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doi:10.2204/iodp.proc.325.104.2011

Hole M0046A

Operations

Site 8, Hole M0046A

The Greatship Maya was on station above Hole M0046A by 0050 h on 11 March 2010 (Table T1). The first three standard rotary corer runs all appeared to skip down a slope, freefalling to 10.8 “mbsf.” However, coring then continued, with seabed corals appearing in the liner at ~10.8 “mbsf” in Run 4. While recovering Run 10 from a depth of 25.2 “mbsf,” there was a partial hole collapse, so the uppermost 1 m of core from Run 11 represents infill. The hole was terminated after Run 13 at 1040 h at 31.20 “mbsf,” with an average recovery of 8.9%. Depth corrections were made at the Onshore Science Party to account for the actual water depth in which the seabed was penetrated and, in turn, the final cored depth. Runs 1R and 2R are thus seen as negative depths, to account for the drill string falling through the water column along the side of a pinnacle, with Run 3R penetrating the seabed for the last 1.2 m of a 3 m run. The final depth cored stands at a corrected depth of 20.4 mbsf. The API pipe was tripped back to Joint 11 in preparation for transit to the next site, and the seabed transponder was recovered. The vessel began the move to Site 12, Hole M0047A, at 1145 h.

Sedimentology and biological assemblages

Hole M0046A is divided into three lithostratigraphic units.

Unit 1: Sections 325-M0046A-1R-1 through 3R-1: modern deposits scraped from the seafloor

The uppermost Unit 1, spanning Sections 325-M0046A-1R-1 through 3R-1, is interpreted (from consideration of drilling logs) as being scrapings from the seafloor down the side of a pinnacle. Unit 1 consists of pebble-sized fragments of bioclastic packstone with corals. This packstone has brown and gray staining. Bioclasts of corals, larger foraminifera, and Homotrema rubrum are also present. A wood fragment covered with a living sponge lies at the base of Section 325-M0044-3R-1.

Large corals are bioeroded Favites and platy Porites, with a few large submassive fragments of Cyphastrea and one ahermatypic Dendrophylliidae.

Unit 2: Sections 325-M0046A-4R-1 through 325-M0046A-9R-CC: coralgal boundstone

Unit 2, spanning Sections 325-M0046A-4R-1 to 9R-1, 13 cm, consists mainly of coral boundstone interlayered with two distinct packstone intervals: 325-M0046A-4R-CC, 0–10 cm, and 5R-CC, 0–10 cm. The boundstone is composed of coralgal framework with a gray packstone matrix (Fig. F68). Cavities of uncertain origin are partly filled with gray fine-grained sediment. No larger foraminifera are present in interval 325-M0046A-6R-CC, 5–10 cm. The upper part of Section 325-M0046A-5R-1 contains ~10 cm of coralline algal–encrusted spiculite (Fig. F69).

Framework corals are massive to submassive Acroporidae, Montipora(?), Porites, and Platygyra(?), mainly in lower parts of the unit. Fragments include submassive to massive Favites, Goniopora, Coscinarea, and branching Acropora, Pocillopora, Seriatopora, and Stylophora.

Unit 3: Sections 325-M0046A-10R-CC through 13R-CC: coral boundstone

The lowermost Unit 3, from Section 325-M0046A-10R-CC through 13R-CC, consists of granule- to pebble-sized (rarely cobble-sized) clasts of corals (Fig. F70) without crusts of nongeniculate coralline algae. This unit is likely to be a coral boundstone that was disturbed by drilling.

The only large coral is a massive Echinopora in Section 325-M0046A-11R-CC. All other corals are fragments that include numerous branching Pocilloporidae, Acropora, and especially Seriatopora, along with some pieces of Porites and Montipora(?).

Physical properties

The total penetration depth for Hole M0046A was 20.40 m DSF-A corrected depth with 2.52 m of recovered core (12.35% corrected recovery). Data acquired as part of the core petrophysics measurement plan for this hole are summarized in Table T2.

Density and porosity

Bulk density multisensor core logger measurements for Hole M0046A cores vary from 1.03 to 2.35 g/cm³ (Fig. F71). Downhole trends are not obvious in this limited data set. Discrete moisture and density measurements were not performed on any cores from Hole M0046A.

P-wave velocity

Offshore P-wave velocity measurements yielded no data, and no samples were selected for discrete P-wave velocity measurement from Hole M0046A.

Magnetic susceptibility

Cores from Hole M0046A have whole core magnetic susceptibility values ranging from –0.12 × 10^–5 to 12.76 × 10^–5 SI. Similar to values obtained for Hole M0044A, the top of the hole (above 5 m CSF-A) is dominated by higher, positive measurements as high to 12.76 × 10^–5 SI, whereas measurements from deeper in the hole (below 5 m CSF-A) are characterized by lower (3.84 × 10^–5 SI) and occasionally slightly negative magnetic susceptibility values (–0.06 × 10^–5 and –0.12 × 10^–5 SI). This change in magnetic susceptibility does not appear to directly correspond with lithostratigraphic changes observed downhole.

Electrical resistivity

Overall, electrical resistivity decreases downhole, with a delineation of values similar to that observed in the corresponding magnetic susceptibility data. Electrical resistivity values range from 0.96 to 29.21 Ωm (Fig. F71). However, Section 325-M0046A-5R-1 presents a range with higher values from 9.14 to 11.93 Ωm.

Digital line scans and color reflectance

Cores from Hole M0046A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0046A varies between 43.82% and 81.24% for L* (Fig. F72). The presence of massive corals in this borehole makes the color reflectance data set less dispersed than in other holes in this transect. The flat surface provided when the massive corals are split is a major factor to consider when evaluating the quality of the data. The variations in a* indicate a value close to zero in sections located between 15 and 21 m CSF-A. Values for b* were more dispersed over the same depths, reaching lower values than the ones at the top and bottom of the borehole.

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. F73). Very low negative (diamagnetic) susceptibilities occur throughout the core, ranging from –1.11 × 10^–8 to –0.44 × 10^–8 m3/kg with an arithmetic mean of –0.74 × 10^–8 m³/kg. Two positive values were recorded in samples taken from 1.71 and 18.46 mbsf with susceptibilities of 0.50 × 10^–8 and 1.50 × 10^–8 m³/kg, respectively, indicating the presence of paramagnetic and/or ferromagnetic minerals.

Chronology

Two calibrated radiocarbon ages (16 cal y BP, Core 325-M0046A-6R; 16 cal y BP, Core 9R) (Fig. F74) and one U-Th age (17 cal y BP, Core 11R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. When rounded to 1 ka, the two calibrated radiocarbon dates give the same age, but the individual age ranges are consistent with their stratigraphic order. The U-Th age is unaffected by corrections for initial ²³⁰Th. Therefore, this hole recovered material from the early deglaciation. There are, however, five shallower cores, so material younger than 16 cal y BP may also be present.

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