Previous   |    Next

doi:10.2204/iodp.proc.342.110.2014

Physical properties

We made physical properties measurements on whole-round sections, section halves, and discrete samples from section halves from Holes U1409A–U1409C. Gamma ray attenuation (GRA) bulk density, magnetic susceptibility, P-wave velocity, and NGR measurements were made on whole-round sections using the Whole-Round Multisensor Logger and NGR Logger. Thermal conductivity measurements were also performed on whole-round sections before they were split. Compressional wave velocity was measured on section halves at a frequency of two in each section (at ~50 and 100 cm) using the P-wave caliper (PWC). For moisture and density (MAD) analyses, one discrete sample was collected in each section (typically at ~35 cm from the top of a section). The Section Half Multisensor Logger was used to measure spectral reflectance and magnetic susceptibility on archive section halves.

Magnetic susceptibility

Overall, magnetic susceptibility ranges from 0 to 160 IU (Fig. F36). From the top of the sediment column at Site U1409 to ~18 mbsf (Lithostratigraphic Unit I; see “Lithostratigraphy”), magnetic susceptibility is characterized by large-amplitude variations. In the upper part of this lithostratigraphic unit, magnetic susceptibility averages ~45 IU, and from 10 to 18 mbsf, values increase from ~8 to 120 IU. Below Unit I, magnetic susceptibility is lower (generally <30 IU) and shows low-amplitude variability. In Unit II, magnetic susceptibility steadily decreases downhole from ~40 to <20 IU. Unit III is characterized by small but high-frequency variations, which correspond to interbedded light greenish gray nannofossil clay and carbonate-rich, white nannofossil ooze. We note several superimposed peaks that correlate among all three holes. The first of these peaks occurs at ~20 mbsf and is associated with a manganese- and sulfide-rich interval. At ~35 mbsf, particularly in Holes U1409A and U1409B, a small peak in magnetic susceptibility values correlates with the downhole transition to the greenish gray sediment. We observe three major peaks in Unit III at ~50, ~72, and ~90 mbsf. Each of these peaks correspond to oxide-rich intervals characterized by yellowish to reddish color. Finally, we note a major peak in magnetic susceptibility (~100 IU) at ~157 mbsf that corresponds to the PETM.

Density and porosity

Two methods were used to evaluate bulk density at Site U1409. The GRA method provided a bulk density estimate from whole-round sections. The MAD methods applied to 134 discrete samples provided a second, independent measure of bulk density, as well as dry density, grain density, water content, and porosity.

Overall, MAD bulk density values vary between 1.37 and 1.95 g/cm³. Changes in MAD bulk density are in general consistent with those in GRA bulk density (Fig. F36). MAD values are ~2.5% lower than GRA values in APC-cored intervals and somewhat higher than GRA values in XCB-cored intervals. In lithostratigraphic Unit I, bulk density increases from 1.4 to 1.75 g/cm³. At the top of Unit II at ~ 18 mbsf, bulk density decreases to 1.4 g/cm³. Bulk density increases downhole throughout the remainder of the sediment section and reaches ~1.9 g/cm³ at ~140 mbsf; this behavior is typical of sedimentary compaction. Bulk density is relatively constant from 150 mbsf to the bottom of Hole U1409A. We observe several downhole trends toward higher densities in individual cores from Holes U1409B and U1409C in these lower intervals; we attribute these trends to artifacts of XCB drilling.

At Site U1409, water content and porosity vary between 25 and 60 wt% and between 45 and 80 vol%, respectively. From the top of the hole to ~140 mbsf, both parameters gradually decrease downhole to 30 wt% water content and 50 vol% porosity. Porosity and water content shift to substantially lower values in the lower part of Unit I, between ~10 and 18 mbsf. The base of this 8 m thick interval correlates to a large stratigraphic hiatus (see “Age-depth model and mass accumulation rates”). Below 140 mbsf, water content and porosity remain relatively constant and average 27 wt% and 50 vol%, respectively. A significant increase to 37 wt% and 61 vol% occurs at ~160 mbsf.

Grain density generally varies between 2.65 and 2.85 g/cm³ throughout Hole U1409A. These data are more variable in lithostratigraphic Units I and II than they are lower in the hole.

P-wave velocity

P-wave velocity was measured using the P-wave logger (PWL) on all whole-round sections and using the PWC on undisturbed section halves from Holes U1409A–U1409C. Whole-round and section half data are generally coherent, but the PWC values are ~20 m/s lower than PWL values in APC-cored intervals (Fig. F37). Overall, P-wave velocity gradually increases downhole from 1500 to 1800 m/s, similar to trends observed in bulk density and water content data; we attribute these trends to downhole compaction of the recovered sediment. A small stepwise decrease from 1570 to 1525 m/s occurs at the transition between lithostratigraphic Units I and II. P-wave velocity data remain relatively constant and average ~1560 m/s throughout Subunit IVa. We did not complete PWL measurements for almost all of Subunits IVb and IVc intervals (between ~125 and ~200 mbsf), but PWC measurements show higher variability than observed uphole. We attribute this greater variability to a change in coring method from APC to XCB at ~125 mbsf.

