IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.347.107.2015

Physical properties

This section summarizes the preliminary physical property results from Site M0063 in Landsort Deep, the deepest basin in the Baltic Sea (437.1 meters below sea level). Five holes were drilled at this site. Hole M0063A was drilled to 115.81 mbsf, Hole M0063B to 29.00 mbsf, Hole M0063C to 96.4 mbsf, Hole M0063D to 86.80 mbsf, and Hole M0063E to 92.80 mbsf. Hole M0063E was designated as a microbiology hole and was extensively subsampled onboard (see “Microbiology”). Strong expansion and methane gas release were encountered at Site M0063, resulting in disturbed core sections (with frequent gas bubbles and cracks) and loss of core material upon removal of the piston for Hole M0063A. Following this, the coring procedure was altered (see “Operations”) to increase recovery. Within the uppermost ~55 mbsf of subsequent holes, recovery is in excess of 100%.

Our interpretations are focused on the physical property data from Hole M0063C, which has the greatest continuous penetration (Fig. F18). Although all physical property measurements described in “Physical properties” in the “Methods” chapter (Andrén et al., 2015) were conducted at Site M0063, whole-round core shipboard P-wave data appear to be highly affected by artifacts. Thermal conductivity data are too sparsely distributed to exhibit any discernible downcore trend. Color reflectance variations do not correspond to other physical properties and lithostratigraphic units.

Natural gamma radiation

High-resolution natural gamma ray (NGR) values are relatively low (~8 cps) in the upper interval of lithostratigraphic Subunits Ia and Ib and then drop to near zero at the Subunit Ib/Ic boundary (Fig. F18). NGR exhibits increasing values toward the base of lithostratigraphic Subunit Ic (~10 cps), with a shift to lower values at the Subunit Ic/Id boundary. NGR progressively decreases in lithostratigraphic Subunit Id to <5 cps. These generally low NGR values within lithostratigraphic Unit I are interpreted as a result of high water content coupled with relatively high organic and low clay content (see “Lithostratigraphy”). NGR values increase sharply below the upper boundary of lithostratigraphic Subunit IIa (>10 cps) and remain high in Subunit IIb (>15 cps). This change could be related to a decrease in water content within lithostratigraphic Unit II. NGR values are relatively constant in lithostratigraphic Unit III (including Subunits IIIa and IIIb), with several negative excursions that might reflect increased silty sand content. NGR values increase within lithostratigraphic Unit IV and remain high (>20 cps) and relatively constant through lithostratigraphic Units V, VI, and VII. Meter-scale peaks throughout Unit VI are likely to correspond to intervals of reduced grain size.

Magnetic susceptibility and noncontact resistivity

The greater water depth at Site M0063 required the use of additional sections of drill string that had not been previously deployed during Expedition 347 (see “Operations”). Rust derived from these sections contaminated the first section of each core run to ~75 mbsf, which is apparent in the raw magnetic susceptibility data (see also “Stratigraphic correlation”) but not in any other physical properties including noncontact resistivity (NCR). Contaminated intervals were identified by high-amplitude, abrupt spikes in magnetic susceptibility and were removed. Both magnetic susceptibility and NCR exhibit an increasing downhole trend and generally low values throughout lithostratigraphic Unit I (Fig. F18). The Unit I/II boundary is characterized by a small peak in magnetic susceptibility and a relative excursion in NCR. Meter-scale peaks in resistivity occur throughout lithostratigraphic Units III, IV, V, and VI. Large excursions in magnetic susceptibility are limited to the lower portion of lithostratigraphic Unit VI.

Density

Dry density increases downhole and is lowest in lithostratigraphic Unit I, with a minimum value of 0.28 g/cm3 recorded in Subunit Ib (Fig. F18). Dry density increases below the Unit I/II boundary to ~0.8 g/cm3. Two excursions in dry density of ~1.5 g/cm3 occur in lithostratigraphic Unit III. Through lithostratigraphic Unit VI, density is generally >1 g/cm3. The greatest density (1.68 g/cm3) in Hole M0063C occurs in lithostratigraphic Unit VII.

Discrete bulk density measurements are well correlated (r2 = 0.83) with high-resolution (2 cm interval) gamma density measured during the offshore phase of Expedition 347 (Figs. F19, F20).

P-wave velocity

Discrete P-wave velocities in the upper ~50 mbsf (Units I–V) are typically ~ 1250 m/s (Fig. F19). Shipboard multisensor core logger (MSCL) measured velocities are anomalously low, likely due to poor core quality. At the bottom of lithostratigraphic Subunit IVb, velocity as measured in discrete measurements increases to ~1500 m/s and the correspondence to the shipboard data generally improves.