At all Leg 208 sites, Global Positioning System (GPS) coordinates from precruise site surveys were used to position the vessel on site. Once the vessel was positioned on a site, the thrusters were lowered and a reference beacon was deployed. Although the automated stationkeeping system of the vessel usually uses GPS data, the beacon provides a backup reference in case of problems with the transmission of satellite data. The final site position was the mean position calculated from the GPS data collected over a significant portion of the time the site was occupied. At the end of operations at a site, the beacon was retrieved and the thrusters were retracted before the vessel proceeded to the next site.
Two coring systems were used during Leg 208: the advanced hydraulic piston corer (APC) and the extended core barrel (XCB). The APC is a "cookie cutter"–type system that cuts cores with minimal coring disturbance. The drill pipe is pressured up until one or two shear pins that hold the inner barrel to the outer barrel fail. The inner barrel strikes out and cuts the core. The driller can detect a successful cut or "full stroke" by the pressure gauge on the rig floor. Whenever possible, the APC was used during Leg 208 because it yields the least disturbed cores.
When "APC refusal" occurs in a hole before the target depth is reached, the XCB is used to advance the hole. The XCB is a rotary system with a small cutting shoe extending below the large rotary bit. The smaller bit can cut a semi-indurated core with less torque and fluid circulation than the main bit and thus optimizes recovery. If the formation becomes even more indurated, the rotary core barrel (RCB) is used. The RCB system is the most conventional drilling system, but it was not needed during Leg 208.
Both the APC and XCB systems use the same bottom-hole assembly (BHA), and switching from one to the other does not require a pipe trip. The standard BHA used at all Leg 208 sites comprised a 97/8-in APC/XCB polycrystalline diamond 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, five 8
-in drill collars, a tapered drill collar, six joints of 5
-in drill pipe, and one crossover sub. The BHA length was 136 m with a total weight of 35,000 lb. When it became necessary to drill over stuck core barrels, the standard steel barrel assembly was used instead of the nonmagnetic barrel. A lockable float valve was used instead of the standard float assembly if the possibility of logging existed.
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, and (2) excess force (>60,000 lb) is required to pull the core barrel out of the formation because of friction. In the case where a full stroke could not be achieved, one or two more attempts were typically made with the APC and each time the bit was advanced by the length of recovered core. After three consecutive strikes with incomplete recovery, the APC was pulled out and the hole was advanced with the XCB. In the cases where full stroke could be achieved but excessive force could not retrieve the barrel, the core barrel was "drilled over" (i.e., after the inner core barrel was successfully shot into the formation, the rotary bit was advanced to total depth to free the APC barrel). This strategy allowed a hole to be advanced much farther with the APC, the preferred drilling tool.
Each cored interval was ~9.5 m long, which is the length of a 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 where there were multiple holes at a site to ensure that coring gaps in one hole were covered by cored intervals in adjacent holes. Adjustments were also necessary to ensure that critical intervals of a few decimeters in length (Paleocene/Eocene [P/E] and Cretaceous/Paleogene [K/P] boundary intervals) were recovered in the middle of a core rather than near the bottom or end.
Drilled and cored intervals are referred to in meters below rig floor (mbrf), which are measured from the dual elevator stool (DES) on the rig floor to the bottom of the drill pipe. Where sediments of substantial thickness cover the seafloor (as at all sites during Leg 208), the mbrf depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first core. If the first core recovered a full barrel of sediment (missing the mudline), the seafloor reference depth of a previous or later hole was used. Water depth is calculated by subtracting the distance between the DES and sea level (typically 10–11 m, depending on the ship's load at a given time) from the mbrf depth. The water depth determined in this way (drill string measurement) usually differs from the precision depth recorder measurements by a few to several meters. The meters below seafloor (mbsf) depths of core tops are calculated by subtracting the seafloor depth in mbrf from the core-top depth in mbrf. The core-top datums from the driller are the ultimate depth reference for any further depth calculation procedures.
As soon as cores arrived on deck, gas void samples and headspace samples were taken by means of a syringe, if applicable, for immediate analysis as part of the shipboard safety and pollution prevention program. Core catcher samples were taken for biostratigraphic analysis. When the core was cut in sections, whole-round samples were taken for shipboard interstitial water examinations. In addition, headspace gas samples were immediately taken from the ends of cut sections and sealed in glass vials for light hydrocarbon analysis.
Before splitting, whole-round core sections were run through the multisensor track (MST) and thermal conductivity measurements were taken. The cores were then split into working and archive halves, from bottom to top, so investigators should be aware that older material could have been transported upward on the split face of each section. The working half of each core was sampled for both shipboard analyses, such as physical properties, carbonate, and X-ray diffraction mineralogy, and shore-based studies. Shipboard sampling was kept at a minimum during Leg 208 to allow completion of a detailed sampling plan after the composite depth scale and sampling splice were built (see "Composite Depth"). The archive halves were photographed with both black-and-white and color film, scanned with the digital imaging system (DIS), described visually and by means of smear slides, measured for color reflectance on the archive-half multisensor track (AMST), and run through the cryogenic magnetometer. Close-up photographs were taken of particular features for illustrations in site summary reports, as requested by scientists.
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 leg, the cores were transferred from the ship into refrigerated trucks and to cold storage at the Bremen Core Repository of the Ocean Drilling Program in Bremen, Germany.
Numbering of sites, holes, cores, and samples follows the standard ODP procedure. A full curatorial identifier for a sample consists of the following information: leg, site, hole, core number, core type, section number, and interval in centimeters measured from the top of the core section. For example, a sample identification of "208-1262A-1H-1, 10–12 cm" would represent a sample removed from the interval between 10 and 12 cm below the top of Section 1, Core 1 (H designates that this core was taken with the APC system) of Hole 1262A during Leg 208.
Cored intervals are also referred to in "curatorial" mbsf. The mbsf depth of a sample 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 datum measured with the drill string. A soft to semisoft sediment core from less than a few hundred mbsf expands upon recovery (typically 10%–20%), and 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 "Composite Depth") (i.e., some cored interval was lost during recovery or was never cut). Thus, a discrepancy exists between the drilling mbsf and the curatorial mbsf. For instance, the curatorial mbsf of a sample taken from the bottom of a core is larger than that of a sample from the top of the subsequent core, where the latter does correspond to the drilled core-top datum.
During Leg 208, multiple APC holes (two to four) were drilled at a site to construct a continuous composite section. This resulted in a meters composite depth (mcd) scale for each site that accommodates core expansion and drilling gaps through interhole correlation of closely spaced measurements of core physical properties (see "Composite Depth").
