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Expedition 336 Scientists2

Core recovery and depths

This chapter presents the methods for our shipboard observations. It also enables the interested investigator to identify data and select samples for further analysis. The information presented here concerns only shipboard operations and analyses described in the site chapters. Methods used by various investigators for shore-based analyses of Integrated Ocean Drilling Program (IODP) Expedition 336 data will be described in individual publications in various professional journals and the “Expedition research results” chapters of this Proceedings volume. This introductory section provides an overview of operations, curatorial conventions, and general core handling and analysis.

Site locations

At all Expedition 336 sites, GPS coordinates from a precruise site survey (Schmidt-Schierhorn et al., 2012) were used to position the R/V JOIDES Resolution 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 confirm the seafloor depth agreed with that 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 336. 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 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 too stiff for the APC system or when drilling harder substrate, such as chert.

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. When full or partial stroke can be achieved but excessive force cannot retrieve the barrel, the core barrel can be “drilled over,” that is, after the inner core barrel is successfully shot into the formation, the drill bit is advanced to total depth to free the APC barrel. When an APC core barrel achieves only a partial stroke, the lowermost portion of the core could be material that is “sucked” into the core barrel. Only standard steel core barrels were used during Expedition 336 coring operations. Most APC/XCB cored intervals were ~9.5 m long, which is the length of a standard core barrel. Core recovery information is provided in the “Operations” section of each site chapter (Expedition 336 Scientists, 2012a, 2012b, 2012c, 2012d). APC cores were not oriented during Expedition 336 coring. Formation temperature measurements were only attempted in IODP Hole U1382B. Downhole logging was conducted in Ocean Drilling Program (ODP) Hole 395A and IODP Holes U1382A and U1383C.

The XCB system was used to advance the hole when APC refusal occurred at the sediment/​basement contact. 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 XCB cutting shoe (bit) extends up to ~30.5 cm ahead of the main bit in soft sediments but retracts into the main bit if hard formations are encountered.

The RCB system was deployed to core basement rocks. The RCB is a conventional rotary drilling system and requires a dedicated RCB bottom-hole assembly and a dedicated RCB drilling bit (outer diameter of 9⅞ inch).

IODP depth conventions

Deep Sea Drilling Project, 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 are combinations of origin of depth (rig floor or seafloor), measurement units (m), and method of construction (composite). The designations evolved over many years to meet the needs of individual science parties.

Over the course of ODP and IODP scientific drilling, issues with 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 so that 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 were problematic, and the requirement to integrate scientific procedures among three IODP implementing organizations amplified the need to establish a standardized and versatile depth framework.

A new classification and nomenclature for depth scale types was defined in 2006–2007 (see IODP Depth Scales Terminology version 2 at​program-policies/) (Table T1). This framework provided the implementing organizations with a basis to address more specific issues of how to manage depth scales, depth maps, and splices. This new depth framework has been implemented in the context of the Laboratory Information Management System (LIMS) database aboard the JOIDES Resolution.

The methods and nomenclature for calculating sample depth in a hole have changed to be method-specific, which will ensure that data acquisition, scale mapping, and composite-scale and splice construction are unequivocal.

The primary scales are measured by the length of drill string (e.g., drilling depth below rig floor [DRF] 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 on the basis of user-defined correlations (such as stratigraphic correlation of cores between holes to create a common CCSF scale from the CSF scale of each hole or for 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.

Unless otherwise noted, all Expedition 336 core depths have been calculated as CSF Method A (CSF-A), and all downhole wireline depths have been calculated as WSF Method A (WSF-A) (see Table T1). To more easily communicate shipboard results in this volume, all depths are reported as mbsf, except where otherwise noted.

Core handling and analysis

Core handling and flow were adjusted to best meet the microbiological priorities of the expedition, which are described in more detail in other sections of this chapter and volume. Fluorescent microspheres were deployed in the core catcher for all cores taken during Expedition 336 (except the mudline core). Hard rock cores were immediately split into ~1.5 m long sections and taken to the splitting room, where they were emptied into sterilized split liners. Immediate review by microbiologists and petrologists allowed hard rock microbiology samples to be selected and taken. These samples were photographed and then removed to the microbiology laboratory for processing. The normal hard rock core flow was then resumed, with the exception that the outer surfaces of whole-round core pieces of sufficient length were also digitally scanned. For sediment cores, the core liner was marked on the catwalk to identify core sections and intensive whole-round samples for microbiology and interstitial water samples. Syringe samples were taken for headspace and microbiological analyses. The core sections remaining after whole-round sampling were taken inside, labeled, and moved to the core refrigerator, where oxygen concentration measurements were made with optodes and interstitial water samples were taken using Rhizon samplers.

At the end of the expedition, the cores were transferred from the ship into refrigerated containers and sent to the IODP Bremen Coast Repository in Bremen, Germany.

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 or rocks, the loose pieces from intervals higher in the hole may be washed down by drilling circulation and accumulate at the bottom of the hole, to 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 core segments within the liner apart. 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, along with gas, out of the liner. Holes may also be drilled in the liner to allow water-filled sections of core to drain. Observed core disturbances are described in “Lithostratigraphy” in the “Sediment and basement contact coring” chapter (Expedition 336 Scientists, 2012a) and graphically indicated on the core summary graphic reports (barrel sheets) (see “Core descriptions”).

Curatorial procedures and sample-depth calculations

Site, hole, core, and sample numbering followed 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, along with the sampling tools and volumes taken. For example, a sample identification of “336-U1382B-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 B of Site U1382 during Expedition 336 (Fig. F1). The “U” preceding hole number indicates the hole was drilled by the United States Implementing Organization (USIO) platform, the JOIDES Resolution.

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. 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 (Table T1), chiefly to avoid confusion during core description and sampling.

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 in the cored interval is not known and should be considered a sampling uncertainty in age-depth analysis and correlation of core data with downhole logging data.

1 Expedition 336 Scientists, 2012. Methods. In Edwards, K.J., Bach, W., Klaus, A., and the Expedition 336 Scientists, Proc. IODP, 336: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.336.102.2012

2 Expedition 336 Scientists’ addresses.

Publication: 16 November 2012
MS 336-102