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doi:10.2204/iodp.proc.325.103.2011

Hole M0036A

Operations

Site 11, Hole M0036A

At 0940 h on 1 March 2010, the seabed transponder was redeployed, and the vessel maneuvered under dynamic positioning to Site 1, Hole M0036A (Table T1). Between 0940 and 1055 h, the vessel undertook a series of small moves (<5 m) to target the top of a coral high and position the API pipe over an area devoid of live coral using the downpipe camera system to image the seabed. By 1100 h, the downpipe camera was recovered and API pipe was being run, tagging the seafloor at 1135 h. The first core (standard rotary corer) was recovered at 1150 h, and operations continued until 0600 h on 2 March, with a total of 22 runs reaching 34 mbsf, with an average recovery of 26.2%. On the penultimate run at 0445 h, the barrel became blocked with sand, requiring the hammer sampler to free it.

At 0600 h, preparations for downhole logging began. Between 0855 and 1115 h, the through-pipe gamma sonde was deployed, logging an interval of 33.26 m WMSF. At 1115 h, circulation and conditioning of the hole commenced prior to three API pipes being tripped, bringing the pipe to 10 mbsf. At 1255 h, lowering of the seabed template began, with the template on the seabed at 1400 h. Progress was slow because of strong currents pushing the electrical umbilical around the underside of the template. At 1510, 1630, 1900, and 2050 h the resistivity, sonic, open-hole gamma, and magnetic susceptibility sondes, respectively, were deployed downhole, logging the intervals 7.5–25.87, 7.45–16.50, 7.45–10.66, and 7.45–7.47 m WMSF, respectively. A gradual collapse of the hole during this period was noted, with each successive tool penetrating to increasingly shallower depths. At 2145 h, the caliper tool was deployed downhole. However, communication problems with the tool meant that at 2330 h a second caliper was tried downhole. Again, communication problems prevented logging using this instrument, and logging operations were terminated at 2340 h on 2 March. Demobilization of the logging equipment continued until 0040 h on 3 March, when the seabed template began to be raised. At 0130 h, two API pipes were tripped and a downpipe camera survey was conducted. Tripping of the API pipe continued from 0220 h and was completed by 0400 h, when the vessel moved to recover the seabed transponder. The Greatship Maya then moved slowly to Site 9, Hole M0037A, while maintenance of the drill floor hydraulics was carried out.

Sedimentology and biological assemblages

Hole M0036A is divided into seven lithostratigraphic units.

Unit 1: Section 325-M0036A-2R-1: coralgal boundstone

The uppermost Unit 1, consisting of Section 325-M0036A-2R-1, is composed of fragments of coralline algae and coral boundstone with serpulid worm tubes, encrusting foraminifers, and bivalves, plus loose particles of gastropods, Halimeda, and larger foraminifera. The boundstone has reddish staining throughout this unit.

This boundstone contains one colonial ahermatypic coral colony and some highly altered coral fragments that are unidentifiable, including one possible solitary Fungiidae.

Unit 2: Sections 325-M0036A-3W-1 through 6R-CC: unconsolidated sediments

Unit 2, consisting of Sections 325-M0036A-3W-1 through 6R-CC, is composed of unconsolidated lime granules and pebbles (in the uppermost 20 cm of the unit) with medium to coarse lime sand that includes granules and pebbles. Major components are fragments of corals, Halimeda, bivalves, gastropods, and bryozoans (Fig. F81). Larger foraminifera are also a major component, with layers of abundant, well-preserved specimens of Alveolinella, Amphistegina, Cycloclypeus, Homotrema, Operculina, and Textulariida in very coarse sand sediments from interval 325-M0036A-3W-1, 30–35 cm, in gravelly very coarse sand sediments from interval 4R-1, 50–55 cm, and in muddy very coarse sand sediments from interval 4R-1, 140–145 cm. Some fragments are clearly derived from the overlying boundstone, and others consist of microbialites. Some mollusk fragments appear fresh and may come from the modern seafloor, so some components may be attributed to downhole contamination during coring.

