| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.342.105.2014 PaleomagnetismWe completed a paleomagnetism study of APC and XCB cores from Holes U1404A–U1404C with the primary objective of establishing a magnetostratigraphy of the site to provide chronostratigraphic age control. The natural remanent magnetization (NRM) of each archive section half was measured at 2.5 cm intervals before and after demagnetization treatment in a peak alternating field (AF) of 20 mT. We processed the archive-half measurement data by removing measurements made within 7.5 cm of section ends and from disturbed intervals described in the Laboratory Information Management System database. Cores 342-U1404A-1H through 23H and 342-U1404B-1H through 27H were azimuthally oriented using the FlexIT tool (Table T11); all other cores were not oriented. We also took 198 discrete samples from working section halves to verify the archive-half measurement data and to measure the anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1404 sediment. Discrete samples were collected and stored in 7 cm3 plastic cubes and typically taken from the least disturbed region closest to the center of each section from Hole U1404A. One discrete sample was collected from Core 342-U1404B-24H. Most of the samples were first subjected to AMS measurements, including bulk susceptibility. Subsequently, NRMs before and after 20 mT AF demagnetization were measured. Twenty-six samples were selected for step-wise demagnetization at 10, 20, 30, 40, and 60 mT; demagnetization up to 80 mT was made in some cases. All discrete sample data are volume corrected to 7 cm3. ResultsDownhole paleomagnetism data after 20 mT demagnetization are presented for Holes U1404A, U1404B, and U1404C in Figures F19, F20, and F21, respectively. Similar to paleomagnetism results from Site U1403 (see “Paleomagnetism” in the “Site U1403” chapter [Norris et al., 2014c]), archive-half measurement data from XCB cores are difficult to interpret because of biscuiting and substantial core disturbance. Therefore, for Site U1404 we chose to interpret only results obtained from APC cores. Also, cores from Hole U1404C span only a ~30 m interval in the upper 50 m of the recovered section and are not oriented; we do not discuss results from Hole U1404C any further. We found the following main features in the paleomagnetism data at Site U1404:
These magnetic characteristics are similar to those of sediments from Site U1403. Low-intensity zonesMagnetic intensity lows are most frequently associated with green-gray sediment (see “Lithostratigraphy”). Magnetic susceptibility values are also very low within these same intervals (less than ~10–4 SI; Figs. F19, F20). These trends may indicate either a lower initial supply of paramagnetic and ferromagnetic minerals or diagenetic loss of these materials in these intervals. Many of these intervals are characterized by disseminated and layered pyrite, as well as glauconite (see “Lithostratigraphy”). Therefore, we favor the interpretation that the magnetic intensity lows are caused by reductive dissolution, which is common in oceanic sediment. Obtaining meaningful paleomagnetism signals from these horizons was often difficult. Inclination biasThe inclination bias indicates that a substantial drilling overprint exists even with the use of nonmagnetic core barrels and cutting shoes and after 20 mT AF demagnetization. Similar to results from Site U1403, we observed only a few horizons which show –60° inclinations (expected for the ~40°N latitude of Site U1404) during reversed magnetozones. Because of this strong inclination biasing, we often cannot identify paleomagnetism polarity solely based on shipboard inclination data. Declination trendsOriented APC cores were recovered downhole to Core 342-U1404A-23H (204.37 mbsf) and for the entire recovered interval in Hole U1404B. We interpret intervals with declination values of ~0° (~180°) to indicate normal (reversed) magnetozones. A magnetozone with a primary normal polarity should not display inclinations less than ~40°, barring sedimentary inclination shallowing biases. Notably, intervals with ~180° declination almost always correspond to inclination values that are shallower than those in the intervals with declination of ~0° (e.g., ~205–210 mbsf; Fig. F20). Thus, the drilling overprint mainly obscured remanent inclination but not declination, similar to the paleomagnetism results from Site U1403. Anomalous declination trendsOriented paleomagnetism declinations from Core 342-U1404A-23H show a systematic offset from expected azimuths. Moreover, the magnitude of this offset increases downhole (Fig. F22A). These observations are best explained by “rifling,” in which the APC drilling assembly rotates as it is fired