Circumstances of the OBEMs on the seafloor and the OBEM drift duration
JM4 and JM6 were not recoverable by conventional means in June 2019 (see “Installation, loss, and recovery of the OBEM” section). There are three possible explanations for why OBEMs JM4 and JM6 did not ascend from the seafloor in June 2019: (1) the release systems did not work, even though they responded normally to the release commands; (2) the weights may have been released, but the glass spheres containing the fluxgate sensors and recorders may have been flooded, preventing the buoyant rise of the OBEMs; or (3) the weights may have been released, but the OBEMs were stuck in sediments and could not ascend. JM4 and JM6 moved about 23.3 m and 58.6 m, respectively, downslope along the seafloor between September 2018 and June 2019 (Fig. 4). In contrast, the other four recovered OBEMs, which were not installed between JM4 and JM6 (Fig. 1c), had moved only 9–15 m. These large differences imply that external forces moved JM4 and JM6. In addition, JM4 was found without its weight and with its two glass spheres still under vacuum and retaining buoyancy. Thus, we reject possibilities (1) and (2) and conclude that JM4 had become stuck in sediments after it moved. Unfortunately, JM6 has not yet been found and we cannot say whether it remains on the seafloor or is adrift in the ocean.
To determine why JM4 got stuck and was later released from the sediments, it is necessary to constrain when JM4 rose off the seafloor. JM4 was confirmed to be on the seafloor on 11 June 2019, and it was found on Iriomote Island on 18 February 2021, 618 days later. However, its data log indicates that it was on the seafloor until at least 24 August 2019 (Fig. 4), when its sensor’s batteries became completely exhausted, because the temperature time series remained at ~ 1.0 °C from the end of February 2019 through the end of August 2019. If JM4 had risen to the surface during this period, the temperature would have increased because ocean temperature increases at shallower depths. Therefore, JM4 must have remained on the seafloor until at least 24 August 2019. Unfortunately, the tiltmeters failed around the end of February 2019, so we cannot use tilt data to confirm this inference. From the recorded data, we can determine only that the longest possible duration of drift for JM4 was 544 days.
In addition to finding JM4 on Iriomote Island, our simulation results with and without horizontal eddy diffusion indicate that 7–10% of the particles around Iriomote Island on 18 February 2021 were transported from around Nishinoshima within the preceding 2 years. Thus, we suggest that sea-surface transport from the Bonin Islands to the Ryukyu Islands and Taiwan is not a rare occurrence. In addition, the shortest transport time we obtained (140 days) is consistent with the lengths of barnacles attached to JM4, which must have been attached for at least 50 days before its retrieval. However, the simulated transport durations varied widely between 140 and 602 days depending on the drift path. Indeed, Nishinoshima and Iriomote Island are situated in the North Pacific Subtropical Gyre, where mesoscale eddy activity is relatively strong (Aoki and Imawaki 1996); a particle transported from Nishinoshima can arrive at Iriomote Island in half a year if it is carried by a relatively strong sea current, whereas one trapped in mesoscale eddies will take much longer. More detailed insight into the path and duration of drift of JM4 will require further comparison between drifting simulations and barnacle growth histories.
Based on our simulations both with and without eddy diffusion, Fig. 3c shows a peak in the 150–180 days bin, in which 2.6% and 3.5% of particles with and without eddy diffusion were achieved, respectively. These transport durations correspond to JM4 rising from the seafloor between 22 August and 21 September 2020. Because Nishinoshima began an eruptive period on 5 December 2019 that ceased at the end of August 2020 (Yanagisawa et al. 2020; Fig. 4), the peak is not consistent with the duration of this volcanic activity. By analyzing the observed data, Baba et al. (2020) inferred that one of five OBEMs deployed between October 2016 and December 2016 moved during the deployment period, which was a quiet period between Nishinoshima eruptions in 2015 and in 2017. The other four OBEMs did not move, but the time series data of the three-component magnetic field and their instrumental tilts showed significant variations in the middle of November (Baba et al. 2020). They were not able to find related volcanic activities on Nishinoshima Island, so it is reasonable to consider the possibility of underwater activity (e.g., underwater volcanic activity or a slope collapse).
On the other hand, if the 180-day drift time corresponds to the release on 22 August 2020, the 180–441 days bins correspond to the release between 5 December 2019 and 21 August 2020, during the volcanic activity. The particles achieved in the 180–441 days bins account for 3.6% and 7.5% of the total number with and without eddy diffusion, respectively, which are larger than those in the 150–180 days bin. Indeed, Yanagisawa et al. (2020) observed a lava flow entering the sea on 20 July 2020 although there is no direct evidence such a lava flow made an OBEM move or release. In the future, to understand both above and under the sea surface volcanic activities, volcanic islands and submarine volcanoes should be studied by using underwater, onshore, airborne, and satellite instruments.
