-
P-modulus (MPa) = VP2 x
bw,
At Site 1231, three deep holes (Holes 1231B, 1231D, and 1231E; a maximum of 121.9 mbsf penetration) and two shallow holes (Holes 1231A and 1231C; <15.0 mbsf penetration) were cored. Physical property measurements in Hole 1231B involved standard-resolution multisensor track (MST), IR camera imaging, thermal conductivity, and discrete samples for moisture and density (MAD). Cores from the remaining four holes were run on the MST only. Additional thermal conductivity measurements were made on cores from Holes 1231D and 1231E to better define an important thermal property transition. High-resolution MST data were acquired on Hole 1231C.
Site 1231 has the same location as DSDP Leg 34, Site 321 (Shipboard Scientific Party, 1976). However, this is the first thorough physical property investigation of this site because of different cruise objectives and advances in instrumentation. At Site 1231 the entire sediment column was cored by APC, providing high-quality core for analysis.
Each section of whole-round core that was analyzed for physical properties was equilibrated to laboratory temperature (2-4 hr) and then run on the MST. The standard-resolution measurements were magnetic susceptibility (spacing = 5 cm, data acquisition scheme [DAQ] = 2 x 1 s), gamma ray attenuation (GRA) densitometer (spacing = 10 cm, count time = 5 s), P-wave velocity (spacing = 10 cm, DAQ = 10), and natural gamma radiation (NGR) (spacing = 30 cm, count time = 15 s). Thermal conductivity measurements were made on the third section of each whole round core in Hole 1231B, where possible, and in three sections from Hole 1231D. Some sections of Holes 1231B and 1231E were removed from the catwalk for microbiology and IW sampling. Physical properties were measured on these sections only if intact parts remained following the sampling. However, the intact record from Hole 1231D provides excellent continuity and spatial resolution of the physical property record.
MAD, P-wave velocity from the digital velocimeter, and resistance data (translated to formation factor as detailed in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter) were collected regularly only from Hole 1231B. MAD samples were taken at a frequency of one per section and at higher resolution in sections with lithologic transitions. MAD samples were co-located with the methane headspace extractions, where possible, to facilitate the volumetric analysis of methane concentrations.
Instrumentation, measurement principles, and data transformations are further discussed in "Physical Properties" in the "Explanatory Notes" chapter.
In general, the stratigraphic section at Site 1231 is characterized by several obvious lithologic changes that are clearly defined by the physical property data. Within individual lithologic zones, the physical property data are relatively constant. A major property change takes place at the Unit II/III boundary (55.4 mbsf) at the siliceous ooze/nannofossil ooze contact. A record of variable terrigenous input is superimposed on these two major biogenic intervals. Changes in the proportion of clay and volcanic glass within the biogenic facies produce distinct zones of physical property variation. Evidence of downhole compaction is weakly present in the resistivity and P-wave velocity data.
In the following sections, we describe the main characteristics of each physical property, relating it to the lithostratigraphic divisions given in "Lithostratigraphy".
The IR scanner was employed on Hole 1231B cores to obtain data from a site where little temperature variation within individual cores was expected. The effects on core liner temperatures of variables such as wireline trip time and drill floor residence time will be investigated postcruise. Measurements of split-core surface temperature variation after equilibration to ambient laboratory temperature were made in order to study the relationship between lithology, water content, and emissivity. These data will be examined after the cruise.
Low-field volume magnetic susceptibility was measured on the MST using the Bartington loop sensor as described in "Magnetic Susceptibility" in "MST Measurements" in "Physical Properties" in the "Explanatory Notes" chapter. Data were collected on WRC sections from Holes 1231A, 1231B, 1231D, and 1231E at standard resolution (spacing = 5 cm, count time = 1.0 s) (Fig. F8). Hole 1231C was run at high resolution (spacing = 1 cm, count time = 0.1 s). Data from all holes match well over the common intervals. A slight vertical offset in some cores on the order of 50 cm is not consistent downhole and is not significant to overall trends.
