The investigation of magnetic properties at Site 1115 included (1) the measurement of bulk susceptibility of whole core sections, (2) point susceptibility and remanent magnetization of archive half core sections, and (3) susceptibility and its anisotropy and remanent magnetization of discrete samples.
Magnetic susceptibility measurements were made on whole core sections as part of the multisensor track (MST) analysis (see "Physical Properties"), and on half core sections as part of the archive multisensor track (AMST) analysis. The MST and AMST susceptibilities (uncorrected for volume) ranged between values on the order of 10-5 and 10-2 SI (Fig. F38A, F38B). In general, susceptibility data from the MST and AMST analyses agreed; differences in magnitude can be attributed to volume differences for the uncorrected data. The apparent constant low values in the AMST data between ~200 and 500 mbsf were caused by technical problems.
The trends of susceptibility and remanent intensity data after AF demagnetization at 20 mT are similar throughout Holes 1115B and 1115C (Fig. F39). The high values below ~580 mbsf and between ~420 and 480 mbsf suggest that the contribution of remanence-carrying ferrimagnetic minerals to the susceptibility and its anisotropy is great. The lower values between ~210 and 420 mbsf, and again between ~480 and 580 mbsf suggest that paramagnetic minerals contribute significantly to the susceptibility. Above ~200 mbsf, both paramagnetic and ferrimagnetic minerals contribute substantially to the susceptibility.
Results of the measurement of susceptibility and its anisotropy (AMS) on discrete samples are shown in Figure F40. The mean magnetic susceptibility, the degree of anisotropy (Pj) and the shape parameter (T) for the susceptibility ellipsoid (Jelinek, 1981), and the inclinations of the maximum (Kmax) and minimum (Kmin) axes of the susceptibility ellipsoid are shown vs. depth.
Mean susceptibilities from discrete samples agreed with long-core susceptibilities. Above 200 mbsf, Pj values were lower than 1.02. Between 200 and 600 mbsf, Pj values increased to between 1.02 and 1.07. Below ~650 mbsf (within lithostratigraphic Unit XII; see "Lithostratigraphic Unit XII"), Pj values increased to a slightly higher range between ~1.04 and 1.08. Above 200 mbsf, T values were scattered between ~-0.5 and 0.5. Between 200 and 600 mbsf, T values were dominantly positive and scattered ~0.5. A slightly greater scattering of T values was observed in lithostratigraphic Units V to X (417-620 mbsf; see "Lithostratigraphy"), in which silt and sand or larger grain size sediments were abundant; T values may be related to the grain-size variations in these units. Below 650 mbsf (lithostratigraphic Unit XII; see "Lithostratigraphic Unit XII"), T values were dominantly at or above 0.5, which suggests that an oblate magnetic fabric dominates in lithostratigraphic Unit XII. The oblateness of the fabrics seems to be enhanced by the higher Pj values of this interval. Inclinations of Kmax and Kmin axes were scattered above 200 mbsf; however, with increasing depth throughout this interval Kmax axes shallowed whereas Kmin axes steepened, so that by ~220 mbsf a clear separation of Kmax and Kmin inclinations was evident: Kmax axes were dominantly very shallow (<20º), while Kmin axes were dominantly very steep (>70º).
In summary, compaction effects increase through the upper 200 mbsf so that a clear compaction signature is evident from the AMS data below ~200 mbsf.
Measurements of remanent magnetization were made on all but the most disturbed sections from archive half cores and on discrete samples from working half core sections. Results are shown in Figures F41 and F42.
A total of 73 discrete samples were subjected to AF demagnetization experiments to assess the stability of the natural remanent magnetization (NRM). The AF demagnetization results of 23 discrete samples of APC cores from Hole 1115B indicated that the NRMs of these samples consisted of two magnetic components. A soft component, probably drilling-induced, with a steep downward direction was generally removed by demagnetization levels up to 10 mT. After removal of the soft component, many samples yielded a stable component that decayed toward the origin of the vector plot between 15 and 25 mT (Fig. F41A, F41B); this component is referred to as the characteristic remanent magnetization (ChRM). Some samples showed curved demagnetization trajectories on vector plots, which indicated a large contribution of a probably drilling-induced component (Fig. F41C); the ChRM from these samples was not isolated.
Nine samples from XCB cores of Hole 1115B (below 216 mbsf) showed weak remanent intensities ~1 × 10-3 A·m-1. All but one sample showed erratic behavior after removal of a drilling-induced component by ~10 mT AF demagnetization (Fig. F41D). One sample showed a soft component with an upward direction, which was removed by 5 mT AF demagnetization, followed by a steeper upward-directed component decaying linearly toward the origin of the vector plot (Fig. F41E).
Among the 41 samples collected from RCB cores of Hole 1115C, a stable magnetic behavior was generally observed in samples from lithostratigraphic Unit V (417-475 mbsf), Unit X (572-619 mbsf), and Unit XII (657-800 mbsf; see "Lithostratigraphy"). These samples showed relatively high initial intensities above 10-2 A·m-1. After removal of a soft component with a shallow to steep downward direction by 10 mT AF demagnetization, samples from Units V and X yielded stable ChRM directions that decayed linearly toward the origin on vector plots (Fig. F41F, F41G). The samples from Unit XII, on the other hand, provided a stable component that did not decay toward the origin of the vector plot (Fig. F41H); this component may represent a remagnetization in the present magnetic field. Samples from the other lithostratigraphic units showed unstable demagnetization behavior after removal of a soft component by 10 mT AF demagnetization.