There are no large corals, but coral fragments are abundant. Although most are too small for identification, they include Seriatopora, Stylophora(?), branching Acropora, foliaceous Leptoseris, small solitary Fungiidae, and possibly Poritidae.

Unit 3: Sections 325-M0036A-7R-1 through 9R-CC: coralgal boundstone

Unit 3, spanning Sections 325-M0036A-7R-1 through 9R-CC, consists of large fragments of coralgal boundstone with minor microbialite. The boundstone is composed mainly of massive corals covered with crusts of nongeniculate coralline algae intergrown with vermetids. The microbialites are thin, digitated crusts lining voids in the boundstone. There is some fine-grained internal sediment, plus fragments of mollusks and bryozoans.

Although there are no large corals in the upper part of Unit 3, massive Isopora (Fig. F82) and thick-branching Acropora dominate the lowest section (Core 325-M0036A-9R). Fragments include (often rounded) pieces of Pocillopora, Isopora, Acropora, Seriatopora, Leptoseris, and Fungiidae. Corals are bioeroded locally.

Unit 4: Sections 325-M0036A-10R-1 to 19R-1, 8 cm: coralgal-microbialite boundstone

Unit 4, spanning Sections 325-M0036A-10R-1 to 19R-1, 8 cm, consists of coralgal-microbialite boundstone with varying proportions of coral, coralline algae, and microbialite arranged in an alternating pattern (Fig. F83). Microbialites are mainly stromatolitic (laminated), with alternating darker and lighter laminae that include trapped bioclasts. Vermetids commonly intergrown with coralline algae occur as thick crusts on top of the corals or as irregular, contorted structures intergrown with microbialites. Internal bioclastic sediments that include larger foraminifera and Halimeda are rare in this unit. There is a large fragment of Tridacna at the top of Section 325-M0036A-13R-1. The boundstone is bioeroded locally by bivalves and sponges. Microbialite crusts are poorly developed in the interval from Sections 325-M0036A-12R-1, 24 cm, through 14R-1.

The coral assemblage is dominated by medium to robustly branching Acropora (Fig. F84), massive Isopora, and fine-branching Seriatopora (Fig. F83). Other common corals are massive Faviidae, Goniopora, and Tubipora. Fragments include all of these taxa, plus Stylophora.

Unit 5: Sections 325-M0036A-19R-1, 8 cm, through 20R-1: dark-colored worm-tube coralgal boundstone

Unit 5, spanning Sections 325-M0036A-19R-1, 8 cm, through 20R-1, consists of a dark-colored boundstone of encrusting coral and thin coralline algae in its upper part and a boundstone of coral, thin foliose coralline algae, and worm tubes in the lowermost 10 cm. The boundstone is intensely bioeroded and “blackened.”

The dominant coral is massive Goniopora, with fragments of massive Faviidae and fine-branching Seriatopora.

Unit 6: Section 325-M0036A-20R-CC: packstone

Unit 6, consisting only of Section 325-M0036A-20R-CC, is composed of dark gray packstone with larger foraminifera, mollusks, and Halimeda (Fig. F85). There are no visible corals.

Unit 7: Sections 325-M0036A-21R-1 through 21R-CC: unconsolidated sediment

The lowermost Unit 7, spanning Sections 325-M0036A-21R-1 through 21R-CC, is composed of unconsolidated coarse lime sand and pebbles in drilling mud. Visible components include larger foraminifera, mollusks, and Halimeda. Well-preserved larger foraminifera Alveolinella, Amphistegina, Elphidiidae, and Operculina are common in muddy, very coarse sand from interval 325-M0036A-21-1R, 0–5 cm. Sandy mud from interval 325-M0036A-21R-1, 20–25 cm, contains fragmented specimens of Elphidiidae, Gypsina, and Operculina, whereas moderately preserved specimens of Amphistegina and Operculina are present (but scarce) in fine-grained sands from interval 325-M0036A-21R-CC, 0–5 cm. There are no visible corals.

Physical properties

Petrophysical data from Hole M0036A are summarized in Table T2. The hole was drilled to 34.00 m DSF-A, and a total of 8.91 m of core was recovered (26.21% recovery).