Unit I extends from 0 to 31 mbsf and is a diatomaceous ooze in which the clay proportion increases steadily from top to bottom. Magnetic susceptibility of the top 11 m varies around an average of ~25 x 10-5 SI units. Decreasing values in the top 1 mbsf may be due to reduction of iron, and this is supported by color changes over the same interval (see "Lithostratigraphy"). Between 11 and 20 mbsf, a zone of gray-green siliceous ooze containing some terrigenous material is characterized by low susceptibility (~5 x 10-5 SI units). The magnetic susceptibility boundary between the two is sharp, whereas the lithologic transition appears to be more gradual. At 20 mbsf a gradual increase in magnetic susceptibility commences coincident with increasing clay content of the sediments. Susceptibility stabilizes at 30 x 10-5 SI units at 24 mbsf and then increases only slightly to ~35 x 10-5 SI units. The Unit I/II boundary, located at 31 mbsf, corresponds to a drop from 35 x 10-5 to 20 x 10-5 SI units. Below this boundary values increase rapidly to 35 x 10-5 SI units and remain relatively stable to 45 mbsf.
The top of the dark brown clay (Subunit IIB) is at 45 mbsf, and the interval over which it is present, from 45-55 mbsf, comprises two magnetic subzones. From 45 to 51 mbsf, the susceptibility increases from ~35 x 10-5 to 50 x 10-5 SI units. At 51 mbsf, there is an abrupt increase to >100 x 10-5 SI units, which is maintained with minor variation to the base of Subunit IIB at 55 mbsf.
The Unit II/III boundary represents the contact between overlying clays and dense nannofossil ooze. The magnetic susceptibility across Subunit IIIA is characteristically and constantly low (~7 x 10-5 to 8 x 10-5 SI units). At the boundary between Subunits IIIA and IIIB (65 mbsf), the susceptibility abruptly increases to ~110 x 10-5 SI units, which is maintained to 68 mbsf. Between 68 and 70 mbsf, the signal drops back to ~15 x 10-5 SI units before returning to ~55 x 10-5 SI units across a narrow interval from 70 to 71mbsf. The peaks in this interval from 65 to 71 mbsf are caused by two dark brown terrigenous layers that are separated by nannofossil ooze.
At the top of Subunit IIIC, magnetic susceptibility falls abruptly to ~5 x 10-5 SI units and remains constant at this level until the base of the subunit at 107 mbsf.
Below 107 mbsf, the magnetic susceptibility gradually increases to a peak at 70 x 10-5 SI units at the bottom of the hole. The lithology at the base of the hole is an orange-brown nannofossil ooze. Its color indicates the presence of oxide minerals and suggests a chemical interaction with basaltic basement.
At Site 1231 we collected 18 discrete samples for paleomagnetic measurements. The sampling frequency was one from each core in Cores 201-1231B-1H through 6H (0.0-50.9 mbsf) and two samples from each core below this interval to the bottom of the hole (Cores 201-1231B-7H through 12H; 50.9-112.3 mbsf). Alternating-field (AF) demagnetization of the natural remanent magnetization (NRM) was conducted up to 40 mT in 10- or 5-mT steps. Anhysteretic remanent magnetization (ARM) was measured to 40 mT in 10-mT steps with a 29-µT direct current-biasing field. AF demagnetization of the ARM was conducted to 40 mT in 10-mT steps.
Hole 1231B shows high magnetic intensity except in Cores 201-1231B-2H through 3H (Fig. F9). In the uppermost sample (201-1231B-1H-3, 14-16 cm), we were able to isolate a stable magnetic direction after 25-mT AF demagnetization, overcoming the drilling-induced overprint (Fig. F10). Cores 201-1231B-4H through 8H show abnormal demagnetization behavior. Volcanic glass-rich clay layers are present within this interval of lithostratigraphic Unit II (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") and these show high NRM intensities and high intensities after 40-mT AF demagnetization (Fig. F9). Intensity decreases at 5-to 30-mT AF demagnetization, then intensity increases at 35- to 40-mT AF demagnetization (Figs. F11).
