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doi:10.2204/iodp.proc.325.104.2011

Hole M0043A

Operations

Site 12, Hole M0043A

The seabed transponder was deployed at 0455 h on 9 March 2010, and by 0515 h the Greatship Maya was on station at Site 12 (Hole M0043A) (Table T1). The seabed template was adjusted in the moonpool, and at 0550 h the API pipe started running to just above the seabed. A downpipe camera survey was conducted at 0740 h, and coring operations (standard rotary corer) started at 0815 h, continuing until 2350 h, when the hole was terminated at 35 mbsf with an average recovery of 17.3%. The API pipe was tripped to 7 m above the seabed, and a downhole camera survey was completed by 0115 h on 10 March 2010. The seabed transponder was recovered at 0115 h and the camera remained inside the pipe while the vessel slowly moved 52 m under dynamic positioning to Site 8, Hole M0044A.

Sedimentology and biological assemblages

Hole M0043A is divided into five lithostratigraphic units.

Unit 1: Sections 325-M0043A-1R-1 through 2R-CC: coralgal boundstone (modern at the top)

The uppermost Unit 1, spanning Sections 325-M0043A-1R-1 through 2R-CC, consists of boundstone dominated by a framework of thin coralline algae and worm tubes (Fig. F45). Internal sediments have fragments of larger foraminifera and mollusks. The boundstone is stained reddish brown, and all components are bioeroded locally. In interval 325-M0043A-1R-1, 0–10 cm, remains of a recent sponge overlie lime mud containing worm tubes and mollusks (Fig. F46). Fragmented specimens of Amphistegina and Heterostegina are rare in muddy gravels from interval 325-M0043A-2R-1, 20–25 cm.

The main corals are medium thickness branching Goniopora(?), submassive Porites, and massive Faviidae. Fragments are mainly fine branching Pocilloporidae (Seriatopora?).

Unit 2: Sections 325-M0043A-3R-1 through 17R-CC: coralgal-microbialite boundstone

Unit 2, spanning Sections 325-M0043A-3R-CC through 17R-CC, consists of coralgal-microbialite boundstone in which coralline algae alternate with microbialite crusts that trap bioclasts (Fig. F47). Coralline algae occur as thick crusts on top of corals or as thinner structures intergrown with microbialites (Fig. F48). Microbialites reach several centimeters in thickness, and they are usually dark colored and poorly laminated (Fig. F49). Some digitate growths occur at the top of microbialite crusts, whereas some internal bioclastic sediments with Halimeda and mollusks occur throughout this unit. All boundstone components are bioeroded by bivalves and sponges.

The main corals forming the framework are massive Isopora, branching Acropora (Figs. F50, F51), massive Faviidae (Fig. F47), and colonies of fine-branching Seriatopora embedded in microbialite (Fig. F48). Associated corals are Porites, Goniopora, Montipora(?), and Faviidae, including branching Echinopora. All of these taxa except Echinopora are common as fragments, which also include Tubipora musica and Agariciidae(?).

Unit 3: Sections 325-M0043A-18R-1 through 21R-CC: unconsolidated sediment

Unit 3, spanning Sections 325-M0043A-18R-1 through 21R-CC, consists of unconsolidated lime granules and pebbles. Major components are coral fragments (as large as 6 cm), coralgal-microbialite boundstone fragments, Halimeda, larger foraminifera, mollusks, bryozoans, echinoids, and worm tubes. Medium to coarse sands from interval 325-M0043A-18R-1, 10–15 cm, contain abundant specimens of abraded Baculogypsina and Calcaria. Fragments of the overlying boundstones and fresh mollusk shells clearly indicate downhole contamination.

A few massive Isopora are associated with encrusting Porites and Agariciidae. Coral fragments are diverse and include Acropora, Isopora, Seriatopora, Porites, Pocillopora(?), Tubipora musica, Fungiidae, Agariciidae, and one small solitary ahermatypic coral.

Unit 4: Section 325-M0043A-22R-CC: packstone overlain by worm tube boundstone

Unit 4, represented only by Section 325-M0043A-22R-CC, is a 5 cm thick blackened boundstone of thin foliose coralline algae and worm tubes overlying a dark packstone (Fig. F52). It also contains larger foraminifera, Halimeda, mollusks, and coral fragments. Corals are fragments of Seriatopora and Acropora.

Unit 5: Section 325-M0043A-23R-CC: lime sand

The lowermost Unit 5, consisting only of Section 325-M0043A-23R-CC, consists of medium to coarse lime sand rich in larger foraminifera and black granules. Interval 325-M0043A-23-CC, 5–10 cm, contains fragmented specimens of Operculina. The black grains probably originate from the overlying deposits, and indicate downhole contamination.

