Kazumasa Oguri

Spatial and temporal variability of surface water in the Kuroshio source region, Pacific Ocean, over the past 21,000 years: evidence from planktonic foraminifera

Abstract The Kuroshio Current is the major western boundary current of the North Pacific Ocean and has had a large impact on surface water character and climate change in the northwestern Pacific region. The Kuroshio Current becomes a distinctive surface flow in the Ryukyu Arc region after diverging from the North Equatorial Current and passing through the Okinawa Trough. Therefore, the Ryukyu Arc area can be called the Kuroshio source region. We reconstructed post-21-ka time–space changes in surface water masses in the Ryukyu Arc region using 15 piston cores which were dated by planktonic δ 18 O stratigraphy and AMS 14 C ages. Our analysis utilized spatial and temporal changes in planktonic foraminiferal assemblages which were classified into the Kuroshio, Subtropical, Coastal, and Cold water groups on the basis of modern faunal distributions in the study region. These results indicate that the Kuroshio Current and adjacent surface water masses experienced major changes during: (1) the Last Glacial Maximum (LGM), and (2) the so-called Pulleniatina minimum event (PME) from ∼4,500 to 3,000 yr BP. The Kuroshio LGM event corresponds to severe global cooling and is marked by decreases in planktonic δ 18 O values and estimated sea-surface temperature (SST) with the dominance of the Cold water group of planktonic foraminifera. Cooling within the Kuroshio source region was enhanced during the LGM event because the Kuroshio Current was forced eastward due to the formation of a land bridge between Taiwan and the southern Ryukyu Arc which prohibited its flow into the Okinawa Trough. Except for the severe reduction and disappearance of the Pulleniatina group, no clear cooling signal was identified during the PME based on δ 18 O values, estimated SST values and variations in the composition of planktonic foraminiferal faunas. The PME assemblages are marked by high abundances of Neogloboquadrina dutertrei , a distinctive Kuroshio type species, along with other species assigned to the Coastal and Central water groups. Subtle ecological differences exist between Pulleniatina obliquiloculata and N. dutertrei ; i.e. P. obliquiloculata exhibits lower rates of reproduction under conditions of lower primary productivity in the central Equatorial Pacific Ocean. El Nino-like conditions in the Equatorial Pacific Ocean result in lower rates of surface transport in the Kuroshio Current. In turn, this response triggers lower rates of primary productivity in central equatorial surface waters as well as in the upstream Kuroshio source region, ultimately resulting in a lower abundance of P. obliquiloculata . Thus, we interpret the PME as a possible proxy signal of El Nino-like conditions and enhancement of the El Nino Southern Oscillation climate system after the PME in the tropical and sub-tropical Pacific Ocean.

Hadal disturbance in the Japan Trench induced by the 2011 Tohoku–Oki Earthquake

In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock.

Generation of Electricity and Illumination by an Environmental Fuel Cell in Deep‐Sea Hydrothermal Vents

Public interest in the generation of power by alternative energy sources in the ocean beyond fossil fuels and nuclear energy has increased in recent years. Power generation in the ocean is also of great interest for the inexpensive and efficient supply of electricity for the survey and exploration of submarine resources. Deep-sea hydrothermal vents are environments that discharge crustal hydrothermal fluids, geologically driven by magmatism and geochemically processed by the high temperatures of rock–seawater interactions and alterations. Hydrothermal fluids enriched with reduced chemicals are mixed with oxidative chemicals in seawater in the vicinity of deep-sea vents. The steep chemical slope between the hydrothermal fluids and ambient seawater has the potential to generate electricity. Therefore, it is likely that electricity is generated in situ between hydrothermal fluids (HF) and seawater (SW), which can be promoted and confirmed by deployment of artificial electrodes and a conductor. Herein, we show the development of an HF–SW fuel cell for deep-sea hydrothermal vents. During the Integrated Ocean Drilling Program (IODP) in 2010, several artificial hydrothermal vents were created by the drilling of wells in the Iheya North hydrothermal field of the Okinawa Trough, Japan. One of the artificial hydrothermal vents (C0014G) had a vigorous discharge with a high fluid temperature (Tmax= 309 8C) at a water depth of 1053 m. In this work, seafloor electrochemical analyses were conducted in the C0014G vent using a remotely operated vehicle (ROV) equipped with a deep-sea potentiostat/galvanostat system (D-Pote), which was covered with an anti-pressure housing and controlled by an onboard computer through data communication. First, the oxidation–reduction potential (ORP) of the ambient seawater and hydrothermal fluids was measured (Figure 1). The average temperature and ORP of the ambient seawater were approximately 4 8C and + 478 mV, respectively, versus the standard hydrogen electrode (SHE), while those of the hydrothermal fluids were approximately 309 8C

Unexpected biotic resilience on the Japanese seafloor caused by the 2011 Tohoku-Oki tsunami