Density and porosity

Bulk density values from whole-core multisensor core logger (MSCL) measurements range from 1.01 to 2.28 g/cm3 (Fig. F86). Bulk density was also measured on 17 discrete samples with values ranging from 1.75 to 2.37 g/cm3 (Fig. F87). Porosity values range from 23% to 58%, whereas grain density fluctuates between 2.74 and 2.82 g/cm3. There are no clear downhole trends in the moisture and density dataset for Hole M0036A. Bulk density from discrete measurements generally falls at the higher end of the range given by the MSCL, which is as expected (the MSCL tends to underestimate gamma density where core quality is poor) (see “Physical properties” in the “Methods” chapter).

P-wave velocity

Whole-core MSCL measurements yielded some P-wave velocity data for Hole M0036A cores (Fig. F86). Values range from 1556.38 to 1657.35 m/s, which are very low values considering the lithology. P-wave measurement on four discrete samples from these cores were also conducted, yielding data in the range of 3520 to 3919 m/s (mean resaturated P-wave) (Fig. F88A). There is no evidence for a systematic change in values downhole. It is difficult to comment on the relationship between bulk density and P-wave velocity owing to the very few data points available (Fig. F88B).

Magnetic susceptibility

There is no downhole trend in the magnetic susceptibility data for Hole M0036A, which may be owing to a combination of core quality and core recovery, as is the case with many of the holes in this transect. However, there is good recovery from Cores 325-M0036A-4R and 5R (3.5–5.23 m CSF-A), which show some variation in magnetic susceptibility values (Fig. F86). Overall, data ranges from diamagnetic –5.78 × 10–5 SI to magnetic 8.09 × 10–5 SI.

Electrical resistivity

Whole-core noncontact resistivity measurements in Hole M0036A yielded data ranging from 0.55 to 26.54 Ωm (Fig. F86). Resistivity values are highly variable downhole. However, from ~3.6 to 5.2 m CSF-A in a zone of good recovery resistivity increases downhole from ~0.5 to 1.1 Ωm.

Digital line scans and color reflectance

All cores from Hole M0036A were digitally scanned and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0036A varies between 43.55% and 84.54% L* units (Fig. F89). Variations in color reflectance parameters show a slight decrease in reflectance with depth. Values in the scale of blue to yellow (b*) approach to zero with depth, with a total variation of 20%. This trend is more evident in the a*/b* ratio, which changes from positive to negative values. The slope in the three parameters (L*, a*, and b*) follows a similar trend. In locations dominated by massive Acropora sp. colonies, color characteristics remain constant throughout.

Thermal conductivity

For Hole M0036A, only one measurement was made at 5.15 m CSF-A, showing a thermal conductivity of 0.826 W/(m·K). The general lack of thermal conductivity measurements in this hole, as well as across transect HYD-01C, is due to the nature of the recovered cores being inappropriate for measurement (see “Physical properties” in the “Methods” chapter).

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. F90). Positive low susceptibilities were recorded throughout the core, with an arithmetic mean value of 0.55 × 10–8 m3/kg. In addition, a cluster of susceptibility measurements located between 3.56 and 5.13 mbsf is characterized by positive values ranging from 0.84 × 10–8 to 1.9 × 10–8 m3/kg. A peak was measured at 18.68 mbsf with a susceptibility value of 1.49 × 10–8 m3/kg, indicating the possible presence of ferromagnetic and/or paramagnetic minerals. The record also shows negative susceptibility recordings at 11.01, 20.58, and 25.03 mbsf with susceptibility values of –2.61 × 10–8, –0.33 × 10–8, and –0.28 × 10–8 m3/kg, respectively.

Chronology

Two calibrated radiocarbon ages (17 cal y BP, Core 325-M0036A-8R; 17 cal y BP, Core 11) (Fig. F91) and one U-Th age (21 cal y BP, Core 18R) (Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The two calibrated radiocarbon dates are the same age, when rounded to 1 cal y BP, but the individual age ranges are still consistent with the stratigraphic order. The U-Th age is only slightly affected by corrections for initial 230Th, the seawater correction making the age 0.7 k.y. younger, thus adding confidence to the interpretation that this hole includes material of Last Glacial Maximum age. Therefore, this hole recovered material from the Last Glacial Maximum and captured the early portion of the deglaciation to ~17 cal y BP.

