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Expedition 318 Scientists2

Core recovery and depths

Information assembled in this chapter will help the reader understand the basis for our shipboard observations and preliminary conclusions. It will also enable the interested investigator to identify data and select samples for further analysis. Information presented here concerns only shipboard operations and analyses described in the site chapters. Methods used by various investigators for shore-based analyses of Expedition 318 data will be described in individual publications in various professional journals. This introductory section provides an overview of operations, curatorial conventions, and general core handling and analysis.

Site locations

At all Expedition 318 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 Syquest Bathy 2010 CHIRP subbottom profiler, which was monitored on the approach to each site to reconfirm the seafloor depths with those from the precruise survey. Once the vessel was positioned at a site, the thrusters were lowered and a positioning beacon was dropped to the seafloor. The dynamic positioning control of the vessel uses navigational input from the GPS and triangulation to the seafloor beacon, weighted by the estimated positional accuracy (Fig. F1). The final hole position was the mean position calculated from the GPS data collected over the time that the hole was occupied.

Drilling operations

The advanced piston corer (APC), extended core barrel (XCB), and rotary core barrel (RCB) systems were used during Expedition 318. These standard coring systems and their characteristics are summarized in Graber et al. (2002). The APC system cuts soft-sediment cores with minimal coring disturbance relative to other Integrated Ocean Drilling Program (IODP) coring systems. After the APC core barrel is lowered through the drill pipe and lands near the bit, the drill pipe is pressured up until the two shear pins that hold the inner barrel attached to the outer barrel fail. The inner barrel then advances into the formation and cuts the core. The driller can detect a successful cut, or “full stroke,” from the pressure gauge on the rig floor. The XCB system is deployed when the formation becomes either too stiff for the APC system or when drilling harder substrate such as chert. The XCB cutting shoe (bit) extends as far as ~30.5 cm ahead of the main bit in soft sediments but retracts into the main bit if hard formations are encountered.

APC refusal is conventionally defined in two ways: (1) the piston fails to achieve a complete stroke (as determined from the pump pressure reading) because the formation is too hard or (2) excessive force (>60,000 lb; ~267 kN) is required to pull the core barrel out of the formation. In the case where full or partial stroke can be achieved but excessive force cannot retrieve the barrel, the core barrel can be “drilled over;” after the inner core barrel is successfully shot into the formation, the drill bit is advanced to total depth to free the APC barrel. This strategy allows a hole to be advanced much farther with the APC, the preferred coring tool. Nonmagnetic core barrels are commonly used during all conventional APC coring, but the APC drillover technique is not typically conducted in the first hole at each site if glacial dropstones or other hard rocks might be encountered. Standard steel core barrels were usually used when utilizing the drillover technique because they are stronger than the nonmagnetic barrels.

Most APC/XCB cored intervals were ~9.5 m long, which is the length of a standard core barrel. In some cases, the drill bit can be drilled or “washed” ahead without recovering sediments to advance the drill bit to a target depth where core recovery can be resumed. Such advances are necessary in multiple holes at a site to ensure that coring gaps in one hole were covered by cored intervals in adjacent holes. The amount of advance is usually 1–4 m and accounts for drilling depth shift caused by tides, heave, and other factors (see “Stratigraphic correlation and composite section”). An alternative method to adjust the offset is to raise the bit 1–4 m off the bottom of the hole before shooting the next APC core. After any hard horizons are established in Hole A at each site by coring and logging, these intervals can often be drilled through using a center bit to obtain piston cores below. Core recovery information is shown in the “Operations” section of each site chapter.

APC cores can be oriented using the FlexIt tool (see “Paleomagnetism”). Formation temperature measurements are usually made in Hole B of each site (see “Downhole logging”) so that we could use the formation information from Hole A to avoid any hard layers that might damage the tools. Downhole logging is usually attempted in one hole at each site.

The XCB system was used to advance the hole when APC refusal occurred in a hole before the target depth was reached and when the formation became either too stiff for the APC system or when drilling hard substrate such as cemented layers and nodules or chert. The XCB is a rotary system with a small cutting shoe extending below the large rotary APC/XCB bit. The smaller bit can cut a semi-indurated core with less torque and fluid circulation than the main bit and thus optimizes recovery.

The bottom-hole assembly (BHA) is the lowermost part of the drill string. The exact configuration of the BHA is reported in the “Operations” section in each site chapter. A typical APC/XCB BHA consists of a drill bit (outer diameter = 11 inches), a bit sub, a seal bore drill collar, a landing saver sub, a modified top sub, a modified head sub, a nonmagnetic drill collar (for APC/XCB), a series of 8 inch (~20.32 cm) drill collars, a tapered drill collar, six joints (two stands) of 5½ inch (~13.97 cm) drill pipe, and one crossover sub. A lockable flapper valve was used so that we could collect downhole logs without dropping the bit when APC/XCB coring.

