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doi:10.2204/iodp.proc.325.106.2011

Hole M0055A

Operations

Site 5, Hole M0055A

The Greatship Maya was on station at Site 5, Hole M0055A, at 1230 h on 1 April 2010, and the seabed transponder was deployed. Once completed, the seabed template was lowered to ~10 m off the seabed and API pipe was run to just above the seabed. The downpipe camera survey was conducted by 1530 h, and the API pipe tagged the seabed at 1545 h. By 1620 h, the API pipe had been washed down 2 m. The API pipe was then disconnected and set on elevators on the drill floor. Following this, HQ rods were run, tagging the bottom (3.29 mbsf) at 1920 h. HQ coring operations continued until 0555 h on 2 April, when the hole was terminated at 31.29 mbsf with an average recovery of 35% (Table T1).

The HQ rods were tripped by 0805 h, and the API pipes were clear of the seabed by 0845 h. The seabed template was lifted above the seabed, and the seabed transponder was recovered. The Greatship Maya departed Hole M0055A at 0920 h.

Sedimentology and biological assemblages

Hole M0055A is divided into five lithostratigraphic units.

Unit 1: 325-M0055A-1R-1, 0–40 cm: unconsolidated mud with sand

The uppermost Unit 1, consisting of interval 325-M0055A-1R-1, 0–40 cm, is composed of unconsolidated mud with sand that includes some bioclasts (e.g., Halimeda, coral, and small benthic foraminifera). Well-preserved specimens of small benthic foraminifera (e.g., Miliolida and Cibicidoides) are common in mud from interval 325-M0055A-1R-1, 10–15 cm. Larger foraminifera and planktonic foraminifera are not present. There are no visible corals.

Unit 2: Sections 325-M0055A-1R-1, 40 cm, through 4R-2: coralgal-microbialite boundstone

Unit 2, spanning Sections 325-M0055A-1R-1, 40 cm, to 4R-2, 150 cm, consists of coralgal-microbialite boundstone (Figs. F67, F68) in which corals are covered with nongeniculate coralline algae or microbialite. Most nongeniculate coralline algae are thick encrusting forms. Locally, microbialite has a laminated fabric. Corals are bioeroded, and borings are partly to fully filled with fine sand. Depressions and spaces between coralgal-microbialite frameworks are filled with unconsolidated or consolidated calcareous sand with bioclasts of Halimeda, coral, and nongeniculate coralline algae.

A diverse coral assemblage is dominated by several branching Acropora species (Figs. F69, F70) to the base of Section 325-M0055A-3R-1. The remainder of the interval is dominated by massive Isopora (Fig. F71), Porites, and Faviidae (Fig. F72); large Tubipora musica (Fig. F73); submassive Goniopora(?); and branching Pocilloporidae, including Stylophora(?) and Seriatopora. Visible fragments are scarce but include Acroporidae and Tubipora musica.

Unit 3: Section 325-M0055A-5R-1: coralgal boundstone

Unit 3, consisting only of Section 325-M0055A-5R-1, is coralgal boundstone containing corals thickly but irregularly encrusted by nongeniculate coralline algae (Fig. F74). Some corals are bioeroded, and the borings are filled with consolidated or unconsolidated sediment. Spaces within the coralgal framework are filled with multigenerational consolidated sediments rich in bioclasts of Halimeda, coral, and mollusks (Fig. F75).

Dominant corals are massive Porites (Fig. F76) with some massive Faviidae (Fig. F77), branching Acropora, and encrusting Montipora(?).

Unit 4: Sections 325-M0055A-5R-2 through 6R-CC: coralgal boundstone with grainstone

Unit 4, spanning Sections 325-M0055A-5R-2, 0 cm, through 6R-CC, is composed entirely of coralgal boundstone in which corals are thickly covered with nongeniculate coralline algae (Fig. F78). Nongeniculate coralline algae are volumetrically more important than corals. Coral skeletons may be diagenetically altered into calcite (Fig. F79). Cavities are filled with bioclastic grainstone/packstone rich in corals, mollusks, Halimeda, and nongeniculate coralline algae fragments. The cavity walls have brown staining throughout the unit (Fig. F80). These features indicate possible subaerial exposure and subsequent meteoric diagenesis before deposition of the grainstone/packstone filling.

The few corals present are diagenetically altered and difficult to identify. They include branching Pocilloporidae (Fig. F81) and Acroporidae (including possibly corymbose Acropora) and an unidentified massive coral.

