Information assembled in this chapter will help the reader understand the basis for our preliminary conclusions and also enable the interested investigator to select samples for further analysis. This information concerns only shipboard operations and analyses described in the site reports in the Initial Reports volume of the Leg 204 Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 204 data will be described in the individual contributions published in the Scientific Results volume and in publications in various professional journals.
The separate sections of the site chapters were written by the following shipboard scientists (authors are listed in alphabetical order; no seniority is implied):
The three-dimensional (3-D) seismic reflection data, collected before the cruise, imaged a 4 km x 11 km volume recorded to 3 s two-way traveltime (Tréhu and Bangs, 2001). The survey used a differential GPS navigation system provided by RACAL Geodetic. Four base stations (Vancouver, British Columbia, Canada; Washington state; northern California; and San Diego, California) provided differential corrections at 1 Hz. Fixes were smoothed with a 15-s running average filter to eliminate ship's motion and determine shot locations in real time. The single 600-m streamer was navigated with compass readings at 150-m intervals along the streamer. Streamer configuration was constructed from compass data for each shot. Tests of navigation accuracy conducted in port show that the ship's position fell within a 2-m radius 95% of the time. During the experiment, the streamer was located by reconstructing the streamer position from the compass readings. Locations are better in the direction parallel to the streamer than perpendicular to it, but both are probably within 5 m of uncertainty.
Horizontal and vertical resolution of the 3-D seismic images is dependent on the frequency content of the data, which becomes lower and more bandwidth limited with depth of penetration. The Hydrate Ridge 3-D seismic data have an approximate bandwidth of 25-200 Hz and a dominant frequency of 125 Hz at the seafloor. The vertical resolution of these data is therefore 3 m based on resolving distinctions of one-quarter of the dominant frequency's wavelength. The common midpoint spacing of 10 m in the in-line and 25 m in the cross-line direction effectively integrates the seismic acoustic impedance data over an estimated first Fresnel zone radius of ~75 m at the seafloor. First-order depth conversions of the seismic reflection data were calculated using velocities obtained from ocean-bottom seismometer data acquired with the 3-D data (Arsenault et al., 2001). These were further refined during the cruise using major seismic horizons correlated to the core and logging data and through velocities obtained from the wireline and core data.
During Leg 204, surface navigation consisted of dynamic positioning at the surface relative to an acoustic beacon placed on the seafloor at each drill site. A Global Positioning System (GPS) was used after the selective availability signal was removed, thereby providing the accuracy of P-code GPS. There is no navigation information for the bottom of the drill string; therefore, its exact position relative to the ship's position is unknown. We assume that the hole's position is directly below the rig floor. During previous drilling, deviation of the hole from the ship's position was determined from cores recovered at shallow depths by the advanced piston corer (APC). Little significant deviation from the vertical was noted.
Three standard coring systems were used during Leg 204: the APC, the extended core barrel (XCB), and the rotary core barrel (RCB). These standard coring systems and their characteristics are summarized in the "Explanatory Notes" chapters of various Initial Reports volumes as well as in Graber et al. (2002). Most cored intervals were ~9.6-m long, which is the length of a standard core barrel. In other cases, the drill string was "washed ahead" without recovering sediments in order to advance the drill bit to a target depth where core recovery needed to be resumed. In addition to these conventional coring tools several pressure coring systems (PCS) were used (see "Downhole Tools and Pressure Coring"). In situ temperature and pressure were measured at ~10 depths in each hole (see "Downhole Tools and Pressure Coring"). Logs of geophysical and geochemical parameters were obtained during logging while drilling (LWD) and using wireline tools (see "Downhole Logging").
Drilled intervals are referred to in meters below rig floor (mbrf), which are measured from the kelly bushing on the rig floor to the bottom of the drill pipe, and meters below seafloor (mbsf), which are calculated. When sediments of substantial thickness cover the seafloor, the mbrf depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first core. Water depth is calculated by subtracting the distance from the rig floor to sea level from the mudline measurement in mbrf. This water depth usually differs from precision depth recorder measurements by a few to several meters. The mbsf depths of core tops are determined by subtracting the seafloor depth (mbrf) from the core top depth (mbrf). The resulting core top data in mbsf are the ultimate reference for any further depth calculation procedures.
