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doi:10.2204/iodp.proc.342.104.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1403A and U1403B with the primary objective of establishing magnetostratigraphy of the site to provide chronostratigraphic age control. The natural remanent magnetization of each archive-half section 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 section 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-U1403A-1H through 16H and 342-U1403B-1H through 9H were azimuthally oriented using the FlexIT tool (Table T11); all other cores were not oriented.

We also took 164 discrete samples from working-half sections to verify the archive-half section measurement data and to measure the anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1403 sediment. Discrete samples were collected and stored in 7 cm³ plastic cubes and were typically taken from the least disturbed region closest to the center of each section. In general, samples were collected at ~1.5 m intervals from Cores 342-U1403A-1H through 16H and 23X through 28X and 342-U1403B-19H through 22H, 23X through 25X, and 28X through 32X. The samples were first subjected to AMS measurements, including bulk susceptibility. Subsequently, they were step-wise AF demagnetized at 10 and 20 mT. Forty samples were further step-wise demagnetized to 60 or 80 mT. All discrete sample data were volume corrected to 7 cm³.

Results

Downhole paleomagnetism data after 20 mT demagnetization are presented for Holes U1403A and U1403B in Figures F18 and F19. Three prominent features are evident:

1. Magnetic intensity lows occur at several horizons (e.g., between ~15 and ~35, ~95 and 130, and ~160 and 170 mbsf),

2. Inclination is biased toward positive values, and

3. ~180° alternations are seen in declination records for APC cores.

Low-intensity zones

Magnetic intensity lows are most frequently associated with distinctive blue-gray sediment intervals in the cores (see “Lithostratigraphy”). Magnetic susceptibility values are also very low within these intervals, indicating either a lower initial supply of paramagnetic and ferromagnetic minerals or diagenetic loss of these materials in these intervals. We favor the interpretation that the magnetic intensity lows are caused by reductive dissolution, which is common in oceanic sediment. We note that it was often difficult to obtain meaningful paleomagnetic signals from these horizons because diagenesis has obscured the primary magnetic signal.

Inclination bias

Inclination bias indicates that a substantial drilling overprint exists even with the use of nonmagnetic core barrels and cutting shoes and after 20 mT demagnetization. Considering the latitude of Site U1403 (~40°N), reversal sequences should be characterized by distinct inclination alternations between ~60° and approximately –60°. However, there are very few horizons in which we observe –60° inclinations. Because of this strong inclination biasing, identifying the paleomagnetic polarity solely based on shipboard inclination data is nearly impossible.

Declination trends

At low to middle latitudes, reversal sequences often can be identified from declination patterns. Indeed, the ~180° alternations in declination observed in Site U1403 cores are consistent with polarity transitions. Declination variations illustrated for the “oriented APC” intervals in Figures F18 and F19 are in geographical coordinates (corrected using the FlexIT core orientation tool). We interpret the intervals with declination 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 often have inclination values that are systematically shallower than those in the intervals with declination of ~0° (e.g., ~50–140 mbsf in Hole U1403A and ~50–70 mbsf in Hole U1403B; Figs. F18, F19). Thus, the drilling overprint mainly obscured remanent inclination but not declination. Shipboard demagnetization to 20 mT can only partially remove this persistent vertical overprint, resulting in positive polarity magnetozones with inclination values ranging from ~50° and higher and negative polarity magnetozones with inclination values from ~40° to –60°. In some cases, shipboard demagnetization of discrete samples verified these trends (see below), but thorough shore-based demagnetization experiments will help clarify intervals with ambiguous or indeterminable polarity.

Comparison between pass-through and discrete sample data

AF demagnetization results for discrete samples are summarized in Tables T12 and T13. Of the 40 samples subjected to step-wise AF demagnetization by 60 or 80 mT, only 11 reveal stable components of magnetization (e.g., Fig. F20A, F20B). For the other samples, drilling overprints are too severe to reveal primary components (Fig. F20C). Nevertheless, these results are still useful for verifying the 20 mT pass-through paleomagnetism data from the archive-half sections. Magnetization intensity and declination are generally consistent between the discrete samples and the archive-half samples (Figs. F18, F19).

We note that inclinations measured in discrete samples are often more shallow than the counterpart values in the archive-half samples. This trend is particularly evident in the “oriented APC” intervals with declinations of ~180° (e.g., ~50–140 mbsf in Fig. F18). These inclination-shallowing trends are consistent with the reversed polarity intervals suggested from the declination. Discordance between inclination measured in the discrete samples and the archive-half sections is substantial in XCB intervals (e.g., ~180–250 mbsf in Fig. F19). We caution that any magnetozone interpretations based on the archive-half section measurement data for the XCB intervals without more thorough demagnetization procedures may be prone to misinterpretations.

In summary, paleomagnetism data from archive-half sections and discrete samples from oriented core intervals generally agree well and reveal a continuous series of magnetozones from Sections 342-U1403A-6H-2 to 16H-2 (46.57–140.30 mbsf) and 342-U1403B-6H-5 to 16H-5 (41.86–136.91 mbsf). Moreover, our shipboard demagnetization results indicate that shore-based measurements are required to interpret archive-half section measurement data to identify and fully characterize magnetozones in nonoriented core intervals.