Downhole measurements

Geophysical wireline operations were completed in Hole M0036A to a total depth of 33.26 m WMSF (seafloor picked from the ANTARES Spectral Natural Gamma Probe [ASGR] log) with the ASGR sonde. Recovery in Hole M0036A was just over 26%; therefore, downhole measurements can be used to fill gaps in the core recovery. Downhole logging was conducted through an Advanced Petroleum Institute (API) hole, the diameter of which is beyond the maximum working size for the acoustic and optical tools (acoustic borehole televiewer [ABI40] and optical borehole viewer [OBI40]). Therefore, a reduced logging suite was run. Chronologically, the tools run were as follows:

  1. ASGR 512 (through API pipe) (the tool experienced a power surge on returning to the surface following logging and was no longer functional),

  2. ALT Induction Conductivity Probe (DIL45) (in open hole),

  3. Sonic (centralizers were undersized so the tool would fit through the API bit; therefore, the tool was not optimally centralized in the hole; hence, errors were introduced into the measurements),

  4. ASGR 256 (back-up Spectral Gamma Ray tool),

  5. Magnetic susceptibility probe (EM51), and

  6. Caliper (attempted deployment).

(See “Downhole logging” in the “Methods” chapter for detailed tool specifications). Following tripping of the pipe to facilitate logging in open hole, the environment downhole became unstable and the drilled hole began to infill, and over the duration of downhole logging the hole continued to infill. The deepest open hole measurement (DIL45) reached 25.72 m WMSF, with Hole M0036A becoming totally infilled by deployment of the caliper (the final tool deployed).

From the logging data, three main logging units were identified in Hole M0036A (Fig. F92):

  1. Unit I (0–11.59 m WMSF; Cores 325-M0036A-2R through 9R) is characterized by (relatively) consistently high total gamma ray (TGR) counts. Values generally range from ~6 to 9.3 cps (through pipe) and 8.7 to 10.3 cps (in open hole); however, at 2.10 m WMSF, counts drop to 3.2 cps. Open-hole data from the DIL45 shows values ranging between 157 and 190 mmho (induction electrical conductivity of greater investigation depth (ILD); deep conductivity measurement). Velocity measurements range from values near to seawater (1500 m/s) to 1893.30 m/s, and magnetic susceptibility hovers around 1.6 mSI. Logging Unit I corresponds to an upper thin unit of coralgal boundstone followed by a 3 m unconsolidated material and a lower 3.35 m unit of coralgal boundstone.

  2. Unit II (11.59–25.31 m WMSF; Cores 325-M0036A-10R through 18R) represents a zone of relatively low gamma ray counts with values ranging between 3.43 and 4.69 cps (through pipe). Within Unit II, conductivity does vary, with two main zones of slightly elevated values. Conductivity ranges from 226 to 343 mmho (ILD; deep conductivity measurement). Higher DIL45 values are found in the depth ranges 12.5–15.66 and 17.98–25.31 m WMSF. Sonic velocity values obtained are not particularly robust, with measured values ranging from values below the P-wave velocity of seawater (1500 m/s) to ~1900 m/s. In this depth range, the associated lithology is a coralgal-microbialite boundstone unit characterized by alternating corals, coralline algae, and microbialites. Areas of higher conductivity are likely to relate to zones of greater rugosity in the formation.

  3. Unit III (25.31–33.26 m WMSF; Cores 325-M0036A-19R through 21R) was solely differentiated on trends in the TGR measured through API pipe. It is characterized by increasing TGR counts, with values starting at 2.73 cps at the top of the logging unit and increasing to 7.54 cps at the base of the hole. The different lithostratigraphic units observed throughout this logging unit are not apparent in the TGR data. The top of the unit is represented by a dark-colored, bioeroded boundstone followed by packstone comprising benthic foraminifers (no corals). The base of the hole comprises unconsolidated coarse lime sand and pebbles containing no corals.