The investigation of magnetic properties for sedimentary units at Site 1109 included the measurement of (1) bulk susceptibility of whole core sections, (2) point susceptibility and remanent magnetization of archive-half core sections, and (3) magnetic susceptibility and its anisotropy and remanent magnetization of discrete samples. For igneous rocks from Hole 1109D, magnetic susceptibility and its anisotropy and remanent magnetization were measured for discrete samples. The Tensor tool was used to orient Cores 180-1109C-3H through 11H (Table T8).
Measured values of magnetic susceptibility made on whole core sections as part of the multisensor track (MST) analysis (see "Magnetic Susceptibility") and on half core sections as part of the archive multisensor track (AMST) analysis are shown plotted vs. depth in Figure F64A and F64B. In general, susceptibility data from the MST and AMST analyses agreed; differences in magnitude can be attributed to volume differences for the uncorrected data.
Magnetic susceptibility and its anisotropy (AMS) were measured on discrete samples from sedimentary units of Holes 1109C and 1109D. The mean magnetic susceptibility, the degree of anisotropy (Pj) and the shape parameter (T) for the susceptibility ellipsoid (Jelinek, 1981), and the inclination of the maximum and minimum axes of the susceptibility ellipsoid (Kmax and Kmin, respectively) are shown vs. depth in Figure F65.
The trend of susceptibility data from discrete samples was consistent with the trends of MST and AMST data on long cores. The very low susceptibilities occurring in the 380-540 mbsf interval suggest that the susceptibility is controlled mainly by paramagnetic minerals. The higher susceptibilities occurring in the 85-380 and 710-750 mbsf intervals, coupled with a similar trend in the remanent intensity data (Fig. F66B), suggest that the contribution of ferromagnetic minerals is large in these intervals.
Between 0 and 120 mbsf, Pj values increased from low values around 1.01 to values between 1.05 and 1.1 (Fig. F65). Below 120 mbsf, values fluctuated around 1.05 with higher values occurring at ~130, 240, 330, and 550 mbsf, and between 670 and 710 mbsf. Low values between 1.01 and 1.03 were observed at the base of the section below 710 mbsf. The range of Pj values reflected an overall low degree of anisotropy.
Values for the shape parameter (T) from Hole 1109C increased with depth from -0.8 to +0.5 between 0 and 85 mbsf (Fig. F65), which corresponded to lithostratigraphic Subunits IA and IB (see "Subunit IA" and "Subunit IB"). Between ~85 and 340 mbsf, T values were scattered with a bias toward values higher than ~0.5. Data from Hole 1109D (below ~350 mbsf) dominantly showed T values above 0.5, which indicated oblate ellipsoids. Negative T values occurred below ~710 mbsf, which indicated prolate ellipsoids.
Inclinations of Kmax and Kmin axes are shown in Figure F65 without structural correction. Structural data indicate horizontal bedding throughout most of Holes 1109C and 1109D, except for the ~35-85 mbsf interval, where bedding dips ranged between ~20º and 70º and were associated with soft-sediment deformation observed in liithostratigraphic Subunit IB (see "Subunit IB" and "Structural Geology"). Most samples between ~85 and 710 mbsf showed very shallow Kmax axes and subvertical Kmin axes, with a small change in the inclinations of Kmax axes corresponding to a larger change in Kmin axes between 250-260 mbsf. Below 710 mbsf, a marked switch occurred between the inclinations of Kmax and Kmin axes; Kmax axes were subvertical and Kmin axes were very shallow.
In summary, the AMS of sediments in Holes 1109C and 1109D is characterized by an oblate magnetic fabric with a subvertical Kmin axis, which is interpreted as a primary fabric related to sediment compaction (Tarling and Hrouda, 1993). Notable are (1) the low mean susceptibilities, (2) the relatively low scatter of all three AMS parameters, (3) a predominance of T values greater than 0.5 indicating an oblate fabric, and (4) the very steep inclinations of Kmin axes associated with data from the 380-560 mbsf interval, which is a dominantly fine-grained clay-rich unit (see "Lithostratigraphic Unit VI"). The AMS data for this interval clearly reflect a primary magnetic fabric related to compaction.
Scatter in Pj and T values between 85 and 380 mbsf may be related to variations in the orientation, volume, and grain size of magnetic minerals related to changing provenance and energy of flow (see "Lithostratigraphy" and "Physical Properties").