Nannofossil ooze samples from lithostratigraphic Subunit IIIC (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") have a high magnetic intensity despite low susceptibility (Fig. F9). All samples from this interval have a downward inclination, which corroborates the result of Ade-Hall and Johnson (1976). Pale orange-yellow semiopaque, mineral-bearing nannofossil ooze of Unit III (Sample 201-1231B-9H-1, 50-52 cm) shows a more stable magnetization (Fig. F12).
Density data were measured on the MST by the GRA densitometer (spacing = 10 cm, count time = 5 s) and were calculated from split-core mass/volume measurements. Porosity was calculated from the split-core samples. Average bulk density values from the two MST runs and the discrete samples show good agreement across the entire downhole section (Fig. F13). A major exception was located across Subunit IIIA (55-66 mbsf), where the two MST runs differ by ~15% (average = 2.15 g/cm3 for Hole 1231B and 1.85 g/cm3 for Hole 1231D). The data from this interval in Hole 1231B are unrealistically high and inconsistent with all other measurements. Therefore, those data points have been eliminated from the figure. The only operational problem at this time was sticking of the MST boat because of a dirty track, but this does not explain the extent of the error. There are a number of other localized exceptions (e.g., 29-31, 47-50, 89-93, and 98-100 mbsf) where the two MST data sets differ. Overall, these do not detract from a description of the general trends, but the values cited for Hole 1231B should be treated with caution.
The top 11 m of Unit I exhibits a slightly curved density profile. Density is 1.2 g/cm3 at 0 mbsf, rises to 1.25 g/cm3 at 5 mbsf, and declines to 1.2 g/cm3 at 11 mbsf. From 11 to 20 mbsf (the zone of green siliceous ooze), density is steady, varying slightly around 1.2 g/cm3. From 20 to 50 mbsf density is slightly more variable, ranging between 1.2 and 1.3 g/cm3. There is not a pronounced density change at the Unit I/II boundary. At 50 and 55 mbsf, there are two steplike increases in density to a high of 1.8 g/cm3 at 55.5 mbsf. This density increase clearly highlights the Unit II/III boundary.
Porosity varies between 85% and 90% from 0 to 45 mbsf. There are five noticeable lows at 3, 12, 30, 38, and 45 mbsf, where the values are ~85% or lower. These are coincident with grain density lows but are not apparent in the bulk density data set. From 45 to 55 mbsf (Subunit IIB), the porosity gradually decreases to 80%. In this interval, a large decrease in the grain density is coincident with a zone of high clay and volcanic glass content and secondary zeolite mineralization. At 55.5 mbsf, the Unit II/III boundary is again clearly marked by a drop in porosity to 55%. The grain density increases to 2.75 g/cm3.
Unit III is a nannofossil ooze. Average bulk density across the whole unit is ~1.7-1.8 g/cm3. Grain density is around 2.75 g/cm3 and porosity remains low, at 58%-59%. These general trends are interrupted by two terrigenous layers at 65-68 and 69-70 mbsf. The terrigenous layers have lower bulk density (~1.5 g/cm3), higher grain density (~2.9 g/cm3), and higher porosity (up to 73%) than the dominant nannofossil ooze of the unit. The top and base of the terrigenous unit, at 66 and 71 mbsf, respectively, coincide with the boundaries of Subunits IIIA/IIIB and IIIB/IIIC. From 107 mbsf to the bottom of Subunit IIID at 114 mbsf, bulk density decreases to 1.65 g/cm3, grain density increases to 2.85 g/cm3, and porosity increases to 70%. It is possible that these trends are linked to chemical interaction with basaltic basement.
P-wave data from the MST P-wave logger (PWL) were recorded at a 10-cm spacing for all available APC cores from Holes 1231A, 1231B, 1231D, and 1231E and at 2-cm spacing for Hole 1231C. The PWS3 velocimeter was used to measure P-wave velocities on split cores from Hole 1231B, with measurements taken at least once per section (Fig. F14A). More closely spaced measurements were made at lithologic boundaries and at intervals marked by possible diagenetic modifications. The PWL-derived velocities are consistently 40-60 m/s slower than the PWS-derived velocities. This offset is consistent with that found at other sites and probably results from core liners incompletely filled with sediment. All velocities in this section are discussed in terms of PWS3 data.
