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doi:10.2204/iodp.proc.325.103.2011

Hole M0031A

Operations

Site 6, Hole M0031A

The Greatship Maya arrived on station at Site 6 at 0730 h on 14 February 2010 (Table T1). While the vessel established a dynamic positioning model, the overshot tools were tested. Running of the API pipe commenced at 0830 h. The downpipe camera was deployed for the precoring survey at 1030 h but was not working; possibly due to a faulty light. The camera was recovered, repaired, and deployed again, but it stuck ~1 m from the end of the bottom-hole assembly because of a bent lifter loop jamming in the narrowest section of the bottom-hole assembly. The problem was rectified, and the precoring survey was completed by 1755 h.

The first standard rotary corer core was recovered at 1825 h on 14 February, and coring operations continued until 0140 h on 16 February. However, problems occurred throughout the operation, including

• Strong currents requiring vessel repositioning five times (each move between 2 and 5 m);

• Hole collapse requiring reaming and flushing after Run 5 and during deployment of the barrel during Run 6 (9–12 mbsf);

• The inner barrel being stuck inside the outer barrel because of compacted sediment after Runs 6, 11, and 12;

• A 2 h downtime between 0530 and 0730 h to fix the mud pump;

• The barrel becoming stuck after Run 9, requiring three attempts to free it and retermination of the overshot wire (1015 and 1155 h) and reaming prior to Run 10;

• Further reaming following a hole collapse after Run 15; and

• High pressure indicating a blocked bit during Run 16, resulting in very limited recovery.

The hole was terminated after 17 runs at 43 mbsf with an average recovery of 13.2%, and preparations were made to begin downhole through-pipe gamma logging. The through-pipe gamma sonde was deployed at 0230 h and was recovered back onto deck at 0755 h. The API pipe was then tripped to 7 m above the seabed by 0855 h, and a postcoring downpipe camera survey was conducted. The camera was left inside the API pipe for the transit. The Greatship Maya departed Hole M0031A at 0945 h on to transit slowly (under dynamic positioning) to Hole M0032A, 15 m away.

Sedimentology and biological assemblages

Hole M0031A is divided into five lithostratigraphic units.

Unit 1: Sections 325-M0031A-2R-1 through 3R-1: coralgal boundstone

The uppermost Unit 1, spanning Sections 325-M0031A-2R-1 through 3R-1, consists of coralgal boundstone fragments with internal sediment and minor stromatolitic microbialite. Bryozoans cooccur with Halimeda in the internal sediment of the boundstone. Coralline algae form complex crusts of contorted plants. Most corals and coralline algae are bioeroded. A distinctive surface in the boundstone at 17 cm in Section 325-M0031A-2R-1 is stained brown and is interpreted as a hardground (Fig. F7).

The dominant corals are massive Isopora (Fig. F8) associated with branching Pocilloporidae. Loose fragments include small pieces of Montipora, Acropora, and Pectiniidae.

Unit 2: Sections 325-M0031A-3R-CC through 8R-CC: coralgal-microbialite boundstone

Unit 2, spanning Sections 325-M0031A-3R-CC through 8R-CC, consists of poorly recovered, coralgal-microbialite boundstone that was highly disturbed during drilling. Fragments contain coral encrusted by thick coralline algae and microbialite, plus a few bryozoans. Internal bioclastic sediment with Halimeda is also visible. Gravel-sized sediment contains a few specimens of the foraminifera Alveolinella and Amphistegina. The boundstone is bioeroded locally by bivalves and sponges.

The coral assemblage is dominated by massive Isopora and medium-thickness, branching (corymbose?) Acropora (Figs. F9, F10). Other corals include Paulastrea and fragments of branching Pocilloporidae, Seriatopora, and possibly Millepora(?).

Unit 3: Sections 325-M0031A-9R-1 to 15R-1, 16 cm: unconsolidated sediment

Unit 3, spanning Sections 325-M0031A-9R-1 to 15R-1, 16 cm, consists of unconsolidated lime sand, granules, and pebbles. Major components include fragments of corals, Halimeda, mollusks, and echinoid spines. The larger foraminifera Amphistegina, Baculogypsina, and Marginopora are sparsely represented in very coarse grained sediments from interval 325-M0031A-13R-1, 45–50 cm, whereas Alveolinella, Amphistegina, Baculogypsina, Planorbulinella, and Sphaerogypsina are common in coarse-grained sediments from interval 13R-1, 115–120 cm.

Some rock fragments probably come from the overlying boundstones. It is difficult to determine whether all loose rock fragments are the result of downhole contamination or whether some of the unconsolidated sand and gravel represent the original in situ sediment within this interval. Section 325-M0031A-11R-1 includes larger fragments of coralgal framestone.