The RCB system was deployed when the APC or XCB coring rates diminished below an acceptable level or if the bit was destroyed by an increasingly hard formation. Because of the expected drilling conditions caused by glacial deposits and glacially eroded sections, we decided to start with RCB coring at all shelf sites. The RCB is a conventional rotary drilling system and it requires a dedicated RCB BHA and a dedicated RCB drilling bit (outer diameter = 9⅞ inches). A typical BHA for RCB coring includes an RCB drill bit, a mechanical bit release, a modified head sub, an outer core barrel, a modified top sub, and a series of drill collars followed by a tapered drill collar and 5½ inch drill pipe.

IODP depth conventions

For the last few decades, Deep Sea Drilling Project, Ocean Drilling Program (ODP), and IODP Phase 1 reports, diagrams, and publications used three primary designations to reference depth: meters below rig floor (mbrf), meters below seafloor (mbsf), and meters composite depth (mcd). These designations were combinations of origin of depth scales (rig floor or seafloor), measurement units (meters), and method of construction (composite). The designations evolved over many years based on the needs of individual science parties.

Over the course of ODP and IODP scientific drilling, issues with the existing depth scale designations and the lack of a consistent framework became apparent. For example, application of the same designation to scales created with distinctly different tools and methods was common (e.g., mbsf for scales measured by drill string tally and those measured with the wireline). Consequently, new scale-type designations were created ad hoc to differentiate the wireline logging scale from the core depth scale depth-mapping procedures and products could be adequately described. Management and use of multiple maps, composite scales, or splices for a hole or a site was problematic, and the requirement to integrate scientific procedures among three IODP implementing organizations amplified the need to establish a standardized and versatile depth framework.

Given the opportunity offered by the hiatus in IODP drilling operations, a new classification and a nomenclature for depth scale types were defined in 2006–2007 (see IODP Depth Scales Terminology at​program-policies/) (Table T1). The framework forms a basis upon which the implementing organizations could address more specific issues of how to manage depth scales, depth maps, and splices. This new depth framework has been implemented within the context of the new Laboratory Information Management System (LIMS;​database/​index.html) aboard the R/V JOIDES Resolution.

The new methods and nomenclature of calculating sample depth in a hole has changed to be method specific. This will ensure that data acquisition, mapping of scales, and construction of composite scales and splices are unequivocal.

The primary scales are measured by the length of drill string (e.g., drilling depth below rig floor and drilling depth below seafloor [DSF]), length of core recovered (e.g., core depth below seafloor [CSF] and core composite depth below seafloor [CCSF]), and logging wireline (e.g., wireline log depth below rig floor [WRF] and wireline log depth below seafloor [WSF]). All units are in meters. Relationships between scales are either defined by protocol (such as the rules for computation of CSF from DSF) or they are defined based on user-defined correlations (such as stratigraphic correlation of cores between holes to create a common CCSF from the CSF of each hole, or core to log correlation). The distinction in nomenclature should keep the user aware that a nominal depth value at two different scales usually does not refer to the exact same stratigraphic interval.

During Expedition 318, unless otherwise noted, all core depths below seafloor were calculated as CSF-A and CCSF-A and all downhole wireline depths calculated as WSF-A (Table T1). For ease of communication of shipboard results, depths are reported in this volume as “mbsf” unless otherwise noted.

Core handling and analysis

As soon as cores arrived on deck, headspace samples were taken using a syringe for immediate hydrocarbon analysis as part of the shipboard safety and pollution prevention program. Core catcher samples were taken for biostratigraphic analysis. Whole-round samples were taken from some core sections for shipboard interstitial water examinations and for microbiological and optical stimulating luminescence age dating. Rhizon interstitial water samples were taken from selected intervals when whole-round samples could not be obtained. (see “Geochemistry and microbiology”). In addition, samples were immediately taken from the ends of cut sections for shore-based microbiological analysis.

For the Adélie Drift cores, whole-round core sections were run through the Special Task Multisensor Logger (STMSL), which collects magnetic susceptibility and gamma ray attenuation (GRA) bulk density immediately after entering the core laboratory to facilitate real-time drilling decisions to maximize stratigraphic overlap between holes (see “Stratigraphic correlation and composite section”). After the cores reached equilibrium with laboratory temperature (typically after 4 h), whole-round core sections were run through the Whole-Round Multisensor Logger (WRMSL; P-wave velocity, density, magnetic susceptibility, and resistivity) and the Natural Gamma Radiation Logger (NGRL). Thermal conductivity measurements were also taken.

