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doi:10.2204/iodp.proc.331.102.2011 Methods1Expedition 331 Scientists2IntroductionThis chapter documents the methods used for shipboard scientific analyses, including sample collection, preparation, and preservation for either shipboard or shore-based analysis. This information can be used to understand how we arrived at preliminary conclusions and interpretations and to provide information for those interested in ancillary analyses, shore-based sampling, or integrative investigations. This chapter also covers coring techniques, core handling, and the numbering of sites, holes, cores, sections, and samples. In most cases, this information mirrors the details described in previous volumes of the Proceedings of the Integrated Ocean Drilling Program. However, the information will differ from previous drilling expeditions undertaken with the D/V Chikyu, as Integrated Ocean Drilling Program (IODP) Expedition 331 is not part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) program. Site locationsExpedition 331 site locations were selected based on preexpedition surveys, including tagging of potential sites using the deep submergence vehicles (DSVs) Shinkai 2000 and 6500. Final sites were selected after remotely operated vehicle (ROV) seafloor surveys during the expedition. Onboard Global Positioning System (GPS) satellite navigation was used to position the vessel using dynamic positioning, and acoustic beacons deployed on the seafloor provided additional means of acquiring and keeping vessel position during operations. When the ROV was deployed, a precise fix on the location of the borehole was obtained upon spud-in. Drilling operationsFour coring systems were employed during Expedition 331:
Drilled intervals are initially referred to in meters drilling depth below seafloor (DSF), which is measured from the kelly bushing on the rig floor to the bottom of the drill pipe and later converted to core depth in meters below seafloor (mbsf). When sediments of substantial thickness cover the seafloor, the mbsf depth of the seafloor is determined using a mudline core combined with the length of the drill string at the time of shooting the first core. The mudline core is taken by tagging the seafloor with the drill bit, lifting the bit to a point <9.5 m off bottom, and shooting a partial HPCS core. Water depth is either estimated from pressure using a sensor on the ROV or calculated by subtracting the distance from the rig floor to sea level from the mudline measurement (DSF). Depths of core tops (mbsf) are determined by subtracting seafloor depth (DSF) from core top depth (DSF). The resulting core top data (mbsf) are the ultimate reference for any further depth calculation. Drilling-induced core deformationX-ray computed tomography (CT) imagery and direct inspection of split cores often reveal significant evidence for disturbance of recovered materials. HPCS-related disturbance includes the concave downward appearance of originally horizontal bedding and liquefaction features related to differential compaction of layers with different competency and pore fluid content, as well as the suction-induced flow of significant amounts of exotic material into the bottom of incomplete HPCS penetrations. ESCS/BHI-related disturbance includes rotation and “spiraling” of sections of cores, “biscuiting” of small sections of core and injection of either drilling mud or finely ground material produced at the drill bit into spaces between individual biscuits, or brecciation or grinding of material before collection into the core liner/core barrel. Core deformation may also result from depressurization, expansion, and thermal equilibration of the core as it travels up the drill string, during handling on deck, and during core cutting and splitting. Where possible, such core disturbance was noted in core descriptions. Numbering of sites, holes, cores, sections, and samplesSites drilled by the Chikyu are numbered consecutively from the first site with a prefix “C” because the site is drilled using the Center for Deep Earth Exploration (CDEX) platform. A site usually refers to one or more holes drilled while the ship is positioned within 300 m of the first hole. However, because of the closeness of several of the Expedition 331 sites, site here also refers to a group of holes with the same set of objectives, a set that differs from those of other groups of holes that may be within 300 m. The first hole drilled at a given site is assigned the site number modified by the suffix “A,” the second hole takes the site number and the suffix “B,” and so forth. These suffixes are assigned regardless of recovery, as long as penetration takes place. During Expedition 331, we drilled at Sites C0013 (proposed Site INH-4D), C0014 (proposed Site INH-5D), C0015 (proposed Site INH-11A), C0016 (proposed Sites INH-1C and INH-1D), and C0017 (proposed Site INH-6B). During Expedition 331, the Chikyu occupied Holes C0013A–C0013H, C0014A–C0014G, C0015A–C0015C, C0016A and C0016B, and C0017A–C0017D. Cored intervals are calculated based on an initial 9.5 m length, which is the standard core barrel length for each coring system. Partial penetration of the HPCS core barrel is generally detectable during coring by measurement of the pressure drop in the hydraulic system that powers the piston core. Partial pressure drop and gradual release of pressure are used to calculate the estimated penetration depth in these cases. In addition, we specified the collection of 4.5 m long coring intervals (half-cores) in areas of extreme interest thought to contain fragile or fractured rocks in fault zones. Expansion of cores in the upper sections (and sucked-in material) and gaps related to unrecovered material result in recovery percentages greater or less than 100%, respectively. Depth intervals are assigned starting from the depth below seafloor at which coring started (mbsf). Because this was the only method used, all depths below seafloor are referred to as mbsf. Short cores (incomplete recovery) are all assumed to start from that initial depth by convention. A core recovered with the HCPS/ESCS/EPCS systems is typically divided into 1.5 m long sections that are numbered serially from 1 beginning at the top. When the BHI system was used, section length was set at 75 cm. For all systems, material recovered from the core catcher was assigned to a separate section, labeled core catcher (CC), and placed at the bottom of the lowermost section of the recovered core. A full identification number for a sample consists of the following information: expedition, site, hole, core number, core type, section number, and top to bottom interval in centimeters measured from the top of the section. For example, a sample identification of “331-C0013D-2H-5, 80–82 cm,” represents a sample removed from the interval between 80 and 82 cm below the top of Section 5 of the second HPCS core from Hole C0013D during Expedition 331 (Fig. F1). All IODP core identifiers indicate core type. The following abbreviations are used:
