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doi:10.2204/iodp.proc.325.103.2011

Hole M0035A

Operations

Site 11, Hole M0035A

The Greatship Maya arrived at Site 11 over Hole M0035A at 2345 h on 21 February 2010 and settled on dynamic positioning (DP) (Table T1). At 0045 h, the moonpool door was opened, and running of API pipe commenced at 0100 h on 22 February. Operations halted between 0200 and 0250 h for repairs to the roughneck hydraulics. Once the API pipe was just above the seabed, the seabed template was lowered 20 m below the sea surface to test the HiPap transponder and was then recovered back to the moonpool. At 0355 h, the API pipe was lowered further, and the downpipe camera was deployed for a precoring survey. The camera was recovered by 0550 h. The API pipe was then run to tag the seabed and pulled back 0.5–1 m, with the aim to have the end of the pipe rest inside the seabed template when deployed but free to move where the template guided it. The template was lowered onto the seabed at 0635 h. However, because of the template settling into the sediment, the first attempt at standard rotary corer (ALN) coring showed that the API pipe and ALN core barrel were now outside the template entry cone. At 0730 h, the ALN barrel and API pipe were tripped, and at 1015 h, the seabed template was recovered. The ALN bit was smashed, and the API bit was cracked in three places. Both were repaired, and the API pipe was rerun starting at 1100 h. The API pipe was in position just above the seabed by 1410 h, and the downpipe camera was deployed. However, at 1445 h, the vessel again lost position with the camera still inside the API pipe. As the HiPap transponder was attached to the seabed template, which had not been deployed yet, the vessel started to drift.

At 1451 h, the API pipe was raised by 2 m to avoid any obstacles on the seabed while the vessel regained station, and the camera was recovered. The GPS came back online at 1513 h. The Master of the Greatship Maya and Dynamic-Positioning Officer stated that they were happy with the DP model at 1555 h and were ready to restart operations, but the ESO Operations Superintendant required that a secondary positioning system be established, henceforth known as the seabed transponder. This transponder was to be deployed over the side of the vessel prior to operations beginning at any site from this point on to prevent further positioning problems occurring during setup and running of coring operations. At 2030 h, the decision was made to abandon Hole M0035A and return to Townsville, where engineers for both the GPS and DP systems would be waiting. The remainder of the API pipe was tripped, and the drill floor secured for transit.

Port call at Townsville

The Greatship Maya departed Site 11 (Hole M0035A) on transect HYD-01C at 2240 h on 22 February 2010 and arrived alongside in Townsville at 2050 h on 23 February. Engineers from Veripos (GPS) and ConverTeam (DP) boarded the vessel. Work to identify and rectify problems with both the DP and GPS systems continued overnight. The Greatship Maya departed Townsville at 1010 h on 24 February to begin sea trials of the DP and GPS systems. At 2350 h, a supply boat arrived alongside the Greatship Maya. Additional core liner tubes and a new seabed frame for the HiPap seabed transponder were taken onboard, and one Veripos surveyor and the ConverTeam engineer departed. One Veripos engineer remained onboard.

Transit to transect HYD-01C

The Greatship Maya departed for Site 11 on transect HYD-01C at 0020 h on 25 February 2010 and arrived at the location of Hole M0035A at 0100 h on 26 February, where she remained waiting on weather until 0405 h on 27 February.

Hole M0035A

At 0330 h on 27 February 2010, the HiPap transponder was attached to the seabed frame in readiness for improvement in the weather conditions. Permission was given to begin operations, and at 0430 h the vessel moved 10 m starboard of Site 11, Hole M0035A, under DP to deploy the seabed transponder. The vessel was back on station over Hole M0035A at 0530 h. The bridge confirmed that drilling operations could begin at 0605 h, and work began changing the API bit to a new stepped impregnated bit. Running API pipe commenced at 0715 h, pausing at 0735 h while the seabed template was suspended in the moonpool before continuing until 0930 h. Between 0930 and 1010 h, a downpipe camera survey was conducted. Additional API pipe was then run, and ALN coring commenced at 1110 h.

At 1250 h, the second core was recovered to deck. However, there were problems with hole collapse, and operations had to cease, as power was lost to the drillers’ consol. Coring operations restarted at 1600 h, with the power problem now restricted to mud pump number 1. The roughneck control box was also found to be leaking hydraulic oil. At 1740 h, the bit became blocked. Pumping and reaming did not free the bit, so at 1915 h the hammer sampler was deployed, freeing the bit at 2215 h.