On March 11th, 2011 the Mw 9.0 2011 Tōhoku-Oki earthquake resulted in a tsunami which caused major devastation in coastal areas. Along the Japanese NE coast, tsunami waves reached maximum run-ups of 40 m, and travelled kilometers inland. Whereas devastation was clearly visible on land, underwater impact is much more difficult to assess. Here, we report unexpected results obtained during a research cruise targeting the seafloor off Shimokita (NE Japan), shortly (five months) after the disaster. The geography of the studied area is characterized by smooth coastline and a gradually descending shelf slope. Although high-energy tsunami waves caused major sediment reworking in shallow-water environments, investigated shelf ecosystems were characterized by surprisingly high benthic diversity and showed no evidence of mass mortality. Conversely, just beyond the shelf break, the benthic ecosystem was dominated by a low-diversity, opportunistic fauna indicating ongoing colonization of massive sand-bed deposits.

High rates of benthic microbial activity at 10.900 meters depth: Results from the Challenger Deep (Mariana Trench)

Microbes regulate the decomposition of organic matter in marine sediments. Measurements at the deepest oceanic site on Earth reveal high rates of microbial activity, potentially fuelled by the deposition of organic matter. Microbes control the decomposition of organic matter inmarine sediments. Decomposition, in turn, contributes to oceanic nutrient regeneration and influences the preservation of organic carbon1. Generally, rates of benthic decomposition decline with increasing water depth, although given the vast extent of the abyss, deep-sea sediments are quantitatively important for the global carbon cycle2,3. However, the deepest regions of the ocean have remained virtually unexplored4. Here, we present observations of microbial activity in sediments at Challenger Deep in the Mariana Trench in the central west Pacific, which at almost 11,000 m depth represents the deepest oceanic site on Earth. We used an autonomous micro-profiling system to assess benthic oxygen consumption rates. We show that although the presence of macrofauna is restricted at Challenger Deep, rates of biological consumption of oxygen are high, exceeding rates at a nearby 6,000-m-deep site by a factor of two. Consistently, analyses of sediments collected from the two sites reveal higher concentrations of microbial cells at Challenger Deep. Furthermore, analyses of sediment 210Pb profiles reveal relatively high sediment deposition in the trench. We conclude that the elevated deposition of organic matter at Challenger Deep maintains intensified microbial activity at the extreme pressures that characterize this environment.

In situ measurement of time-series two dimensional O2 distributions at sediment-water interface using a planar O2 optode system connected with a submarine cable

The first long time monitoring of O2 dynamics at sediment-water interface (SWI) was conducted using with a newly developed planar O2 optode system for in situ measurement. The monitoring was carried out connecting the optode system with Hatsushima permanent station with a 100 meter-long extension cable under supporting by the operation of ROV Hyper-dolphin. The monitoring was successfully carried out for 113.3 hours. During the experiment, 3,141 two dimensional O2 profiles and the corresponding pseudo-grayscale images across SWI were obtained, respectively. From the results, further understanding of new insights on dynamic interactions between fauna activities and environmental changes are expected.

Potential technique for improving the survival of victims of tsunamis

We investigated a method for surviving tsunamis that involved the use of personal flotation devices (PFDs). In our work, we succeeded in numerically demonstrating that the heads of all the dummies wearing PFDs remained on the surface and were not dragged underwater after the artificial tsunami wave hit them. In contrast, the heads of all the dummies not wearing PFDs were drawn underwater immediately; these dummies were subsequently entrapped in a vortex. The results of our series of experiments are important as a first step to preventing the tragedies caused by tsunamis.

Polysaccharide hydrolase of the hadal zone amphipods Hirondellea gigas

Abstract Hirondellea species are common inhabitants in the hadal region deeper than 7,000 m. We found that Hirondellea gigas thrived in the Challenger Deep possessed polysaccharide hydrolases as digestive enzymes. To obtain various enzymes of other H. gigas, we captured amphipods from the Japan Trench, and Izu-Ogasawara (Bonin) Trench. A phylogenetic analysis based on the cytochrome oxidase I gene showed close relationships among amphipods, despite the geographic distance between the localities. However, several differences in enzymatic properties were observed in these H. gigas specimens. We also carried out RNA sequencing of H. gigas from the Izu-Ogasawara Trench. The cellulase gene of H. gigas was highly homologous to cellobiohydrolase of Glucosyl Hydrolase family 7 (GH7). On the other hand, enzymatic properties of H. gigas’s cellulase were different from those of typical GH7 cellobiohydrolase. Thus, these results indicate that hadal-zone amphipod can be good candidates as the new enzyme resource. The deepest sea amphipod, Hirondellea gigas digests plant debris with GH7 cellobiohydrolase, GH9 ß-1. 4 glucanase, and amylase to obtain nutrients in oligotrophic sea bottom.