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Hole M0041A


Site 10, Hole M0041A

The Greatship Maya was on station over Hole M0041A by 0915 h on 6 March 2010, and a precoring downpipe camera survey was conducted (Table T1). The camera was back on deck by 0925 h, and the API pipe tagged the seabed at 0955 h. Coring operations ran smoothly for 12 standard rotary corer runs before the hole was terminated at 1650 h at 22.1 mbsf, with an average recovery of 45.5%.

The API pipe was tripped until it sat ~5 m above the seabed, and the downpipe camera was deployed. However, the camera became stuck in the bottom-hole assembly at 1720 h and could not be recovered. At 1745 h, the decision was taken to cut the camera cable and trip the API pipe, passing the camera cable through each pipe successively in order to retain the camera should it become free while tripping the API pipe. At 2110 h, the camera became free in the pipe above the collars and was recovered. By 2130 h, all API pipe was in the rack/slips and retermination of the camera cable had begun. The seabed transponder was recovered, and the vessel began to transit 6 km landward to Site 2, Hole M0042A, at 2220 h.

Sedimentology and biological assemblages

Hole M0041A is divided into five lithostratigraphic units.

Unit 1: Sections 325-M0041A-1R-1 through 1R-2: unconsolidated mud and very fine sand

The uppermost Unit 1, spanning Sections 325-M0041A-1R-1 through 1R-2, consists of 200 cm of unconsolidated mud and very fine sand, with rare bioclasts of larger foraminifera and mollusks. The uppermost 3 cm in Section 325-M0041A-1R-1 is rich in molluscan shells. Interval 325-M0041A-1R-1, 14–19 cm, contains a few well-preserved specimens of Amphistegina and Miliolidae. There are no visible corals in Unit 1.

Unit 2: Sections 325-M0041A-1R-CC to 4R-1, 8 cm: fragmented boundstone/packstone and unconsolidated lime sand and gravel

Unit 2, spanning Sections 325-M0041A-1R-CC, 0 cm, to Section 4R-1, 8 cm, consists of angular, pebble-sized lithoclasts of coralgal boundstone and bioclastic packstone, with sand- to pebble-sized bioclasts of corals, mollusks, and, to a lesser extent, larger foraminifera (Fig. F15). Sediments from interval 325-M0041A-1R-2, 70–75 cm, only contain small benthic foraminifera. Lithoclasts are stained dark gray.

Except for one massive Porites, most corals are broken pieces of encrusting to massive Acroporidae and Poritidae. Smaller fragments include Porites, Acropora(?), Isopora(?), Montipora(?), and Pocilloporidae.

Unit 3: Sections 325-M0041A-4R-1, 8 cm, to 7R-1, 21 cm: microbialite boundstone

Unit 3, spanning Sections 325-M0041A-4R-1, 8 cm, to 7R-1, 21 cm, mainly consists of (coralgal) microbialite with varying amounts of coralline algae, but its nature is difficult to determine because the recovered material is broken into many fragments, possibly by drilling disturbance. A well-preserved sample of microbialite from interval 325-M0041A-6R-1, 13–31 cm, has both thrombolitic and laminated fabrics (Fig. F16). Laminar microbialites also form columns (~1–2 cm in diameter and ~6 cm in height), and cavities in the microbialites are filled with bioclastic packstone.

Pieces of Acroporidae(?) and Poritidae(?) dominate the upper parts of Unit 3, whereas Agariciidae and Faviidae dominate toward the base. Coral fragments are largely unidentifiable in the upper parts of the unit, but fragments toward the base include Seriatopora, Tubipora musica, Acroporidae, Agariciidae, Poritidae(?), and free-living Heteropsammia cochlea.

Unit 4: Sections 325-M0041A-7R-1, 21 cm, to 10R-1, 8 cm: very coarse lime sand with coral clasts

Unit 4, spanning Sections 325-M0041A-7R-1, 21 cm, to 10R-1, 8 cm, consists of very coarse lime sand with pebble-sized clasts of corals (Fig. F17). The very coarse lime sand is composed mainly of bioclasts of corals along with mollusks, with lesser amounts of Halimeda and larger foraminifera. Muddy medium sands from interval 325-M0041A-8R-1, 40–45 cm, include many well-preserved specimens of Operculina and Alveolinella and abraded specimens of Amphistegina. Fragmented specimens of Operculina, Alveolinella, Amphistegina, and Elphidiidae are common in muddy medium sands from interval 325-M0041A-10R-1, 34–39 cm.

