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doi:10.2204/iodp.proc.325.105.2011

Hole M0049B

Operations

Site 4, Hole M0049B

Coring recommenced at Site 4, Hole M0049B, at 0400 h on 22 March 2010 and continued for 13 runs (extended nose corer Runs 1–3; standard rotary corer Runs 4–13) until the hole was terminated at 15.6 mbsf at 1520 h with an average recovery of 17.9% (Table T1). The API pipe was tripped to just above the seabed, and a downpipe camera survey was conducted between 1550 and 1610 h.

Sedimentology and biological assemblages

Hole M0049B is divided into two lithostratigraphic units.

Unit 1: Sections 325-M0049B-1X-1 to 10R-1, 11 cm: coralgal-microbialite boundstone

The uppermost Unit 1, spanning Sections 325-M0049B-1X-1 to 10R-1, 11 cm, consists of different sized fragments of slightly bioeroded coralgal-microbialite boundstone. The proportions of corals, coralline algae, and microbialites in the boundstone vary. The boundstone forming the uppermost part of Unit 2 includes a framework of thin foliose coralline algae overgrowing a large bryozoan fragment (Fig. F8). Microbialite crusts are more abundant in the interval formed by Sections 325-M0049B-5R-1 through 6R-CC. They are brown and poorly laminated (Fig. F9) or structureless. Internal bioclastic sediments are lime sand with Halimeda and mollusk fragments. Geopetal fabrics are common in small cavities in Section 325-M0049B-9R-CC (Fig. F10). Small stalactitic and stalagmitic columns also occur in some coral cavities (Fig. F11). There are no larger foraminifera in intervals 325-M0049B-4R-CC, 4–9 cm, and 8R-CC, 10–15 cm.

The coral assemblage is diverse. Larger corals are increasingly abundant toward the base of the unit and are dominated by massive Porites (Fig. F12), submassive to massive Astreopora(?) and Faviidae, robustly branching Lobophyllia or Euphyllia, and encrusting (and/or platy) Montipora(?). Associated corals are branching Acropora and Seriatopora, submassive Porites and/or Montipora, Faviidae, Agariciidae, and Mussidae(?). Fragments include Acropora, Pachyseris, Leptoseris, Porites, Montipora, and unidentifiable Agariciidae, Faviidae, and Mussidae(?).

Unit 2: Sections 325-M0049B-10R-1, 11 cm, through 12R-1: coralgal boundstone with complex microbialite

The lowermost Unit 2, spanning Sections 325-M0049B-10R-1, 11 cm, through Section 12R-1, consists of slightly bioeroded coralgal-microbialite boundstone in which the proportion of microbialite in relation to coral and coralline algae is high throughout the unit. The microbialite has finger- to dome-like shapes and is brown with distinct laminations (stromatolitic). Microbialite crusts appear to be interlayered in several places with different types of internal sediment varying from Halimeda rudstone to floatstone to laminated grainstones. Major components of the internal sediment include fragments of Tubipora musica, bryozoans, and mollusks. Cavities are partially filled with geopetal fabrics and “microstalactitic” microbialite (?) cement (Fig. F13). The surface of the cement lining some cavities is stained red.

The main corals are submassive to massive Porites(?) or Montipora(?), which are often difficult to distinguish when embedded in microbialite. Smaller corals and fragments include Tubipora musica, Seriatopora, Acropora, and Millepora(?).

Physical properties

Hole M0049B had a penetration depth of 15.60 m DSF-A, of which 2.79 m of core was successfully recovered (17.88% recovery). Table T2 summarizes the petrophysical properties of this core.

Density and porosity

Multisensor core logger bulk density varies from 1.06 to 2.46 g/cm³ in cores from Hole M0049B (Fig. F14). Owing to core recovery and the fact that the recovered cores are generally short, there is not a continuous record of bulk density downhole. Hole lithologies are dominated by lime pebbles of coralgal-microbialite boundstones. Nine discrete samples from Hole M0049B were measured for moisture and density (Fig. F15). Porosity fluctuates between 17% and 35% between 0 and 13.36 m core depth below seafloor (CSF-A). The minimum value (17%) corresponds to two core plugs taken at 14 and 14.16 m CSF-A. In the entire hole, bulk density varies between 2.17 and 2.48 g/cm³. Grain density varies across the samples from 2.75 to 2.79 g/cm³ (Fig. F15).

P-wave velocity

P-wave velocity measurements taken on whole cores offshore yielded no data because of core quality issues (Fig. F14). Two 20 mm core plugs were taken from boundstone lithologies. Velocities measured on these samples were 4560 and 4993 m/s (mean resaturated values) (Fig. F16), which are appropriate values for well-lithified, porous formations such as these microbialites. The two available data points on the bulk density/P-wave velocity cross-plot indicate that velocity decreases with increasing density (Fig. F16).

Magnetic susceptibility

Magnetic susceptibility values from core recovered in Hole M0049B range from –0.64 × 10^–5 to 4.29 × 10^–5 SI (Fig. F14). Recovery is such that it is impossible to comment on downhole trends or interesting intervals.

Electrical resistivity

Electrical resistivity measured on whole cores is highly variable and ranges from 6.17 to 78.34 Ωm (Fig. F14). Owing to recovery and the fact the cores are generally short and either rubbly or biscuited, resistivity data are greatly compromised. This is evidenced in the erratic nature of the multisensor core logger data summary plots (Fig. F14).

Digital line-scans and color reflectance

Cores from Hole M0049B were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance varies from 48.34% to 82.98% L* (Fig. F17). In the uppermost few centimeters of the hole, a coralgal boundstone is present. Color reflectance measurements for this interval range from 60% to 70% for L* (Fig. F17) and are variable in terms of all the color reflectance indexes. Values of a* and b* are positive for all depths, indicating a predominance of red and yellow color. Measurements taken from 2 to 4 m CSF-A and from 5 m CSF-A to the base of the hole show a dispersion of ~20% for all sections measured because of the presence of lime pebbles and cobbles. Between 3.7 and 4 m CSF-A, the presence of a massive coral creates an increase in reflectance to 80% with less dispersion in the measurements than in other parts of the borehole. No general trend is observed in this hole.

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. F18). Positive susceptibility samples range from 0.09 × 10^–8 to 4.15 × 10^–8 m³/kg and have an arithmetic mean of 1.90 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. In addition, there are four negative (diamagnetic) susceptibility measurements for samples located at 2.22, 13.57, 15.13, and 15.41 mbsf with values of –0.37 × 10^–8, –0.06 × 10^–8, –0.12 × 10^–8, and –1.50 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (4 calibrated years before present [cal y BP; years before 1950 AD], Core 325-M0049B-2X; 12 cal y BP, Core 4R) (Fig. F19) and one U-Th age (16 cal y BP, Core 9R) (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 0.6 k.y. younger). This hole recovered material from the early to middle deglaciation, and the Holocene.

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