James M. Brooks

Biogeography and Potential Exchanges Among the Atlantic Equatorial Belt Cold-Seep Faunas

Like hydrothermal vents along oceanic ridges, cold seeps are patchy and isolated ecosystems along continental margins, extending from bathyal to abyssal depths. The Atlantic Equatorial Belt (AEB), from the Gulf of Mexico to the Gulf of Guinea, was one focus of the Census of Marine Life ChEss (Chemosynthetic Ecosystems) program to study biogeography of seep and vent fauna. We present a review and analysis of collections from five seep regions along the AEB: the Gulf of Mexico where extensive faunal sampling has been conducted from 400 to 3300m, the Barbados accretionary prism, the Blake ridge diapir, and in the Eastern Atlantic from the Congo and Gabon margins and the recently explored Nigeria margin. Of the 72 taxa identified at the species level, a total of 9 species or species complexes are identified as amphi-Atlantic. Similarity analyses based on both Bray Curtis and Hellinger distances among 9 faunal collections, and principal component analysis based on presence/absence of megafauna species at these sites, suggest that within the AEB seep megafauna community structure is influenced primarily by depth rather than by geographic distance. Depth segregation is observed between 1000 and 2000m, with the middle slope sites either grouped with those deeper than 2000m or with the shallower sites. The highest level of community similarity was found between the seeps of the Florida escarpment and Congo margin. In the western Atlantic, the highest degree of similarity is observed between the shallowest sites of the Barbados prism and of the Louisiana slope. The high number of amphi-atlantic cold-seep species that do not cluster according to biogeographic regions, and the importance of depth in structuring AEB cold-seep communities are the major conclusions of this study. The hydrothermal vent sites along the Mid Atlantic Ridge (MAR) did not appear as “stepping stones” for dispersal of the AEB seep fauna, however, the south MAR and off axis regions should be further explored to more fully test this hypothesis.

CPT Stinger - An Innovative Method to Obtain CPT Data for Integrated Geoscience Studies

A new deepwater static cone penetrometer system, “CPT Stinger”, was used to investigate subsurface conditions at a number of production sites in the deepwater Gulf of Mexico. Same site high-resolution geophysical data and long cores obtained with a Jumbo Piston Core (JPC) system illustrate the excellent correlation obtained with continuous geotechnical and geophysical data for defining the spatial variation in soil properties. The high cost of drilling deepwater borings and sampling at widely spaced intervals imposes a significant constraint on obtaining a sufficient quantity of high quality soils data. Thus, improved methods are desirable for more quickly assessing requisite soil properties without sacrificing accuracy. The “CPT Stinger” system is a new tool that can fill this role. This new system allows the same general suite of JPC coring equipment to be modified for CPT testing and can be deployed from an oilfield supply vessel. By simply replacing the standard piston core liner with a CPT system containing thrusting rods, a power/control module, and CPT data logging system, the field operation can quickly be converted from sampling to in situ testing mode. The results show that the new system provides continuous soundings with centimeter-depth accuracy and stratigraphic consistency. In addition to the acquisition of high-quality static CPT data, the sampling rate of the CPT logger allows the acquisition of dynamic CPT data during free fall that can be adjusted for velocity differences to emulate static data A particularly effective way to use the system is in conjunction with nearby, continuous sampling with JPC cores and subbottom seismic profiles. This allows the correlation of the CPT results with strengths from continuous samples over a significant depth of overlap and with the geophysical cross sections. Correlations are presented in the paper for undrained shear strength data from long cores with in situ CPT data. Following the premise that more information acquired for a given budget tends to reduce the risks associated with foundation design and installation planning, the economic benefits of rapidly acquiring the geotechnical data from a lower cost vessel are also illustrated. Introduction The high cost of deepwater oil and gas developments has focused much attention on fast tracking these projects from discovery to production. An integrated geologic/geotechnical study including the “CPT Stinger” system provides an opportunity to characterize subsurface conditions in a cost effective manner. In contrast, the high cost of drilling deepwater rotary borings that sample at widely spaced intervals tends to limit the opportunity to acquire a sufficient quantity of high quality soils data to fully understand the site. Background The importance of conducting integrated geoscience (geologic/geotechnical) studies for deepwater developments has been clearly described by Campbell et al. (1988) and Young and Kasch (2011). The three-dimensional integrated geoscience (geologic/geotechnical) model must be defined early to serve as a basis for planning the architecture of production facilities. The model serves to better understand the constraints imposed by: (1) geologic conditions and geo-hazards, (2) subsurface stratigraphy and its spatial variability, and (3) variable soil profiles and geotechnical properties. The resulting goal of the integrated geoscience model is to select facility sites with favorable conditions such as those with uniform geologic/geotechnical conditions and those most conducive to safe operations of planned seafloor supported structures.