into the sediment. Rifling may explain the brittle shear features (P-bands and Reidel shears) observed near the unusually sharp lithostratigraphic contact at interval 342-U1404A-23H-4, 120–122 cm (Fig. F22B, F22C), and throughout Core 342-U1404A-23H. The amount of displacement, compaction, or both along this contact is unknown, but it is probably minor. Comparison between pass-through and discrete sample dataAF demagnetization results for the discrete samples are summarized in Table T12. Of the 26 samples treated with peak AF demagnetization fields of 60 or 80 mT, 17 reveal relatively stable components of magnetization (e.g., Fig. F23A). These samples have remanent magnetizations that are high enough to be measured by the onboard JR-6A spinner superconducting rock magnetometer. The remaining samples usually displayed NRM intensities that decreased by an order of magnitude following AF demagnetization in 10 or 20 mT fields, with components above this treatment level characterized by poor direction stability (e.g., Fig. F23B). This behavior indicates that the combination of drilling overprint and low magnetic intensity described above has obscured the primary magnetic signal in these stratigraphic intervals. Nevertheless, these results are useful for verifying the 20 mT pass-through paleomagnetism data from the archive section halves. Magnetization intensity and declination are generally consistent between the discrete samples and the archive-half samples (Figs. F19, F20). We note that inclinations measured in discrete samples are often more shallow than their counterpart values in the archive-half samples. Discrete samples were collected from the central part of the cores, which is generally least affected by mechanical and magnetic drilling overprint (Acton et al., 2002). Therefore, discrete samples are often considered to preserve the least-biased primary remanences. Moreover, discrete samples are not affected by signal smoothing, which is an inherent feature of pass-through measurements (Roberts, 2006; Xuan and Channell, 2009). An exception to this trend is observed in the interval between 30.0 and 62.5 cm in Section 342-U1404B-24H-4 (~207.0–207.4 mbsf). In this interval, archive-half measurement data yielded ~90° inclination and ~0° declination even after 20 mT AF demagnetization, resulting in an apparent normal polarity magnetozone (Fig. F24). We collected a discrete sample from this interval to test the superconducting rock magnetometer results. AF demagnetization of the sample shows that the drilling overprint could not be removed even with a peak demagnetization field of 80 mT (Fig. F23C). The lower one-third of this interval has a reddish hue (Fig. F24), suggesting that it contains oxidized iron, such as hematite. We conclude that this interval contains high-coercivity iron oxides, which can easily acquire a remanence overprint but resist AF demagnetization treatments. Shore-based coercivity experiments can easily verify this interpretation. Because we interpret this interval as a drilling overprint, we exclude it from further analysis. In summary, paleomagnetism data from archive section halves and discrete samples from oriented core intervals generally agree well and reveal a semicontinuous series of magnetozones from Sections 342-U1404A-23H-4 to 32H-3 (200.67–268.49 mbsf) and 342-U1404B-24H-1 to 26H-3 (203.07–218.49 mbsf). Downhole plots indicate that a discontinuous series of magnetozones is probably recorded higher in the recovered section in both Holes U1404A and U1404B, but shore-based studies are necessary to identify and fully characterize magnetozones in this low–magnetic intensity interval. MagnetostratigraphyIdentification of magnetozones and correlation to the geomagnetic polarity timescale (GPTS) was straightforward for azimuthally oriented APC intervals in lithostratigraphic Units III and IV in Hole U1404B. Unoriented APC cores in Hole U1404A also yielded a distinct magnetostratigraphy. Our general strategy was to identify magnetozones first by using systematic changes in inclination, but more frequently by using declination reversals from oriented cores. Intervals with downward and northerly (~0°) magnetizations are assigned normal polarity, whereas intervals with low-to-upward and southerly (~180°) declinations indicate reversed polarity. This approach yields several normal and reversed magnetozones throughout Holes U1404A and U1404B (Fig. F25). In Hole U1404A, we identified magnetozones in Sections 342-U1404A-23H-4 through 32H-3 (~200.67–268.49 mbsf). In Hole U1404B, we identified magnetozones from Section 342-U1404B-24H-1 through 26H-4 (~203–220 mbsf). By utilizing radiolarian and nannofossil biostratigraphic datums from Holes U1404A and U1404B (see “Biostratigraphy”), we