The cores were then split lengthwise, from bottom to top, into working and archive halves. Investigators should be aware that older material could have been transported upward on the split face of each section during splitting. The working half of each core was sampled for shipboard biostratigraphy, physical properties, carbonate, paleomagnetism, bulk X-ray diffraction (XRD) mineralogy, and inductively coupled plasma spectroscopy (ICP) and organic geochemical studies. Shipboard sampling was kept to a minimum during Expedition 318 to allow construction of a scientifically appropriate sampling plan at the end of the expedition. Archive-half sections were scanned on the Section Half Multisensor Logger (SHMSL), measured for color reflectance on the Section Half Imaging Logger (SHIL), described visually and by means of smear slides, and finally run through the cryogenic magnetometer.

No core sampling was done on board other than for shipboard analyses and personal samples for research focusing on ephemeral properties. Both halves of the core were then shrink-wrapped, put into labeled plastic tubes, sealed, and transferred to cold storage space aboard the ship. At the end of the expedition, the cores were transferred from the ship into refrigerated trucks and then to cold storage at the IODP Gulf Coast Repository in College Station, Texas (USA).

We modified the core flow for the Adélie Drift cores because of the unique nature of these laminated, potentially high organic content, high sedimentation rate sediments. All whole-round core sections were put though the normal track systems but, when possible, were also put through the cryogenic magnetometer. We only split and processed sections from the first hole. As necessary, these were split one section at a time so that they could be imaged before any alteration occurred as a result of relatively high organic content. Following complete core processing, we subsampled the split sections to prevent any dissolution from acidification of the pore waters. These samples were air dried for postcruise analyses. Once the shipboard analyses were completed, we sealed the core sections along with oxygen absorbers in two layers of shrink-wrap.

Drilling-induced core deformation

Cores may be significantly disturbed and contain extraneous material as a result of the coring and core handling process. In formations with loose sand layers, the sand from intervals higher in the hole may be washed down by the drilling circulation, accumulate at the bottom of the hole, and be sampled with the next core. The top 10–50 cm of each core must therefore be examined critically for potential “fall-in” during description. Common coring-induced deformation includes concave-downward appearance of originally horizontal bedding. In APC cores, the motion of the piston may result in fluidization (flow-in) at the bottom of the cores. Retrieval from depth to the surface may result in elastic rebound. Gas that is in solution at depth may become free and drive apart core segments within the liner. When gas content is high, pressure must be relieved for safety reasons before the cores are cut into segments. This is accomplished by drilling holes into the liner, which forces some sediment out of the liner along with gas. Observed core disturbances are described in the “Lithostratigraphy” section in each site chapter and graphically indicated on the core summary graphic reports (“barrel sheets”).

Curatorial procedures and sample depth calculations

Numbering of sites, holes, cores, and samples followed the standard IODP procedure. A full curatorial identifier for a sample consists of the following information: expedition, site, hole, core number, core type, section number, and interval in centimeters measured from the top of the core section and also the sampling tools and volumes taken. For example, a sample identification of “318-U1355A-1H-2, 10–12 cm” represents a sample taken from the interval between 10 and 12 cm below the top of Section 2 of Core 1 (“H” designates that this core was taken with the APC system) of Hole A of Site U1355 during Expedition 318 (Fig. F1). The “U” preceding hole numbers indicates the hole was drilled by the United States Implementing Organization (USIO) platform, the JOIDES Resolution. Other core types are designated by “R” for cores taken with the RCB system and “X” for cores taken by the XCB system.

Core intervals are defined by the length of drill string, the seafloor depth, and the amount the driller advances the core barrel, reported in DSF. Once a core is recovered on board, the length of core is measured and this is the “curated” length. The depth of a sample below seafloor (CSF) 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 depth measured with the drill string (DSF). A soft- to semisoft-sediment core from less than a few hundred meters below seafloor expands upon recovery (typically a few percent to as much as 15%), so the recovered interval does not match the cored interval. In addition, a coring gap typically occurs between cores, as shown by composite depth construction (see “Stratigraphic correlation and composite section”). Thus, a discrepancy can exist between the DSF depth and the CSF depth.

For instance, when a recovered core measures >100% of the cored interval, the CSF depth of a sample taken from the bottom of that core will be deeper than that from a sample from the top of the subsequent core. For this expedition, we report all results in the core depth below seafloor method allowing these overlaps (CSF-A) (Table T1), chiefly to avoid confusion during core description and sampling. The primary focus of the Adélie Drift site was to obtain complete sections, so multiple APC holes were drilled to construct a continuous composite sections; these are reported as CCSF. CCSF depths were reported in two manners. CCSF-A is the depth based on a simple alignment of cores from multiple adjacent holes. CCSF-A is generally longer than CSF-A. CCSF-B is the CCSF-A depth divided by a linear scaling factor to “compress” the depth scale to approximate CSF-A.

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.

1 Expedition 318 Scientists, 2011. Methods. In Escutia, C., Brinkhuis, H., Klaus, A., and the Expedition 318 Scientists, Proc. IODP, 318: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.318.102.2011

2Expedition 318 Scientists’ addresses.

Publication: 2 July 2011
MS 318-102