The scatter in T values and inclinations of Kmax and Kmin axes between 0 and 85 mbsf may be the result of the low degree of compaction suggested by the low Pj values; however, soft-sediment deformation (see "Lithostratigraphy") was observed between ~35 and 85 mbsf, which probably contributed to the scatter in the deformed zone.
Magnetic fabrics of samples from altered sediments below 710 mbsf (see "Lithostratigraphic Unit IX") showed prolate ellipsoids with subvertical Kmax axes. The AMS data suggest a vertical alignment of magnetic grains, which may be related to migration of fluids that might have played a role in the alteration of the sediments. The samples were from cores that showed the presence of goethite, based on XRD analysis, occurring as concretions (lithostratigraphic Unit IX; see "Lithostratigraphic Unit IX"). The susceptibility of goethite is very low relative to that of magnetite or maghemite (Tarling and Hrouda, 1993), which suggests that goethite probably does not account for the observed high susceptibilities.
Measurement of remanent magnetization was made on all but the most disturbed archive-half sections from sediment cores. A total of 114 discrete samples from working-half sections of sediment cores were measured.
Demagnetization behavior of discrete samples generally showed two components of magnetization. The soft component, which was removed by 20 mT alternating field (AF) demagnetization, showed directions that were moderately steep to steep downward in XCB cores (Fig. F67A, F67B); RCB cores showed directions associated with the soft component that ranged between shallow to steep downward (Fig. F67C, F67D). The downward directions associated with the soft component probably represent an overprint acquired from the drill string.
Most XCB samples showed stable directions after AF demagnetization at 25 mT (Fig. F67A, F67B); but many samples from RCB cores showed erratic behavior during AF demagnetization with no stable endpoint reached (Fig. F67E).
At Holes 1109A and 1109B, intensities were on the order of 10-2 A·m-1, consistent with the intensities observed at Hole 1109C for comparable depths (Fig. F66A, F66B). Intensity of remanent magnetization after AF demagnetization increased sharply from values on the order of 10-3 A·m-1 to maximum values on the order of 10-2 A·m-1 at ~85 mbsf; below ~100 mbsf the intensity decreased gradually to values on the order of 10-3 A·m-1 with a few narrow horizons showing higher values (Fig. F66B). Another abrupt increase in intensity occurred at ~270 mbsf, followed by a gradual decrease to low values on the order of 10-5 -10-4 A·m-1 by ~377 mbsf, below which values remained very low. Between ~540 and 590 mbsf, values increased by about one order of magnitude, after which very low values (10-5-10-4 A·m-1) persisted until ~710 mbsf. Below 710 mbsf values increased abruptly up to 10-3 to 10-2 A·m-1. Intensities of discrete samples were consistent with those from long core data for the same AF demagnetization levels.
Reliable core orientation data from the Tensor tool was limited to two cores (180-1109C-10-H and 11-H). Tensor tool results for the other oriented cores showed a large amount of scatter in data used to determine the mean orientation angle (MTF; see Table T8). Therefore, declinations corrected for core orientation between 84 and 102 mbsf were consistent with the expected declination, but elsewhere Tensor corrected declinations were questionable. Declinations from XCB and RCB cores were highly scattered, which precluded their use for magnetostratigraphic interpretation (Fig. F66B).
The polarity of the remanent magnetization after AF demagnetization at 20 mT for Hole 1109C and at 25 mT for Hole 1109D was determined primarily from the inclinations. Although scatter was relatively high, trends within the inclination data from long cores, corroborated by discrete sample analysis and intensity data, facilitated the polarity interpretation. Only sedimentary sections were used for the magnetostratigraphy.
From Holes 1109A and 1109B, only 10 and 15 m, respectively, of sediment were recovered to meet the demand for on ship high resolution sampling. Sediments from these holes recorded part of the Brunhes normal polarity chron, which is consistent with the paleontologic data (Fig. F66A).
Figure F66C shows the variation of inclination with depth at Holes 1109C and 1109D, as well as the polarity zones that are listed in Table T9. Magnetostratigraphic interpretation of the polarity zones described below are consistent with the paleontologic data (see "Biostratigraphy").
The transition that occurs at the base of N1 (35.8-36 mbsf) is marked by a decrease in intensity and a rapid change in the polarity of inclination data (Fig. F66D). This boundary represents the Brunhes/Matuyama Chron boundary (0.78 Ma).