The few large corals in this unit are medium thickness, branching (corymbose?) Acropora or massive Poritidae. Fragments of other corals include Seriatopora, Pocillopora, Montastrea, Pavona turbinaria(?), Tubipora, Montipora(?), and Palauastrea(?).

Unit 4: interval 325-M0031A-15R-1, 16–23 cm: grainstone

Unit 4, consisting of interval 325-M0031A-15R-1, 16–23 cm, is composed of a grainstone of larger foraminifera, coralline algae, and mollusks (Fig. F11). Coral fragments include Turbinaria, Poritidae, Faviidae, and Coscinaraea(?).

Unit 5: Sections 325-M0031A-16R-1 through 17R-CC: unconsolidated sediment

The lowermost Unit 5, spanning Sections 325-M0031A-16R-1 through 17R-CC, consists of unconsolidated coarse lime sand, granules, and pebbles. The coarse sand sediment has horizontal laminae. Major components are fragments of coral, Halimeda, mollusks, larger foraminifera (black-stained specimens of Rotallidae [Amphistegina or Elphidiidae]), and echinoids. The uppermost part of Unit 5 contains a large fragment of Tridacna (Fig. F12). Because some of these fragments are clearly derived from the overlying boundstones, coarser particles in this unit can be interpreted partly as downhole contamination.

There are no large corals, but coral fragments include parts of branching colonies of Seriatopora, Acropora, and unidentified Acroporidae and Pocillporidae.

Physical properties

Hole M0031A was drilled to 43 m DSF-A with a total of 5.68 m of core recovered equating to 13.21% recovery. Physical property data acquired in this hole are summarized in Table T2.

Density and porosity

Bulk density values from whole-core multisensor core logger (MSCL) measurements range from 1.03 to 2.37 g/cm³ (Fig. F13). Bulk density from discrete samples varies between 1.79 and 2.40 g/cm³, and porosity varies between 23% and 55% (Fig. F14). Grain density is higher (as much as 2.83 g/cm³) in the bottom of the borehole (20–40 m CSF-A) than in the top (as much as 2.73 g/cm³; 0–20 m CSF-A). These measurements are difficult to compare with the MSCL bulk density measurements because of core quality (see “Quality assurance and quality control” in the “Methods” chapter). However, discrete density measurements are, overall, higher than whole-core measurements, as might be expected. Rocks recovered from the borehole are very heterogeneous, with many lithologies changing rapidly with depth (for example, lime pebbles, coral, boundstone, lime granules, framestone, lime sand, etc.). It is therefore difficult to relate porosity directly with observed lithology.

P-wave velocity

P-wave velocity measurements taken on whole cores offshore ranged from 1675.08 to 1736.35 m/s (Fig. F13). These velocities are very close to that of seawater (1500 m/s) and are therefore indicative of unconsolidated sediments. Discrete samples from this hole were not available for P-wave analysis.

Magnetic susceptibility

For Hole M0031A cores, MSCL magnetic susceptibility was measured using a 1 cm sampling interval with the 80 mm loop (Fig. F13). Values range from –1.28 × 10^–5 to 49.67 × 10^–5 SI, with the majority of values falling within the 0 to 10 × 10^–5 SI range. Owing to core recovery, it is difficult to comment on downhole trends. However, it is clear that in Section 325-M0031A-10R-1 at 20.56 m CSF-A, magnetic susceptibility is elevated relative to the cores above and below. In this section, magnetic susceptibility decreases downsection from a high of 49.67 × 10^–5 to 11.82 × 10^–5 SI at the section base.

Electrical resistivity

Electrical resistivity is primarily affected by lithology, pore fluid, and salinity, as well as core liner saturation. Resistivity is highly variable from low values of 0.74 Ωm to high values of 37.51 Ωm (Fig. F13). Notable intervals of high resistivity occur in Sections 325-M0031A-2R-1, 7R-1, and 11R-1 (~3, ~12, and ~23.6 m CSF-A, respectively). The lowest resistivity is registered at ~28–29.60 m CSF-A.

Digital line scans and color reflectance

All cores from Hole M0031A were measured using a digital line scan system with all data recorded at a resolution of 150 pixel/cm as both images and RGB values. Color reflectance L* varies between 30.81% and 82.37% (Fig. F15). In general, reflectance values of coral framework were higher than those for carbonate sediments. Lime pebbles and fragments of microbialite and coral were common in this hole, and where sections were composed only of these fragments, no color reflectance measurements were taken because of the lack of flat surfaces (see “Physical properties” in the “Methods” chapter). The variation in the color of the microbialites is not due to measurements being taken on an uneven surface but rather to the frequent natural changes in color of this type of material. Deeper samples showed lower values for a* entering the negative range of green color and also lower values of b*, close to zero in some cases and very close to the blue color scale. The presence of a slight change in the colors is more easily observed in the ratio a*/b*, which is a better indicator of cyclic variations (Blum, 1997).