The velocities increase slightly from 1520 to 1550 m/s from 0 to 50 mbsf (Unit I and Subunit IIA). In the dark brown terrigenous layer from 50 to 55 mbsf (Subunit IIB), velocities decrease steadily to a low of 1510 m/s. This decrease coincides with increasing bulk density and decreasing porosity (Fig. F13).
The abrupt sedimentological transition at the Unit II/III interface is apparent in the abrupt P-wave velocity increase to 1530 m/s at the boundary. Between 55 and 64 mbsf P-wave velocity drops to 1480 m/s. At 64 mbsf, P-wave velocities increase from 1480 to 1560 m/s over a 2-m interval. At the Subunit IIIA/IIIB boundary P-wave velocity drops back to ~1500 m/s across 1 m. The underlying dark brown terrigenous sediment of Subunit IIIB is characterized by velocity variations between 1510 and 1540 m/s. Velocities in the upper 20 m of Subunit IIIC trend upward, from 1540 to 1560 m/s, coincident with a bulk density increase and a porosity decrease. A sudden velocity increase at 90 mbsf (to ~1590 m/s) is followed by a consistent but variable velocity of ~1580 m/s to the base of Subunit IIID. The 20-m/s oscillation around this trend appears to be related to porosity variation.
At first inspection, the sequence of velocity changes and the overall velocity increase from 1520 to ~1600 m/s could be indicative of mechanical compaction. We cannot constrain the actual in situ mechanical state, however, without consolidation testing or wireline velocity/density information. The following analysis examines the shipboard measurements in an attempt to isolate relict signatures of burial and/or diagenesis. To isolate the effects of density and porosity variation, we utilize the compressional wave modulus (P-modulus) transform, a combination of the effects of shipboard P-wave velocity and density profiles, as a proxy for mechanical state:
bw,
where,
bw = (wet) bulk density (g/cm3).
The plot of the P-modulus transform (Fig. F14B) shows a constant mechanical state (~2.8 Mpa) with small variations (±0.1 MPa), reflecting minor porosity oscillation in Units I and II. The steplike transition at the Unit II/III boundary is clearly reflected in P-modulus increase (from 2.8 to 4.4 MPa) and encompasses the intrinsic sediment fabric difference (most notably, the porosity drop of 22%) between the siliceous ooze and nannofossil ooze compositions. The ~0.1-MPa P-modulus increase to the top of Subunit IIID (108 mbsf) probably indicates a small relict burial compaction signature. The arrow-highlighted transitions (Fig. F14B) are porosity controlled and show the sensitivity of the depressurized, dominantly nannofossil sediment fabric state to non-nannofossil content (decreasing opaque or semiopaque minerals over 65-84 mbsf, with the reverse trend over 108-114 mbsf).
NGR was measured on the MST for all Site 1231 holes (spacing = 30 cm, count time = 15 s), except for Hole 1231C, which was run at a higher spatial resolution (spacing = 15 cm, count time = 30 s). The data are plotted in Figure F15 as uncorrected average counts per second (cps).
NGR is relatively high at the seafloor (~70 cps), but declines rapidly over the first 6 mbsf to the Unit I/II average of ~22 cps. This transition may reflect redox-related uranium enrichment resulting from high organic content (see "Natural Gamma Ray Emmission" in "Physical Properties" in the "Site 1226" chapter). At this time, shipboard organic carbon data are not available to test this hypothesis. The NGR baseline of ~22 cps is consistent to near the base of Unit II. Increased emission intervals at the top of Subunit IIA and the bottom of Subunit IIB coincide with minima in grain density transitions that mark maximum terrigenous content. The combination of low porosity and grain density with high gamma radiation, particularly in Subunit IIB, suggests that these narrow zones are clay-rich concentrations.
At the Unit II/III boundary, natural gamma radiation drops to 10-12 cps, except for the clay-rich interval (up to 22 cps) that comprises Subunit IIIB. The increased NGR of Subunit IIIB is consistent with the clay enrichment signature in Unit II. The stable NGR signal over Subunit IIID further strengthens the supposition that changing color and apparent consolidation result from diagenetic processes rather than a change in clay content.
