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doi:10.2204/iodp.proc.325.103.2011

Hole M0033A

Operations

Site 6, Hole M0033A

The vessel was on station at 1353 h, 17 February 2010 (Table T1), after an additional 2 m move northeast from the original location to avoid the presence of live coral, observed during the precoring downpipe camera survey. At 1445 h, the standard rotary corer core barrel and additional API pipe were run. Run 1 commenced at 1510 h after a slight delay due to a Differential Global Positioning System dropout. Coring operations continued until 0000 h, and then the compressors and power packs were refueled until 0025 h on 18 February. Coring then restarted until 0530 h, when the barrel stuck on Run 16. It was recovered after 30 min but had a broken latch lug on recovery. Run 17 was recovered at 0745 h after the water pressure spiked. The liner tube was crushed and stuck inside the barrel, with an extruded lug halfway down the barrel, reducing operations to one fully functional core barrel while the liner was removed. Coring continued for another six runs before the hole was terminated at 1335 h at 32.8 mbsf, with an average recovery of 40.9%.

API pipe was tripped to just above the seabed before the downpipe camera was deployed at 1435 h. However, the camera would not pass through the final 3–4 m of the bottom-hole assembly, and the wireline became twisted. The wireline was quickly untwisted from the top of the derrick, but the camera would still not pass through the bottom-hole assembly, so the camera survey was aborted at 1540 h without running a postcoring survey. The camera was recovered to deck by 1600 h. By 1725 h, the remainder of the API pipe was tripped, and the vessel began a slow (1 kt) transit to Site 3 in order to allow for repairs to the roughneck slips.

Sedimentology and biological assemblages

Hole M0033A is divided into five lithostratigraphic units.

Unit 1: Sections 325-M0033A-1R-1 through 1R-CC: coralgal boundstone

The uppermost Unit 1, spanning Sections 325-M0033A-1R-1 through 1R-CC, consists of fragments of coralgal boundstone. The uppermost interval 325-M0033A-1R-1, 0–45 cm, consists of heavily bored, brown-stained boundstone with coral and internal sediment. Coralline algae form an open framework of thin plants on corals (Porites), which, in turn, overlie or are surrounded by packstone with coral, Halimeda, mollusk, benthic foraminiferal, and coralline algal fragments. These deposits are bioeroded with brown stains.

This unit is dominated by a massive Porites colony with associated pieces of massive Montipora(?) and Goniopora(?).

Unit 2: Sections 325-M0033A-2R-1 to 11R-1, 19 cm: coralgal-microbialite boundstone

Unit 2, spanning Sections 325-M0033A-2R-1 to 11R-1, 19 cm (Fig. F30), consists of coralgal-microbialite boundstone (Fig. F31). Relative proportions of corals (Fig. F32), coralline algae, and microbialites in the boundstone vary considerably throughout the unit. Coralline algae alternate with microbialite crusts containing trapped bioclasts. Vermetids intergrown with coralline algae are common (Fig. F33). Bryozoans and serpulids are visible locally. Coralline algae occur as thick crusts overgrowing corals or as irregular, contorted structures intergrown with microbialites. Microbialites are usually dark colored with poorly defined laminae or alternating darker and lighter laminae. Some internal bioclastic sediment with Halimeda occurs throughout this unit. The boundstone is bioeroded locally by bivalves and sponges. The larger foraminifera Amphistegina and Operculina are present, but scarce, in muddy gravels from interval 325-M0033A-3R-1, 11–16 cm.

The dominant corals are massive Isopora colonies associated with medium-thickness branching (digitate) Acropora (Figs. F31, F34) and fine-branching Pocilloporidae. Coral fragments include Isopora, Pocillopora, Tubipora musica, and Fungiidae.

Unit 3: Sections 325-M0033A-11R-1, 19 cm, through 19R-CC: coralgal-microbialite boundstone with geopetal fabrics

Unit 3, spanning Sections 325-M0033A-11R-1, 19 cm, through 19R-CC, also consists of coralgal-microbialite boundstone. Although the components and main features of this boundstone are similar to those of Unit 2, Unit 3 has a higher proportion of internal sediment and also contains geopetal fabrics in borings, intraskeletal voids, and small cavities (Fig. F35). The internal sediment consists of wackestone/packstone to floatstone rich in mollusks, Halimeda, bryozoans, echinoids, larger foraminifera, and coralline algae that locally form small rhodoliths. As in Unit 2, relative proportions of corals, coralline algae, and microbialites vary substantially, but the three components seem to cycle repeatedly throughout the unit. Microbialites are mainly stromatolitic and commonly contain trapped bioclasts (Fig. F36). Bioerosion tends to be concentrated in the outer parts of corals but also affects some other components of the boundstone.

The coral assemblage is diverse and dominated by Faviidae (mainly Favia) (Figs. F37, F38), branching Acropora, fine-branched Seriatopora, and thicker Pocillopora branches (Fig. F39). Other common corals are Montipora, Porites, Tubipora musica (Fig. F40), Fungiidae, Psammocora(?), and Agariciidae(?). Fragments include all of the above corals plus Cyphastrea and Siderastreidae.

Unit 4: Core 325-M0033A-20R through Section 22R-1: unconsolidated sediment

Unit 4, spanning Core 325-M0033A-20R through Section 22R-1, consists of unconsolidated lime granules and pebbles with minor quantities of sand. Major components include coral fragments, Halimeda, and mollusks (Fig. F41). Larger foraminifera (Alveolinella, Amphistegina, and Soritinae) are common in interval 325-M0033A-22R-1, 40–45 cm, and echinoid spines also occur throughout this interval. Some fragments of the overlying boundstones were identified and are clearly the result of downhole contamination.

