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doi:10.2204/iodp.proc.317.210.2018

Introduction

Hoyanagi et al. (2014) reported oxygen and stable carbon isotope ratios of the benthic foraminifer Nanionella flemingi in cores from Hole U1352B, which was drilled on the upper slope offshore Canterbury Basin (New Zealand) during Integrated Ocean Drilling Program (IODP) Expedition 317. The oxygen isotope record was correlated with the LR04 stack (Lisiecki and Raymo, 2005) with the support of shipboard bioevent ages (see the “Site U1352” chapter [Expedition 317 Scientists, 2011b]) to generate an age model for the upper 500 m of Hole U1352B.

Although the nannofossil bioevent highest occurrence (HO) of Pseudoemiliania lacunosa (0.44 Ma) was used for the age model in Hoyanagi et al. (2014), the following problems have since been identified:

Pseudoemiliania ovata and Pseudoemiliania pacifica were found above the HO of P. lacunosa datum (see the “Site U1352” chapter [Expedition 317 Scientists, 2011b]).

The benthic foraminiferal bioevent, HO of Bolivinita pliozea (0.6 Ma), was placed above the HO of P. lacunosa (0.44 Ma) in Hole U1352B (i.e., the older age datum was placed above the younger age datum). Such a depth reversal for these two bioevent datums was also recognized in Holes U1351B and U1354B.

The HO of P. lacunosa defines the nannofossil Zone NN20/NN19 boundary and coincides with marine isotope Stage (MIS) 12. However, Hoyanagi et al. (2014) did not find a significant positive oxygen isotopic peak, which would be expected at MIS 12, near the core position originally chosen for the HO of P. lacunosa during Expedition 317 (i.e., between 155.99 and 164.18 m; all depths are core depth below seafloor, Method A [CSF-A]) (see the “Site U1352” chapter [Expedition 317 Scientists, 2011b]). Hoyanagi et al. (2014) measured oxygen and carbon isotopes every ~1.5 m in this interval, resulting in samples <5500 y apart, assuming an average sedimentation rate of 28 cm/1000 y (see below). Therefore, we should be able to recognize δ¹⁸O and δ¹³C fluctuations with cycle periods exceeding 11,000 y, which is sufficient to resolve MISs.

Sea level is considered to have risen 135 m from glacial MIS 12 to interglacial MIS 11 (Miller et al., 2005), so the absence of a positive oxygen isotopic ratio peak at the Zone NN20/NN19 boundary in the Hoyanagi et al. (2014) age model is troubling. Therefore, we have reexamined the occurrence of P. lacunosa and have made additional isotopic measurements of benthic foraminifer N. flemingi in Hole U1352B cores. These new data indicate that the shipboard depth of the HO of P. lacunosa should be revised for the following reasons.

Genus Pseudoemiliania was divided into two species, P. lacunosa and P. ovata, by Young (1998). Specimens of P. ovata larger than 6.5 µm were further defined as P. pacifica by de Kaenel et al. (1999). Usually, the HO of P. lacunosa is defined by the last occurrences of the genus Pseudoemiliania (i.e., any of these species), and the HO of P. lacunosa occurs in almost same horizon as the HO of P. ovata (Expedition 315 Scientists, 2009). Expedition 317 Scientists placed the HO of P. lacunosa at the horizon between 155.99 and 164.18 m. However, samples at 141.14 m yield P. ovata and P. pacifica in the cores from Hole U1352B (see the “Site U1352” chapter [Expedition 317 Scientists, 2011b]); the HO of P. lacunosa should therefore be no deeper than 141.14 m.

Reexamination of the depth of the HO of P. lacunosa in Hole U1352B and additional measurements of benthic foraminiferal oxygen and stable carbon isotope ratios presented in this report lead to a revised correlation with the LR04 stack and a new age model for Hole U1352B. Furthermore, Hoyanagi et al. (2014) did not evaluate the vital effect of N. flemingi. We therefore have conducted new measurements of oxygen and stable carbon isotopic ratios of the benthic foraminifer Uvigerina perigrina, which occurs rarely in the cores, to ensure that correlation of isotopic measurements made on N. flemingi with the LR04 stack is justified.

Our new results suggest that the original age model (Hoyanagi et al., 2014) should be modified in the intervals between 115 and 215 m and between 255 and 305 m. The former age model is unchanged outside these intervals. All data presented in Hoyanagi et al. (2014) are duplicated in this report.

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