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Expedition 320/321 Scientists2


The Pacific Equatorial Age Transect (PEAT) program, which includes Integrated Ocean Drilling Program (IODP) Expeditions 320 and 321, was the first scientific program following the complete refit of the R/V JOIDES Resolution. During this process, scientific facilities on the ship were replaced with a completely new structure and all major drilling and ship systems were completely refurbished. All laboratory equipment used on these first two expeditions was new, refurbished, upgraded, or replaced. This was also the first scientific use of the new software controlling the instruments and new database system.

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 320/321 data will be described in individual publications in various professional journals and the "Expedition research results" 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 320/321 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 depth 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) and the extended core barrel (XCB) were used during Expedition 320/321. 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 either too stiff for the APC system, or when drilling harder substrate such as chert. The XCB cutting shoe (bit) extends ~30.5 cm ahead of the main bit in soft sediments but retracts into the main bit if hard formations are encountered.

The standard bottom-hole assembly (BHA) used at Expedition 320/321 sites was composed of an 11 inch (~29.05 cm) drill bit, a bit sub, a seal bore drill collar, a landing saver sub, a modified top sub, a modified head sub, a nonmagnetic drill collar, seven 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.

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 were used during all conventional APC coring, but the APC drillover technique was not used in the first hole at each site because chert was expected at these sites. Standard steel core barrels were 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 string was drilled or "washed" ahead without recovering sediments to advance the drill bit to a target depth where core recovery needed to be resumed. Such advances were 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 was typically 1–4 m and accounted for drilling depth shift caused by tides, heave, and other factors (see "Stratigraphic correlation and composite section"). After chert horizons were established in Hole A at each site by coring and logging, these intervals were often drilled through using a center bit, so as to obtain piston cores below. Core recovery information is shown in the "Operations" section of each site chapter.

All APC cores taken during Expedition 321 and some APC cores taken during Expedition 320 were oriented using the FlexIt tool (see "Paleomagnetism"). Formation temperature measurements were made in Hole B of each site (see "Downhole measurements") so we could use the formation information from Hole A to avoid any chert layers that might damage the tools. Downhole logging was conducted in one hole at each site.

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 (m), 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 (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) aboard the 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 [DRF], drilling depth below seafloor [DSF]), length of core recovered (e.g., core depth below seafloor [CSF], core composite depth below seafloor [CCSF]), and logging wireline (e.g., wireline log depth below rig floor [WRF], wireline log depth below seafloor [WSF]). All units are in meters.

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 waters for postcruise analyses. Rhizon interstitial water samples were taken for selected intervals in addition to whole rounds (see "Geochemistry"). In addition, samples were immediately taken from the ends of cut sections for shore-based microbiological analysis.

To facilitate real-time drilling decisions to maximize stratigraphic overlap between holes, 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 (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 loggers. Thermal conductivity measurements were also taken, typically in Section 3 of each core.

The cores were then split lengthwise into working and archive halves, from bottom to top. 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 shore-based studies. Shipboard sampling was kept to a minimum during Expedition 320/321 to allow construction of a detailed sampling plan after the composite section was built. 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 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).

Drilling-induced core deformation

When cores are split significant sediment disturbance may be observed. This might include a concave-downward appearance of originally horizontal bedding, haphazard mixing of lumps of different lithologies (mainly at the tops of cores), fluidization, and flow-in (primarily at the bottoms of cores). The "Lithology" section in each site chapter presents the observations of core disturbance. 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.

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 "320-U1331A-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 U1331 during Expedition 320. The "U" preceding hole numbers indicates the hole was drilled by the United States Implementing Organization 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 (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 with allowing these overlaps (CSF) (Table T1), chiefly to avoid confusion during core description and sampling. The primary focus of Expedition 320/321 was obtaining complete sections, so multiple APC/XCB holes (typically three) were drilled at a site 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. CCSF-B is the CCSF-A depth divided by a linear scaling factor to "compress" the depth scale to approximate CSF.

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.

1Expedition 320/321 Scientists, 2010. Methods. In Pälike, H., Lyle, M., Nishi, H., Raffi, I., Gamage, K., Klaus, A., and the Expedition 320/321 Scientists, Proc. IODP, 332/321: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.320321.102.2010

2Expedition 320/321 Scientists' addresses.

Publication: 30 October 2010
MS 320321-102