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doi:10.2204/iodp.proc.314315316.132.2009 Expedition 316 methods1Expedition 316 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 the means by which 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, because of the relationship between this expedition and the other expeditions that comprise the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and the large number and volume of sample requests, there are significant differences that will be explained below. Site locationsIntegrated Ocean Drilling Program (IODP) Expedition 316 site locations were selected based on precruise surveys. 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 remotely operated vehicle (ROV) was deployed, a precise fix on the location of the borehole was obtained upon spud-in. When operations took place without ROV support, hole position was determined based on the vessel GPS fix. Drilling operationsFour coring systems were employed during Expedition 316:
The ESCS punch coring method was tested for the first time during Expedition 316, following the determination that all scientific objectives achievable during the allotted time had been met (after nominal total depth had been reached in Hole C0008C). The ESCS punch system was designed to extend the range of nonrotational coring ahead of the bit into formations that are too consolidated for HPCS coring yet not competent to support rotary coring (either ESCS or RCB). The system utilizes a combination of an ESCS lower outer barrel assembly, an RCB upper outer barrel assembly, an HPCS lower core barrel and cutting shoe, and an ESCS upper core barrel. The HPCS lower core barrel and cutting shoe extends beyond the coring bit, similar to the configuration of the standard ESCS system; however, this cutting shoe does not rotate and is latched in place within the bottom-hole assembly (BHA). The system takes advantage of the downward pressure exerted by the weight of the drill string, controlled via the drawworks winch, to force the extended cutting shoe into the formation ahead of the coring bit. Cores are collected and retained using a core catcher assembly similar to that employed on standard HPCS piston coring systems. Three coring runs with this prototype system proved extremely promising, delivering high-quality cores at high rates of recovery in material that often proves problematic to core with the HPCS, RCB, or standard ESCS systems. Further testing and development of this system are ongoing and will be the subject of upcoming Center for Deep Earth Exploration (CDEX) technical notes. Drilled intervals are initially referred to in drillers depth below rig floor (DRF), which is measured from the kelly bushing on the rig floor to the bottom of the drill pipe, and later converted to core depth below seafloor (CSF). When sediments of substantial thickness cover the seafloor, the DRF depth of the seafloor is determined using either the ROV as an absolute reference or 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 directly measured (using the ROV) or calculated by subtracting the distance from the rig floor to sea level from the mudline measurement (DRF). CSF depths of core tops are determined by subtracting seafloor depth (DRF) from core top depth (DRF). The resulting core top data (CSF) are the ultimate reference for any further depth calculation procedures. 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/RCB-related disturbance includes the 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 D/V Chikyu are numbered consecutively from the first site with a prefix “C.” A site refers to one or more holes drilled while the ship is positioned within 300 m of the first hole. 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 suffix “B,” and so forth. These suffixes are assigned regardless of recovery, as long as penetration takes place. During Expedition 316, we drilled at Sites C0004 (proposed Site NT2-01I), C0006 (proposed Site NT1-03B), C0007 (proposed Site NT1-03A), and C0008 (proposed Site NT2-10A). During Expedition 316, the Chikyu occupied Holes C0004C and C0004D; Holes C0006C, C0006D, C0006E, and C0006F; Holes C0007A, C0007B, C0007C, and C0007D; and Holes C0008A, C0008B, and C0008C. 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 is used to calculate the estimated penetration depth in these cases. In addition, we specified the collection of 4.5 m long coring intervals when using the RCB 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 (IODP coring depth scale calculated using Method A [CSF]). Short cores (incomplete recovery) are all assumed to start from that initial depth by convention. Core expansion is corrected during final processing of core measurements by subtracting void spaces, subtracting exotic material, and accounting for expansion (IODP coring depth scale calculated using Method B [CSF-B]). A recovered core is typically divided into 1.5 m long sections that are numbered serially from 1 beginning at the top. During this expedition and IODP Expedition 315, whole-round core samples removed for collection of time-sensitive interstitial water sampling were often assigned their own section number in order to allow for rapid X-ray CT scanning of time-sensitive samples. 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 “316-C0004C-2H-5, 80–85 cm,” represents a sample removed from the interval between 80 and 85 cm below the top of Section 5 of the second HPCS core from Hole C0004C, during Expedition 316 (Fig. F1). All IODP core identifiers indicate core type. The following abbreviations are used:
Core handlingThe following sections describe in detail the flow of core from the drill floor through the laboratory. See Figure F2 for a step-by-step flow chart of the entire process. Core cutting areaAs soon as a core is retrieved on deck, the core catcher is delivered to the core cutting area. A sample is taken for paleontological analysis, and the remainder of the core catcher sample is packed into the core liner and curated. The whole core is delivered to the core cutting area, small (5 cm3) plugs of sediment are removed from the bottom of appropriate core sections for headspace gas analysis, and (primarily in shallower depths) the core is scanned with an infrared camera to identify negative temperature anomalies associated with gas hydrate or hydrate-rich intervals. Any such intervals are immediately cut out of the core, curated, and delivered to the quality assurance (QA)/quality control (QC) laboratory for processing. The recovered core length and the total length of void space are measured, and core identification, length, drilling advance, and depth information are entered into the J-CORES database. If cores contain gas in void spaces, void gas samples are collected at this time. The core is divided into sequentially numbered sections, and the sections are cut. At this time, sections chosen for interstitial water analysis by the inorganic