Natural gamma radiation

NGR values range between 3 and 35 cps at Site U1409, and two major trends are observed. First, NGR values average ~25 cps between 0 and 100 mbsf, and the profile is characterized by many superimposed peaks and troughs within this interval that can be correlated among all three holes. Second, at ~100 mbsf NGR values decrease abruptly to ~5 cps and remain low downhole to the bottom of the hole. NGR is anticorrelated with calcium carbonate content, with low NGR counts in the high-carbonate lithostratigraphic Unit IV and high NGR counts in the low-carbonate content Units I–III (see “Geochemistry”).

Color reflectance

The standard operating resolution of data acquisition was decreased from 2.5 to 5 cm for Holes U1409A and U1409C to speed up processing time during periods of very rapid core recovery. From the top of the sediment column to ~100 mbsf, color reflectance parameters a* and b* follow similar trends and features in all three holes (Fig. F38). In lithostratigraphic Unit I, values average ~1.9 for a* and ~10 for b*. At the contact between Units I and II, a* increases from 1.9 to 4.4 and b* increases from 10 to ~16. This change in color reflectance values reflects the sharp contact between the brown, red, and gray-banded Pleistocene sediment and the underlying yellowish brown Oligocene-age silty clay (see “Lithostratigraphy”). In the lower intervals of Unit II, a* and b* values decrease sharply (a* from 4.4 to –1 and b* from 10 to 1.3). This decrease corresponds to a downhole transition in sediment color from light yellow to greenish gray (see “Lithostratigraphy”). Throughout Unit III, a* and b* remain relatively constant with average values of –0.2 and –0.3, respectively. The b* record shows distinctive peaks at ~50, ~72, and ~90 mbsf in all three holes. These peaks correlate to peaks in the magnetic susceptibility record and probably signify Fe oxide–rich horizons (Fig. F36). The a* record shows a pronounced downhole step increase (from –0.6 to 4) at the contact between Units III and IV (~100 mbsf). Values for a* and b* both increase downhole throughout Subunit IVa. In Subunits IVb and IVc, a* remains relatively constant but b* continues to increase. Peaks in both parameters occur at 155 mbsf, which is consistent with peaks observed in nearly all the other physical properties; this is the PETM interval.

L* generally follows pronounced lithostratigraphic changes and can be readily correlated among all three holes (Fig. F38). L* averages 50 in Unit I and increases to 56 at the contact with the underlying Unit II. L* decreases slightly from ~60 to 55 throughout Unit II and increases sharply from 55 to 65 at the boundary with Unit III (~35 mbsf). In Unit III, L* values average ~64, but some superimposed cyclic variations occur at ~38, 47, and 70 mbsf. At the top of Unit IV, L* increases abruptly to 84; this interval correlates with the transition from interbedded gray and white nannofossil clay and ooze to pinkish white carbonate-rich nannofossil ooze with foraminifers (see “Lithostratigraphy”). Throughout Unit IV, L* decreases downhole from ~84 to 68. In Holes U1409A and U1409B at the Subunit IVb/IVc boundary (~155 mbsf), L* values decrease from 70 to 57; this interval also correlates with the lithostratigraphic expression of the PETM. The major variations in L* recorded at Site U1409 are consistent with the magnetic susceptibility and NGR data series (Fig. F37) and appear to correlate mainly with changes in calcium carbonate content (see “Geochemistry”).

Thermal conductivity

Fifty-one measurements were completed on whole-round sections from Holes U1409A–U1409C (Table T22). Thermal conductivity values increase in magnitude downhole from 0.9 to 1.4 W/(m·K) near the top and bottom of the holes, respectively, and generally correlate among all three holes (Fig. F39). Thermal conductivity measurements show a good correlation (R² = ~0.84) with GRA bulk density (Fig. F40). Thermal conductivity data are more scattered in lithostratigraphic Unit I, probably because of higher water content and the presence of foraminifer sand in these intervals. Throughout Units II and III and Subunit IVa, thermal conductivity values increase gradually from ~1.1 to 1.2 W/(m·K) at ~115 mbsf. Across the Subunit IVa/IVb boundary (~135 mbsf), thermal conductivity increases abruptly to 1.45 W/(m·K). This increase correlates to the transition from ooze to chalk (see “Lithostratigraphy”). Thermal conductivity remains relatively constant at ~1.4 W/(m·K) in Subunits IVb and IVc.

Top of page   |    Previous   |    Next