When cores are split, many show signs of significant sediment disturbance, including the concave-downward appearance of originally horizontal bedding, haphazard mixing of lumps of different lithologies (mainly at the tops of cores), fluidization, and flow-in. 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. These changes were particularly important during Leg 204 because temperature and pressure variations induce exsolution of gas from interstitial waters (IWs) and dissociation of gas hydrate, which also releases large amounts of gas. The "Lithostratigraphy" section in each site chapter discuss characteristics of cores that indicate this type of disturbance and can, therefore, serve as proxies for the presence of gas hydrates in situ.
Numbering of sites, holes, cores, and samples follows the standard Ocean Drilling Program (ODP) procedure (Fig. F1). A full curatorial identifier for a sample consists of the 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 204-1244A-1H-1, 10-12 cm, represents 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 1244A during Leg 204. Cored intervals are also referred to in "curatorial" mbsf. The mbsf 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 sediment core from less than a few hundred mbsf may, in some cases, expand upon recovery (typically 10% in the upper 300 mbsf), and its length may not necessarily match the drilled interval. In addition, a coring gap is typically present between cores. Thus, a discrepancy may exist between the drilling mbsf and the curatorial mbsf. For instance, the curatorial mbsf of a sample taken from the bottom of a core may be larger than that of a sample from the top of the subsequent core, where the latter corresponds to the drilled core top datum.
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 facies with downhole log signals.
Cores were generally handled according to the ODP standard core-handling procedures as described in previous Initial Reports volumes and the Shipboard Scientist's Handbook, with modifications required to quickly identify intervals containing gas hydrates and to maintain approximately sterile conditions for microbiological sampling. Precautions were also taken to identify and safely deal with hydrogen sulfide.
To identify gas hydrates, cores were scanned with both handheld and track-mounted infrared (IR) cameras. Intervals with significant thermal anomalies were marked with magic marker, and whole-round cores were removed and stored in liquid nitrogen or pressure vessels. This is explained further in "Physical Properties," and in individual site chapters. Some suspected gas hydrate-bearing intervals were cut and immediately dropped into liquid nitrogen-filled dewars on the catwalk. Others were rapidly frozen. Others were placed in pressure chambers. Others were allowed to dissociate for various shipboard IR calibration experiments or geochemical analyses.
Certain cores were identified ahead of time as being candidates for microbiological sampling and tagged with tracers (see "Microbiology"). When these cores were brought on board, appropriate sections for microbiological study were identified by the microbiologist on shift and taken immediately to the microbiology laboratory, which was located in a refrigerated van aft of the drilling floor.
Gas samples for routine shipboard safety and pollution-prevention samples were collected on the catwalk (see "Organic Geochemistry"). Whole rounds (10 to 20 m long) were taken for IW analysis (see "Interstitial Water Geochemistry"). The cores were then compressed with a plunger to remove large voids and cut into nominally 1.5-m sections. The remaining cut sections were transferred to the core laboratory for further processing.
Whole-round core sections not used for microbiological sampling were run through the multisensor track (MST), and thermal conductivity measurements were performed (see "Physical Properties"). The cores were then split into working and archive halves (from bottom to top). Investigators should be aware that older material may have been transported upward on the split face of each section.
Visual core descriptions (VCDs) were prepared of the archive halves augmented by smear slides and thin sections (see "Lithostratigraphy"). The archive halves were photographed with both black-and-white and color film. In addition, close-up photographs were taken of particular features for illustrations in site chapters, as requested by individual scientists. All sections of core not removed for microbiological sampling were additionally imaged using a digital imaging track system equipped with a line-scan camera. A few cores of special interest were measured for color reflectance and high-resolution magnetic susceptibility (MS) using the archive multisensor track (AMST) (see "Lithostratigraphy"), but this was not standard procedure for most cores because of time limitations. No paleomagnetic studies were done on board because of limited personnel. Moreover, shipboard biostratigraphy indicates that most cores were too young for magnetostratigraphy to have provided significant temporal resolution.
The working half was sampled both for shipboard analysis, such as physical properties, carbonate, and bulk X-ray diffraction (XRD) mineralogy and for shore-based studies (see "Physical Properties"). Both halves of the core were then put into labeled plastic tubes, sealed, and placed in cold storage space aboard the ship. At the end of the leg, the cores were transferred from the ship into refrigerated containers and shipped to the ODP Gulf Coast Core Repository in College Station, Texas.
1Examples
of how to reference the whole or part of this volume can be found under "Citations"
in the preliminary pages of the volume.
2Shipboard
Scientific Party addresses can be found under "Shipboard
Scientific Party" in the preliminary pages of the volume.
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