Unit 5: Sections 325-M0055A-7R-1 through 10R-1: grainstone

The lowermost Unit 5, spanning Sections 325-M0055A-7R-1 through 10R-1, consists mainly of bioclastic grainstone with pale brown staining. The grainstone contains abundant bioclasts of Halimeda, mollusks, and corals (Fig. F82). The corals are diagenetically altered, possibly indicating subaerial exposure. Recognizable corals include pebbles of Acroporidae at the top of the unit and larger pieces of massive Faviidae and one piece of Porites lower in the unit.

Physical properties

Hole M0055A was cored to a total depth of 31.29 m DSF-A, of which 10.00 m was successfully recovered (35% recovery). Petrophysical data are summarized in Table T2.

Density and porosity

Bulk density multisensor core logger (MSCL) measurements vary from 1.00 to 2.41 g/cm³ in cores from Hole M0055A (Fig. F83). Despite core recovery being relatively high in comparison with other holes and the fact that core length is also higher (average = 0.76 m), the quality of the bulk density data is not very good because core quality is poor (see “Physical properties” in the “Methods” chapter), with many of the cores composed of either biscuited or very fragmented material. Overall, no evidence for a downhole trend or any intervals where data are significantly different exists. Twelve measurements were taken on discrete samples (Fig. F84), with bulk density data ranging from 1.95 to 2.44 g/cm³. Figure F84 illustrates the fact that poor core quality leads to an underestimate of bulk density from the MSCL measurement. As in Hole M0053A, coralgal-microbialite and coralgal boundstone units dominate between 4 and 16 m CSF-A, and the porosity varies between 13% and 48%. Grain density for these discrete samples varies between 2.66 and 2.83 g/cm³.

P-wave velocity

MSCL P-wave velocity data are only available for the first core in Hole M0055A, and values range from 1510 to 1647 m/s (Fig. F83). As with many of the other holes, these values are very close to the velocity of seawater, which may be indicative of poor core quality giving an underestimate of the actual core density. Six core plugs were taken from Hole M0055A cores and were measured with the P-wave logger. Velocities measured on these samples range from 3591 to 4256 m/s (Fig. F85A). In discrete samples measured from Hole M0055A, P-wave velocity exhibits no correlation with bulk density (Fig. F85B).

Magnetic susceptibility

The top of Hole M0055A (Section 325-M0055A-1R-1) can be distinguished from the remainder of the hole by elevated magnetic susceptibility values reaching a maximum of 395.32 × 10^–5 SI (Fig. F83). This section corresponds to a lithostratigraphic unit composed of unconsolidated mud and sand. From Section 325-M0055A-2R-1 to total depth, values remain in the range of –1 × 10^–5 to 10 × 10^–5 SI.

Electrical resistivity

Noncontact resistivity, measured on whole cores, is highly variable and ranges from 0.33 to 77.41 Ωm (Fig. F83). Despite the questionable core quality (see “Physical properties” in the “Methods” chapter), the quality of some of the data is good, with smooth curves being evident in downcore sections. The central portion of this hole yields higher resistivity values than the top and bottom intervals (a similar trend to the magnetic susceptibility data). This central interval, between ~6 and ~17 m CSF-A, corresponds to the dominantly coralgal-microbialite and coralgal boundstone units.

Digital line scans and color reflectance

Cores from Hole M0055A were digitally scanned and, where appropriate, measured for color reflectance. Hole M0055A has a range of reflectance from 46.36% to 83.07% for L* (Fig. F86). In the upper sections (3.30–4.10 m CSF-A), values evolve from lower reflectance to higher reflectance downhole because of changes in lithology from lime mud to sandy mud and then to boundstone pebbles. From 6.30 to 16.71 m CSF-A, values obtained for reflectance are dispersed. This dispersion appears to be related to the presence of a coralgal-microbialite boundstone unit with heterogeneous color. In this area, some measurements indicate a high value for red color (11.14 m CSF-A) coincident with the presence of Tubipora sp. The last two sections measured (top depths of 19.5 and 22.5 m CSF-A) correspond to grainstones with Halimeda sp.

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. F87). Positive low susceptibilities occur for most of the core, ranging from 0.06 × 10^–8 to 11.64 × 10^–8 with an arithmetic mean value of 2.14 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. In addition, two negative susceptibilities were measured at 16.31 and 28.31 mbsf with values of –0.39 × 10^–8 and –0.35 × 10^–8 m³/kg, respectively.

Chronology

Two calibrated radiocarbon ages (14 cal y BP, Core 325-M0055A-1R; 23 cal y BP, Core 4R) (Fig. F88) and one U-Th age (25 cal y BP, Core 5R) (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 1.0 k.y. younger). This hole recovered material from the Last Glacial Maximum, as well as the early deglacial. This hole has a substantial amount of recovery from the Last Glacial Maximum and potentially older material because there are five cores below Core 325-M0055A-5R, which is dated to 25 cal y BP.

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