Foraminifera are adapted to a wide range of environments, and environmental DNA (eDNA) metabarcoding of foraminifera should facilitate development of new environmental indicators. In this study, we used eDNA metabarcoding to evaluate the discrepancy between planktic and benthic foraminifera molecular communities identified in bottom water and short sediment cores. The molecular community was compared to foraminiferal shells in sediment traps set on the seafloor. Samples were collected in June and August around the Takuyo-Daigo Seamount in the western subtropical Pacific Ocean. Approximately 40% of amplicon sequence variants (ASVs) pertained to unknown foraminiferal lineages in sediment samples, compared with only 22% in bottom water. Bottom water contained benthic foraminifera and taxonomically unassigned lineages, which were attributed to resuspended particles. In bottom water, 100 ASVs were assigned to planktic foraminifera. ASVs assigned to Candeina nitida were most abundant and accounted for 36%–86% of planktic foraminiferal ASVs. In sedimentary DNA, Globigerinita glutinata was the most abundant among 33 ASVs of planktic foraminifera. However, transparent shells in sediment traps contained more spinose species, such as Globigerinoides ruber , whereas C. nitida was not found and few G. glutinata were detected. This discrepancy between the three samples may be due to the species-specific preservation, to polymerase chain reaction biases, and/or to low abundance of planktic foraminifers. In sedimentary DNA, 893 ASVs were assigned to high-level foraminiferal taxa. Among benthic foraminiferal lineages, monothalamids were most abundant, as reported in other deep-sea regions. Molecular communities formed one cluster above the boundary at which ASVs sharply decrease across the three cores. Our results suggest that depth within the sediment core can affect foraminiferal ASVs, but the distance between sites up to 200 m did not strongly affect ASVs of sedimentary DNA at least above the boundary at which ASVs sharply decrease. Sequences of foraminiferal DNA in sediment decreased linearly in core PC02-A1, but exponentially in core PC03-B3. The decline of foraminiferal ASVs may reflect both the decreases in numbers of living foraminifera and degradation of DNA in sediment, related to the particle mixing depth.
The focus of this work is the change in sediment properties and chemical characteristics that occur after land reclamation projects. The results indicate a higher sedimentation rate in Lake Hachirogata after reclamation, with the rate increasing with proximity to the agricultural zone. In the west-side water samples, higher levels of dissolved total nitrogen and dissolved total phosphorus (DTP) were found in both surface and bottom waters. The increase in P (39–80%) was generally greater than that for N (12–16%), regarding the nutrient supply from reclaimed farmland in the western part of the lake. In the eastern part of the lake, the pore-water Cl− profile showed a decreasing vertical gradient in the sediment core. This indicates desalination of the lake water after construction of a sluice gate in 1961. In the western sediment-core sample, a uniform Cl− profile indicates the mixing of lake water and pore water after reclamation. Considering the sedimentation of P in the last 100 years, there is a trend of increasing accumulation of P and P-activities after the reclamation project. This appears to be an impact from change in the lake environment as a result of increased agricultural nutrients, desalination, and residence. A large amount of mobile phosphorus (42–72% of TP in the western core sample) trapped in sediment increases the risk of phosphorus release and intensification of algal blooms. High sediment phosphorus and phosphorus mobility should be considered a source of pollution in the coastal environment.
Since October 2010, JAMSTEC has developed two autonomous uderwater vehicles (AUVs); a cruising-AUV “Jinbei” and a working-AUV “Otohime” (Fig.1). Their major purposes are to observe underwater CO 2 distribution around carbon dioxide capture and storage field, and to explore seabed mineral resources in Japan's EEZ. The cruising-AUV performs wide area survey with sonars and chemical sensors. According to the survey results, the working-AUV “Otohime” accesses feature points and observes seafloor in detail, with its chemical sensors, cameras, and a manipulator. In this paper, we introduce the newly developed AUV “Otohime” and its sea trials at Sagami Bay. Through two dives at the depth of 80-120m for total 3 hours operation, we verified the performance of the overall hardware/software systems.
We investigated characteristics of benthic macrofaunal communities and geochemical parameters in and around microbial mat-covered sediments associated with a methane seepage on Sakata Knoll in the northeastern Japan Sea. A depression on top of the knoll corresponds to a gas-hydrate-bearing area with seepage of methane-rich fluid, and microbial mats patchily cover the seafloor sediments. Sediment cores were collected at three sites for this study: one within a microbial mat, a second a few meters outside of the microbial mat, and a third from a reference site outside the gas-hydrate-bearing areas.The profile of porewater sulfate ion concentrations below the microbial mat showed linear decreases from near seawater values close to the sediment-water interface to zero concentration at approximately 7 cmbsf. Compared to the reference site, total sulfur (TS) contents were significantly higher and δ13C of total inorganic carbon were strongly negative values in the entire core below the microbial mats and in sediments below 10 cm depth outside mat, respectively. These results indicate the presence of the authigenic carbonates and sulfide minerals produced by the anaerobic oxidation of methane (AOM) in these sediments. There is no fall in sulfate concentration in the whole core collected outside microbial mat, which may be explained by the higher upward flow of methane gas inside than outside the mat. Both the 18S rRNA genes and morphological analyses showed that the surface sediment inside the microbial mat noticeably favored annelids, with dorvilleid Ophryotrocha sp. and ampharetid Neosabellides sp. identified as major constituents. The sulfidic sediment conditions with concentrations of H2S up to 121 µM resulting from AOM likely resulted in the predominance of annelids with tolerance to sulfide. In addition, the higher contents of Mo in whole cores collected inside the microbial mat than that at the reference site indicate strong enrichments of molybdenum in the sediment inside the mat. The positive correlation of Mo with TS contents suggests that the AOM-derived H2S favored the capture of Mo on sulfide minerals such as pyrite. Because of the absence of enrichments of redox-sensitive trace elements other than Mo in the Sakata Knoll sediments, molybdenum may be efficiently transported into the AOM-induced sulfidic seafloor by absorption on the particulate Fe-Mn hydroxides that are well preserved in oxic water column of the Japan Sea Proper Water. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).
Abstract Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80 kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4.
