Drift of an ocean bottom electromagnetometer from the Bonin to Ryukyu Islands: estimation of the path and travel time by numerical tracking experiments

Ocean bottom electromagnetometers (OBEMs) installed on the seafloor around Nishinoshima Island (Bonin Islands) were missing after a December volcanic eruption. In February 2021, one was found on a beach on Iriomote Island (Ryukyu Islands), implying that it drifted westward for 1700 km. The reason(s) for the disappearance of the OBEMs and the path followed by the recovered OBEM while drifting are important information for future ocean bottom observations and seafloor volcanology in general. We conducted particle drifting simulations with and without horizonal eddy diffusion to estimate the possible drift path and duration of the recovered OBEM. Our simulations show that particles arriving at Iriomote Island have a 7–10% probability of having been transported from Nishinoshima; thus, such transport is not a rare occurrence. Transport durations in our simulations varied widely between 140 and 602 days depending on the drift paths. More detailed insight into the path and duration of drift of the OBEM will require further comparison between drifting simulations and growth histories of barnacles attached on the OBEM. A similar drift duration and path was reported for pumices that erupted from Fukutoku-Oka-no-Ba submarine volcano (southern Bonin Islands) during 18–21 January 1986 and arrived in the Ryukyu Islands in late May 1986. Such drifting simulations may prove useful for identifying the sources of drift pumices, and thus otherwise undetectable eruptions. Finally, the Fukutoku-Oka-no-Ba submarine volcano erupted on 13 August 2021, producing abundant pumice rafts that, based on our results, would likely arrive in the Ryukyu Islands. In fact, the beginning of October 2021, they began to arrive in the Ryukyu Islands. Graphical Abstract

Triangulation supports agricultural spread of the Transeurasian languages

The origin and early dispersal of speakers of Transeurasian languages—that is, Japanese, Korean, Tungusic, Mongolic and Turkic—is among the most disputed issues of Eurasian population history1–3. A key problem is the relationship between linguistic dispersals, agricultural expansions and population movements4,5. Here we address this question by ‘triangulating’ genetics, archaeology and linguistics in a unified perspective. We report wide-ranging datasets from these disciplines, including a comprehensive Transeurasian agropastoral and basic vocabulary; an archaeological database of 255 Neolithic–Bronze Age sites from Northeast Asia; and a collection of ancient genomes from Korea, the Ryukyu islands and early cereal farmers in Japan, complementing previously published genomes from East Asia. Challenging the traditional ‘pastoralist hypothesis’6–8, we show that the common ancestry and primary dispersals of Transeurasian languages can be traced back to the first farmers moving across Northeast Asia from the Early Neolithic onwards, but that this shared heritage has been masked by extensive cultural interaction since the Bronze Age. As well as marking considerable progress in the three individual disciplines, by combining their converging evidence we show that the early spread of Transeurasian speakers was driven by agriculture.

Last glacial temperature reconstructions using coupled isotopic analyses of fossil snails and stalagmites from archaeological caves in Okinawa, Japan

We applied a new geoarchaeological method with two carbonate archives, which are fossil snails from Sakitari Cave and stalagmites from Gyokusen Cave, on Okinawa Island, Japan, to reconstruct surface air temperature changes over the northwestern Pacific since the last glacial period. Oxygen isotope ratios (δ18O) of modern and fossil freshwater snail shells were determined to infer seasonal temperature variations. The observational and analytical data confirm that δ18O values of fluid inclusion waters in the stalagmite can be regarded as those of spring waters at the sites where snails lived. Our results indicate that the annual mean, summer, and winter air temperatures were lower by 6–7 °C at ca. 23 thousand years ago (ka) and 4–5 °C at ca. 16–13 ka than those of the present day. Our reconstruction implies that surface air cooling was possibly two times greater than that of seawater around the Ryukyu Islands during the Last Glacial Maximum, which potentially enhanced the development of the East Asian summer monsoon during the last deglaciation. Considering the potential uncertainties in the temperature estimations, the climatic interpretations of this study are not necessarily definitive due to the limited number of samples. Nevertheless, our new geoarchaeological approach using coupled δ18O determinations of fossil snails and stalagmite fluid inclusion waters will be useful for reconstructing snapshots of seasonally resolved time series of air temperatures during the Quaternary.

Ocean Bottom Electromagnetometer Carried From Bonin to Ryukyu Islands by Sea Currents

