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Expedition 311 Scientists2
Information assembled in this chapter will help the reader understand the basis for our preliminary conclusions and also enable the interested investigator to identify data and select samples for further analysis. This information concerns only shipboard operations and analyses described in the site chapters. Methods used by various investigators for shore-based analyses of Expedition 311 data will be described in individual publications in various professional journals and the "Research results" section of this Proceedings volume. This introductory section provides an overview of operations, curatorial conventions, and general core handling and analysis.
At all Expedition 311 sites, Global Positioning System (GPS) coordinates from precruise site surveys were used to position the vessel on site. The only seismic system used during the cruise was the 3.5 kHz profiler, which was monitored on the approach to each site to determine water depth prior to spudding. Once the vessel was positioned at a site, the thrusters were lowered and a reference beacon was deployed. Although the automated stationkeeping system of the vessel usually uses GPS data, the beacon provides a backup reference in case of problems with the transmission of satellite data. The final site position was the mean position calculated from the GPS data collected over the time that the site was occupied.
Two standard coring systems were used during Expedition 311: the advanced piston corer (APC) and the extended core barrel (XCB). These standard coring systems and their characteristics are summarized in the "Explanatory Notes" chapters of various Ocean Drilling Program (ODP) Initial Reports volumes. Most APC/XCB cored intervals were ~9.6 m long, which is the length of a standard core barrel. In a few cases, the drill string was "washed ahead" without recovering sediments to advance the drill bit to a target depth, where core recovery was resumed. In addition to these conventional coring tools, several pressure coring systems were used (see "Pressure coring"). In situ temperature was measured at prescribed depth intervals at each site (see "Physical properties"). Logs of geophysical parameters were obtained during logging while drilling (LWD) and measurement while drilling (MWD) and using wireline tools (see "Downhole logging").
Drilled intervals are referred to in meters below rig floor (mbrf), which is measured from the kelly bushing on the rig floor to the bottom of the drill pipe, and meters below seafloor (mbsf), which is calculated. When sediments of substantial thickness cover the seafloor, the meters below rig floor depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first core. Water depth is calculated by subtracting the distance from the rig floor to sea level from the mudline measurement in meters below rig floor. This water depth usually differs from precision depth recorder measurements by a few to several meters. The meters below seafloor depths of core tops are determined by subtracting the seafloor depth (meters below rig floor) from the core top depth (meters below rig floor). The resulting core top data in meters below seafloor are the ultimate reference for any further depth calculation procedures.
When cores are split, many show signs of significant sediment disturbance, including the concave-downward appearance of originally horizontal bedding, haphazard mixing of lumps of different lithologies (mainly at the tops of cores), fluidization, and flow-in. Core deformation may also occur during retrieval because of changes in pressure and temperature as the core is raised and during cutting and core handling on deck. These changes were particularly important during Expedition 311 because temperature and pressure variations induce exsolution of gas from interstitial water (IW) and dissociation of gas hydrate, which also releases large amounts of gas. The "Lithostratigraphy" section in each site chapter discusses characteristics of cores that indicate this type of disturbance and can, therefore, serve as proxies for the presence of gas hydrate.
Numbering of sites, holes, cores, and samples follows standard Integrated Ocean Drilling Program (IODP) conventions (Fig. F1). A full curatorial identifier for a sample consists of the expedition, site, hole, core number, core type, section number, and interval in centimeters measured from the top of the core section. For example, a sample identification of 311-U1325A-1H-1, 10–12 cm, represents a sample removed from the interval between 10 and 12 cm below the top of Section 1 of Core 1 (H designates that this core was taken with the APC system) in Hole U1325A during Expedition 311. Cored intervals are also referred to in "curatorial" meters below seafloor. The mbsf of a sample is calculated by adding the depth of the sample below the section top and the lengths of all higher sections in the core to the core top datum measured with the drill string.
In general, sediment core from less than a few hundred meters below seafloor may, in some cases, expand upon recovery (typically 10% in the upper 300 m), and its length may not necessarily match the drilled interval. Such core expansion can be greater for gassy cores like the ones recovered during Expedition 311. In addition, a coring gap is typically present between cores. Thus, a discrepancy may exist between the drilling mbsf and the curatorial mbsf. For instance, the curatorial mbsf of a sample taken from the bottom of a core may be larger than that of a sample from the top of the subsequent core, whereas the latter corresponds to the drilled core-top datum.