Larger coral clasts are pieces of massive Faviidae (including Favites). Smaller clasts and fragments include Plesiastrea(?), Montipora or Porites, Seriatopora, and Turbinaria.

Unit 5: Sections 325-M0041A-10R-1, 8 cm, through 12R-CC: unconsolidated fine to medium sand

The lowermost Unit 5, spanning Sections 325-M0041A-10R-11, 8 cm, through 12R-CC, consists mainly of unconsolidated fine to medium lime sand with larger foraminifera (e.g., Operculina), molluscan shell fragments, and rare coral clasts. Some interlayered lithologies consist mainly of pebble-sized bioclasts of mollusks, larger foraminifera, and smaller benthic foraminifera (including Homotrema rubrum). There are well-preserved and fragmented specimens of Operculina (Fig. F18) in muddy medium-grained sands from intervals 325-M0041A-11R-1, 55–60 cm, 12R-1, 64–69 cm, and 12R-2, 24–29 cm.

Corals are scarce but include small fragments of Porites, Montipora(?), Seriatopora, free-living Heterocyanthus cochlea, and solitary ahermatypic corals.

Physical properties

Hole M0041A was cored to a total depth of 22.10 m CSF-A, of which 10.06 m was successfully recovered (45.52% recovery). As with other holes from this transect, physical property measurements for this hole are compiled in Table T2.

Density and porosity

Bulk density varies from 1.13 to 2.41 g/cm3 in cores from Hole M0041A (Fig. F19). The uppermost 2 m and bottommost 5 m of the hole are characterized by relatively stable bulk density values in the range of 1.92 to 2.21 g/cm3. The middle part of the hole has more erratic bulk density values, which are likely a function of core length and core quality (see “Physical properties” in the “Methods” chapter). Twelve samples were taken from Hole M0041A for discrete moisture and density analysis (Fig. F20). High porosity (52% and 53%) sandy lime mud and lime sand is present between 0 and 1.5 m CSF-A. From 3.5 to 13.20 m CSF-A, there are fragmented units (lime sand and lime pebbles) where the porosity ranges from 27% to 37%. Below 13.20 m CSF-A, there is a lime sand unit of higher porosity (42%–50%). Beyond this, porosity changes with depth in a similar manner to that found in Hole M0040A. This is probably due to the fact that the boreholes are in close proximity to one another and at a similar water depth. Similar to Hole M0040A, grain density varies from 2.74 to 2.79 g/cm3, whereas bulk density ranges from 1.83 to 2.32 g/cm3. There are no downhole trends evident from the density data.

P-wave velocity

In Hole M0041A, whole-core P-wave velocity measurements taken offshore range from 1508.59 to 1700.41 m/s (Fig. F19). The lower values are too close to the value of P-wave velocity of water, indicating a problem with the validity of these data points. This problem is probably due to core quality issues (see “Physical properties” in the “Methods” chapter). Multisensor core logger P-wave velocity data are restricted to the uppermost 2 m and bottommost 5 m, with the lower interval exhibiting a decrease in value with depth. The absence of data from the central portion of Hole M0041A is a function of core quality and the potential for acoustic coupling across the P-wave velocity transducers. One discrete P-wave velocity measurement was taken on a sample from Hole M0041A cores, giving a value of 3841 m/s (mean value for resaturated core).

Magnetic susceptibility

In Hole M0041A, magnetic susceptibility values range from –1.00 × 10–5 to 67.34 × 10–5 SI, with most values falling between –1 × 10–5 and 1 × 10–5 SI (Fig. F19). Magnetic susceptibility highs occur in the bottom half of the hole at 12.98 CSF-A (15.57 × 10–5 SI), 16.24 CSF-A (22.92 × 10–5 SI), 17.77 CSF-A (67.34 × 10–5 SI), and 19.19 CSF-A (43.05 × 10–5 SI). No evidence of a downhole trend in the values exists.