Ocean bottom electromagnetometers (OBEMs) installed on the seafloor around Nishinoshima Island (Bonin Islands) were missing after a December volcanic eruption. In February 2021, one was found on a beach on Iriomote Island (Ryukyu Islands), implying that it drifted westward for 1,700 km. The reason(s) for the disappearance of the OBEMs and the path followed by the recovered OBEM while drifting are important information for future ocean bottom observations and seafloor volcanology in general. We conducted particle drifting simulations with and without horizonal eddy diffusion to estimate the possible drift path and duration of the recovered OBEM. Our simulations show that particles transported from Nishinoshima have a 7-10 % probability of arriving at Iriomote Island, which is thus not a rare occurrence. Transport durations in our simulations varied widely between 140 and 602 days depending on the drift paths. The most likely drift duration in our simulation was 150 – 180 days, with or without eddy diffusion, corresponding to the release from the seafloor of the OBEM between 22 August and 21 September 2020. These dates follow shortly after intensifying eruptions at Nishinoshima, which may have affected the seafloor around the island. A similar drift duration and path was reported for pumices that erupted from Fukutoku-Oka-no-Ba submarine volcano (northern Bonin Islands) during 18-21 January 1986 and arrived in the Ryukyu Islands in late May 1986. Such drifting simulations may prove useful for identifying the sources of drift pumices, and thus otherwise undetectable eruptions. Finally, the Fukutoku-Oka-no-Ba submarine volcano erupted on 13 August 2021, producing abundant pumice rafts that, based on our results, will likely arrive in the Ryukyu Islands in the coming months.

Callogobius falx, a new species of goby from southern Japan

Callogobius falx n. sp. (Gobiidae) is described on the basis of two specimens (14.4 and 23.1 mm in standard length: SL) from the Ryukyu Islands, southern Japan. The new species is characterized by the following combination of characters: dorsal-fin rays VI-I, 8 or 9; anal-fin rays I, 7; pectoral-fin rays 18 or 19; lateral scale rows 20–22; body depth moderate, neither particularly slender nor deep (depth at pelvic-fin origin 18.3–20.0% of SL); pelvic-fin base with frenum; caudal-fin length moderate, margin rounded; cephalic sensory-canal pores absent; interorbital papillae row separated, forming two rows (one row in 14.4 mm paratype); postorbital papillae row not continuous with upper cranial papillae row; preopercular and transverse opercular papillae rows connected, forming single transverse row; body with three oblique black bars, angled upwards anteriorly and continuous with black bars on dorsal fins, anteriormost bar extending from 1st dorsal fin to midline of body, posterior two bars extending from second dorsal fin to dorsal surface of body and caudal peduncle, respectively; pectoral fin white, upper half widely (ca. 1/2 length of rays) margined with black; caudal-fin base with narrow sickle-like bar.

Redescription of the brackish water crab Ptychognathus glaber Stimpson, 1858 based on the topotypic material from the Ogasawara Islands and new record of P. lipkei N.K. Ng, 2010 from Japan (Decapoda: Brachyura: Varunidae)

Examination of material from various Japanese localities clarified that the estuarine crab Ptychognathus glaber Stimpson, 1858 is possibly restricted to the Ogasawara Islands, Japan. Specimens from the Ryukyu Islands, thought to represent P. glaber, actually represent P. lipkei N.K. Ng, 2010 instead, previously known only from Cebu, the Philippines. Ptychognathus glaber is redescribed in detail, and differentiating characters between it and P. lipkei are reassessed. Previous literature records of P. glaber are reviewed in the light of our findings. Genetic analysis using partial sequences of the mitochondrial 16S rRNA gene suggests that P. glaber is genetically closer to P. ishii Sakai, 1939 than to P. lipkei, and thus P. ishii is also compared morphologically with P. glaber.

Higher Genetic Diversity of the Common Sea Cucumber Holothuria (Halodeima) atra in Marine Protected Areas of the Central and Southern Ryukyu Islands

Recently, sea cucumbers (Echinodermata: Holothuroidea) have been over-exploited in many areas of the world, including in the Ryukyu Islands, southern Japan, due to increases in their economic importance. Nevertheless, management and protection of sea cucumbers are insufficient worldwide. The black sea cucumber Holothuria (Halodeima) atra Jaeger, 1833, inhabits a large range across the Indo-West Pacific Ocean and is a widely harvested species. Here we conducted population genetic analyses on H. atra using partial mitochondrial DNA sequences of cytochrome c oxidase subunit I (COI) and 16S ribosomal RNA (16S) to examine 11 different populations around three island groups in the middle Ryukyus; Okinawajima Island, the Kerama Islands, and the Sakishima Islands, all within Okinawa Prefecture. We found 27 haplotypes for COI and 16 haplotypes for 16S. Locations within national and quasi-national parks (Zamami Island, Keramas, and Manza, Okinawajima; managed by the national Ministry of Environment and Okinawa Prefecture, respectively) had the highest number of haplotypes, whereas locations with less management and more anthropogenic pressure had lower numbers The mean of all samples' genetic diversity indices was moderate with regards to both haplotype and nucleotide diversity. According to our results, Zamami Ama was the most genetically diverse location based on both markers used, likely because it is located within Kerama-Shoto National Park with comparatively stricter regulations than most other locations. Based on our COI sequences, three-quarters of the locations with the highest haplotype diversity were found to be distant from Okinawajima Island, indicating that the genetic diversity of H. atra was reduced around Okinawajima Island. Our results possibly reflect negative impacts from anthropogenic pressures such as over-harvesting and coastal development, although future comprehensive research including sequences of nuclear loci is needed to confirm this hypothesis.

Characteristics of seismicity in the southern Okinawa Trough and their relation to back-arc rifting processes

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region.