In Hole 1115A, intensity of remanent magnetization of long cores after AF demagnetization at 20 mT ranged from values on the order of 10-3 A·m-1 up to values on the order of 10-2 A·m-1 (Fig. F42A). In Holes 1115B and 1115C, values were on the order of 10-3 and 10-2 A·m-1 throughout the upper 210 m. Low intensities on the order of 10-4 A·m-1 were found between ~210 and 420 mbsf, and again between ~470 and 520 mbsf (Fig. F42B). Below ~420 mbsf, intensities increased to values on the order of 10-2 A·m-1, where they generally persisted to the bottom of the hole.
The polarity of the remanent magnetization after AF demagnetization at 20 mT for Site 1115 was determined from the directions of APC cores and from the inclinations of XCB and RCB cores. Tensor tool data (Table T6) was used to orient directions for Cores 180-1115B-3H through 23H. Declinations corrected for core orientation between ~17 and 217 mbsf were consistent with the expected directions and were used in conjunction with the inclinations to interpret polarities. Declinations from XCB and RCB cores were highly scattered, which precluded their use for magnetostratigraphic interpretation. Directions from long cores were corroborated by discrete sample analysis. Downcore variations in remanent intensity, inclination, and declination are shown in Figure F42A and F42B.
Figure F43A shows downcore variations in intensity, inclination, declination, and magnetostratigraphic interpretation for Hole 1115B. Figure F43B shows downcore variations in the inclinations for Holes 1115B and 1115C, along with the magnetostratigraphic interpretation. Poor recovery between ~400 and 650 mbsf and high scatter in the data below ~400 mbsf precluded interpretation of the directions in Hole 1115C in terms of the magnetic polarity time scale.
The transition that is found between 33.5 and 36 mbsf represents the Brunhes/Matuyama boundary (0.78 Ma; Berggren et al., 1995). Using estimated sedimentation rates of 34 m/m.y. for the past 0.46 Ma and 59 m/m.y. for the period from 0.46 to 0.78 Ma (see Fig. F37), the Brunhes Chron should span ~34.5 m of section, which is reasonably consistent with the observed span of ~36 m.
The boundary at ~162 mbsf represents the Matuyama/Gauss polarity transition (2.58 Ma; Berggren et al., 1995), which is consistent with the paleontologic data (see "Biostratigraphy"). Using estimated sedimentation rates of 59 m/m.y. for the period between 0.78 and 1.95 Ma, and 79 m/m.y. for the period between 1.95 and 2.58 Ma (see Fig. F37), the Matuyama Chron should span ~119 m of section, which is reasonably consistent with the observed span of ~126 m.
The Gauss/Gilbert boundary (3.58 Ma; Berggren et al., 1995) occurs at ~386-387 mbsf. Using estimated sedimentation rates of 79 m/m.y. for the period between 2.58 and 3.04 Ma and 284 m/m.y. for the period between 3.04 and 3.58 Ma (see Fig. F37), the Gauss Chron should span ~190 m of section, which is not consistent with the observed span of ~224 m. The estimated sedimentation rate during the early Gauss Chron (between 3.11 and 3.58 Ma) was 393 m/m.y.
The Jaramillo Subchron (C1r.1n; 0.99-1.07 Ma, Berggren et al., 1995) is between ~43.5 and 47 mbsf. Sedimentation rates of ~59 m/m.y. have been estimated for this depth range (see Fig. F37), which suggests that the Jaramillo Subchron should span ~4.7 m of section and is reasonably consistent with the observed span of ~3.5 m.
The Olduvai Subchron (C2n; 1.77-1.95 Ma, Berggren et al., 1995) is between ~90.5 and 102.5-103.5 mbsf. Sedimentation rates of ~63 m/m.y. have been estimated for these depths (see Fig. F37), which suggests that the Olduvai Subchron should span ~11.3 m of section and is consistent with the observed span of ~13 m.
The termination of the Kaena Subchron (C2An.1r; 3.04-3.11 Ma, Berggren et al., 1995) is found at ~192.5 mbsf; the onset of the Kaena Subchron is poorly defined by shallowed inclinations between 202 and 217 mbsf, but is well constrained by the declinations to ~202 mbsf.
The Mammoth Subchron (C2An.2r; 3.22-3.33 Ma, Berggren et al., 1995) does not show up in long-core data; however, data from discrete samples suggest that it may be present between ~320 and 335 mbsf, where core recovery was low. This position is consistent in age with the paleontologic datums above and below, but the high estimated sedimentation rate of ~284 m/m.y. suggests that the Mammoth Subchron should span ~31 m of section, which is not consistent with the 15 m observed. Further study is needed to support this interpretation.
Evidence for excursions of the magnetic field is found between 53 and 54.5 mbsf, which probably represents the Cobb Event (C1r.2r.1n; 1.20-1.21 Ma, Berggren et al., 1995). Using the estimated sedimentation rate of ~59 m/m.y. (see Fig. F37), this event is expected to span ~0.6 m, which is not consistent with the observed span of ~1.5 m.
Evidence for the Reunion Event (C2r.2r.2r; 2.14-2.15 Ma, Berggren et al., 1995) is found between ~118 and 119.5 mbsf. Using the estimated sedimentation rate of ~79 m/m.y., this event is expected to span ~0.79 m of section, which is not consistent with the observed span of ~1.5 m.
No evidence for excursions within the Brunhes Chron was observed.