Electrical resistivity

Similar to the density and P-wave data sets, resistivity data are relatively good and continuous at the top and bottom of the hole but erratic in the central section. Electrical resistivity measured on whole cores varies from 0.58 to 5.03 Ωm (Fig. F19). Resistivity values are most reliable in the upper and lower portions of the borehole, where the values are the lowest. Overall, there are no obvious downhole trends.

Digital line scans and color reflectance

All cores from Hole M0041A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0041A ranges from 44.71% to 73.7%. Hole M0041A exhibits a similar trend to Hole M0040A (Fig. F21). The uppermost 2 m of the hole is characterized by color reflectance values between 50.05% and 63.64% where muddy lime and pebbles were detected. A slightly higher reflectance was detected below 3 m CSF-A, reaching a maximum of 66.88% at 5.7 m CSF-A. Below this depth, the reflectance of the materials decreases with a higher dispersion of data corresponding to lime-sand areas. The presence of mollusk shells in certain areas could explain a higher dispersion of the data at ~13 m CSF-A. Where lime and muddy sand was present, values for the index a* became negative (green color), and values were positive when coarser sediments, corresponding to broken coral fossils, were present. The value of b* was positive in all cases, showing higher dispersion in depth within the same range. This indicated a yellow color in the samples.

Thermal conductivity

Three thermal conductivity measurement points were taken for Hole M0041A, with values ranging from 1.06 to 1.09 W/(m·K).


Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on all samples taken from the working half of the recovered core using both 1 cm3 and paleomagnetic standard samples (Fig. F22). Very low to low negative (diamagnetic) susceptibilities occur throughout the entire core, ranging from –0.03 × 10–8 to –2.87 × 10–8 m3/kg with an average of –1.21 × 10–8 m3/kg. In addition, positive susceptibilities range from 0.00 to 58.18 × 10–8 m3/kg with an arithmetic mean of 3.94 × 10–8 m3/kg. Two prominent susceptibility peaks are located at 14.15 and 15.63 mbsf with values of 9.16 × 10–8 and 58.18 × 10–8 m3/kg, respectively. These positive susceptibilities may indicate the presence of paramagnetic and/or ferromagnetic minerals.

Preliminary results obtained from the paleomagnetic study of a U-channel taken from Section 325-M0041A-12R-1 (transect HYD-02A) are also presented (Figs. F23, F24). The noisy natural remanent magnetization (NRM) demagnetization paths are attributed to the relatively low intensity of magnetizations (1.08 × 10–9 to 2.19 × 10–7 A/m with a mean of 2.02 × 10–8 A/m). A limited number of samples are characterized by high NRMs, and these are associated with lithologic layers of high magnetic susceptibility. Consistency of NRM inclinations of the discrete cubes measured can also be correlated with the intensity of magnetization results.

The generally positive and high inclination values obtained for Expedition 325 samples are not what is expected for the low paleolatitude of the sampling sites (latitude = 15°–20°S) with corresponding geocentric axial dipole values of ~28° to 36°S. One interpretation of the results is that a significant portion of the drilling overprint remains on the majority of samples studied. Alternatively, there may be a pervasive present-field overprint that was not possible to remove with alternating-field demagnetization experiments.

Anhysteretic remanent magnetization (ARM) values (n = 134) have an arithmetic mean of 5.16 × 10–8 A/m (Fig. F23D). Values >3 × 10–8 A/m occur in interval 325-M0041A-12R-1, 3–20 cm, with an average of 2.69 × 10–7 A/m. The rest of the core shows an arithmetic mean of 2.16 × 10–8 A/m. Therefore, the uppermost ~20 cm of Section 325-M0041A-12R-1 has ARM of over an order higher than the rest of the section. This interval at the top of the section testifies that depositional or preservation conditions are different here than in the rest of the section. We do not exclude the contamination of this interval during the drilling operations, which may have biased the magnetic properties.


This hole has a single calibrated radiocarbon age of 17 cal y BP from Core 325-M0041A-2R (Fig. F25). The dated sample is from the top of the hole, indicating that this hole may have captured part of the early deglaciation. There are 10 cores beneath this 17 cal y BP section, indicating that this hole may have also recovered material from the Last Glacial Maximum, and possibly older.