At 2325 h, the decision was made to switch to HQ coring. Preparations for this continued until 0205 h on 28 February, including fitting a collar to the API pipe above the seabed template suspended in the moonpool. It was observed that the API pipe and seabed template were very unstable within the moonpool. This was attributed to strong currents pushing the API pipe and the API pipe not reacting against the hole. At 0335 h, the decision was made to return to API coring. Repairs were made to the winch cable at 0425 h, damaged during setting up for HQ coring, with coring operations restarting at 0440 h. Between 0800 and 1025 h, operations were halted because of a problem with the mud pumps caused by air entering the suction valve in the moonpool. Between 1200 and 1235 h, and again between 1600 and 1620 h, operations halted as repairs were made to the compressor because of overheating. At 1625 h, the bit became blocked during Run 14, but it was freed by pumping water.

The bit became blocked again, accompanied by high pressure at 0240 and 0415 h on 1 March. The ALN barrel was recovered on both occasions. Sand had jammed the inner and outer barrel. The barrel was stripped down, fully cleaned, and reassembled. Coring continued until 0740 h when Hole M0035A was terminated at 29.9 mbsf, with an average recovery of 40.9%. The API pipe was tripped to 5 m above the seabed, and a downpipe camera survey was conducted, finishing at 0850 h. The camera was left inside the API pipe for the move to the next coring location. The seabed transponder was recovered and a fully charged beacon attached to the frame.

Sedimentology and biological assemblages

Hole M0035A is divided into four lithostratigraphic units.

Unit 1: Core 325-M0035A-1R through Section 2R-CC: modern coralgal boundstone

The uppermost Unit 1, consisting of the interval from Core 325-M0035A-1R through Section 2R-CC, consists of modern reef deposits in which major constituents are coral fragments encrusted by coralline algae and bioclastic lime sands with coral, coralline algae, Halimeda, and larger foraminifera (abundant and well-preserved or stained specimens of Alveolinella, Amphistegina, Cycloclypeus, Elphidiidae, Heterostegina, and Lenticulina). There are no hermatypic (zooxanthellate) corals.

Unit 2: Sections 325-M0035A-3R-1 to 7R-1, 16 cm: coralgal boundstone

Unit 2, spanning Sections 325-M0035A-3R-1, to 7R-1, 16 cm, is a coral boundstone composed largely of massive corals, with bioclastic lime sands containing shells and coral fragments, Halimeda, coralline algae, and rare fragmented foraminifera Amphistegina, Heterostegina, and Sphaerogypsina. The benthic foraminifera Amphistegina and Operculina (fragmented and scarce) are present in interval 325-M0035A-1R-1, 20–25 cm, and in sandy gravel sediments from interval 5R-1, 25–30 cm.

There are a few unidentified ahermatypic (nonzooxanthellate) corals at the top of the unit. The dominant coral is massive Isopora (Fig. F64), with some Pocillopora and Montipora(?). Coral fragments are diverse, including Isopora, Tubipora musica, Seriatopora, Acropora, Montipora, Agariciidae, Faviidae, and Fungiidae.

Unit 3: Sections 325-M0035A-7R-1, 16 cm, to 23R-1, 6 cm: coralgal-microbialite boundstone

Unit 3, spanning Sections 325-M0035A-7R-1, 16 cm, to 23R-1, 6 cm, is a coralgal-microbialite boundstone (Fig. F65) that contains massive and branching corals and coralline algal crusts up to several centimeters thick. The algal crusts contain vermetid gastropods and, less often, the encrusting foraminifer Homotrema rubrum. Microbialites are usually dark gray and weakly laminated; in rare cases, they are columnar. The succession in this unit, from bottom to top, is: coral, coralline algae, microbialite. The lower part of this unit contains bioclastic rudstone associated with the microbialite (Fig. F66).

Stained and fragmented specimens of Amphistegina, Elphidiidae, and Heterostegina are scarce in interval 325-M0035A-12R-1, 20–25 cm, whereas the very coarse sand from interval 13R-1, 20–25 cm, contains fragmented specimens (scarce) of Amphistegina, Elphidiidae, and Gypsina. Very coarse sediments from interval 325-M0035A-21R-1, 40–45 cm, contain abundant well-preserved specimens of Cycloclypeus, Operculina, Alveolinella, Homotrema, Sphaerogypsina, and Textulariida.

The uppermost part of Unit 3 (Cores 325-M0035A-7R through 14R) is dominated by massive Isopora (Fig. F67) and branching colonies of Seriatopora (Fig. F68), Acropora, and Pocillopora. The main associated corals are Montipora, Goniopora(?), branching Acropora (Fig. F69), and Tubipora musica. Lower in the hole, downward from Core 325-M0035A-15R, Unit 3 is dominated by branching Acropora, massive Faviidae (including Favia pallida and Cyphastrea) (Fig. F70), Agariciidae, and Tubipora musica (Fig. F71). Associated corals include branching Acropora (Fig. F72), Montipora, Seriatopora, Porites, and Pectiniidae(?). Fragments are similar to the dominant and associated corals in each section.

