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doi:10.2204/iodp.proc.325.106.2011

Hole M0056A

Operations

Site 5, Hole M0056A

The Greatship Maya transited slowly to Site 5 (Hole M0056A), 66 m away from Hole M0055A, with the seabed template suspended in the water column. At 0935 h on 2 April 2010 the seabed transponder was deployed, and at 0945 h the template was lowered over Hole M0056A to 72 mbsl. Between 0955 and 1035 h, the API pipe and seabed template were lowered to just above the seabed, at which point the downpipe camera survey was conducted. The seabed template was lowered onto the seabed, and the API pipe tagged bottom at 1120 h. The API was then washed in 1.3 m. At 1230 h, the API pipe was broken and the elevators set on the slips prior to running HQ rods.

Coring operations started when the HQ rods tagged bottom at 1415 h and continued until 2100 h, when the HQ crossover and saver sub had to be replaced following failure of a weld. Coring restarted at 2140 h and continued until 0635 h on 3 April, when the hole was terminated at 41.29 mbsf with an average recovery of 30.8% (Table T1). The HQ rods were tripped by 0840 h and the top drive reconnected to the API pipe. The seabed template and API pipe were lifted to ~9 m above the seabed, and a downpipe camera survey was conducted. At 0935 h, the API pipe was tripped back to the bumper sub, and the seabed template was lifted into the moonpool in preparation for transit. At 1040 h, the seabed transponder was recovered, and the Greatship Maya departed Site 5 at 1050 h.

Sedimentology and biological assemblages

Hole M0056A is divided into four lithostratigraphic units.

Unit 1: Sections 325-M0056A-1R-1 to 2R-1, 127 cm: coralgal boundstone

The uppermost Unit 1, spanning Sections 325-M0056A-1R-1 to 2R-1, 127 cm, unconformably overlies Unit 2 and is coralgal boundstone composed mainly of corals thickly covered with encrusting nongeniculate coralline algae (Fig. F89). In situ fruticose (shrubby branched) nongeniculate coralline algae occur in interval 325-M0056A-2R-1, 87–94 cm (Fig. F90). The uppermost part (~110 cm) of Unit 1 is stained brown. Cavities with brown-stained walls occur throughout Unit 1 and are partly filled with unconsolidated sand dominated by Halimeda segments. Interval 325-M0056A-2R-1, 76–95 cm, contains cavities filled with gravelly internal sediment (Figs. F91, F92).

The upper part of Unit 1 is dominated by abundant encrusting Montipora colonies (Fig. F90) and possibly some encrusting Porites(?). This pattern transitions into dominance by massive Faviidae (e.g., Cyphastrea) (Fig. F93), submassive Porites, corymbose(?) Acropora, and encrusting Montipora(?) lower in the unit (intervals 325-M0056A-1R-2 through 2R-1).

Unit 2: Sections 325-M0056A-2R-1, 127 cm, to 7R-1, 47 cm: packstone with grainstone interlayers

Unit 2, spanning Sections 325-M0056A-2R-1, 127 cm, to 7R-1, 47 cm, unconformably underlies the coralgal boundstone of Unit 1. Unit 2 is packstone composed mainly of bioclasts of coral, Halimeda, nongeniculate coralline algae, and mollusks (Fig. F94). Dissolved segments of Halimeda create moldic porosity in some places, especially in intervals 325-M0056A-4R-1, 20–60 cm, and 4R-1, 60–65 cm, and Section 7R-1, 47 cm (Fig. F95). Cobble-sized lithoclasts of coralgal boundstone occur in some places, especially interval 325-M0056A-4R-1, 132–140 cm. Grainstone interlayers occur in two places: intervals 325-M0056A-4R-1, 108–141 cm, and 5R-1, 46–55 cm. The packstone in interval 325-M0056A-3R-1, 10–26 cm, has brown staining.

Coral dissolution increases in severity with increasing depth in this unit, and few corals are identifiable, apart from Section 325-M0056A-5R-1, which contains several encrusting to massive Porites and Faviidae. Identifiable corals include a few pieces of Porites, Montipora(?), Acroporidae, Agariciidae, and Fungiidae.

Unit 3: Sections 325-M0056A-7R-1, 47 cm, to 10R-1, 16 cm: coralgal-microbialite boundstone

Unit 3, spanning Sections 325-M0056A-7R-1, 47 cm, to 10R-1, 16 cm, is composed mainly of coralgal-microbialite boundstone (Fig. F96) in which thick microbialites with laminated fabrics cover corals. The corals also appear to be diagenetically altered (Figs. F97, F98). Nongeniculate coralline algae are a minor component. Depressions in microbialite surfaces are filled with bioclastic packstone containing Halimeda segments and molluscan shells (Fig. F99). Dissolution cavities filled by light gray, consolidated silt-sized internal sediment occur throughout Unit 3.

There are almost no corals in the uppermost and lowermost sections of Unit 3. Altered corals in the middle of Unit 3 are mainly encrusting Porites(?) or Montipora(?), with a few massive Faviidae (Fig. F100) and Porites(?).

