poeciliopsis gracilis, and Pseudoxiphophorus bimaculatus [syn.=Heterandria bimaculata]) with Gyrodactylus cichlidarum, a monogenean parasite of cichlid fishes, which has been co-introduced globally with its translocated, African “tilapia” hosts. This tilapia pathogen was found on poeciliid fishes both within their native distribution range in the Gulf of Mexico slope, as well as on invasive species artificially introduced to the Mexican highlands, to rivers draining into the Pacific Ocean. Identity of G. cichlidarum was confirmed by morphological and molecular analyses. Prevalence and abundance of infection were low, but this is the first record of G. cichlidarum infecting poeciliids (Cyprinodontiformes), which are distantly related to this parasite’s primary cichlid fish hosts (Perciformes). This study provides evidence that G. cichlidarum, a recognized pathogen which has been co-introduced globally with its cichlid fish hosts for aquacultural purposes, is able to infect non-related poeciliid fishes inhabiting water bodies adjacent to tilapia farms, thereby potentially increasing its ability to disperse between farms and different river basins. It is of particular concern that G. cichlidarum was found on poeciliids, as these invasive fishes have been introduced worldwide and could act as carriers for this parasite known to induce significant mortality of farmed tilapias − globally, the second most important freshwater aquaculture fish group, after the carps.

Morphological and molecular description of eight new species of Gyrodactylus von Nordmann, 1832 (Platyhelminthes: Monogenea) from poeciliid fishes, collected in their natural distribution range in the Gulf of Mexico slope, Mexico

Eight new species of Gyrodactylus are described from Poecilia mexicana, Poeciliopsis gracilis, Pseudoxiphophorus bimaculatus [syn. = Heterandria bimaculata], and Xiphophorus hellerii collected in the Nautla and La Antigua River Basins in Veracruz, and in the Tecolutla River Basin in Puebla, Mexico. Analyzing the morphology of the marginal hooks, Gyrodactylus pseudobullatarudis n. sp. and Gyrodactylus xtachuna n. sp. are both very similar to Gyrodactylus bullatarudis; Gyrodactylus takoke n. sp. resembles Gyrodactylus xalapensis; Gyrodactylus lhkahuili n. sp. is similar to Gyrodactylus jarocho; and both Gyrodactylus microdactylus n. sp. and Gyrodactylus actzu n. sp. are similar to Gyrodactylus poeciliae in that all three species possess extremely short shaft points. A hypothesis of the systematic relationships of the eight new Gyrodactylus species and some of the known gyrodactylids infecting poeciliids was constructed with sequences of the Internal Transcribed Spacers (ITS1 and ITS2) and the 5.8S ribosomal gene of the rRNA. Phylogenetic trees showed that the new and previously described species of Gyrodactylus infecting poeciliid fishes do not form a monophyletic assemblage. Trees also showed that the eight new species described morphologically correspond to well-supported monophyletic groups; and that morphologically similar species are also phylogenetically close. Additionally, we correct previous erroneous records of the presence of Gyrodactylus bullatarudis on wild Poecilia mexicana and Xiphophorus hellerii collected in Mexico, as re-examination of the original specimens indicated that these corresponded to Gyrodactylus pseudobullatarudis n. sp. (infecting Poecilia mexicana and Xiphophorus hellerii) and to Gyrodactylus xtachuna n. sp. (on Xiphophorus hellerii). Finally, given the widespread anthropogenic translocation of poeciliid fishes for the aquarium trade and mosquito control programs, as well as the existence of invasive, feral poeciliid populations worldwide, we discuss the possibility that gyrodactylid parasites could be introduced along with the fish hosts-this work provides taxonomic information to assess that possibility, as it describes parasites collected from poeciliid fishes within their native distribution range.

Concordant biogeographic patterns among multiple taxonomic groups in the Mexican freshwater biota.