In Core 325-M0035A-21R, there are 90 cm of coarse to very coarse lime sand with Halimeda, benthic foraminifera, coral (including Seriatopora), and mollusks, as well as microbialite clasts. This sand, which occurs between the coralgal-microbialite boundstones, appears to be downhole contamination and, therefore, an artifact of drilling.

Unit 4: Section 325-M0035A-23R-1, 6 cm, to the base of Hole M0035A: packstone to grainstone

The lowermost Unit 4, spanning Section 325-M0035A-23R-1, 6 cm, to the base of Hole M0035A, is composed of a dark gray bioclastic packstone to grainstone (Figs. F73, F74) that contains fragments of coral (including Porites, Faviidae, and Pocilloporidae), coralline algae, Halimeda, mollusk shells, and benthic foraminifer tests.

Physical properties

A total of 12.23 m of core was recovered from Hole M0035A, which was drilled to 29.90 m DSF-A. This equates to 40.90% recovery. Physical property data for this hole are summarized in Table T2.

Density and porosity

Bulk density values from whole-core multisensor core logger (MSCL) measurements range from 1.00 to 2.50 g/cm³ (Fig. F75). Bulk density was also measured on 15 discrete samples and ranges from 1.92 to 2.43 g/cm³ (Fig. F76). Porosity ranges from 19% to 49%. Grain densities in Hole M0035A are similar to those found in Hole M0033A (ranging from 2.74 to 2.80 g/cm³). Because of the heterogeneity in the formations and/or the disturbed nature of the core material, it is difficult to compare it with the MSCL data and/or stratigraphy.

P-wave velocity

Whole-core MSCL P-wave measurements were unsuccessful because of underfilled core liners (no acoustic coupling). However, seven discrete samples from Hole M0035A were measured (Fig. F77A) using the discrete P-wave logger. P-wave values (resaturated) from discrete samples range from 3207 to 4321 m/s (mean resaturated P-wave). There is no evidence from these seven measurements for any downhole trend in P-wave data. However, bulk density and P-wave measured on discrete samples show similar trends (Fig. F77B).

Magnetic susceptibility

Hole M0035A cores yielded magnetic susceptibility values ranging from –3.90 × 10^–5 to 248.66 × 10^–5 SI (Fig. F75). The majority of the data falls within the –1 × 10^–5 to 5 × 10^–5 SI range. There is no clear downhole trend; however, there is one significant magnetic susceptibility high at the top of Section 325-M0035A-12R-1 (15.65 m CSF-A) that decreases to “normal” hole values downcore. There is no indication in the core description for this high magnetic susceptibility coinciding with a particular lithology.

Electrical resistivity

Noncontact resistivity measurements taken on whole cores yielded values ranging from 0.45 to 11.06 Ωm (Fig. F75). There appear to be potentially three intervals of higher resistivity (~12.5–13, ~17–18.2, and ~23.8–24.2 m CSF-A). However, owing to the often poor core quality (see “Physical properties” in the “Methods” chapter) in Hole M0035A, resistivity data should be treated with a degree of caution.

Digital line scans and color reflectance

Similar to the other holes in this transect, all cores from Hole M0035A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance L* in Hole M0035A varies between 47.18% and 82.84%. Variations in the measurements of color reflectance parameters show a slight decrease in reflectance with depth (Fig. F78). Values in the scale of blue to yellow (b*) get closer to zero with depth. This trend is more clear in the a*/b* ratio, which changes from positive to negative values downhole. Coral fragments of massive Acropora sp. show consistent repetitive values for the parameters L*, a*, and b*. Increases in the dispersion of the data in each core section, mainly represented by L* and b* values, likely result from the complex nature of the lithologies that includes coralgal and coralgal-microbialite boundstone, broken fragments, and unlithified sediments. In this borehole, there is again a clear relationship between the presence of Tubipora musica and the increase in the red parameter (a*) to positive numbers.

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. F79). Positive low susceptibilities occur throughout the core, with an arithmetic mean value of 0.46 × 10^–8 m³/kg. There is also a peak located at 15.93 mbsf with a relatively high susceptibility value of 1.80 × 10^–8 m³/kg, indicating the possible presence of ferromagnetic and/or paramagnetic minerals. The record also shows four negative (diamagnetic) susceptibilities at 12.75, 21.08, 22.77, and 28.04 mbsf with values of –0.09 × 10^–8, –0.06 × 10^–8, –0.10 × 10^–8, and –0.20 × 10^–8 m³/kg, respectively.

Chronology

Two calibrated radiocarbon ages (15 cal y BP, Core 325-M0035A-3R; 18 cal y BP, Core 12R) (Fig. F80) and one U-Th age (21 cal y BP, Core 20R) (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 recovered material from the Last Glacial Maximum and captured the early portion of the deglaciation to ~15 cal y BP.

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