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doi:10.2204/iodp.proc.325.103.2011

Hole M0038A

Operations

Site 8, Hole M0038A

The seabed transponder was deployed at 0320 h on 4 March 2010 prior to moving onto station by 0340 h (Table T1). However, a mud pump suction failure at 0330 h prevented operations commencing until 1400 h. API pipe was run to just above the seabed by 1430 h. A downpipe camera survey was completed by 1510 h, and the API pipe tagged the seabed at 1535 h. However, on recovery of the first standard rotary corer core at 1540 h, the bit was found to have been completely destroyed. The decision was made to lift the drillstring, move 2 m, and begin a new hole to avoid coring any remnants of the bit. As this area was within the field of view of the camera survey for Hole M0038A, no new survey was required prior to coring operations at the hole.

Sedimentology and biological assemblages

Hole M0038A contains a single lithostratigraphic unit.

Unit 1: Sections 325-M0038A-1R-1 through 1R-CC: modern seafloor sediment

Unit 1, spanning Sections 325-M0038A-1R-1 through 1R-CC, contains all sedimentary material from Hole M0038A. The section is composed of pebble-sized fragments of corals, coralline algae, and bryozoans in addition to coral rudstone with a bioclastic packstone matrix, all of which have brown staining. These may be derived from the modern seafloor. Cavities, possibly formed by dissolution, occur in the rudstone and are partly to fully filled with gray, well-consolidated lime mud.

There were no identifiable corals in Section 325-M0038A-1R-1, but Section 1R-CC consists exclusively of coral fragments with brown staining, including large pieces of massive Porites that were part of a single in situ colony until fractured during drilling.

Physical properties

A total of 0.18 m of core was recovered from Hole M0038A, which was drilled to 1.5 m DSF-A (12% recovery). Owing to the very short hole and the low recovery, it was not possible to carry out the full suite of petrophysical measurements (Table T2) on the recovered core.

Density and porosity

In Hole M0038A, whole-core multisensor core logger measurements were not taken because of low core recovery, and only one sample was taken for discrete measurement. Bulk density for this sample is 2.33 g/cm³, grain density is 2.75 g/cm³, and porosity is 24%.

Digital line scans and color reflectance

Cores from Hole M0038A were digitally scanned and, where appropriate, measured for color reflectance. Only three color reflectance spectrophotometry measurements in Hole M0038A were possible because of the lack of recovery. L* values had a minimum of 72% and a maximum of 74.69%. The graph is not shown for this borehole because of the lack of measurements.

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on one sample taken from the working half of the recovered core. Very low negative (diamagnetic) susceptibility was measured in this sample, taken at 0.03 mbsf, with a value of –0.23 × 10^–8 m³/kg.

Chronology

This hole has only one calibrated radiocarbon age of 13 cal y BP from Core 325-M0038A-1R (Fig. F100). This age is from the top of the hole, indicating that it may have captured part of the early deglaciation.

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