In this paper we analyse the degree of concordance in species richness and taxonomic distinctness (diversity) patterns among different freshwater taxonomic groups in order to test three long held patterns described in Mexican freshwater biogeography: 1. The aquatic biota of Mexico includes two distinct faunas, a rich Neotropical component in the south and a south-eastern region and a less rich Nearctic component towards central and northern latitudes of the country. 2. A hotspot of species richness and diversity has been recorded in the Usumacinta, including the Yucatan Peninsula. 3. The presence of two distinct biotas in Mexico, an eastern one distributed along the Gulf of Mexico slope, and a western one associated to the Pacific versant. We use species richness and taxonomic distinctness to explore patterns of diversity and how these patterns change between zoogeographical regions. This paper points out a clear separation between Neotropical and Nearctic drainage basins but also between eastern (Gulf of Mexico) and western (Pacific) drainage basins. Present data gives additional empirical support from freshwater biota for three long held beliefs regarding distributional patterns of the Mexican biota. The neotropical basins of Mexico are generally host to a richest and more diversified fauna, that includes more families, genera and species, compared to the less rich and less diverse fauna in the nearctic basins.

Mineralogical and stable isotopic characterization of authigenic carbonate from a hydrocarbon seep site, Gulf of Mexico slope: Possible relation to crude oil degradation

One of the characteristic features on the cold deep seafloor on Atwater Valley (AT) Block 425 (1930 m water depth) in the Gulf of Mexico is the occurrence of seepage of crude oil and hydrocarbon gases associated with thermogenic gas hydrate and authigenic carbonate deposits. Mineralogical, petrographic and isotopic analyses revealed that the precipitation of these carbonates was related to the microbial oxidation of hydrocarbons. Carbonate deposits are composed of heavily oil-stained, small nodules to large cobbles of dense limestones of fine-grained textures. Other carbonate fragments are composed of small intraclasts, pellets and some bivalve shell fragments cemented together by aragonite cement. XRD analysis and petrographic investigations revealed that the authigenic carbonate is predominantly composed of aragonite that occurs as microcrystalline matrix and cement. Microcrystalline, sparitic and botryoidal acicular aragonite crystals are observed as void-filling cement. The δ 18O values of the authigenic carbonate (3.80 to 4.16‰ V-PDB) suggest that the precipitating fluid has equilibrated with the cold bottom seawater. The calculated δ 18O values (0.18 to 0.54‰ SMOW) of the precipitating fluid show subtle deviation from the values reported for the present bottom seawater. This is attributed possibly to the precipitation from pore water with temperature ranged from 3.3 to 1.7 °C, which is less than the measured bottom seawater (4 °C). However, the mixing of the marine pore water with some deep hydrocarbon-rich brines or hydrate-water would provide a source of 18O-enrichment fluid for the precipitation of the authigenic carbonates. Based on the predominance of aragonite with botryoidal and clotted fabric, the occurrence of pyrite and from the oxygen isotopic data, it is plausible that the authigenic carbonates may have formed near the sediment/water interface in an oxygen-depleted environment with low temperature. The narrow range of the δ 13C values of the authigenic carbonate (− 23.88 to − 28.62‰ V-PDB) suggests that a single source of carbon is predominant. Based on the moderately low δ 13C values and the occurrence of spots of crude oil coatings of the carbonate samples, in addition to the application of a carbon isotope mass balance model, the carbon involving in the formation of the carbonates of the AT 425 seep site could be mainly derived from microbial degradation of crude oil. Microbial oxidation of non-methane thermogenic hydrocarbon gases such as ethane, propane and isobutane would play some role as a source of carbonate-carbon.

