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doi:10.2204/iodp.proc.325.103.2011

Hole M0037A

Operations

Site 9, Hole M0037A

The seabed transponder was deployed at 0640 h on 2 March 2010, and by 0700 h, the vessel was in position above Site 9, Hole M0037A (Table T1). Running of API pipe commenced at 0745 h, but repairs to a hydraulic hose on the elevator after a collision with the roughneck halted operations between 0820 and 0950 h. Running of the API pipe restarted at 0950 h, and by 1100 h, the pipe was situated 10 m above the seabed and the downpipe camera was being deployed. The camera survey was completed by 1140 h, and preparations to core began. The first standard rotary corer core was recovered at 1330 h, and coring continued successfully until 0055 h on 4 March, when the hole was terminated at 21 mbsf with an average recovery of 35.8%.

The API pipe was tripped to 10 m above the seabed between 0055 and 0230 h, when the vessel moved to recover the seabed transponder. At 0250 h, the Greatship Maya began to move slowly, under dynamic positioning, to Site 8, Hole M0038A.

Sedimentology and biological assemblages

Hole M0037A is divided into three lithostratigraphic intervals.

Unit 1: Sections 325-M0037A-1R-1 to 8R-1, 60 cm: unconsolidated sediment

The uppermost Unit 1, spanning Sections 325-M0037A-1R-1 to 8R-1, 60 cm, consists of unconsolidated lime granules and pebbles with minor amounts of sand. Major components include fragments of Halimeda, mollusks, corals, and bryozoans, with minor coralline algae and echinoids. Fragmented specimens of the larger foraminifera Alveolinella, Amphistegina, Elphidiidae, Heterostegina, Operculina, and Soritinae are common in muddy gravel from interval 325-M0037A-1R-1, 70–75 cm, whereas the muddy gravel from interval 11R-2, 10–15 cm, commonly contains well-preserved specimens of Amphistegina, Alveolinella, Elphidiidae, and Soritinae. Some grains appear to be microbialite boundstone and packstone. The boundstone fragments are typical of the present-day seafloor samples recovered in Section 325-M0037A-6R-1 and are indicative of some downhole contamination.

Coral fragments of Seriatopora are abundant in most sections. Other fragments include Acropora, Isopora, Tubipora, and Pocillopora(?).

Unit 2: interval 325-M0037A-8R-1, 60–70 cm: grainstone

Unit 2, consisting only of interval 325-M0037A-8R-1, 60–70 cm, is composed of gray grainstone (Fig. F93) with well-sorted grains of larger foraminifera, mollusks, and corals. Coral fragments include Seriatopora, Acropora, and Pocilloporidae.

Unit 3: Sections 325-M0037A-8R-CC through 13R-1: lime sand and pebbles

The lowermost Unit 3, spanning Sections 325-M0037A-8R-CC through 13R-1, consists of lime pebbles and granules in a fine to coarse sand matrix. Major components are larger foraminifera. Well-preserved and stained specimens of Alveolinella, Amphistegina, Baculogypsina, and Operculina are common in interval 325-M0037A-9R-1, 30–35 cm, and well-preserved and stained specimens of Alveolinella, Amphistegina, Elphidiidae, Miliolida, and Operculina are common in interval 10R-1, 70–75 cm. Specimens of Alveolinella, Elphidiidae, and Operculina are rare in the medium to very coarse sand-sized sediments from interval 325-M0037A-13R-1, 25–30 cm.

Bivalve/gastropod shells and Halimeda were also major components of Unit 3. Echinoid fragments are minor elements, and black grains, probably microbialite, are also present. Foraminiferal tests and bivalve shells have a distinct subhorizontal orientation in undisturbed portions of the cores (Fig. F94). Accumulations of pebbles of diverse composition at the top of all cores in this unit clearly indicate downhole contamination (see Fig. F9 in the “Methods” chapter).

Seriatopora fragments are present throughout Unit 3, with some Pocilloporidae near the top.

