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doi:10.2204/iodp.proc.325.103.2011

Hole M0032A

Operations

Site 6, Hole M0032A

At 0952 h on 16 February 2010, the Greatship Maya was settled on station over Hole M0032A and a downpipe camera survey was conducted (Table T1). At 1050 h, an additional API pipe was added, and coring operations commenced at 1115 h using metal splits. The string overpressurized on the first run, and the standard rotary corer was tripped to free it. There was also evidence of hole caving and overpressurizing on the second run. However, coring then progressed steadily until 2355 h, when the bit became blocked with sediment during Run 12 (liner tubes were used for Runs 5–15 instead of the metal splits). Runs 14–16 (metal splits used for Run 16) took from 0155 to 0515 h on 17 February to complete. Between 0515 and 0555 h there was a delay in operations for refueling the hydraulic power packs. Coring then continued to the end of the hole at 0925 h at 36.7 mbsf (average recovery = 16.3%).

The API pipe was tripped to 5 m above the seabed, and at 1015 h the downpipe camera was deployed for a seabed survey. Once this was completed and the camera was recovered at 1110 h, the hydraulic power packs were shut down for maintenance until 1330 h, when the vessel departed for Site 6, Hole M0033A.

Sedimentology and biological assemblages

Hole M0032A is divided into four distinct lithostratigraphic units.

Unit 1: Section 325-M0032A-1R-1: coralgal boundstone

The uppermost Unit 1, consisting only of Section 325-M0032A-1R-1, is composed of fragments of coralline algae and coral bindstone with serpulid worm tubes, bivalves, gastropods, and Halimeda. Recently living coralline algae, now dead, are present at the very top of the section. Reddish staining is common within the bindstone.

The dominant corals are submassive to massive colonies of Montipora (Fig. F19) plus fragments of Montipora and some foliaceous coral fragments.

Unit 2: Sections 325-M0032A-1R-CC through 14R-CC: coralgal-microbialite boundstone

The thick Unit 2, spanning Sections 325-M0032A-1R-CC through 14R-CC, consists of coralgal-microbialite boundstone (Fig. F20). The boundstone is composed mainly of massive corals covered with crusts of nongeniculate coralline algae, which, in turn, are covered by thick microbialites. In some cases, coralline algae alternate with microbialite crusts (Fig. F21). Microbialites are dark colored and generally weakly to clearly laminated, with alternating darker and lighter laminae (Fig. F20). Bioclasts (Halimeda segments, molluscan shell fragments, and foraminifera) are imbedded in the microbialites. Some internal bioclastic sediment is also visible. Reddish-stained cavities are scattered through Section 325-M0032A-11R-1 (Fig. F22).

The dominant corals are massive Isopora (Fig. F23) and Faviidae, followed by branching Pocillopora, Acropora, and Seriatopora (Fig. F24). Associated corals include submassive Porites, Faviidae, Tubipora musica, Leptoseris(?), and Montipora(?). The same taxa are also abundant as small fragments. Corals are bioeroded locally.

Unit 3: Sections 325-M0032A-15R-1 through 16R-CC: unconsolidated sediment

Unit 3, spanning Sections 325-M0032A-15R-1 through 16R-CC, consists mostly of unconsolidated lime sand with lime granules and pebbles. Major components are fragments of coral, some with coralline algal crusts. The larger foraminifera Baculogypsina, Calcarina, Gypsina, and Sphaerogypsina are common in sandy gravel sediments from interval 325-M0032A-16R-1, 0–5 cm. Mollusks, Halimeda, and echinoid spines also occur. Some of these components may represent downhole contamination during the coring operation.

Corals are present only as fragments and include Seriatopora, Pocillopora, Acropora, Leptoseris, Tubipora musica, and Pachyseris(?).

Unit 4: Sections 325-M0032A-18R-1 through 19R-CC: packstone/grainstone

The lowermost Unit 4, spanning Sections 325-M0032A-18R-1 through 19R-CC, consists mainly of packstone/grainstone to rudstone with mollusks, coral, Halimeda, echinoderms, and bryozoans. Gravelly sand sediments from interval 325-M0032A-18R-CC, 0–5 cm, contain abundant well-preserved specimens of the larger foraminifer Amphistegina. However, Amphistegina specimens became scarce in the muddy sand from interval 325-M0032A-19R-1, 12–17 cm. An interval in Section 325-M0032A-19R-1 is composed of fragments of corals encrusted by coralline algae and is intercalated with the packstone/grainstone.