Unit 4: Sections 325-M0056A-10R-1, 16 cm, through 16R-CC: packstone

The lowermost Unit 4, spanning Sections 325-M0056A-10R-1, 16 cm, to 16R-CC, 7 cm, consist of bioclastic packstone composed mainly of bioclasts rich in Halimeda segments (Fig. F101). Corals are common in Cores 325-M0056A-11R through 13R. Other bioclasts include nongeniculate coralline algae and mollusks. A grainstone interlayer occurs near the base of Hole M0055A in interval 325-M0055A-14R-1, 25–40 cm. Many bioclasts have dissolved, creating moldic porosity (Figs. F101, F102). Dissolution cavities may be filled with consolidated, gray, silt-sized internal sediment (Fig. F103).

Corals are diagenetically altered and vary in identity and abundance. Massive Porites and Galaxea are common in Sections 325-M0056A-13R-1, 13R-CC, and 15R-CC. Scarce corals in Sections 325-M0056A-11R-1 and 14R-1 are mainly encrusting Montipora(?), Porites, and Faviidae; other fragments include Isopora and Agariciidae.

Physical properties

Hole M0056A was cored to a total depth of 41.29 m DSF-A, of which 12.73 m was successfully recovered (30.83% recovery). Table T2 summarizes the physical property data for this hole.

Density and porosity

Multisensor core logger bulk density in Hole M0056A cores varies from 1.00 to 2.52 g/cm³ (Fig. F104). Similar to Hole M0055A, gamma density data quality is compromised by the fragmented nature of much of the recovered core, yielding data which often zig-zags down section. As such, it is extremely difficult to identify any downhole patterns. Measurements taken on 10 discrete samples (Fig. F105) indicate a range of porosity from 18% to 42% and variation in grain density from 2.68 to 2.79 g/cm³. As expected, bulk density from the discrete samples is at the high end of the range given by the multisensor core logger measurements, with values falling between 2.02 to 2.42 g/cm³.

P-wave velocity

There are no whole-core P-wave velocity data available for Hole M0056A. However, three discrete samples were taken from the hole for measurement on the P-wave logger. These samples have P-wave velocities in the range of 3715 to 4168 m/s (mean resaturated values) (Fig. F106A). These values are appropriate for well-lithified porous formations such as these. With this limited data set, no clear relationship exists between bulk density and velocity (Fig. F106B).

Magnetic susceptibility

Magnetic susceptibility values in Hole M0056A range from –1.22 × 10^–5 to 308.18 × 10^–5 SI, with the majority of values falling in the range of –1 × 10^–5 to 5 × 10^–5 SI. Two magnetic susceptibility highs occur at 26.14 m CSF-A (308.18 × 10^–5 SI) and 29.22 m CSF-A (36.63 × 10^–5 SI) (Fig. F104). Both of these highs occur within the packstone unit. No obvious downhole trends exist.

Electrical resistivity

Hole M0056A appears to identify a series of packages with decreasing resistivity downhole (Fig. F104). The most obvious of these packages can be observed in Cores 325-M0056A-1R, 4R, 8R, and 9R. Overall, electrical resistivity measurements range from 0.58 to 38.20 Ωm, and no obvious downhole trend exists.

Digital line scans and color reflectance

Cores from Hole M0056A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0056A shows no obvious trend with depth. Values are highly scattered for all sections. L* varies between 51.4% and 80.83% (Fig. F107). The first two sections measured (0–7 m CSF-A) are the ones with largest range of reflectance values. They correspond to the coralgal boundstone unit and bioclastic packstone interval. The range for L* narrows downsection, with values constrained between 60% and 80%, with the exception of two outliers with lower reflectance. The heterogeneity of the recovered core, in terms of both lithology and degree of fragmentation, makes it impossible to identify any pattern without a more detailed examination of the data. No relevant trends were found in the color indexes (a* and b*).

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on samples taken from the working half of the recovered core (Fig. F108). Positive susceptibilities were recorded in the majority of samples, ranging from 0.01 × 10^–8 to 3.68 × 10^–8, with an arithmetic mean of 0.91 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. In addition, nine negative susceptibilities (diamagnetic) were measured, present in two main intervals from 0 to 6 mbsf and 26 to 37 mbsf, ranging from –0.13 × 10^–8 to –1.92 × 10^–8, with an arithmetic mean of –0.67 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (34 cal y BP, Core 325-M0056A-2R; 39 cal y BP, Core 5R) (Fig. F109) and one U-Th age (84 cal y BP, Core 13R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is only slightly affected by corrections for initial ²³⁰Th (the seawater correction makes the age 0.5 k.y. younger). Therefore, this hole recovered material older than the Last Glacial Maximum from marine isotope Stages 5a and 3. It is likely that this hole may also have recovered material from marine isotope Stage 4 between Cores 325-M0056A-5R and 13R.

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