Tephrochronology
Based on the geochemical composition of the sampled tephras at C0018 and comparison with on-land tephras, the ages of the tephras were determined (Fig. 6; Table 3). Tephras Ata-Th (0.238 Ma; Kuwae et al. 2002; Nagahashi et al. 2004), Aso-1 (0.249 Ma; Kuwae et al. 2002; Nagahashi et al. 2004), BT72 (0.349 Ma; Kuwae et al. 2002; Nagahashi et al. 2004), and Hkd-Ku (0.76 Ma; Nagahashi et al. 2015; Suzuki et al. 2005) occur at depths of 24.9, 29.2, 57.1, and 114.3 m, respectively, (22.02, 26.4, 46.9, and 77.9 mf MTD corrected depth, respectively).
δ18O isotope stratigraphy
Oxygen isotope values of C0018 for the studied sequence (upper 200 m of C0018A) range between −0.31 and 2.39‰. While the uppermost 75 m of C0018A shows high variability in δ18O data, with values between minima of around 1‰ and maxima of 2.5‰, the lowermost 100 m display a more regular pattern (Fig. 2). For the lower part, a comparison of the C0018A δ18O dataset with the stack reference curves is not straightforward because of the difference in δ18O values between the planktonic and benthic foraminifera datasets. However, we observe broadly similar patterns between the two curves and therefore compare the trends between warm and cold phases.
Age–depth model and dating of MTDs
The final age–depth model of C0018A is based on 14C age data, tephrochronology, and isotope stratigraphy. The matching points (X1–X4) are dated at 125, 135, 328, and 400 ka on the LR04 reference curve, respectively (Fig. 7, before matching). The matching between peaks of C0018A and LR04 is based on visual correlation with uncertainties of ~5000 years due primarily to the sampling interval in C0018A. The uncertainties related to the tephrochronology can be up to several tens of thousands of years (e.g., Hkd-Ku; Suzuki et al. 2005). To take these uncertainties into account, we introduced an average error of ~2000 years for the tephras (Table 4). A comparison between the reference datasets of Lisiecki and Raymo (2005) and Bassinot et al. (1994) shows several offsets in the isotope stratigraphy.
The ages of the MTDs can be extracted from the age–depth model. MTD 1 is essentially based on the results of radiocarbon dating and is dated to 13.0–14.2 cal kyr BP (Fig. 5).
The ages of MTDs 2, 3, 4, and 5 are based on tephrochronology and isotope stratigraphy and are dated at 323–339, 372–384, 394–413, and 508–521 kyr, respectively (Fig. 8). The age of MTD 6 is based on sedimentation rates calculated between Hkd-Ku and Ss-Az and yield an age of 857–867 kyr.
Furthermore, MTD 1, which is dated at 13.0–14.2 kyr cal BP, can be placed within a phase of decreasing δ18O values (Fig. 7). The onset of the termination of the last glacial cycle has been dated at ~16.5 kyr BP in the Intermediate Pacific (Stern and Lisiecki 2014). Thus, MTD 1 can be placed within Marine Isotope Stage 1 (MIS 1) (Fig. 7). The decreasing trend in δ18O values above MTD 1 (from 4.7 to 3.2‰) correlates with the trend in δ18O values of the Intermediate Pacific (Stern and Lisiecki 2014). Thus, MTD 1 can be placed within the warming trend of MIS 1 (increases in temperature and sea level), and it occurred ~2000–3000 years after the onset of MIS 1. Similarly, MTDs 2 and 6 can be placed within the warming trends of MIS 9 and 21, respectively. MTD 5 can be placed within the phase of decreasing δ18O values of MIS 13, while MTDs 3 and 4 are also placed within an interglacial (MIS 11), albeit not within the transition to heavier δ18O values. Although uncertainties of around 10,000–20,000 years exist, the dating of C0018A provides a better constraint on the timing of the MTD. Furthermore, the use of the isotope stratigraphy illustrates that the MTDs are all placed within odd-numbered marine isotope stages, thus raising the possibility of a climatic influence on the preconditioning of MTD-related slope failures.
Climate as a preconditioning factor?
The data of the present study show that the MTDs recorded at site C0018 in the slope basin of the Nankai accretionary prism occur within interglacials, suggesting that the MTDs were emplaced during phases of relative high sea level and temperatures. This implicates the role of climate as a primary preconditioning factor for slope failures in this highly active tectonic region. If we assume the result to be representative, it is necessary to discuss this possible relationship. As numerous articles have discussed the relationship between climate and submarine landslides (e.g., Owen et al. 2007; Maslin et al. 2010), here, we consider the climate-influenced factors (e.g., sea level, water temperature, and related factors such as gas hydrate destabilization and sedimentation rates) that could precondition slope failures and only briefly discuss how these factors may be applicable in the studied setting. The discussion will mainly focus on the example of MTD 1 in the context of the last interglacial cycle.
Sea level changes
Rising sea level during interglacial periods results in enhanced water loading on continental margins and overpressure in sediments. This has been suggested as a trigger for the Holocene Storegga Slide and tsunami (e.g., Smith et al. 2013). The timing of MTD 1 coincides with a concentration of passive margin deposits aged between 8 and 15 kyr (Brothers et al. 2013). Following Brothers et al. (2013), the eustatic sea level rise during this phase had the potential to cause bending stresses that could lead to enhanced seismicity, thereby establishing a direct relationship between sea level rise and slope failure on passive margins. However, Urlaub et al. (2013) instead showed that the ages of landslides on passive margins can be described by a temporally random Poisson distribution and therefore do not show a positive correlation between the frequency of major slope failures on passive margins and sea level during the past 180,000 years. Thus, it would be speculative to state that glacial-to-interglacial sea-level variations caused the slope failures of the present study.