Larger corals are Isopora and Faviidae, associated with fragments of Acropora, Isopora, Montipora, Seriatopora, Faviidae, Tubipora musica, and Stylasterinidae(?).

Physical properties

Hole M0032A was drilled to 36.70 m DSF-A. A total of 5.99 m was recovered (16.32% recovery). Table T2 summarizes the physical property measurements taken on this core.

Density and porosity

Bulk density values from whole-core multisensor core logger measurements range from 1.04 to 2.54 g/cm³ (Fig. F25). The bulk density of discrete samples varies between 2.04 and 2.41 g/cm³, and porosity varies between 20% and 41% (Fig. F26). Grain density is relatively constant with a range of 2.74 to 2.79 g/cm³. The heterogeneity downhole is significant and is similar to the previous hole (M0031A). However, the density and porosity parameters have lower variability in this borehole.

P-wave velocity

P-wave velocity measurements taken on whole cores offshore ranged from 1780.70 to 1937.94 m/s (Fig. F25). Only one discrete P-wave velocity measurement was possible in Hole M0032A, sampled at 16.38 to 16.40 m CSF-A. The drilled plug is a mixture of coral and coralline algae and has a mean resaturated velocity of 5037 m/s.

Magnetic susceptibility

In Hole M0032A, multisensor core logger magnetic susceptibility values range from –1.90 × 10^–5 to 12.07 × 10^–5 SI, with most values being close to zero. There are two clear exceptions to this where there are magnetic susceptibility highs. The first of these is encountered in Section 325-M0032A-4R-1 (7.95 m CSF-A), where values climb to 5.69 × 10^–5 SI. The second is deeper in the hole (17.61 m CSF-A) in Section 325-M0032A-10R-1, where the maximum magnetic susceptibility value for the hole is reached (12.07 × 10^–5 SI).

Electrical resistivity

Electrical resistivity is affected by pore fluid and salinity, as well as core liner saturation and core quality. In Hole M0032A, resistivity ranges from low values of 0.59 Ωm to higher values of 12.10 Ωm (Fig. F25). The highest resistivity occurs in Sections 325-M0032A-1R-1 and 8R-2. The lowest resistivity is registered at ~12–13 m CSF-A.

Digital line scans and color reflectance

All cores from Hole M0032A were measured using the digital line scan system with all data recorded at a resolution of 150 pixel/cm as both images and RGB values. All appropriate cores were also scanned for color reflectance. Color reflectance L* in Hole M0032A varies between 45.03% and 82.69% (Fig. F27). Variations in color reflectance parameters slightly decrease in reflectance with depth. The first unit (0–5 m CSF-A), which presented a heterogeneous lithology, showed a wider spectrum of values in the color reflectance spectrophotometry. Lithologies in the deepest part of Hole M0032A (18–33 m CSF-A) were very homogeneous in color reflectance characteristics, despite the occurrence of several different lithostratigraphic units (e.g., coralgal-microbialites, unconsolidated sediments, and packstones). The presence of a slight change in color is clearer in the ratio a*/b*, which is a better indicator of cyclic variations (Blum, 1997).

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. F28). Low positive magnetic susceptibilities occur throughout the core, ranging from 0.019 × 10^–8 to 0.68 × 10^–8 m³/kg. The measured arithmetic mean was 0.27 × 10^–8 m³/kg. Four negative susceptibilities occur at 5.06, 17.62, 23.90, and 32.81 mbsf with susceptibility values of –0.34 × 10^–8, –0.44 × 10^–8, –0.02 × 10^–8, and –0.15 × 10^–8 m³/kg, respectively.

Chronology

Two calibrated radiocarbon ages (13 cal y BP, Core 325-M0032A-1R; 15 cal y BP, Core 3R) (Fig. F29) and two U-Th ages (20 cal y BP, Core 8R; 61 cal y BP, Core 18R) (Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The older U-Th age is unaffected by corrections for initial ²³⁰Th, adding to the confidence in this age interpretation. However, the younger age of 20 cal y BP from Core 325-M0031A-8R is made significantly younger by these corrections. This hole recovered material from the Last Glacial Maximum interval and captured the early portion of the deglaciation to ~13 cal y BP. The deeper portions of this hole recovered older Pleistocene material.

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