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doi:10.2204/iodp.proc.325.103.2011

Hole M0039A

Operations

Site 8, Hole M0039A

Hole M0039A tagged seabed at 1625 h on 4 March 2010, and the first standard rotary corer core was on deck at 1700 h (Table T1). Coring continued until 2340 h, when the driller noted high mud pressure, suggesting the barrel was blocked. On recovery, the latch head assembly was found to be blocked with sediment. This occurred on three consecutive deployments, resulting in no advance in the drilled depth. At 0300 h on 5 March, the barrel advanced slowly during Run 10, and coring continued until 1400 h (with a break between 1130 and 1220 h for repairs to the latch head dog), when the hole was terminated at 28.4 mbsf with an average recovery of 36.6%. By 1420 h, the API pipe was tripped to just above the seabed in order to run a downpipe camera survey. Tripping the API pipe then continued from 1455 to 1650 h, when the seabed transponder was recovered.

Sedimentology and biological assemblages

Hole M0039A is divided into five lithostratigraphic units.

Unit 1: interval 325-M0039A-1R-1, 0–18 cm: modern sediments

The uppermost Unit 1, consisting of interval 325-M0039A-1R-1, 0–18 cm, is modern sediment composed of mainly of unconsolidated, predominantly pebble sized coral clasts (some with brown staining) associated with calcareous gravelly sand. Some coral clasts are covered with nongeniculate coralline algae. Well-preserved specimens of Textulariida are present but scarce in muddy gravels from interval 325-M0039A-1R-1, 5–10 cm.

The top of Unit 1 contains pieces of massive Faviidae above colonial ahermatypic corals, whereas an unidentified, medium thickness, branching coral is common below 15 cm.

Unit 2: interval 325-M0039A-1R-1, 18–38 cm: floatstone

Unit 2, consisting of the short interval 325-M0039A-1R-1, 18–38 cm, is composed of bioclastic floatstone (Fig. F101) containing gravel-sized bioclasts of bryozoans, mollusks, gastropods, and smaller proportions of bivalves. The matrix of this floatstone has a packstone fabric in which some cavities, of uncertain origin, are partly filled with well-lithified lime mud. There are no visible corals in Unit 2.

Unit 3: Sections 325-M0039A-1R-1, 38 cm, to 4R-1, 52 cm: coralgal boundstone

Unit 3, spanning Sections 325-M0039A-1R-1, 38 cm, to 4R-1, 52 cm, is a coralgal boundstone (Fig. F102). Massive corals are common throughout this unit. Nongeniculate coralline algae are usually a minor component of the sediment, but thick crusts of nongeniculate coralline algae occur in interval 325-M0039A-3R-1, 26–31 cm. There is a thin (~5 mm) layer of microbialite covering a massive coral at Section 325-M0039A-4R-1, 32 cm. Two intervals of bioclastic packstones are intercalated at 325-M0039A-4R-1, 9–16 cm, and 4R-1, 20–40 cm (Fig. F103). These packstones are composed mainly of mollusk bioclasts, fragments of corals (encrusted by nongeniculate coralline algae), and nongeniculate coralline algae. Halimeda segments are abundant in interval 325-M0039A-4R-1, 20–40 cm.

Massive, probably in situ, Porites colonies (Fig. F104) dominate Unit 3, and associated corals include massive Isopora (Fig. F105), medium-thickness branching Acropora (Fig. F106), submassive Goniopora, and branching Pocilloporidae. Fragments include Porites, Isopora, Seriatopora, Pocillopora(?), and some ahermatypic corals.

Unit 4: Sections 325-M0039A-4R-CC to 21R-1, 11 cm: coralgal-microbialite boundstone

Unit 4, spanning Sections 325-M0039A-4R-CC through 21R-1, 11 cm, is composed of coralgal-microbialite boundstone associated with bioclastic packstone (Fig. F107). The boundstone consists of corals thinly covered with nongeniculate coralline algae and capped by thick microbialite envelopes. Massive corals dominate, with some associated branching forms. Nongeniculate coralline algae are not as important volumetrically as corals and microbialites. The microbialites are gray and weakly to clearly laminated (Fig. F108). Depressions and layers of bioclasts such as Halimeda segments, molluscan shell fragments, and coral clasts occur within the microbialites, and these portions have a packstone fabric (Fig. F109). Cavities of uncertain origin are common, and some are partly filled with unconsolidated and/or consolidated internal sediments (calcareous silt). Some coral clasts in the lowermost part of interval 325-M0039A-21R-1 0–11 cm, are stained dark gray. Fragments of Homotrema were found in gravelly coarse sands from interval 325-M0039A-12R-CC, 5–10 cm, whereas well-preserved or abraded specimens of Amphistegina, Baculogypsina, Calcarina, Marginopora, and Miliolida are common in medium to coarse sands from interval 21R-1, 15–20 cm.