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. F16). Low positive susceptibility values occur throughout the core, with an arithmetic mean of 0.84 × 10^–8 m³/kg. Two peaks were also recorded, at 8.70 and 20.64 mbsf, with maximum values of 4.82 × 10^–8 and 5.38 × 10^–8 m³/kg, respectively. At 18.16 and 31.06 mbsf, two samples recorded low negative susceptibility values of –1.09 × 10^–8 and –0.17 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (13 calibrated years before present [cal y BP; years before 1950 AD], Core 325-M0031A-2R; 17 cal y BP, Core 8R) (Fig. F17) and one U-Th age (25 cal y BP, Core 16R) (Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is unaffected by corrections for initial ²³⁰Th, adding to the confidence in this age interpretation. This hole has recovered material from the Last Glacial Maximum interval and has captured the early stage of the deglaciation to ~13 cal y BP.

Downhole measurements

Geophysical wireline operations were completed in Hole M0031A to a depth of 36.37 m wireline matched depth below seafloor (WMSF) (seafloor picked from the log data) with the ANTARES Spectral Natural Gamma Probe (ASGR), one of the slimhole tools from the available logging tool suite (see “Downhole logging” in the “Methods” chapter). Recovery in Hole M0031A was just over 13%. Therefore, wireline logging was performed to identify main boundaries in the borehole sequence and provide a continuous dataset. The ASGR log was acquired through the API pipe. In carbonate lithologies, where counts are normally very low, data were further attenuated by the presence of the drill pipe. Ideally, following logging through pipe the ASGR probe would be run in open-hole conditions and the signals compared and depth-matched for quality assurance/quality control. However, because of poor hole stability, this was not possible at this site.

Total natural gamma radiation (TGR) is very low (~12.2 cps) despite logging speeds of 1 m/min (Fig. F18). Although there is no clear differentiation between contributions of the different elements, changes in the concentration of uranium appear to coincide with the majority of variations in TGR. In order to identify statistically accurate results, negative component values (i.e., negative concentrations of potassium, uranium, and thorium) were removed from the dataset. When interpolated (assuming a gap between data points no greater than 10 m), both the original and cleaned datasets exhibit similar trends.

Three (or four) logging units can be identified in Hole M0031A (Fig. F18):

1. Unit I/Unit II (0–14.48 m WMSF; Cores 325-M0031A-2R through 7R) is characterized by TGR counts as high as 8 cps. The number of counts fluctuates throughout and reaches a low of ~3 cps at 5.25 m WMSF. The minimum TGR (cps) is where a tentative divide has been made between Unit I and Unit II; however, it is more logical that these units are grouped as one. The TGR low may reflect the change from a coralgal boundstone unit to a coralgal-microbialite boundstone unit. These two logging units fit well with the described lithostratigraphic units and hence link a relatively high TGR signal with both coralgal and coralgal-microbialite boundstones.

2. Unit III (14.48–25.31 m WMSF; Cores 325-M0031A-8R through 11R) represents a zone of consistently low (relatively) TGR counts ranging from ~2.3 to 5.28 cps. This relates to a large zone described as unconsolidated sediments. Recovered core material comprises lime sand, granules, and pebbles. It is difficult to be certain whether all the observed loose fragments are a result of downhole contamination or if some of this material is in situ. Some areas of larger fragments are possibly derived from the coralgal boundstone units.

3. Unit IV (25.31–36.37 m WMSF; Cores 325-M0031A-12R through 17R) is characterized by a zone of increasing TGR counts with depth. Values range from ~2.9 to ~10 cps. The lithology in this zone is typified by a very thin grainstone unit at ~31 m CSF-A surrounded by unconsolidated sediments. The bottom portion of this hole comprises some coarser sand sediment with horizontal laminae. The sediment consists of benthic foraminifera, coral fragments, and echinoids. There is uncertainty regarding whether some of the larger particles/fragments in these sediments represent contamination from higher in the borehole.

Note that the total hole depth for Hole M0031A from the logging data is ~4 m shallower than that from the drilling depth below seafloor. Downhole logging represents a method of continuous data acquisition uphole. The location/depth of the seafloor is picked from a spike/positive shift in the gamma ray curve (as done here for Hole M0031A). Often there are small differences in core and wireline depth. In this instance, it is noted that there is a discrepancy of –3 m between the lithostratigraphic unit boundary depths identified in the TGR data. Further work may be able to refine the core-log depth relationship, resulting in a closer correlation between an area of consistently relatively high TGR counts with the boundstone units.

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