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Expedition 307 Scientists2


Information assembled in this chapter will help the reader understand the basis for our preliminary conclusions and 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 Expedition Reports section of the Expedition 307 Proceedings of the Integrated Ocean Drilling Program (IODP) volume. Methods used by various investigators for shore-based analyses of Expedition 307 samples will be described in the individual contributions published in the Research Results section of the Expedition 307 Proceedings volume and in publications in various professional journals.

Authorship of site chapters

The separate sections of the site chapters were written by the following shipboard scientists (listed in alphabetical order, no seniority is implied):

  • Expedition summary and principal results: Expedition 307 Scientists
  • Background and objectives: Timothy Ferdelman, Akihiro Kano
  • Operations: Derryl Schroeder, Trevor Williams
  • Lithostratigraphy: Miriam Andres, Morten Bjerager, Boris Dorschel, Jay Gregg, Xianghui Li, Ivana Novosel, Keiichi Sasaki, Chizuru Takashima, Jürgen Titschack
  • Biostratigraphy: Kohei Abe, Emily Browning
  • Paleomagnetism: Anneleen Foubert, Yuji Fuwa
  • Geochemistry and microbiology: Barry Cragg, Tracy Frank, Jim Gharib, Philippe Léonide, Kai Mangelsdorf, Saburo Sakai, Vladimir Samarkin, Arthur Spivack
  • Physical properties and stratigraphic correlation: Veerle Huvenne, Ben De Mol, Xavier Monteys, Akiko Tanaka
  • Downhole logging: Philippe Gaillot

Drilling operations

Three standard coring systems were used during Expedition 307: the advanced piston corer (APC), the extended core barrel (XCB), and the rotary core barrel (RCB). These standard coring systems and their characteristics are summarized in the “Methods” chapters of previous Proceedings volumes as well a number of Ocean Drilling Program (ODP) Technical Notes. Most 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 resume.

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 from the length of pipe deployed less estimated seafloor depth. 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 meters, though the difference can be larger in areas of steep seafloor slopes. The mbsf depths of core tops are determined by subtracting the seafloor depth (mbrf) from the core-top depth (mbrf). The resulting core-top datums in mbsf are the ultimate reference for any further depth calculation procedures.

Drilling deformation

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. The latter three disturbances can be particularly severe during XCB and RCB drilling. 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 follows standard IODP procedure. A full curatorial identifier for a sample consists of the expedition, 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 307-U1316A-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) of Hole U1316A during Expedition 307. 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 sediment core from less than a few hundred mbsf may, in some cases, expand upon recovery (typically 10% in the uppermost 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. See the discussions in “Downhole measurements” and “Physical properties” on the prospects for correlation of the curatorial depths (mbsf) or meters composite depths (mcd) with logging information to obtain a “true” depth interval.

Core handling and analysis

The original drilling strategy of Expedition 307 was to first drill Hole A using APC and XCB drilling for stratigraphy and develop a coarse geochemical framework (sulfate and methane) for making drilling decisions on holes assigned for microbiological sampling. After downhole logging of Hole A, Hole B would be dedicated to high-resolution microbiological and geochemical sampling and drilled to depth of APC refusal. Hole C would be drilled by RCB and dedicated to high-resolution stratigraphy, filling any coring gaps identified by stratigraphic correlation of the first two holes (see “Composite section development”) and extending the microbiology and geochemical sampling studies. However, after Site U1316, drilling contingencies forced us to alter our plan. For Site U1317, microbiological and geochemical sampling proceeded first and at relatively high resolution in Hole U1317A. Although sections for microbiological analysis were missing, opening cores from Hole U1317A allowed the sedimentologists to gather immediate information concerning the mound facies. This critical information further led to our decision to freeze and saw coral-bearing sections from subsequent holes (see “Frozen cores”).

Drilling during Expedition 307 entailed two changes to the normal core flow in the laboratories. First, the presence of coral, coral rubble, and dropstones meant that cores could not be split using the normal core cutting techniques employed for nonlithified sediments. Second, a strong microbiological and geochemical component of Expedition 307 necessitated the adoption of a core flow scheme to deal with sensitive microbiological samples. Upon arrival on the catwalk, routine shipboard safety and pollution prevention samples along with acridine orange direct count (AODC) syringe samples and methane gas samples were collected (see “Geochemistry and microbiology”). A sample was removed from the core catcher and taken to the micropaleontology laboratory for rapid biostratigraphic determination. When the core was cut into sections, whole-round samples were taken for shipboard interstitial water examinations. On nonmicrobiological core sections, whole cores were run through a “Fast Track” multisensor core logger (MSCL) equipped with two magnetic susceptibility loops to facilitate real-time drilling decisions concerning the presence of corals and to maximize stratigraphic overlap between holes (see “Composite section development”). Whole-round core sections were then run through the multisensor track (MST), and thermal conductivity measurements were taken. At Site U1317, the presence of corals meant that a large fraction of cores were not split on board (see “Frozen cores”). Because these sediments were likely to contain magnetic sulfide minerals that can rapidly decompose in air, cores were measured on the cryogenic magnetometer as whole rounds.

