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doi:10.2204/iodp.proc.325.104.2011

Hole M0044A

Operations

Site 8, Hole M0044A

The seabed transponder was deployed at 0140 h on 10 March 2010, and the vessel was on station over Hole M0044A by 0155 h (Table T1). The downpipe camera survey was completed by 0225 h, and the barrel was prepared as API pipe was run to tag the seabed. The first core (standard rotary corer [ALN]) was recovered at 0320 h. On recovering the second core at 0415 h, the ALN polycrystalline diamond bit was found to be damaged and the stabilizer housing was deformed. The bit was changed for a short impregnated ALN bit, and coring continued. At 0800 h, the vessel lost a forward thruster for 5 min without affecting coring operations. At 0905 h, the seabed template was lowered in the moonpool in an attempt to dampen drill string vibration. At 1120 h, the ALN barrel became stuck in the bottom-hole assembly (BHA) during Run 9. Attempts were made to recover the barrel, but at 1310 h, the wireline failed. In order to recover the barrel and overshot, the API pipe had to be tripped, terminating Hole M0044A at 11 mbsf with an average recovery of 15.2%. The ALN was recovered from the BHA at 1645 h. The barrel and BHA were checked, and the overshot wire was repaired.

The API pipe was run in to just above the seabed by 1850 h, and a downpipe camera survey was conducted. The vessel repositioned over ~2.5 m under dynamic positioning to Hole M0045A.

Sedimentology and biological assemblages

Hole M0044A is divided into two lithostratigraphic units.

Unit 1: Section 325-M0044A-1R-1: fragments of bioclastic packstone

The uppermost Unit 1, consisting only of interval 325-M0044A-1R, 0–21 cm, is composed of a packstone ranging in color from buff to gray (Fig. F58). The packstone is composed of bioclasts of corals, bryozoans, encrusting foraminifera, nongeniculate coralline algae, and mollusks. Some fragments have brown (to gray) staining. Cavities of unknown origin are common in the gray fragments, and some of the cavities are partly to fully filled with silt-sized gray internal sediment. The few coral clasts are extensively altered and unidentifiable.

Unit 2: Sections 325-M0044A-2R-CC through 9R-CC: coralgal-microbialite boundstone

The lowermost Unit 2, spanning Sections 325-M0044A-2R-CC through 9R-CC, is composed mainly of microbialites (Figs. F59, F60). Corals are covered with nongeniculate coralline algae, which are, in turn, encrusted with laminated microbialites. Nongeniculate coralline algae are usually thin (<1 mm) and volumetrically less abundant than microbialites and corals. Corals are common, and some are bioeroded by bivalve mollusks with borings partly filled with unconsolidated silt-sized internal sediment (Fig. F61).

Dominant corals are Acropora with thick bases and medium thickness branches (Fig. F62), branching Porites and Pocillopora, and massive Isopora and Faviidae. Most fragments are branching Acropora, with some Porites and Pocillopora.

Physical properties

Total penetration depth for Hole M0044A was 11 m DSF-A with 1.67 m of recovered core (15.18% recovery). Core petrophysical data for this hole are summarized in Table T2.

Density and porosity

Multisensor core logger bulk density measurements from Hole M0044A cores vary from 1.00 to 2.28 g/cm³ (Fig. F63). Downhole trends are not obvious in this limited data set. Discrete moisture and density measurements were taken for two samples, which gave bulk density values of 2.46 and 2.32 g/cm³. Porosity values of the same samples were 19% and 26%, respectively. Grain density values were, respectively, 2.79 and 2.77 g/cm³.

P-wave velocity

Offshore multisensor core logger P-wave velocity measurements yielded no data. However, two discrete P-wave velocity measurements were taken on cores from Hole M0044A with mean values for the samples of 3874 and 4870 m/s (mean resaturated values) (Fig. F64).

Magnetic susceptibility

Cores from Hole M0044A have magnetic susceptibility values ranging from –0.86 × 10^–5 to 13.36 × 10^–5 SI (Fig. F63). Measurements from the top of the hole (0–5.5 m CSF-A) are dominantly positive (~3 × 10^–5 to 13 × 10^–5 SI), whereas measurements from the bottom of the hole are dominantly negative. This delineation of magnetic susceptibility values corresponds to the division of the hole into two distinct lithostratigraphic units: an upper bioclastic packstone (fragmented) and a lower coralgal-microbialite boundstone.

Electrical resistivity

Electrical resistivity values range from 2.39 to 44.84 Ωm (Fig. F63). A systematic change in resistivity downhole is not obvious, but this may be due to poor recovery. However, the highest values are present toward the base of the hole (an opposite trend to the magnetic susceptibility values).

Digital line scans and color reflectance

Cores from Hole M0044A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0044A varies between 53.37% and 80.34% for L* (Fig. F65). Cores from the top of Hole M0044A yielded the lower values in this range, which are associated with a bioclastic packstone. The presence of fractured Acropora sp. boundstone in the remainder of the hole produced dispersed values for all color reflectance spectrophotometry parameters.

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. F66). Low negative (diamagnetic) susceptibilities occur throughout the core, ranging from –3.70 × 10^–8 to –0.10 × 10^–8 m³/kg with an arithmetic mean value of –1.17 × 10^–8 m³/kg. Two positive sample measurements, taken from samples located at the same depth of 5.61 mbsf, display susceptibility values of 1.45 × 10^–8 and 0.48 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals.

Chronology

Two calibrated radiocarbon ages (14 cal y BP, Core 325-M0044A-2R; 17 cal y BP, Core 8R) (Fig. F67) are consistent with their stratigraphic positions. This hole has recovered material from the early deglaciation to 14 cal y BP.

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