Implications for drifting pumices in the Ryukyu Islands
Just as JM4 was found on the beach on Iriomote Island, many pumices are found on beaches in the Ryukyu Islands (e.g., Kato 1980, 1988; Nakano and Kawanabe 1992). Kato (1988) reported that many pumices had been stranded on the Ryukyu Islands since late May 1986. Based on the mineralogy and geochemistry of the pumices, he concluded that they were effused during the 18–21 January 1986 eruption of Fukutoku-Oka-no-Ba (Fig. 1), thus indicating a distance and duration of drifting similar to those determined for OBEM JM4. This similarity indicates that in both cases, drifting was mainly affected by the westward current of the Kuroshio recirculation (Fig. 5). In addition to the dominant westward flow in the area between the Bonin and Ryukyu Islands, mesoscale eddies and the eastward-flowing subtropical counter current (STCC) may sometimes affect the drift paths and transport times of pumice (Fig. 5). In our experiments, some particles were trapped in eddies or transported eastward from Nishinoshima for a certain period. On the other hand, depending on the location of mesoscale eddies and the STCC while it was drifting, the OBEM could have reached the Ryukyu Islands more quickly.
It is generally difficult to specify the origin of drift pumices. For example, the origins of most drift pumices on Iriomote Island cannot be identified, although one type has been correlated to Fukutoku-Oka-no-Ba based on color, clast size, mineralogy, and geochemistry (Nakano and Kawanabe 1992). Our drifting simulation might therefore be used to quantitatively evaluate possible drift pumice origins. The tracking paths obtained in our simulation (Fig. 4a) reach Taiwan, the Philippines, and the Mariana Islands, which were mentioned as possible pumice origins by Nakano and Kawanabe (1992). Moreover, such drifting simulations might be useful in constraining the duration of volcanic activities and detecting otherwise undetectable eruptions of submarine volcanoes. Nakano and Kawanabe (1992) also reported that marine organisms were attached to drift pumices. As we have demonstrated, integrating biomarker analyses with drifting simulations can further narrow the possible origins and drift paths/durations of pumices.
On 13 August 2021, a submarine eruption occurred at Fukutoku-Oka-no-Ba, and a new horseshoe-shaped island 1 km in diameter was observed on 16 August. Aerial photographs and satellite images show numerous pumice rafts drifting to the northwest (Japan Meteorological Agency 2021a). Pumice rafts are known to cause various hazards, such as blocking harbors, ports, and marine traffic, and damaging hulls and propellers (e.g., Oppenheimer 2003; Jutzeler et al. 2014). Drifting simulations could also be used to predict possible paths of pumice rafts, and thus potential risks (Jutzeler et al. 2014, 2020). Indeed, Jutzeler et al. (2020) recently tried to predict the ongoing dispersal of pumice rafts from the Tonga Arc (southwestern Pacific Ocean) using the latest-monitored ocean current data for forecast simulations.
We stress, however, that the drifting simulation presented here does not provide a forecast; it is a hindcasting simulation. In addition, though the wind effect was included as Ekman transport (Talley et al. 2011) in the velocity field that we used, the simulation did not consider the effect of direct wind drag on drifting pumice. Nonetheless, these simulation results and observational evidence that many pumices from Fukutoku-Oka-no-Ba, as well as the JM4 OBEM from Nishinoshima, arrived in the Ryukyu Islands suggest that rafts of pumice erupted in August 2021 should move westward and drift ashore in the Ryukyu Islands in several months.
In fact, in the beginning of October 2021, pumice rafts from the 2021 eruption of Fukutoku-Oka-no-Ba began to arrive in the Ryukyu Islands, where they have greatly interfered with the operation of ships and harbor functions, as reported by various social media outlets and news media. There is also concern that pumice rafts carried by the Kuroshio current will drift onto the coasts of the Honshu arc in the near future. The drift duration of pumice rafts from the 2021 Fukutoku-Oka-no-Ba eruption that drifted to Okinawa and the Amami Islands in the Ryukyu Islands was about 2 months. This drift duration is much shorter than that following the 1986 Fukutoku-Oka-no-Ba eruption (about 4 months) and also that of the drifting OBEM from Nishinoshima to Iriomote Island (longer than four months) as estimated by our hindcasting simulation.
Potential causes of these large differences are the effects of surface windage and the STCC (Additional file 1: Fig. S3). The force driving drifting pumice depends on sea currents, windage, and waves (Jutzeler et al. 2014, 2020), but our drifting simulation for the OBEM did not take into account the effect of windage. The seasonal difference between the 1986 and 2021 eruptions (which occurred in January and August, respectively) might also have caused the overall direction of the windage to be different; in particular, typhoon No. 16 (Mindulle) in the area the drift path during September 2021 (Japan Meteorological Agency 2021b) might have sped up the westward drifting of the pumices. In addition, mesoscale eddy activity and the STCC, which show drastic seasonal and interannual variation (White et al. 1978; Kazmin and Rienecker 1996; Qiu and Chen 2010), are likely to be other important causes of the large difference in transport duration (Additional file 1: Fig. S3). Detailed investigations of actual observational cases and comparison of their results with numerical simulations of drifting pumices would improve understanding of the effect of windage and small sea currents and make it possible to construct better prediction models.