There are no large corals, but identifiable fragments include Seriatopora, Porites, and possibly Montipora.

Unit 5: Sections 325-M0033A-22R-CC through 23R-CC: packstone/grainstone

The lowermost Unit 5, spanning Sections 325-M0033A-22R-CC through 325-M0033A-23R-CC, is composed mainly of a dark gray packstone/grainstone. The major components are benthic foraminifera, gastropods, bivalves, coral, Halimeda, larger foraminifera, echinoderms, and bryozoans. It also includes local occurrences of worm tubes and possible calcareous sponges (Fig. F42). The top of the cemented interval is heavily bored by worm tubes and sponges. The uppermost interval of Section 325-M0033A-23R-1, from 0 to 25 cm, contains lime granules to pebbles, similar in composition to the overlying interval, intercalated with the packstone/grainstone, and probably represents downhole contamination during coring.

There are no large corals, but identifiable fragments include Seriatopora, Porites, and possibly Montipora.

Physical properties

Hole M0033A was drilled to 32.80 m DSF-A. A total of 13.41 m of core was recovered (40.88% recovery). Petrophysical data acquired from this core are compiled in Table T2.

Density and porosity

Gamma density values from whole-core multisensor core logger (MSCL) measurements range from 1.00 to 2.53 g/cm³ (Fig. F43). The bulk density of discrete samples varies between 1.84 and 2.45 g/cm³, and porosity varies between 18% and 54% (Fig. F44). As expected, discrete measurements resulted in values at the higher end of the range exhibited by MSCL measurements. Grain density is nearly constant with a range of 2.76 to 2.84 g/cm³. Petrophysical properties on the discrete plug samples do show fluctuations. However, it is difficult to compare these values with the MSCL data and/or lithostratigraphic results because of the heterogeneity and/or disturbed nature of the recovered core material.

P-wave velocity

P-wave velocity offshore MSCL measurements on whole cores were unsuccessful owing to poor acoustic coupling (function of poor core quality). However, eight discrete samples were collected from Hole M0033A and measured onshore (Fig. F45A). P-wave velocity values from discrete samples range from 3340 to 4712 m/s (resaturated). There are two main trends downhole. Between 3.24 and 12.90 m CSF-A velocity decreases from 4080 m/s to the minimum measured here. Velocity then increases to the maximum value (4712 m/s) at 14.61 m CSF-A and with depth decreases to 3865 m/s (base measurement at 23.91 m CSF-A). Neither of these packages of decreasing velocity appear to coincide with lithologic changes downhole. Bulk density and P-wave velocity measured on discrete samples show similar trends: as bulk density increases, P-wave velocity also increases (Fig. F45B).

Magnetic susceptibility

The majority of the MSCL magnetic susceptibility data for Hole M0033A is in the range of –1 × 10^–5 to 0 SI (Fig. F43). The overall range is –1.29 × 10^–5 to 7.17 × 10^–5 SI. Two core sections yielded high values of magnetic susceptibility relative to the rest of the hole. These were Sections 325-M0033A-5R-2 (highest value at 7.61 m CSF-A) and 8R-1 (highest value at 9.89 m CSF-A), and in both cases, values were highest at the top of the section, decreasing downsection.

Electrical resistivity

Electrical resistivity for Hole M0033A cores ranges from low values of 0.77 Ωm to higher values of 28.86 Ωm (Fig. F43). The highest resistivity occurs in Sections 325-M0033A-6R-1 and 9R-1. The lowest resistivity is recorded at ~20 m CSF-A.

Digital line scans and color reflectance

All cores from Hole M0033A were measured using the digital line scan system, with all data recorded at a resolution of 150 pixel/cm as both images and RGB values. All appropriate cores were also scanned for color reflectance. Color reflectance in Hole M0033A varies between 30.86% and 83.96% L* units (Fig. F46). Variations in color reflectance parameters slightly decrease (6.4%) in reflectance with depth. The alternation of coralgal-microbialite boundstones and unconsolidated carbonate sediments from 3.3 to 7.2 m CSF-A produces a greater fluctuation in the values of the three color reflectance parameters (L*, a*, and b*). A more homogeneous area of carbonate sediment from 25.3 to 29.2 m CSF-A reflects a slight decrease in reflectance values. In the bottom few meters of Hole M0033A, the presence of a packstone with worm tubes is visible in the a*/b* values.

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. F47). Four low negative (diamagnetic) susceptibilities were recorded at 0.01, 6.03, 15.86, and 16.04 mbsf, with susceptibility values of –0.39 × 10^–8, –0.06 × 10^–8, –0.16 × 10^–8, and –0.01 × 10^–8 m³/kg, respectively. Measured positive core susceptibilities range from 0.07 × 10^–8 to 1.90 × 10^–8 m³/kg (with a salient peak at 7.53 mbsf). The arithmetic mean susceptibility for M0033A cores was 0.38 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (14 cal y BP, Core 325-M0033A-3; 15 cal y BP, Core 7R) (Fig. F48) and one U-Th age (31 cal y BP, Core 15R) (Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is made significantly younger by corrections for initial ²³⁰Th and is thus less reliable. It is therefore unlikely that Core 325-M0033A-15R has a true age older than the Last Glacial Maximum. This hole recovered material from the Last Glacial Maximum interval and captured the early portion of the deglaciation to ~14 cal y BP.

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