can correlate these series of magnetozones to a nearly continuous earliest early Oligocene to late Eocene magnetostratigraphy. Correlation of magnetozones between Holes U1404A and U1404B is corroborated by lithostratigraphic horizons and physical property features (see “Stratigraphic correlation”). Our correlation is presented in Table T13 and shown in Figures F19, F20, and F25. In Hole U1404A, we identified the Chron C12r/C13n boundary (33.157 Ma) at ~200.67 mbsf. This correlation is based on the identification of nannofossil Zone NP21 at ~200.06 mbsf. The Chron C13r/C15n boundary (34.999 Ma) is located at ~206.42 mbsf in Section 342-U1404A-24H-2. This correlation, which is based on nannofossil Zone NP19/NP20, indicates that the Chron C13n–C13r transition is missing in the core gap between Cores 342-U1404A-23H and 24H, a gap that is consistent with stratigraphic correlation. The Chron C16n.1r/C16n.2n, C16n.2n/C16r, and C16r/C17n.1n boundaries are recorded in Core 342-U1404A-25H. This correlation is based on the identification of nannofossil Zone NP19/NP20 in the bottom of this core and requires a substantial core gap between Cores 342-U1404A-24H and 25H to explain the absence of the Chron C15n/C15r boundary in either of these cores. We identified the Chron C17n.1r/C17n.2n boundary at ~222.22 mbsf in Core 342-U1404A-26H based on nannofossil Zone NP19/NP20 and NP18 datums in the reversed polarity magnetozone at the top of this core. A distinctive blue lithostratigraphic horizon characterized by a large decrease in magnetic intensity and a possible normal polarity excursion occurs ~40 cm above this magnetozone boundary, serving as a strong tie point between Holes U1404A and U1404B. We assigned the long normal magnetozone in Core 342-U1404A-27H and the top of Core 28H to Chron C18n.1n. This correlation requires that Chrons C17n.2r, C17n.3n, and C17r, representing ~0.522 Ma, are in the core gap between Cores 342-U1404A-26H and 27H. Two short reversed polarity intervals observed in the declination record in Core 342-U1404A-27H may correspond to cryptochrons, such as Chron C18n.1n-1. We refrain from correlating these declination anomalies to the GPTS until we can confirm the robustness of the paleomagnetic signal. These potential magnetozones are currently based on only 1 or 2 data points from pass-through shipboard measurements. We correlate the normal-reversed-normal polarity signal in Core 342-U1404A-28H to the base of Chron C18n.1n, C18n.1r, and the top of C18n.2n. The boundary between Chrons C18n.2n and C18r occurs at ~248.89 mbsf in Core 342-U1404A-29H. The long, reversed magnetozone in Cores 342-U1404A-30H and 31H and the top of Core 32H is correlated to Chron C18r, which is consistent with Zone NP16 nannofossils found in Core 31H. The reversed-normal-reversed magnetostratigraphy observed in Core 342-U1404A-32H is correlated to the Chron C18r/C19n and C19n/C19r boundaries. We refrained from correlating magnetozones observed in XCB cores below Core 32H, but note that the pattern is consistent with Chrons C19r and C20n. In Hole U1404B, we correlated our magnetostratigraphy to a nearly continuous series of earliest Oligocene to latest late Eocene chrons. We identified the Chron C12r/C13n and C13n/C13r boundaries in Core 342-U1404B-24H. Nannofossil Zones NP21 and NP19/NP20 indicate that the series of well-resolved magnetozones in Core 342-U1404B-25H are the bottom of Chrons C13r, C15n, C15r, C16n.1n, C16n.1r, and most of C16n.2n. The Chron C16n.1r/C16n.2n boundary (36.051 Ma) at ~212.96 mbsf in Section 342-U1404B-25H-3 provides a tie point to Core 342-U1404A-25H-1. We correlate the normal-reversed-normal magnetozonation observed in Hole U1404B to Chrons C17n.1n, C17n.1r, and C17n.2n, providing tie points to Core 342-U1404A-26H. Our correlations imply that Chron C16r is missing in the core gap between Cores 342-U1404B-25H and 26H. Although distinct magnetozones are evident above these described intervals in lithostratigraphic Subunit IIb, we cannot correlate them to the GPTS at this time. The series is intermittent because of the low magnetization intensity of this reduced sediment and the sensitivity of shipboard magnetometers. In many intervals in Subunit IIb, a strong drilling-induced overprint has also obscured the primary magnetostratigraphy. The magnetostratigraphic age model for Site U1404 allows for precise dating of carbonate accumulation events in lithostratigraphic Units III and IV (see “Lithostratigraphy”) and precise identification of the Eocene/Oligocene boundary. The Eocene/Oligocene boundary (33.89 Ma) occurs just prior to the Chron C13n/C13r boundary (33.705 Ma). We identified this chron boundary at interval 342-U1404B-24H-3, 110–120 cm (206.30–206.40 mbsf) (Fig. F24). Site U1404 