Formation factor (longitudinal and transverse) was determined for Hole 1231B as described in "Formation Factor" in "Physical Properties" in the "Explanatory Notes" chapter with a minimum sample interval of one per section. All formation factor data were co-located (within 3 cm) with discrete (PWS3) P-wave velocity measurements. As expected (except in Subunit IB), formation factor data (Fig. F16) show inverse proportionality to porosity (i.e., Archie's Law). Consistent with the P-wave velocity analysis, formation factor data exhibit no indication of significant consolidation.
Longitudinal formation factors range between 1.7 and 2.15 in the clayey siliceous oozes of Unit I, with a decreasing trend that reflects a small porosity increase over the interval of ~2-27 mbsf (Fig. F13). Over Unit II, formation factors range between 1.6 and 3.1, with oscillations about the indicated trend lines (Fig. F16). These oscillations represent porosity variation of ±2%-5% about the interval trend (88%-85% for Subunit IIA and 85%-80% for Subunit IIB). The counterintuitive formation factor decrease in Subunit IIB probably reflects a gradational change in pore structure that corresponds to a rapidly increasing clay mineral and metal oxide content (see "Lithostratigraphy"). The abrupt lithologic change at the Subunit IIB/Unit III boundary results in a sharp increase in formation factor that is correlated with a porosity reduction from 80% to 57%. Below the Discoaster interval (55.3-65 mbsf), formation factors range between 2.8 and 3.9, with the variation controlled by porosity. The final ~0.3 m above basement shows a large increase in formation factor, jumping from ~3.2 to 6.8. Such a rapid change probably indicates a significant increase in diagenetic deposition in pore throats, given the minor total porosity decrease from 70% to 65% and small bulk density increase (Fig. F13).
Thermal conductivity measurements were made in Hole 1231B at a rate of one per core (usually the third section, at 75 cm, if this was available). In order to better resolve the conductivity change associated with the lithologic transition at the Unit II/III boundary, additional measurements were made in Holes 1231D and 1231E (Fig. F17A). Values range between 0.70 and 1.26 W/(m·K) (average = 0.95 W/[m·K]). The abrupt transition between the siliceous sediments to nannofossil ooze is clear in the thermal conductivity increase (from ~0.73 to ~1.20 W/[m·K]) between 54.5 and 55.3 mbsf. Within the two lithology-differentiated intervals (Units I/II and Unit III, respectively), thermal conductivity changes are controlled by water content (Fig. F17B).
At Site 1231 the direct relationship between physical properties and lithologic variation is more obvious than at any other site during the leg. The principal sedimentary types overlying basaltic basement are a nannofossil ooze overlain by a siliceous ooze, with the boundary (Unit II/III interface) at 55 mbsf. The nannofossil ooze is characterized by low magnetic susceptibility, very low gamma radiation, and high bulk density. The high bulk density is related to a markedly lower porosity in the nannofossil unit compared with the siliceous ooze. We suggest this difference is a result of the dissimilarity between the constituent microfossils making up the two units. Small flattened nannofossil plates allow a much tighter initial packing and lower porosity than the larger open diatom frustules. The only significant evidence for burial compaction or diagenesis occurs in the basal 0.3 m of the sedimentary section. There is little grain density variation across the entire cored section, reflecting the small density difference between dominantly quartz/silica above and calcite below.
Above 55 mbsf (Units I and II) the sedimentary section is characterized by higher gamma radiation and magnetic susceptibility signatures and lower bulk density and P-wave velocity. The gamma radiation increase is probably a result of variable clay content in the sequence. The base of the clays in the sediment sequence is at 70 mbsf, with the peak of input between 55 and 50 mbsf. Above this level, clay content gradually declines but is still present up to the seafloor.
Of note in the sequence is the sharp-bounded magnetic susceptibility low between 11 and 20 mbsf. This zone lies within an otherwise gradationally changing lithologic sequence. This suggests the anomalously low magnetic signature here is not of primary depositional origin.