The transitions observed between R1-N2 and N2-R2 (47.5-48 mbsf and 53.5-54 mbsf) were marked by a rapid change in polarity and a decrease in intensity (Fig. F66D). These transitions represent the upper and lower boundaries, respectively, of the Jaramillo Subchron (C1r.1n; 0.99-1.07 Ma).
A normal polarity interval occurs between 70.5 and 74 mbsf (N3; Fig. F66C), which probably represents the Cobb Mountain Subchron (C1r.2r.1n; 1.20-1.21 Ma).
The apparent transitions between ~35 and 85 mbsf, described in the preceding paragraphs, occurred in sections where soft-sediment deformation was observed and overturned beds within this interval were noted (see "Structural Geology"). Correction of the paleomagnetic data for tilted and overturned beds changed the polarity and removed the limited transition data indicated by the inclinations. However, interpretation of precisely which horizons were overturned was not clear, so that correction for tilt was viewed with caution, and tilt-corrected data were not shown in Figure F66A, F66B, F66C, and F66D.
The boundary between R5 and N4 (290-291.5 mbsf) shown in Figure F66C represents the Matuyama/Gauss polarity transition (2.58 Ma).
Mixed polarities were observed between ~213 and 255 mbsf (M1) and again between 266 and 269 mbsf (M2), above the Matuyama/Gauss boundary. These data, which admittedly were from a zone of poor core recovery and were highly scattered, suggest that the Olduvai Subchron (C2n; 1.77-1.95 Ma) and the Reunion Subchron (C2r.1n; 2.14-2.15 Ma), respectively, were recorded at these depths. The magnetostratigraphic interpretation of the Olduvai and Reunion is supported by seismic stratigraphic correlation between Sites 1109 and 1118 (see Fig. F40 and "Vertical Seismic Profiling, Depth Conversion, and Site Correlation" in "Thematic Overview," both in the "Leg 180 Summary" chapter).
The termination of the Kaena Subchron (C2An.1r; 3.04 Ma) occurs at the boundary between N4 and R6 (331 mbsf) in Figure F66C; the beginning of the Kaena (3.11 Ma) is not clearly defined but occurs within M3 (342-353 mbsf), where mixed polarities were observed. This interpretation is consistent with the paleontologic data, which dates the interval between ~314 and 334 mbsf at 3.09 Ma (see "Biostratigraphy").
The termination and beginning of the Mammoth Subchron (C2An.2r), the ages of which are 3.22 and 3.33 Ma, respectively, occur at the boundaries between N5-R7 (358 mbsf) and R7-N6 (384.5 mbsf) in Figure F66C; the termination of the Mammoth is found in Hole 1109C at ~355-357 mbsf. Paleontologic data indicate an age of 3.35 Ma between ~362 and 370 mbsf in Hole 1109C, and between ~359 and 368 mbsf in Hole 1109D, which is reasonably consistent with the magnetostratigraphic interpretation.
The Gauss/Gilbert transition (3.58 Ma) occurs at the boundary between N6 and R8 (470-472 mbsf) in Figure F66C. Paleontologic data places an age of 3.58 Ma at depths between 434.70 and 444.49 mbsf, which is reasonably consistent with the magnetostratigraphic interpretation.
The remainder of Hole 1109D below 472 mbsf spans part of the Gilbert reversed polarity chron (Fig. F66C). Within the Gilbert reversed polarity chron, mixed polarity zone M4 (698-715 mbsf) reflected unreliable data associated with very low intensities.
A total of 43 minicore and cut cube samples were taken from igneous rocks found below 750 mbsf in Hole 1109D. The rocks were identified as dolerite and interpreted as a sill, which was divided into two components based on petrographic observations (see "Igneous and Metamorphic Petrology"). The boundary between the two components is present at ~20 cm in Section 180-1109D-51R-4 (800.59 mbsf). A fine-grained zone was observed at the base of the upper component between ~800 mbsf (at ~100 cm in Section 180-1109D-51R-3) and the boundary in Section 51R-4. Downward from the boundary, a coarsening of grain size was observed in the lower component (see "Igneous and Metamorphic Petrology"); however, the coarsest grain size observed in the lower component was finer than that observed in the upper component. At the boundary, a chilled glassy margin with thin veins of glassy material intruded into the upper component, which suggested the intrusion of the lower component into the upper one. Out of 43 samples, five samples were collected from the fine-grained base of the upper component and four from the lower component.