Temperature and gas hydrates
Changes in water temperature may alter the stability of gas hydrates. Furthermore, large amounts of gas hydrates have been detected in the present study region (Bangs et al. 2010) and at drilling site C0008, located directly in the upslope region of our studied MTD site (Kinoshita et al. 2011; Fig. 1), as indicated by the presence of a bottom-simulating reflector (BSR) in seismic data (Fig. 2) and chemical anomalies in the borehole (Kinoshita et al. 2011), respectively. Most of the 250–350 m thick sedimentary succession overlying the accretionary prism is currently located within the stability zone of gas hydrate. However, during glacial terminations, bottom-water temperature can increase by as much as 2 °C, as documented in the deep waters of the western Pacific (Siddall et al. 2010). Indeed, a theoretical methane hydrate stability calculation (Sloan and Koh 2008), assuming a 2 °C increase in bottom-water temperature, diffusive heat transport through sediments, and an average heat flow 60 mW/m2 (Yamano et al. 2014), indicates that changes in bottom-water temperatures can result in an upward shift of the gas hydrate stability zone by up to 11 m over 10,000 years. This may result in hydrate dissociation, producing weak zones within sediments and therefore initiating or preconditioning slope failure (Sultan et al. 2004). In contrast, increasing pressure at the seafloor due to rising sea level would shift the base of the hydrate stability zone downward.
The Kuroshio Current off Japan is one of the strongest warm currents in the northwest Pacific Ocean (Iwatani et al. 2016; Fig. 1), as it transports heat northward from the tropics. The Kuroshio Current might therefore play an important role in increasing bottom-water temperatures in the study region. However, sea surface temperature (SST) reconstructions have shown that the area south of Japan was occupied by cold water masses during the period between the LGM and the start of the Holocene (Ikehara et al. 2011 and references therein), associated principally with a southward shift of the Kuroshio path during the last glaciation–deglaciation (Sawada and Handa 1998). Thus, sea surface temperatures were lower at the time of MTD 1.
Sedimentation rates
During glaciations and associated relative sea-level lowstands, sediment delivery to slopes is expected to be higher, thus possibly causing unstable slopes (Brothers et al. 2013). IODP drill site C0018 is separated from the shore, shelf, and upper slope areas by the morphologic high of the megasplay fault and only receives remobilized sediments from slopes that are susceptible to fail at water depths of 2000–2500 m (Figs. 1 and 2). There is no direct sediment source into the basin and the main depocenters nearby are located in the Kumano basin and downslope in the Nankai Trough (Buchs et al. 2015; Usman et al. 2014). There is possibly sediment input from the NNE, from the Tenyu or Fuji rivers (Usman et al. 2014), but as the sediments deposited in the studied basin are very fine grained, this represents only the very distal to pelagic part of the supplied sediment. The sedimentation rates at C0018A determined in this study vary by ~100 cm/kyr (which is the same order of magnitude as estimated by Henry et al. 2012). A significant increase in sedimentation rate is only observed before MTDs 3 and 4 of up to 300 cm/kyr. Thus, although the basin is isolated, we cannot exclude the possibility that sediment availability does not influence MTD occurrence. However, the exact timing of this change is not well constrained by the age model, as the sedimentation rate is based on linear interpolation between neighboring dating points.
Overall, this discussion has shown that the data are not sufficient to explicitly determine how climate is affecting slope stability. In particular, MTDs at C0018A do not coincide with every interglacial during the past 1 Ma. Thus, further studies are needed to better understand the preconditioning of mass movements.
Climate vs. local effects vs. tectonic factors
As mentioned above, MTDs at C0018 do not coincide with every interglacial during the past 1 Ma. However, we have to consider that C0018 does not record all MTDs of the slope basin, as the source and deposition of MTDs might vary through time (e.g., Yamada et al. 2010). Thus, C0018 may not be fully representative of the entire basin. However, as the 3D reflection seismic data in the region around C0018 do not image any other thick MTDs (Strasser et al. 2011), C0018 might therefore be representative in terms of its ability to capture the largest MTD events, although we cannot exclude the potential for lower-volume, higher-frequency MTDs that cannot be resolved in the seismic data. If we assume that, at least for the large landslides, C0018 is representative of the entire basin, the data suggest that MTDs within odd-numbered marine isotopic stages occur during three phases (MTD 1 at ~13–14.2 cal kyr, MTDs 2–5 between 0.3 and 0.5 Ma, and MTD 6 at 0.86 Ma). The first MTD (MTD 6) occurs close to the end of a period of major tectonic uplift of the forearc high between 0.9 and 1.3 Ma (Gulick et al. 2010). This uplift resulted in slope steepening, thereby creating the slopes necessary for the initiation of slope failure and perhaps explaining the onset of mass transport deposition at ~0.86 Ma with MTD 6. For the period around 0.5 Ma, as inferred from the reconstructed flow directions of MTDs and bathymetric analyses (Kanamatsu et al. 2014), seamount subduction could have affected the study area (Fig. 1; Kimura et al. 2011) and resulted in a change in slope orientation from NE–SW to NW–SE (the present slope). This seamount subduction might also have been responsible for the occurrence of MTDs between 0.3 and 0.5 Ma. For the youngest MTD phase (MTD 1), which begun with the end of the last glacial, no plausible tectonic activity has yet been described, but we speculate that it may relate to the current phase of recent megasplay fault activity (Moore et al. 2007). Importantly, the reconstructed tectonic uplift phases (before 0.9 Ma and at ~0.5 Ma) would also have affected the gas hydrate stability zone and may therefore have preconditioned the slopes, making them more prone to submarine landsliding.