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doi:10.2204/iodp.proc.327.109.2011

Work in progress

As the science involving CORK platforms evolves and expands, tools and techniques for monitoring and/or experimentation continue to become more sophisticated. Here, we describe some developments poised to expand the utility of subseafloor observatories.

DEBI-t (Deep Exploration Biosphere Investigative Tool)

To date, methods developed to detect microbial life in the deep biosphere involve ex situ analysis of recovered materials from boreholes. Microbial communities hosted on the recovered materials are analyzed by extracting cells or cellular components, applying dyes to visualize cells with microscopy, or by employing other analyses (Orcutt et al., 2011). Cell visualization is a standard technique, yet requires time- and labor-intensive hands-on counting of fluorescent cells under a microscope. Because of problems associated with the interference of fluorescent minerals and staining, researchers often resort to physically removing cells from surfaces to stain and count them separately. This too is a laborious and inefficient process that results in the loss of cells and information about the mineralogical context that may have influenced the microbial ecology.

To meet challenges associated with detecting and quantifying microbial life within the subsurface, a novel in situ downhole logging (wireline) tool to detect microbial life in subseafloor boreholes is in development. This tool utilizes deep ultraviolet (“deep UV,” or DUV) radiation to detect cells. DUV is an optical method that enables detection and imaging of single bacterial cells on natural and opaque surfaces, including assessment of bacterial density and distribution of single cells to biofilms (Bhartia et al., 2010). DUV induces and detects native fluorescence of organic components intrinsic to the cell or spore while avoiding autofluorescence interference from the substrate, enabling detection of bacteria at spatial scales ranging from tens of centimeters to micrometers (i.e., both communities of microbes and single cells). The Deep Exploration Biosphere Investigative Tool (DEBI-t) is a wireline tool that uses DUV to map the distribution and abundance of cells in a native borehole environment. The first in situ use of this tool will occur during Expedition 336 in Deep Sea Drilling Project (DSDP) Hole 395A. This technology could become widely used to detect and map remote life, becoming a “satellite imager” application for borehole environments.

Oxygen sensor

One of the limitations of present downhole sampling capabilities is the lack of dissolved oxygen measurements. Dissolved oxygen readily exchanges through PTFE coils and can be consumed in copper coils. Measurements of dissolved oxygen would provide a gauge of the rate of oxidation within basement, particularly as the hole rebounds from drilling processes that introduce surface and bottom seawater into the formation. For example, Wheat et al. (2010) documented the consumption of nitrate during the rebound period after installation of the Hole U1301A CORK, presumably from microbial processes. Furthermore, dissolved oxygen may be quantifiable in some basaltic formation fluids, possibly in basaltic basement at the sites planned for Expedition 336.

To obtain dissolved oxygen data from the borehole, an Antares oxygen sensor has been coupled to a data logger and battery package that fits within the confines of a 6 cm diameter pressure vessel (RBR Ltd., Ottawa, Canada). Given the duration of the experiment, the small diameter of the pressure vessel, and the collection of two samples per day, the primary constraint is the length of the pressure housing. Long, narrow tubes used as pressure housings are not as strong as short, larger diameter ones. Packaging for this sensor allows it to easily mate to OsmoSampler packages and minimizes vibration to the instrument. This development represents the first geochemical sensor to be deployed in a subseafloor observatory. An alternative approach for obtaining oxygen data is to develop an OsmoSampler with glass sample coils. Glass coils are used with high-pressure liquid chromatography to separate compounds for subsequent detection. Such a system may include the addition of a biocide (e.g., BOSS package), but no system has yet been tested.

Gas-rich fluid sampling in gas hydrates

Plans are being devised to deploy three new CORKs in 2013 along the Cascadia Subduction Zone to assess gas compositions and fluxes (IODP Proposal 553-Full2). These CORKs will be positioned within a gas hydrate field, building upon earlier drilling, CORK installation, and sampling attempts (Westbrook, Carson, Musgrave, et al., 1994; Carson et al., 2003). On the basis of these experiences, the new CORKs are being designed to sample dissolved gases (e.g., methane and other hydrocarbon fluxes) and ions and determine pore water seepage speeds and direction well below the bottom-simulating reflector (BSR) and the hydrate stability zone. Because of complications from hydrate formation in earlier operations, the present plan is to isolate the lower section, depriving the stability zone of dissolved gases needed for hydrate formation. This opens up the possibility of sampling two horizons as well as at the seafloor on the wellhead.

Developing samplers for this environment is not trivial. Major concerns include minimizing the annular volume within the area of interest to minimize dilution of formation fluids with seawater from drilling operations. Another major concern is the disturbance of gas-rich samples when they are exposed to conditions on the ship, where the change in pressure will likely result in the loss of sample, similar to the Hole 1200C experience. Thus, a system that automatically seals the sample tubing at pressure is being developed. Such systems typically use a mechanical spring, which is a concern for a multiyear deployment in this environment. Other closing mechanisms are being investigated.

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