magnetostratigraphy verifies the low accumulation rates (<1 m/m.y.) across the Eocene/Oligocene boundary and the high accumulation rates (>13 m/m.y.) in the middle Eocene implied by the radiolarian and nannofossil biostratigraphy (Fig. F20). These results indicate that high sedimentation at Site U1404 persisted through the late middle Eocene, whereas downslope at Site U1403, clay accumulation waned in the early middle Eocene. The distinctive gray-blue interval observed at Site U1403 in intervals 342-U1403A-6H-2, 80–120 cm, and 342-U1403B-6H-5, 50–90 cm, is found in intervals 342-U1404A-26H-1, 85–115 cm, and 342-U1404B-26H-3H, 110–150 cm, at Site U1404. Site U1404 magnetostratigraphy indicates that this interval was deposited during Chron C17n.1r (37.753–37.872 Ma). This correlation suggests that we have misinterpreted a dissolution/overprint horizon in Holes U1403A and U1403B as magnetozones and that our Site U1403 magnetostratigraphy may need revision. Thus, this distinctive blue horizon probably is not the Chesapeake Bay Impact event during Chron C16n.1n (35.706–35.892 Ma) as previously considered and may instead be another, earlier impact event (Coccioni et al., 2009) or volcanic ash. Detailed shore-based geochemical and rock magnetic experiments should help clarify the origin of this distinctive stratigraphic horizon. We note, however, that Chron C16n.1n is represented in the Hole U1404B magnetostratigraphy in interval 342-U1404B-25H-3, 30.0–92.5 cm. This stratigraphic interval is characterized by a narrow horizon with the highest magnetic susceptibility values observed in all recovered sediment at Site U1404, in what is otherwise homogeneous reddish brown clay-rich nannofossil ooze (see “Physical properties”). Magnetic susceptibility and anisotropy of magnetic susceptibilityBulk magnetic susceptibility measured on 197 discrete samples is summarized in Table T14. Downhole variation in raw whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1404A are shown in Figure F19. We multiplied the WRMS data, which are in instrument units, by a factor of 0.577 × 10–5 to convert to approximate SI volume susceptibilities (see “Paleomagnetism” in the “Methods” chapter [Norris et al., 2014b]). WRMS and DSMS data agree very well after this conversion, and we attribute small absolute differences to the fact that the conversion factor applied to the WRMS data is not constant downhole because of changes in core diameter and density; only discrete samples provide calibrated susceptibility values in SI units. Noise in the WRMS data is easily attributed to section edge-effects, core disturbance, or shear-pin fall-in, none of which have been trimmed from the raw WRMS data presented in Figure F19. Nevertheless, both magnetic susceptibility data sets show the same first- and second-order cyclic trends, as well as prominent steps, indicating that these trends are robust features of Site U1404 sediment. AMS results for the discrete samples are also summarized in Table T14 and shown in Figure F26. The eigenvalues associated with the maximum (τ1), intermediate (τ2), and minimum (τ3) magnetic susceptibilities at Site U1404 show prominent downhole trends. Divergence between τ1 and τ3 increases from the top of Hole U1404A to the base of lithostratigraphic Subunit IIa (0–22.38 mbsf) (see “Lithostratigraphy”), indicating magnetic anisotropy increases with depth. This divergence increases substantially at the top of Subunit IIb (33.20 mbsf) and then gradually decreases to isotropic values with depth. τ1 and τ3 gradually diverge from the top of Unit III (200.60 mbsf) to the base of Unit IV (299.82 mbsf), with a noticeable increase in divergence beginning at 271.00 mbsf with the start of XCB core recovery. These eigenvalue trends have a close inverse correlation with bulk density and positive correlation with porosity (see “Physical properties”), counterintuitively suggesting that high magnetic anisotropy is related to low density (i.e., low compaction) and high interstitial water content in clay-rich Subunit IIa. The inclination of the minimum eigenvector (V3) is more variable in APC cores than it is in XCB cores, which is similar to AMS results from Site U1403. Moreover, V3 shows the greatest variability in lithostratigraphic Subunit IIb. In contradistinction to results from Site U1403, the degree of anisotropy (P; τ1/τ3) is much higher in APC cores than in XCB cores, with a prominent peak in the upper part of Subunit IIb. Collectively, AMS results suggest that magnetic anisotropy is largely controlled by lithology rather than coring method in APC-recovered intervals. Although subhorizontal oblate fabrics are evident in intervals recovered by XCB coring, they are most pronounced in the least lithified part of Subunit IIb. |
|||