Core handlingThe following section describes in detail the core flow from the drill floor through the laboratory to storage (Fig. F2). Four different coring systems were deployed during Expedition 331, which required two different core cutting areas and different retrieval procedures, as well as different procedures for sectioning of cores and handling of fast-tracked samples. The use of aluminum liners for HPCS/EPCS/ESCS coring also lead to adjustment of core handling. HPCS/ESCS/EPCS core handlingThe standard methods of coring used during Expedition 331 were changed little or not at all from previous procedures. As soon as a core was retrieved on deck, the core catcher was delivered to the core cutting area. A sample was taken for paleontological analysis, and the remainder of the core catcher sample was packed into the core liner and curated. The whole core was then delivered to the core cutting area. At the core cutting area, the recovered core length and the total length of void space were measured, and core identification, length, drilling advance, and depth information were entered into the J-CORES database. If cores contained gas in void spaces, void gas samples were collected at this time through a hole drilled into the liner. Collection was not possible when ship-standard coring systems were used with aluminum liners. The core was divided into sequentially numbered sections, and the sections were cut. Pairs of small (5 cm3) plugs of sediment were removed from the bottom of appropriate core sections for headspace gas analysis. All sections were then taken to the core processing deck and scanned by X-ray CT for choosing whole-round samples for microbiology and interstitial water squeezing. Oversight by a sedimentologist or geochemist acting as a “watchdog” was required before the interstitial water whole round was squeezed. If the watchdog rejected a sample, a new interstitial water sample was chosen. Each section was then sealed at the top (blue cap) and bottom (white cap); yellow caps indicate removed whole-round core samples (with the sample code marked on the end cap). All sections were marked and labeled, data were entered into the J-CORES database, and sections were moved to the core processing deck. BHI 4 inch coreBHI cores were brought up in barrels of 9–27 m lengths at a time. If cores longer than 9 m were taken, they consisted of two to three 9 m aluminum inner tubes and were cut into 9 m sections that were processed one at a time. These cores were processed at a separate core cutting area behind the derrick of the Chikyu. Because the cores were laid down behind the derrick and handled by BHI core techs, a team consisting of the Curator and a science party member trained for H2S conditions assisted in this core cutting area in labeling the liner, taking headspace gas samples, and evaluating the potential for whole-round sampling based on the exposed cross sections of the otherwise opaque aluminum core liners. Sections of BHI core are nominally 0.75 m long, which made them manageable for a single person and which fit in the onboard storage racks at two per rack. Cores were between 9 and 18 m in length. Each section was cut by pushing the core under a waxed diamond-blade rotary saw, starting from the top. Based on the quality of the cross section, sections were scored for whole-round potential and set aside in a cage. When all sections were cut, the highest ranked sections were carried to the regular core cutting area, bagged if measured H2S exceeded safe levels, and taken into the laboratory. Shipboard general measurement planCore flows for 2.44 and 4 inch core sections were similar from the time the sections entered the laboratory, except that some multisensor core logger for whole-round samples (MSCL-W; Geotek Ltd., London, UK) measurements could not be run on the 4 inch cores because of size and on the aluminum-lined 2.44 inch cores because of liner material. Core catcher material was sampled for paleontology studies. At Site C0013 no fossils were found, so core catcher samples were used to support and elaborate on findings by the visual core description (VCD) group using scanning electron microscopy. Because the sampling plan called for frequent paired whole-round samples for microbiology and pore water geochemistry, all sections were brought to the X-ray CT and scanned. Based on these images, whole-round samples were cut and sent either to squeezing in the sample preparation room or to the microbiology laboratory for further sampling. The remaining sections were scanned using the MSCL-W for magnetic susceptibility, P-wave velocity (PWV), noncontact resistivity, gamma ray attenuation (GRA) density, and natural gamma radiation (NGR). For the 4 inch cores, only GRA and NGR could be run. Scans were completed after equilibration to room temperature of 20°–22°C, which usually meant allowing the cores to cool. Next, unsplit cores were subjected to thermal conductivity measurements, followed by splitting into archive and working halves. The working halves were further sampled for X-ray diffraction (XRD) and moisture and density (MAD) measurements. Archive halves went to the MSCL imaging system (MSCL-I) and then to the VCD group of petrologists and sedimentologists. After core description, archive halves were taken to the MSCL 3-D gantry (MSCL-XYZ) and scanned spectrophotometrically. Both archive and working halves were then wrapped in double layers of cling film, bagged, sealed, and stored at 4°C in the core reefer. H2S precautionsMany cores released H2S gas in concentrations >200 ppmv (detection limit) when sectioned. Rather than let them vent, these were often taken straight to the laboratory after being wrapped in airtight bags. These sections were scanned in the bags on the X-ray CT scanner and cut for whole-round sampling in a fume hood if necessary. These cores were then allowed to vent in the draft chamber before further processing. The areas where H2S was released were the whole-round cutting area and the core splitting room. When split, the freshly exposed longitudinal sections at times gave off enough gas to require self-contained breathing apparatus (SCBA) packs to be worn during splitting. Split sections were then allowed to sit in the draft room to degas. Downhole loggingTwo advanced piston corer temperature tool (APCT3) probes were available for Expedition 331. These were calibrated for the range 0°–55°C and were not designed to survive for long at much higher temperatures. We used these probes only for the uppermost two cores at Site C0014 and the uppermost seven cores at the cooler Site C0017. At higher temperatures we used commercial thermoseal (Nichiyu Giken Co., Ltd.) strips taped to the outer surface of the core liner. These adhesive strips have embedded, chemically impregnated wafers that turn black when exposed to the designated temperature. Each strip contained five wafers calibrated in 5° or 10°C increments and covered one of several limited ranges between 75° and 210°C. |
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