Magnetostratigraphy

Identification of magnetozones and correlation to the geomagnetic polarity timescale (GPTS) was most straightforward for azimuthally oriented APC intervals (Cores 342-U1403A-1H through 16H and 342-U1403B-1H through 9H) (Fig. F21). Our general strategy was to identify magnetozones first by using inclination reversals 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 U1403A and U1403B, including a continuous series from Sections 342-U1403A-6H-1 to 16H-2 (~46–140 mbsf) and 342-U1403B-6H-5 to 16H-5 (~41–137 mbsf).

By utilizing robust radiolarian and nannofossil biostratigraphy from Core 342-U1403A-16H (see “Biostratigraphy”), we can correlate the continuous series of magnetozones uphole through Site U1403 to an early to late Eocene magnetostratigraphy. This correlation is corroborated by radiolarian biostratigraphy in Cores 342-U1403A-8H through 15H. In Hole U1403A, we identified Chrons C16n.2n (36.051–36.700 Ma) to C22n (48.566–49.344 Ma) from 46.57 to 140.30 mbsf. In Hole U1403B, we identified Chrons C16n.1r (35.892–36.051 Ma) to C22n (48.566–49.344 Ma) from 41.86 to 136.91 mbsf. Although distinct magnetozones are evident above these intervals, we cannot correlate them to the GPTS because the series does not match the GPTS reversal pattern, and we lack biostratigraphic age control to independently anchor these series in time. Strong drilling-induced overprinting has obscured the primary magnetostratigraphy in cores recovered using the XCB coring system, so we refrain from correlating these intervals to the GPTS until we have completed thorough demagnetization experiments on these cores. The magnetostratigraphic age model for Site U1403 is summarized and presented in Table T14 and Figures F18, F19, and F21.

The magnetostratigraphic age model for Site U1403 allows for precise dating of clay-rich and fossiliferously poor lithostratigraphic Units II and III (see “Lithostratigraphy”). The magnetostratigraphy at Site U1403 verifies the high clay sedimentation rates implied by the radiolarian biostratigraphy (see “Biostratigraphy”). These results indicate that from the beginning of Chron C22n to the end of Chron C20n (49.344–43.301 Ma), clay accumulated at Site U1403 at over twice the rate seen during Chrons C19r to C16n.1r (43.301–35.892 Ma).

Site U1403 magnetostratigraphy also suggests that the distinctive gray-blue interval at 342-U1403A-6H-2, 80–120 cm, and 342-U1403B-6H-5, 50–90 cm, was deposited during Chron C16n.1n (35.706–35.892 Ma). This stratigraphic interval is characterized by low magnetic susceptibility, a peak in NGR, and the presence of euhedral silicate minerals that have been tentatively identified as feldspars (Fig. F14). We tentatively conclude that this interval marks the Chesapeake Bay impact event, which occurred within Chron C16n.1n (Coccioni et al., 2009). We note that a lithologically similar gray-blue bed is present at Site U1404, but at that site the magnetochronology and biostratigraphy are consistent with deposition just above the Chron C17n.1r/C17n.2n boundary. We are more confident in the magnetostratography at Site U1404 than that at Site U1403. Therefore, the gray-blue bed at Site U1403 is either a separate event from that at Site U1404, or the identification of Chron 16n at Site U1403 is incorrect.

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on discrete samples is summarized in Tables T15 and T16. Downhole variation for whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) are shown in Figure F18. 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 well, 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. Nevertheless, both magnetic susceptibility data sets show the same first- and second-order cyclic trends, indicating that these trends are robust features of Site U1403 sediment.

AMS results for the discrete samples are also summarized in Tables T15 and T16 and are shown in Figures F22 and F23. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) magnetic susceptibilities at Site U1403 are tightly clustered downhole to ~120 mbsf, indicating an isotropic magnetic fabric. Magnetic anisotropy increases and becomes more oblate with depth in both Holes U1403A and U1403B. We note that the inclination of the minimum eigenvector (V₃) is highly variable in APC cores but is clustered toward near-vertical values in XCB cores. Similarly, the degree of anisotropy (P; τ₁/τ₃) is generally highest in XCB cores, with a notable exception in Cores 342-U1403A-13H through 16H and 342-U1403B-19H through 22H, in which P is remarkably high. In Cores 342-U1403A-13H through 16H, the high P values correspond to intervals with lower clay content, lowest bulk density, and greatest porosity (Fig. F22). These observations suggest that magnetic anisotropy is largely controlled by lithology (i.e., grain size) rather than coring method in APC-recovered intervals. However, subhorizontal oblate fabrics are most evident in intervals recovered by XCB in Holes U1403A and U1403B, even in clay-rich horizons at these depths. We attribute this strong magnetic anisotropy to the compounded effects of burial, drilling-induced compaction, and biscuiting characteristic of XCB coring.

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