Nineteen minicores were subjected to AF demagnetization up to 30 mT: 17 samples from the upper component, one from the fine-grained base, and one from the lower component. The natural remanent magnetization (NRM) intensity of the samples ranged between 1 × 100 and 4 × 100 A·m-1. A soft component with a steep downward direction was largely removed between 2 and 5 or 10 mT after removal of a viscous component between 0 and 2 mT (Fig. F68A, F68B, F68C, F68D). For most samples, less than 50% of the initial intensity remained after AF demagnetization at 5 mT. The soft component with the steep downward direction was considered a drilling-induced overprint carried by large grains of ferrimagnetic minerals (probably multidomain titanomagnetite). Demagnetization results between 10 and 30 mT generally indicated the presence of two components. Four samples provided a stable component between 20 and 30 mT (Fig. F68, examples shown in B and D), which is referred to as the characteristic remanent magnetization (ChRM). However, isolation of the ChRM in other samples was prevented by a large overlap of coercivities, which was indicated by the curved trajectory of demagnetization results or by unstable magnetic behavior related to a very weak intensity of remanence at high demagnetization steps (Fig. F68, examples shown in A and C).
The ChRM directions of four samples are shown in Figure F68E. Steep upward inclinations (range: -53º to -62º) were indicated for two samples from the upper component and for one sample from the fine-grained base; the one sample from the lower component showed a steep downward direction. The difference in the polarity indicated by the ChRM of these four samples suggests different ages of emplacement between the two components of the sill (or dike). Furthermore, inclinations from all four samples were much steeper than expected for the present site latitude (~18.5º) and were much steeper than those of the sediments overlying the dolerite. The ChRM directions preserved in the dolerite implied a tilting of the sill after emplacement, either as a coherent unit or as fragmented blocks. Tilting of the dolerite may be the result of block rotations related to faulting; many fractures observed in the dolerite were interpreted as faults (see "Structural Geology"). Block rotation probably preceded the formation of the overlying sedimentary sequences, because structures observed in the sediments indicated no large scale tilting of the sedimentary basin.
The mean susceptibility, the degree of anisotropy (Pj) and the shape parameter (T) for the susceptibility ellipsoid (Jelinek, 1981), and the inclinations of the maximum (Kmax), intermediate (Kint), and minimum (Kmin) axes of the susceptibility ellipsoid for discrete samples are shown in Figure F69. Mean susceptibilities were on the order of 10-2 SI, which suggested that ferrimagnetic minerals dominated the susceptibilities. The fine-grained base and the lower component showed relatively weaker susceptibilities than the upper component. The Pj values generally ranged between 1.01 and 1.10, reflecting an overall low degree of anisotropy. Almost all samples showed T values between ~0 and -0.8, indicating prolate ellipsoids that reflect a predominant lineation to the magnetic fabric. The degree of prolateness appeared to increase with increasing values of Pj. Magnetic fabrics of the upper component generally showed steep Kmax axes with subhorizontal Kint and Kmin axes, whereas horizontal Kmax axes with vertical Kmin axes are observed in samples from the fine-grained base and the lower component.
Studies have shown that magnetic lineations in igneous rocks may be either parallel or perpendicular to the flow direction, depending on magma viscosity and flow rate (Tarling and Hrouda, 1993). This suggests that the magnetic fabric of the upper component of the dolerite unit of Hole 1109D may have been controlled by the magma flow.
Interpretation of grain-size variations (see "Igneous and Metamorphic Petrology") suggested that the fine-grained base and the lower component underwent rapid cooling near the contact plane, which may have caused the different magnetic fabrics determined for the two components. Magnetic fabrics near a contact plane generally reflect an inward-directed cooling stress, which results in Kmin axes normal to the contact plane (Tarling and Hrouda, 1993); alternatively, irregular flow of magma near the contact could have caused the difference in the magnetic fabrics determined for the two dolerite components in Hole 1109D.
Because the orientations of the dolerite cores were unknown and the ChRMs implied a tilting of the dolerite unit, directions associated with the magnetic fabrics do not reflect the true flow direction. Postcruise paleomagnetic studies may enable further interpretation of the directions of the magnetic fabrics.