geochemist are removed from the core, taken to the core processing deck, and scanned by X-ray CT. Oversight by a structural geologist, sedimentologist, or geotechnical specialist acting as a “watchdog” (see below) is required before the interstitial water whole round is squeezed. If a sample is rejected by the watchdog, a new interstitial water sample is chosen. Each section is then sealed at the top (blue cap) and bottom (white cap); yellow caps indicate removed whole-round core samples (with sample code marked on the end cap). All sections are marked and labeled, data are entered into the J-CORES database, and sections are moved to the core processing deck. Core processing deckAll sections are scanned using X-ray CT, and each shift has a structural geologist or sedimentologist acting as a watchdog to oversee the collection and selection of whole-round core samples, to identify sections or intervals of special interest or unique character, and to prevent destruction of any critical samples. Whole-round core samples for microbiology and anelastic strain recovery analyses are taken immediately following X-ray CT imaging, with approval of the watchdog. Following X-ray CT scanning and time-sensitive whole-round core sampling, core sections are allowed to equilibrate to ambient temperature (2–3 h), after which they are run through the whole-round multisensor core logger (MSCL-W). During Expedition 316, measurements of gamma ray attenuation (GRA) bulk density, P-wave velocity, resistivity, magnetic susceptibility, and natural gamma radiation were collected using the MSCL (Geotek Ltd., London, UK). Thermal conductivity measurements were also made on whole-round core sections for soft-sediment cores. Following thermal conductivity measurement, a second round of whole-round core sampling for shore-based geotechnical and hydrogeologic analyses is conducted under the guidance of the watchdog. Whole-round core samples for interstitial water and geotechnical analysis had associated “cluster” samples taken from the whole round before processing or storage: this group of samples was taken adjacent to each whole-round core and consists of material for X-ray diffraction (XRD), moisture and density (MAD), and carbonate analyses. Following completion of whole-round core sampling, sections are split axially into working and archive halves. The archive half is subjected to nondestructive visual core description (VCD), digital photo image scanning, color spectroscopy, and paleomagnetic measurement, after which it is covered in plastic film, shrink-wrapped in plastic, and stored in either the cold-store refrigerator or the refrigerated containers at 4°C. The working half is subjected to structural analysis and sampling and measurement for physical properties, including moisture and density, and is then sampled for shipboard and postcruise analyses. Following completion of sampling, these sections are also wrapped, sealed, and stored at 4°C in preparation for shipping to the core repository. All samples collected are labeled, packaged, stored, and shipped to their final destinations according to standard practice or special instructions. Special handlingExpedition 316 cores collected across fault zones or inversions of paleontological age that potentially recovered well-preserved individual faults, fault rocks, and damage zones were subjected to intense X-ray CT image investigation. These cores were withheld from the standard processing procedure and were treated as follows (Fig. F3): cores were scanned by X-ray CT, critical intervals were identified, and cores were stored in the 4°C refrigerator. Shipboard scientists analyzed the CT imagery to identify critical sections to preserve or sample appropriately. Cores in noncritical intervals were test-split to determine the degree of disruption of fabric and structure and the best methods for description and sampling. Agreement on particular critical intervals was reached, and selected whole rounds were removed for physical properties and structural investigation. The remainder of the cores were split, described, and sampled during shift crossover to ensure that all interested parties were present for consultation and to collect material and data. Specified samples of particular interest to the physical property specialists were sampled in whole-round form. Core sections were oriented relative to the fault zones identified (orientation of the split surface was chosen to be parallel to the direction of maximum dip of foliations or fault planes). The liners of these whole-round cores were cut along the determined orientation and lifted off the whole-round sample. The sample was then wrapped in plastic wrap and heat-sealed in two layers of plastic sleeve designed for archival core storage. This procedure was designed to stabilize the fragile material and prevent water from the rock saw from infiltrating the core material. After splitting, the samples were described and photographed and an archive and working half were designated. Both the original core orientation and the structurally determined orientation of the core relative to the liner-based reference frame were recorded. The working half was covered in plastic wrap and stored. The archive half was dried at 30°C for 2 days and then stabilized using several applications of epoxy resin to the half-round section. The archive half was then cut parallel to the split surface to create a true archive half for preservation and an ~2 cm thick slab for structural, microtextural, and point chemical analyses. Splitting was carefully and slowly carried out using a standard rock saw cooled with alcohol rather than water to prevent excess hydration of clays or erosion of the fine-grained fault material. The fresh surface of the archive half and slab were then coated with a thin layer of epoxy to prevent excessive dehydration. The slab material was set aside for thin sectioning according to sample requests and description needs (both scientific and for location and contextualization of selected samples from the working half). After evaluating the above method for successful preservation, the working half of the core sample was divided lengthwise into two quarter-round pieces available for sampling for chemical, sedimentological, XRD, X-ray fluorescence (XRF), or other bulk analyses (QRND-A) and for experimental friction, consolidation, permeability, microchemistry, and potential dating experiments (QRND-E). Sample requests, potential conflicts, and other sampling arrangements were worked out by the shipboard science party and the Sample Allocation Committee in accordance with the sample requests made prior to the start of the expedition. Authorship of site chaptersThe separate sections of the site chapters and methods chapter were written by the following shipboard scientists (authors are listed in alphabetical order; no seniority is implied):
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