Interstitial Light Hydrocarbon Gases in Jumbo Piston Cores Offshore Indonesia: Thermogenic or Biogenic?

Abstract Interstitial light hydrocarbon gases were measured to 12 m below seabed in two Jumbo piston cores acquired in deepwater sediments offshore Indonesia. Determinations were made for dissolved methane, ethene, ethane, propene, propane, iso-butane, n-butane, iso-pentane, and n-pentane, as well as for carbon dioxide. Stable carbon isotopic ratios of methane and ethane were also measured. Such gas measurements are typically performed in deepwaters around the world as elements of surface geochemical exploration programs for the purpose of distinguishing biogenic from thermogenic sources in seep gases. Various gas source models have been developed in the literature to aid in accurate interpretation for systems that have been biodegraded, fractionated, and/or mixed during migration. In these models, assumptions are made about the ranges of composition of the end-member gas types and maturities. The compositional ranges of the thermogenic and biogenic end members have been empirically derived from a large database of drilled-hole gas data and various theoretical considerations. In addition to this literature base, tens of thousands of near-surface marine sediment cores have been acquired worldwide and analyzed for interstitial gases on a proprietary SGE basis over the last 35-40 years. Interpretation of these results has helped to fine-tune the accepted end-member compositions of the source models. However, essentially all of the SGE sediment cores have reached a maximum 5 m below the seabed. Our acquisition and analysis of cores down to 12 m has revealed information about gas compositions not apparent from the former SGE coring efforts. In particular, ethane concentrations are in the accepted thermogenic range, but their stable carbon isotopic compositions are not. Near-surface gases formerly interpreted to have an unambiguous thermogenic component are here shown to be purely biogenic. Our conclusions may change the boundaries of the biogenic end member used for the indication of thermogenic gas traces in near-surface sediments.

Thermophilic endospores associated with migrated thermogenic hydrocarbons in deep Gulf of Mexico marine sediments

Dormant endospores of thermophilic bacteria (thermospores) can be detected in cold marine sediments following high-temperature incubation. Thermospores in the cold seabed may be explained by a dispersal history originating in deep biosphere oil reservoir habitats where upward migration of petroleum fluids at hydrocarbon seeps transports viable cells into the overlying ocean. We assessed this deep-to-shallow dispersal hypothesis through geochemical and microbiological analyses of 111 marine sediments from the deep water Eastern Gulf of Mexico. GC-MS and fluorescence confirmed the unambiguous presence of thermogenic hydrocarbons in 71 of these locations, indicating seepage from deeply sourced petroleum in the subsurface. Heating each sediment to 50 °C followed by 16S rRNA gene sequencing revealed several thermospores with a cosmopolitan distribution throughout the study area, as well as thermospores that were more geographically restricted. Among the thermospores having a more limited distribution, 12 OTUs from eight different lineages were repeatedly detected in sediments containing thermogenic hydrocarbons. A subset of these were significantly correlated with hydrocarbons (p < 0.05) and most closely related to Clostridiales previously detected in oil reservoirs from around the world. This provides evidence of bacteria in the ocean being dispersed out of oil reservoirs, and suggests that specific thermospores may be used as model organisms for studying warm-to-cold transmigration in the deep sea.