Correction [to “Alvin explores the Deep Northern Gulf of Mexico Slope”

Correction [to “Alvin explores the Deep Northern Gulf of Mexico Slope”

Day-night activity rhythm of the cold seep shrimp Alvinocaris stactophila (Caridea: Alvinocarididae) from the Gulf of Mexico

The activity rhythm of the cold seep shrimp Alvinocaris stactophila from the Gulf of Mexico slope (650 m depth) was investigated in the laboratory in relation to an artificial 12 h light-dark regime. Animals were sampled with a submersible and transferred into individual aquaria where their activity was monitored for 5 d by taking digital video snapshots every 30 s. An observer analysed the footage by counting the number of times an animal crossed two perpendicular lines drawn on a PC screen per 30 min interval. Resulting time series were represented over consecutive days and the waveform analysis was used to precisely assess the phase (i.e. the peak timing) and its limits (i.e. onset and offset) in relation to light ON and OFF. The majority (73%) of animals showed a marked nocturnal pattern of activity with number of movements close to zero during the photophase. Waveform analysis showed that the behavioural transition from activity to inactivity after light ON occurred within approximately 1 h. Considering the general lack of knowledge on the regulation of activity rhythms of crustaceans from subtidal areas, the present data provide a new insight on the role played by light in the regulation of animal activity rhythm in deep-water environments such as those of the cold seeps in the Gulf of Mexico.

Checklist of helminth parasites of freshwater fishes from Mexico

Using published records and original data derived from my research, a checklist was generated of the helminth parasite fauna in 194 native and 18 introduced freshwater fish species from 30 families from Mexico. The checklist contains 262 nominal species, from 152 genera and 59 families of helminth parasites. It includes 37 species of adult trematodes, 52 metacercariae, 49 monogeneans, 15 adult cestodes, 22 metacestodes, 7 adult acanthocephalans, 5 cystacanths, 56 adult nematodes and 19 larval nematodes. Most of these species (150, 57%) are Neotropical, 35 (13%) are Nearctic, and 28 (11%) are either anthropogenically introduced into Mexico (21 of them), or are world-wide in their distribution (7 species). Insufficient data preclude the determination of the status of the rest of the species. Only 18 of these species are tentativelly pointed out as endemic to Mexico, although 55 species are recognized as being endemic to Mesoamerica. The data demonstrate that the helminth fauna of freshwater fish in Mexico is divided into Neotropical and Nearctic components with no apparent transition zone between them. The Nearctic fauna mainly consists of allogenic generalist species capable of invading Neotropical hosts and environments, but the Neotropical fauna, mostly autogenic and consisting of specialists to certain host families, is limited to Neotropical environments and hosts. This Neotropical fauna forms part of a Mesoamerican zone that extends from southeast Mexico along the Gulf of Mexico slope and into Central America. Fish families have typical groups of helminth species which allows the spatial distribution of helminths to follow that of their hosts. Together with their allogenic generalist characteristics, this gives this helminth fauna a broad distribution that covers Mexico’s Neotropical and Nearctic basins.

Reply to Comment on ‘Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope,’ by I.R. MacDonald, L.C. Bender, M. Vardaro, B. Bernard, and J.M. Brooks

Reply to Comment on ‘Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope,’ by I.R. MacDonald, L.C. Bender, M. Vardaro, B. Bernard, and J.M. Brooks

Comment on “Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope,” by I.R. MacDonald, L.C. Bender, M. Vardaro, B. Bernard, and J.M. Brooks[Earth Planet. Sci. Lett. 233 (2005) 49−59

Comment on “Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope,” by I.R. MacDonald, L.C. Bender, M. Vardaro, B. Bernard, and J.M. Brooks[Earth Planet. Sci. Lett. 233 (2005) 49−59

Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope

Ambient temperature is a critical factor determining the stability of gas hydrate deposits on continental margins. To study this process directly under varying conditions, a monitoring array comprising a time-lapse camera and in-situ temperature probes was deployed at a hydrocarbon seep known as Bush Hill, where gas hydrates deposits are exposed at the seafloor in a water depth of 570 m. For intervals of 91 days and 30 days, the digital camera recorded several daily images of a prominent gas hydrate mound consisting of structure II gas hydrate. The temperature probes were constructed with one autonomous thermistor at each end of a 50-cm PVC wand and recorded temperatures with precision of better than 0.1 °C at 30-min intervals over 327 days. One probe was implanted with a tight seal into a drill hole about 7 cm deep in the top of the gas hydrate mound. The second was inserted about 50 cm deep into the adjacent sediments. For each probe, the top thermistor recorded the ambient water temperature, while the bottom thermistor synoptically recorded the internal temperature of the hydrate or sediment. The bottom water temperatures ranged from 6.64 to 9.73 °C with a mean of 7.90 °C and standard deviation of 0.437. Photographic results showed no dramatic changes in the size, shape or gas venting from the mound. By comparing the temperatures recorded at the tips of the probes with the synoptic water temperature, we estimate that thermal diffusivity for the gas hydrate deposit was 7.14·10 −8 to 8.33·10 −8 m 2 s −1 and was 1.77·10 −7 to 3.01·10 −7 m 2 s −1 for the sediments at the sites where temperatures were recorded. The diffusivity measured in gas hydrate was lower than that measured under laboratory conditions. Stability of gas hydrate in this setting is not likely to be affected by short-term changes in bottom water temperature within the range observed.

Distinct side-scan sonar, RADARSAT SAR, and acoustic profiler signatures of gas and oil seeps on the Gulf of Mexico slope

We examined bubble streams from four natural seep sites on the upper continental slope of the Gulf of Mexico. Synthetic aperture radar images verified surface oil slicks over sites with oily bubbles but not over those with non-oily bubbles. Non-oily bubbles produced high backscatter on side-scan sonar records, but were difficult to detect with acoustic profilers. Oily bubbles produced clear signatures extending from the seafloor to the near-surface on acoustic profiles and produced acoustic shadows on side-scan sonar records. We hypothesize that the bubbles’ oily coating causes the different signatures, since all bubbles were resonant at the tested frequencies.

Lipid and carbon isotopic evidence of methane-oxidizing and sulfate-reducing bacteria in association with gas hydrates from the Gulf of Mexico

An integrated lipid biomarker–carbon isotope approach reveals new insight to microbial methane oxidation in the Gulf of Mexico gas-hydrate system. Hydrate-bearing and hydrate-free sediments were collected from the Gulf of Mexico slope using a research submersible. Phospholipid fatty acids consist mainly of C16–C18 compounds, which are largely derived from bacteria. The phospholipid fatty acids suggest that total biomass is enhanced 11–30-fold in gas-hydrate–bearing sediment compared to hydrate-free sediment. Lipid biomarkers indicative of sulfate-reducing bacteria are strongly depleted in 13C (δ13C = −48‰ to −70‰) in the hydrate-bearing samples, suggesting that they are involved in the oxidation of methane (δ13C = −47‰ for thermogenic methane and −70‰ for biogenic methane). Isotopic properties of other biomarkers suggest that sulfur-oxidizing bacteria ( Beggiatoa ) may also contribute to the lipid pool in hydrate-bearing samples, which are characterized by less negative δ13C values (to −11.2‰). In the hydrate-free sample, fatty acid biomarkers have δ13C values of −27.6‰ to −39.6‰, indicating that crude oil (average ∼−27‰) or terrestrial organic carbon (average ∼−20‰) are the likely carbon sources. Our results provide the first lipid biomarker–stable isotope evidence that sulfate- reducing bacteria play an important role in anaerobic methane oxidation in the Gulf of Mexico gas hydrates. The coupled activities of methane-oxidizing and sulfate-reducing organisms contribute to the development of ecosystems in deep-sea environments and result in sequestration of carbon as buried organic carbon and authigenic carbonates. These have implications for studying climate change based on carbon budgets.

Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?

Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.

Abstract: Fluid and gas expulsion on the Northern Gulf of Mexico Slope: Effects of frequency and magnitude on seafloor impacts with special emphasis on gas hydrates

Abstract: Fluid and gas expulsion on the Northern Gulf of Mexico Slope: Effects of frequency and magnitude on seafloor impacts with special emphasis on gas hydrates