The working half of each core was sampled for basic shipboard analysis (i.e., physical properties, carbonate, and bulk X-ray diffraction mineralogy) and for ephemeral minerals such as greigite for shore-based studies. In general, however, shipboard sampling was kept to a minimum during Expedition 307 to wait for opening and description at the IODP Bremen Core Repository (BCR). When short pieces of sedimentary rock were recovered, the individual pieces were split with the rock saw and placed in split liner compartments created by sealing spacers into the liners with acetone.

The archive-half sections were scanned on the digital imaging system, measured for color reflectance on the archive multisensor track (AMST), described visually and by means of smear slides, run through the cryogenic magnetometer, and finally photographed with color film one whole core at a time. Digital 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 expedition in Mobile, Alabama (USA), the cores were transferred to the IODP Gulf Coast Core Repository (GCR) and then on to BCR, where core description and sampling parties took place in October 2005.

Frozen cores

Coral-bearing cores pose problems for conventional core splitting in that the wire drags coral pieces up the core face or breaks them into fragments. Sawing unfrozen cores produced a surface layer of grime, coral fragments, and matrix that could not be scraped off without damaging the sedimentary structures. Depending on the coral preservation, freezing the cores before splitting results in undamaged split cores with a clean split-core face (this method had been successful for short cores from a nearby location collected by the Marion Dufresne). Approximately 67 cores were coral-bearing and thus suitable for frozen splitting (Site U1317, 0–155 mbsf; Site U1316, 50–55 mbsf):

  • Cores from Hole U1317A were split conventionally with the saw to establish lithostratigraphy and coral content and to check the state of the split core face.
  • Core U1316B-6H was the first test of the freeze and split procedure.
  • Cores U1317C-1H through 6H were frozen and split during the expedition.
  • Cores U1317C-7H through 16H were frozen and split during the transit from Ponta Delgada, Azores (Portugal), to Mobile.
  • Cores from Holes U1317B and U1317E were frozen and split at GCR in August 2005.

The procedure for frozen-core splitting is as follows. A core patch is placed around the liner because the butyrate liner becomes brittle and fragile after freezing. The cores are frozen at between –20° and –50°C. The temperature range of the onboard freezers was between –50° and –86°C. A balance has to be struck: too warm and the core starts to melt during splitting, too cold and the butyrate liner becomes more brittle and the risk of sediment damage increases. The cores were split with a saw and transferred to new liners while frozen, in which state the cores can generally be lifted without breaking. The cores were warmed to about –10°C in the walk-in freezer, and the core surface was scraped clear of grime.

Some adjustments to conventional core flow were required. Cores frozen and split on board had to be cut into 1.2 m lengths in order to fit into the upright freezer and were run through the MST and cryogenic magnetometer (see “Paleomagnetism”) before freezing. Cores reentered the core flow after splitting and out of sequence because of the >24 h freezing and splitting process. Cores left unopened on board for freezing and splitting at GCR were cut into conventional 1.5 m lengths and measured on the shipboard MST and cryogenic magnetometer. Minimum required measurements (core photos, color reflectance, and lithological description) were done postcruise.

Microbiological sampling

Two additional objectives of sampling during Expedition 307 were to obtain (1) high-resolution interstitial water geochemical profiles to delineate microbial reaction zones and evaluate potential zones of hydrocarbon flow and (2) corresponding samples for a variety of microbiological and biogeochemical analyses to be performed primarily in shore-based laboratories. Since microorganisms existing at deepwater seafloor temperatures (2°–4°C) can be acutely sensitive to elevated temperature (>10°C) and oxygen, we recognized a critical need to prevent thermal equilibration and exposure of the cores to oxygen after recovery. Sampling strategies, core flow, and subsampling for shore-based laboratories generally followed a modified scheme developed during Ocean Drilling Program (ODP) Leg 201. To minimize equilibration of the cores, the core barrel was extracted from the drill string and immediately transferred to the catwalk and marked by the IODP curatorial staff into 1.5 m sections. Shipboard microbiologists identified one or more 1.5 m sections (hereafter referred to as the MBIO sections) for rapid microbiological processing. Once the MBIO sections were selected, they were labeled with a red permanent marker with orientation and section number and removed from the core. Ends of the removed sections were covered with plastic caps but not sealed, and the sections were carried into the hold refrigerator, which was set to ~4°C and served as a microbiology cold room. Multiple sections were moved to the cold room to ensure that sufficient undisturbed material was available for microbiology and geochemistry sampling. At Site U1318, the sections identified for microbiology were first run through the Fast Track MSCL (a 5–10 min delay). Because of the short duration of scientific operations, the primary emphasis was on cleanly cutting and properly sampling the requisite number of whole-round cores for shore-based experimentation and analysis, as described in “Whole-round core sampling in the cold room.” Unsampled microbiological subsections and the remainder of the core on the catwalk were processed as described above. Frozen and cooled microbiology and geochemistry samples were shipped to shore-based laboratories from Ponta Delgada.

1 Expedition 307 Scientists, 2006. Methods. In Ferdelman, T.G., Kano, A., Williams, T., Henriet, J.-P., and the Expedition 307 Scientists. Proc. IODP, 307: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.307.102.2006

2 Expedition 307 Scientists’ addresses.

Publication: 14 October 2006
MS 307-102