3. Explanatory Notes¹

Shipboard Scientific Party²

INTRODUCTION

In this chapter, we have assembled information that 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 Proceedings of the Ocean Drilling Program, Initial Reports, volume. Methods used by various investigators for shore-based analyses of Leg 184 data will be described in the individual contributions published in the Scientific Results volume and in publications within various professional journals.

Two coring systems were used during Leg 184: the advanced hydraulic piston corer (APC) and the extended core barrel (XCB). These systems were applied to maximize core recovery in the sediments being cored. Each cored interval was ~9.5 m long, which is the length of a core barrel. In some 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. Coring systems and their characteristics, such as drilling-related deformation, are summarized in the "Explanatory Notes" chapters of various previous Initial Reports volumes. The Leg 139 Initial Reports volume includes a particularly detailed description.

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 (as at all sites during Leg 184), 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 from the core-top depth in mbrf. The core-top datums (in mbrf) from the driller are the ultimate depth reference for any further depth calculation procedures.

Numbering of sites, holes, cores, and samples follows the standard Ocean Drilling Program (ODP) procedure. 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 184-1143A-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 1143A during Leg 184.

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 m), and its length may not necessarily match the drilled interval. In addition, a coring gap typically occurs between cores, as shown by composite depth construction (see "Composite Section"). 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.

During Leg 184, multiple APC holes (typically three) were cored at each 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 using closely spaced measurements of core physical properties (see "Composite Section").

General core-handling procedures are described in previous Initial Report volumes and the Shipboard Scientist's Handbook and are only summarized here. As soon as cores arrived on deck, gas void 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 obtained 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 extracted 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 described visually and by means of smear slides and then sampled both for shipboard analysis—such as physical properties, carbonate, and bulk X-ray diffraction mineralogy—and for shore-based studies. Shipboard sampling was kept at a minimum during Leg 184 to allow for construction of a detailed sampling plan after the composite section was built. The archive-half sections were measured for color reflectance, run through the cryogenic magnetometer, and photographed with both black-and-white and color film. Close-up photographs were taken of particular features for illustrations in site summaries, as requested by individual scientists.

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.