There are two intervals of sand with pebbles and common to abundant bioclasts of Halimeda, benthic foraminifera, and mollusks in addition to corals and limestone clasts: intervals 325-M0039A-20R-1, 0–73 cm, and 21R-1, 11–22 cm (Fig. F110). These intervals probably formed during coring, and, therefore, indicate downhole contamination.

A diverse coral assemblage is dominated by massive Porites (Fig. F111), submassive to massive Faviidae (Fig. F112), massive Isopora (Fig. F113), and branching Acropora (including tabular colonies) (Fig. F114). Associated corals include massive Astreopora (Fig. F115), Cyphastrea, Montipora, Pocillopora, Echinopora (Fig. F116), Goniopora, and Tubipora. One section contains an apparently alcyonarian spiculite (Fig. F117).

Unit 5: Section 325-M0039A-21R-CC: grainstone

Unit 5, the lowermost unit in Hole M0039A, consists of material from Section 325-M0039A-21R-CC and contains a grainstone with well-sorted bioclasts of corals, benthic foraminifers, Halimeda, and mollusks (Fig. F118). All coral fragments are tiny; only one Acropora fragment was identified.

Physical properties

A total of 10.39 m of core was recovered from Hole M0039A, which was drilled to 28.4 m DSF-A (36.58% recovery). The corresponding petrophysical data are summarized in Table T2.

Density and porosity

In Hole M0039A, whole-core multisensor core logger (MSCL) measurements range from 1.02 to 2.45 g/cm³ (Fig. F119). The nature of the core quality and the short core lengths mean that the data quality is compromised (see “Physical properties” in the “Methods” chapter). However, eight discrete samples were analyzed for moisture and density, giving bulk density values in the range of 2.02 to 2.39 g/cm³. Porosity values for the same samples range from 20% to 43% (Fig. F120), and grain density fluctuates between 2.75 and 2.81 g/cm³. There are no clear downhole trends in the density and porosity datasets and equally no clear relationship between the two bulk density measurements (MSCL and discrete).

P-wave velocity

P-wave velocity MSCL measurement of Hole M0039A cores yielded a solitary data point (6.23 m CSF-A) with a value of 1636.52 m/s (Fig. F119). Two samples were measured using the discrete P-wave logger. Mean resaturated measurement values were 3131 and 4701 m/s (Fig. F121). Owing to the limited P-wave data points available in Hole M0039A, it is impossible to comment on downhole trends.

Magnetic susceptibility

The magnetic susceptibility data range for this hole is –4.13 × 10^–5 to 9.79 × 10^–5 SI (Fig. F119). However, the majority of magnetic susceptibility values fall between –1 × 10^–5 and 1 × 10^–5 SI. Core recovery impacts the ability to ascertain any downhole trends in Hole M0039A.

Electrical resistivity

Whole-core noncontact resistivity measurements on Hole M0039A cores yielded data ranging from 0.59 to 12.97 Ωm (Fig. F119). Similar to the other datasets for this hole, it is difficult to comment on trends or notable features with respect to the resistivity dataset owing to short core length, core quality, and recovery.

Digital line scans and color reflectance

All cores from Hole M0039A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance L* in Hole M0039A varies between 51.59% and 84.24% (Fig. F122). Variations in color reflectance parameters show a slight decrease in reflectance with depth, as well as a decrease in the dispersion of the measurements. This can be attributed to the alternation between coralgal and coralgal-microbialite boundstone, broken fragments, and unlithified sediment lithologies in the higher and medium parts of the borehole. With the exception of the basal packstone unit, the lowermost 6 m of the borehole presented a more homogeneous composition that corresponds to the less dispersed measurements of color spectroscopy. The outliers in the red wavelength, high a* value, were due to Tubipora sp.

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. F123). Very low positive susceptibilities occur throughout the core, with five negative peaks at 0.08, 7.69, 13.47, 14.12, and 15.55 mbsf characterized by susceptibilities of –0.22 × 10^–8, –0.38 × 10^–8, –0.12 × 10^–8, –0.08 × 10^–8, and –0.53 × 10^–8 m³/kg. An arithmetic mean value of 0.20 × 10^–8 m³/kg was calculated for the positive values recorded in this core.

Chronology

Two calibrated radiocarbon ages (14 cal y BP, Core 325-M0039A-1R; 17 cal y BP, Core 9R) (Fig. F124) and one U-Th age (19 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 (the seawater correction makes the age only 0.5 k.y. younger), adding to the confidence in this age interpretation. This hole recovered material from the early portion of the deglaciation to 14 cal y BP, and because there are five cores below the 19 cal y BP dated sample, the hole may also contain older material.

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