Apart from a few fragments of Isopora(?) in the uppermost parts of the unit, there are no visible corals.

Physical properties

Hole M0043A yielded 6.04 m of core from a total penetration depth of 35.0 m DSF-A (17.26% recovery). Table T2 summarizes physical property data collected for this hole.

Density and porosity

Bulk density measured on Hole M0043A cores varies from 1.05 to 2.40 g/cm³ (Fig. F53). Similar to Hole M0042A, recovery and core length has impacted the amount and quality of gamma density data for this hole (see “Physical properties” in the “Methods” chapter). Again, it is impossible to comment on trends downhole. However, it was possible to measure 16 samples from Hole M0043A for discrete moisture and density. Bulk density data from these analyses vary from 1.83 to 2.39 g/cm³, whereas porosity values in the hole range from 22% to 53% (Fig. F54). Disturbed lime pebbles lie between 0 and 8 m CSF-A, with porosity decreasing from 52% to 32% downhole. Between 10 and 21 m CSF-A, where the lithology is principally coralgal-microbialite boundstone, porosity varies between 22% and 43%. Below 21 m CSF-A, lime pebbles have a porosity varying between 24% and 53%. Similar to Holes M0040A and M0041A, grain density varies from 2.74 to 2.82 g/cm³.

P-wave velocity

Offshore P-wave velocity measurements range from 1526.84 to 1895.75 m/s (Fig. F53). The minimum values are too low for the type of sediment observed, likely because of the poor core quality (see “Physical properties” in the “Methods” chapter). The multisensor core logger P-wave data come from only one of the recovered core sections, with data absent from the rest of the hole. There were no discrete P-wave velocity measurements taken on core from Hole M0043A. The combination of an extremely limited P-wave velocity data set with questionable validity of those data renders it impossible to make any sort of interpretation.

Magnetic susceptibility

Cores from Hole M0043A have magnetic susceptibility values ranging from –12.76 × 10^–5 to 9.82 × 10^–5 SI (Fig. F53). Similar to Hole M0042A, data are limited; therefore, identifying any trends is difficult. Three sections suggest intervals of relatively high magnetic susceptibility: Sections 325-M0043A-2R-1 (with a value of 9.82 × 10^–5 SI at 2.34 CSF-A), 6R-1 (with a magnetic susceptibility peak value of 5.27 × 10^–5 SI at 8.76 CSF-A), and 12R-1 (with a value of 4.87 × 10^–5 SI at 17.13 CSF-A).

Electrical resistivity

Similar to the other data sets, short core lengths and discontinuous recovery downhole means the resistivity profile makes little sense on its own. Electrical resistivity measured on whole cores varies from 0.75 to 93.27 Ωm (Fig. F53).

Digital line scans and color reflectance

All cores from Hole M0043A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0043A varies between 41.17% and 83.95% L* (Fig. F55). Reflectance values were highly dispersed for most of the borehole sections. The more disperse sections correspond to the thick coralgal-microbialite boundstone unit, mainly between 10 and 20 m CSF-A. Less dispersed values were present in the lowermost 5 m of the hole, corresponding to blackened boundstone, unconsolidated, and packstone units. In the uppermost 1 m of the hole, values were negative where bindstone and lime pebbles were present. Below 2 m CSF-A, color values showed a downhole trend, from positive a* values (red) to negative ones (green). Lower b* values were also found in the lowermost few meters, although they were always in the positive range (yellow). The trend in color is more obvious in the a*/b* ratio.

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. F56). Low negative (diamagnetic) susceptibilities occur throughout the core, ranging from –2.76 × 10^–8 to –0.17 × 10^–8 m³/kg with an arithmetic mean of –0.92 × 10^–8 m³/kg. Four positive measurements were taken from samples located at 0.07, 2.28, 8.79, and 15.15 mbsf with susceptibilities of 0.245, 3.36, 0.14, and 0.01 × 10^–8 m³/kg, respectively, indicating the presence of paramagnetic and/or ferromagnetic minerals.

Chronology

Two calibrated radiocarbon ages (15 cal y BP, Core 325-M0043A-2R; 18 cal y BP, Core 9R) (Fig. F57) and one U-Th age (20 cal y BP, Core 18R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is unaffected by corrections of initial ²³⁰Th, adding to the confidence in this age interpretation. This hole recovered material from the Last Glacial Maximum and early deglaciation to 15 cal y BP. There are five cores below the 20 cal y BP section, indicating that there may be more material representing the Last Glacial Maximum or even older Pleistocene deposits in this hole.

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