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doi:10.2204/iodp.proc.317.210.2018

Results

Nannofossil datum reexamination

Results of microscopic observation of nannofossil occurrences are shown in Table T1. P. lacunosa is found from 128.35 to 180.06 m. P. ovata is found from 130.34 to 159.47 m. P. pacifica was observed in two samples at 157.86 and 175.07 m (Table T1; Fig. F1). Therefore, we confirmed that the occurrences of P. lacunosa and/or P. ovata are as shallow as 131.98 m (Table T1). The isolated occurrence of P. lacunosa var. lacunosa at 128.35 m is considered to be reworked. Therefore, we propose that the HO of P. lacunosa (Zone NN20/NN19; 0.44 Ma) in Hole U1352B be placed between 129.77 and 130.34 m (Table T4).

Impact of new isotopic measurements

The measured interval between 123 and 151 m includes our revised depth of the HO of P. lacunosa (131.98 m). Oxygen isotope ratios were relatively high throughout this interval in Hoyanagi et al. (2014). In contrast, our new measurements resolve two additional positive peaks and one new negative excursion within this interval (Fig. F2). However, the new measurements reveal no new peaks outside this interval.

Although the average sedimentation rate in the upper 500 m of Hole U1352B is 28 cm/ky, a lower rate (17 cm/ky) is estimated for the interval between the HO of P. lacunosa and the lowest occurrence (LO) of Emiliania huxleyi (121.1 m). Hoyanagi et al. (2014) measured isotopic ratios at 1.5 m intervals. This interval represents ~5400 y with a sedimentation rate of 28 cm/ky but ~8800 y with a rate of 17 cm/ky. Hoyanagi et al. (2014) could therefore have missed several oxygen isotopic ratio peaks in the interval where the sedimentation rate is low. The addition of 41 new measurements in the key 123–151 m interval to the original 58 measurements of Hoyanagi et al. (2014) increases measurement resolution to ~3000 y with a 17 cm/ky sedimentation rate and led to resolution of the previously undetected peaks.

Comparison of oxygen and stable carbon isotopic ratios for N. flemingi and U. perigrina is shown in Figure F2. The oxygen isotope ratios of N. flemingi have a strong correlation with those of U. perigrina (Fig. F3). The relationship between oxygen isotopic ratios of N. flemingi (Y) and U. perigrina (X) is expressed by the equation Y = X + 0.27. Therefore, oxygen isotope ratios of N. flemingi and U. perigrina can essentially be used interchangeably. The stable carbon isotopic ratios of the two species also show a positive correlation (Fig. F4). Because U. perigrina is considered to have the same oxygen and stable carbon isotopic ratios as the surrounding seawater (Shackleton and Hall, 1984), N. flemingi must also have the same or very similar oxygen and carbon isotopic ratios as those of ambient seawater and can therefore be correlated with the LR04 stack, which is based on U. perigrina.

Correlation with the LR04 stack and revised age model

The HO of P. lacunosa marks the top of Zone NN19 (Martini, 1971) and the top of Subzone CN14a (Okada and Bukry, 1980). Previous studies have demonstrated the worldwide synchroneity of this datum, which corresponds to MIS 12 (e.g., Thierstein et al., 1977; Wei, 1993; Raffi et al., 2006). Our new oxygen isotope measurements show new positive peaks at ~130 m (δ¹⁸O = 4.023‰ at 129.90 m) and ~137 m (δ¹⁸O = 3.825‰ at 137.08 m) and a new negative peak at ~135 m (δ¹⁸O = 3.059‰ at 134.84 m) (Fig. F2; Table T2). Using our revised HO of P. lacunosa (0.44 Ma) at 131.98 m (Table T4), we correlate these new positive peaks with MIS 12 and 14 and the negative peak with MIS 13. Our new correlations show a clear positive peak in δ¹⁸O at the HO of P. lacunosa that is concordant with the timescale of Lourens et al. (2004).

Expedition 317 Scientists suggested that a hiatus from 1.8 to 2.7 Ma lies somewhere between 491.74 and 525.34 m. Therefore, we use the eight Pleistocene nannofossil datum levels shallower than 491.74 m (Table T4; see the “Site U1352” chapter [Expedition 317 Scientists, 2011b]) to correlate the δ¹⁸O record (Fig. F5A) with the global benthic foraminiferal δ¹⁸O LR04 stack of Lisiecki and Raymo (2005) (Fig. F5B) and to plot the Expedition 317 δ¹⁸O record versus age from 1.8 Ma to present.

We made revised correlations relative to Hoyanagi et al. (2014) for the intervals between MIS 11 and 17 and between MIS 25 and 29 (Fig. F5C). N. flemingi tests were present in the interval between 62 and 66 m but were not present between 10.7 and 62 m. Therefore, we cannot correlate our record to MIS 4 of the LR04 stack. Other correlations of Hoyanagi et al. (2014) between the Hole U1352B oxygen isotope record and the LR04 stack were left unchanged (Fig. F5).

Figure F6 shows the sedimentation rates (age model) for Hole U1352B. The ages within the intervals between 115 and 215 m and between 255 and 305 m are shifted relative to the ages in Hoyanagi et al. (2014). Maximum age differences between the two models is ~0.2 my for the same depth. The new age model (Fig. F6) indicates that the average sedimentation rate in the upper ~500 m of Hole U1352B is ~28 cm/1000 y. However, sedimentation rates fluctuate; the maximum rate is ~76 cm/1000 y between 0.8 and 0.7 Ma and the minimum rate is ~3 cm/1000 y between 0.9 and 0.8 Ma.

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