Physical properties

Hole M0037A was drilled to 21.00 m DSF-A with a total of 7.52 m of core recovered (35.81% recovery). Petrophysical data are compiled in Table T2.

Density and porosity

In Hole M0037A, bulk density values from whole-core multisensor core logger (MSCL) measurements range from 1.17 to 2.54 g/cm³ (Fig. F95). Owing to reasonable core recovery and better core quality in this hole, the dataset is much more complete than for many of the holes in this transect. Bulk density was also measured on 12 discrete samples with values ranging from 1.94 to 2.40 g/cm³ and porosity values ranging from 23% to 48% (Fig. F96). Grain density fluctuates between 2.73 and 2.81 g/cm³. MSCL density data correspond well with the discrete density measurements.

P-wave velocity

Whole-core MSCL measurements give P-wave velocity data for Hole M0037A ranging from 1500.34 to 1872.03 m/s (Fig. F95). The minimum values obtained indicate unrealistic values for sediment (1500 m/s is the velocity of seawater) (see “Physical properties” in the “Methods” chapter). The P-wave record in Hole M0037A is incomplete, with data only available from the top of the hole (0–1.2 m CSF-A) and from 11.2 to 15.6 m CSF-A. It is therefore difficult to identify any downhole trends in the dataset. No P-wave measurements were taken on discrete samples from Hole M0037A.

Magnetic susceptibility

Similar to other holes in this transect, the majority of magnetic susceptibility measurements for Hole M0037A are close to zero (Fig. F95). There are three distinct highs in the bottom half of this hole that show smooth curves peaking at 55.97 × 10^–5 SI (Section 325-M0037A-9R-1; 12.69 m CSF-A), 38.27 × 10^–5 SI (Section 10R-1; 14.15 m CSF-A), and 46.56 × 10^–5 SI (Section 11R-1; 15.66 m CSF-A). The data range for this hole is –1.31 × 10^–5 to 55.97 × 10^–5 SI.

Electrical resistivity

Noncontact resistivity measurements on whole cores from Hole M0037A generate data ranging from 0.65 to 2.54 Ωm (Fig. F95). Although the dataset is patchy (a consequence of core recovery), there are some zones of better recovery where there are intervals that have clear trends of increasing resistivity with depth. Most notably, these trends occur in the intervals 0–1.8 m CSF-A (from 0.9 to 1.8 Ωm) and 14–15.4 m CSF-A (0.6 to 1.4 Ωm).

Digital line scans and color reflectance

All cores from Hole M0037A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance L* in Hole M0037A varies between 35.12% and 73.35% (Fig. F97). The cores were more homogeneous than those from other boreholes in this transect. Several areas were identified by visual description as being disturbed by drilling and, as such, contained a higher quantity of loose broken fragments. In general, these intervals were not measured for color reflectance because of the lack of flat, smooth surfaces in the core. Toward the top of Hole M0037A, a wide distribution of color reflectance values is coincident with the coarser, fragmented intervals. In core sections deeper in the hole where fragmentation is less prevalent, the deviation from the mean is generally less.

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. F98). Low positive susceptibilities were measured throughout the core, with an arithmetic mean value of 0.33 × 10^–8 m³/kg. At the top of the core, four positive susceptibilities were recorded between 0.1 and 1.39 mbsf. A higher susceptibility value of 1.83 × 10^–8 m³/kg, recorded at 7.57 mbsf, indicates the possible presence of ferromagnetic and/or paramagnetic minerals.

Chronology

Two calibrated radiocarbon ages (8 cal y BP, Core 325-M0037A-4R; 16 cal y BP, Core 6R) (Fig. F99) and one U-Th age (13 cal y BP, Core 7R) (Table T10 in the “Methods” chapter) are inconsistent with the stratigraphic order of the samples. This may indicate that the material dated represents a reworked deposit, disturbance by drilling, or inaccuracy in dating. The true age of the U-Th dated coral is unlikely to be older (the initial ²³⁰Th seawater correction actually makes the ages younger by 1.2 k.y.). This hole appears to consist of deposits of deglacial age but may include reworked material.

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