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doi:10.2204/iodp.proc.303306.208.2010

Results

Magnetic concentration

Susceptibility (k) and ARM represent concentrations of magnetic minerals in a sample. Wide variations in the k and ARM profiles are observed in the upper sedimentary interval (0–15 mcd) with larger amplitudes and longer periods of fluctuation; smaller amplitudes and shorter periods show less fluctuation in the lower interval (15–65 mcd) (Fig. F3). The interval between 15 and 47 mcd shows gradual decreases in the concentrations of magnetic minerals. Fairly low and unclear cycle patterns of k and ARM are observed in sections below 29 mcd.

The fluctuating pattern of magnetic concentration possibly reflects carbonate content, which might be correlated to the δ¹⁸O pattern as L* (see the “Site U1312” chapter). The fluctuating pattern between 0 and 10 mcd might correlate to the benthic δ¹⁸O pattern (Lisiecki and Raymo, 2005), but the pattern of the interval between 10 and 15 mcd is not clear. The interval from 15 to 29 mcd can be correlated to the δ¹⁸O pattern again according to onboard correlation (see the “Site U1312” chapter) and the paleomagnetic data below. Fairly low and indistinct patterns in magnetic concentrations below 29 mcd differ from δ¹⁸O patterns. Most portions of cores are not ideal for RPI study because of the highly fluctuating magnetic mineral contents. One proposed criteria for RPI study is that the maximum concentration must be no more than 10× the minimum concentration (e.g., Tauxe, 1993).

Paleomagnetic direction

Because each U-channel sample was measured separately, data from both ends of the U-channel (top and bottom 10 cm) were eliminated from analysis in order to avoid the edge effect of superconducting quantum interference device (SQUID) sensors. Maximum angular deviation (MAD) values calculated by PCA are usually <4°, but intervals at polarity boundaries are larger, as high as 45.6° (Fig. F4). The inclination profile obtained after PCA indicates normal and reversed polarity intervals (Fig. F4). Inclinations above 37 mcd are clearly biased to the normal or reversed sides, but below this horizon the inclination pattern is unclear. Particularly the interval between 37 and 50 mcd involves prominent short intervals of positive inclination in the dominantly reversed interval. Because the inclination pattern in the interval between 55 and 59.5 mcd presumably correlates to Chron C2An (Gauss) according to the onboard biostratigraphy results (see the “Site U1312” chapter); the spiky normal intervals from 46.13 to 46.86 mcd could correspond to Chron C2n (Olduvai). The identified chrons and subchrons are given in Table T1.

Paleointensity proxy

Relative paleointensity proxies are calculated only in the interval between 16.91 and 46.13 mcd (from the Brunhes/Matuyama chron boundary to the possible top of the Olduvai Chron), which shows relatively smaller variations in magnetic mineral concentrations. NRM intensity at 20 mT demagnetization is normalized by ARM intensity at 20 mT and k (NRM20mT/ARM20mT and NRM20mT/k) (Fig. F5), and both are compared with the paleointensity data at Ocean Drilling Program Site 984 in the North Atlantic (Channell et al., 2002). The decrease in the proxies just below the bottom of the Jaramillo event (23 mcd) in the data produced from Site U1312 isdistinctly different from Site 984 data. Although the cause of this decrease is unclear so far, the inclination profile reveals two short zones of normal polarity between 23 and 25 mcd (Fig. F4). Thus, it is supposed that the low is related to a short subchron such as the Cobb Mountain event. The paleointensity proxy patterns in the interval between 25 and 36 mcd are similar to the interval from ~1200 to 1470 ka at Site 984. The proxy values at Site U1312 below 36 mcd drastically decrease, but the pattern seems tocorrelate to the profile of Site 984.

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