If a core has incomplete recovery, all cored material is assumed to originate from the top of the drilled interval as a continuous section for curation purposes. The true depth interval within the cored interval is not known. This should be considered as a sampling uncertainty in age-depth analysis and correlation of core data with downhole logging data.
Cores were generally handled according to ODP/IODP standard core handling procedures as described in previous ODP Initial Reports volumes and the Shipboard Scientist's Handbook (ODP Science Services, 2006), with modifications required to quickly identify intervals containing gas hydrates and to maintain approximately aseptic conditions for microbiological sampling. Precautions were also taken to identify and safely deal with hydrogen sulfide gas.
To identify gas hydrates, cores were scanned with both handheld and track-mounted infrared (IR) cameras to identify intervals with significant thermal anomalies. This is explained further in "Physical properties" and in individual site chapters. Some suspected gas hydrate–bearing intervals were cut and immediately stored in liquid nitrogen–filled dewars on the catwalk. In addition, some gas hydrate samples that self-extruded from the liner were collected for various shipboard geochemical analyses.
Cores identified for microbiological sampling were cored with drilling fluid spiked with contamination tracers (see "Microbiology"). When these cores were brought on board, appropriate sections for microbiological studies were taken on the catwalk and then transferred immediately to the microbiology laboratory on the forecastle deck. In intervals of more intense microbiological interest, an entire 1.5 m section was identified on the catwalk for rapid microbiological processing. Once the section was selected, it was labeled with a red permanent marker with orientation and section number and removed from the core. Ends of the removed section were covered with plastic caps, but not sealed, and the section was carried into the hold refrigerator, which was set to ~4°C and served as a microbiology cold room for microbiology and geochemistry sampling.
Gas samples for routine shipboard safety and pollution-prevention purposes were collected on the catwalk for shipboard and shore-based studies (see "Organic geochemistry"). Whole rounds (10–30 cm long) were taken for interstitial water analysis (see "Interstitial water geochemistry"). Additional whole-round samples were also collected for shore-based physical properties and organic geochemistry studies. The cores were then compressed with a plunger to remove large voids and cut into 1.5 m sections. The remaining cut sections were transferred to the core laboratory for further processing.
Whole-round core sections not used for microbiological sampling were run through the multisensor track (MST), and thermal conductivity measurements were performed (see "Physical properties"). The cores were then split into working and archive halves (from bottom to top). Investigators should be aware that older material may have been transported upward on the split face of each section.
The archive-half sections were scanned on the digital imaging system (DIS) and measured for color reflectance on the archive multisensor track (AMST). Select intervals also were measured with the AMST point susceptibility meter. Visual core descriptions (VCDs) of the archive halves were prepared, augmented by smear slides and thin sections (see "Lithostratigraphy"). The archive halves were then run through the cryogenic magnetometer (see "Physical properties") and photographed with color film one whole core at a time. Digital close-up photographs were taken of particular features for illustrations in site summary reports, as requested by scientists.
P-wave velocity, shear strength, and electrical resistivity were measured on split-core sections of the working half. The working half was then sampled both for shipboard analyses, such as physical properties, carbonate, and bulk X-ray diffraction (XRD) mineralogy, and for shore-based studies (see "Physical properties"). Both halves of the core were then put into labeled plastic tubes, sealed, and placed in cold storage space aboard the ship.
Two 20 ft refrigerated containers were mounted on the vessel (on the lab stack roof and on top of the core technician shop) during Expedition 311 for pressure core processing and analyses. The lab stack van (hereafter referred to as the pressure coring system [PCS] van) contained the PCS degassing manifold, a vertical gamma ray attenuation (GRA) density logger, and an Agilent microgas chromatograph. The core technician shop container (hereafter referred to as the HYACINTH van) housed the HYACINTH transfer chamber and a P-wave velocity, GRA, and linear X-ray core logger.
At the end of the expedition, the cores were transferred from the ship into refrigerated containers and shipped to the IODP Gulf Coast Core Repository in College Station, Texas, USA. Gas hydrate samples (preserved in liquid nitrogen or pressure vessels) and HYACINTH pressure cores were transferred to the Pacific Geoscience Centre of the Geological Survey of Canada, Sidney, British Columbia, for inventory and shore-based analyses prior to distribution to approved investigators.