Reconstructing the chronology of the natural and anthropogenic uranium isotopic signals in a marine sediment core from Beppu Bay, Japan.

The long-lived U isotopes, 233U and 236U, have been used increasingly in recent years as marine circulation tracers and for identifying sources of uranium contamination in the environment. The sedimentation histories of these two U isotopes in combination with natural 238U were reconstructed for an anoxic sediment core collected from Beppu Bay, Japan, in the western North Pacific Ocean showing good time resolution (less than 2.6 y/sample). The 233U/236U atom ratio showed a prominent peak of (3.20 ± 0.30) × 10-2 around 1957 which can be attributed to the input from atmospheric nuclear weapons tests including thermonuclear tests conducting in the Equatorial Pacific. The integrated 233U/236U ratio of (1.64 ± 0.08) × 10-2 for the sediment was found to be in relatively good agreement with the representative ratio published for global fallout (∼1.4 × 10-2). A prominent increase in the authigenic ratio of 233U/238Ua,s in the leached fraction (1.39 ± 0.11 × 10-11) and the bulk digestion (1.36 ± 0.10 × 10-11) was also observed around 1957. This reflects the input supply of 233U to the seawater which is known to have a relatively constant 238U content. The authigenic 236U/238Ua,s ratio (0.18 ± 0.02 × 10-9) obtained for 1921 increased from the early 1950's to a maximum of (6.59 ± 0.60) × 10-9 around 1962. The variation in this ratio represents well the introduction history of U into the surface environment without site-specific U contamination and the time profile is also consistent with the 137Cs signature. This work thus provides a benchmark for the long-term use of the isotopic U composition as an input parameter for seawater circulation tracers and as a chronological marker for anoxic sediments and sedimentary rocks. Especially the 233U/236U ratio may serve as a key-marker for the new geological age Anthropocene.

A 75-year history of microplastic fragment accumulation rates in a semi-enclosed hypoxic basin.

Plastic budgets in the marine environment and their long-term trends are yet to be fully understood. Measuring the accumulation rates in bottom sediments is crucial to solving the riddle of missing ocean plastics. Previous studies based on coastal sediment cores have found that accumulation rates have increased with increases in plastic production and/or regional populations. However, the correlations between the rates and bioactivities or ocean dynamics, which are crucial for modeling the microplastic sinking process, have not been examined. We revealed a 75-year microplastic fragment (0.3-5.0 mm) accumulation rate history in a hypoxic basin, Beppu Bay, Japan, based on multi-core analysis and 210Pb dating of the sediment which was cross-checked by time control with 137Cs radioactivity peaks. We found that a long-term linear increasing trend with an approximately 20-year variation overlapped with significant peaks around 1990 and 2014 with the first polypropylene microplastic fragment detected from a 1958.8-1961.0 CE sediment layer. The maximum rate was 203 pieces m-2 y-1 with an abundance of 86 pieces kg-1-dry in 2014. Smaller fragments in the size range of 0.3-2.0 mm have been consistently dominant in terms of the accumulation rate throughout the 1955-2015 period, accounting for 85.3 % of the total accumulation rate. The three major polymers (polyethylene, polypropylene, and polystyrene) accounted for 96.6 % of the total rate. The rate was highly and positively correlated with the chlorophyll-a accumulation rate and concentration in the sediment. Based on the microplastic accumulation rates and concentration in the seawater, the mean sinking velocity of microplastics was estimated to be in the order of 101 m d-1. Our results will contribute to significant progress in modeling the microplastic sinking process by offering the first field measurement-based mean sinking velocity and significant correlations between the rate and bioactivity-related signals.

Plutonium isotopes in the North Western Pacific sediments coupled with radiocarbon in corals recording precise timing of the Anthropocene.

Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure 239Pu and 240Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher isotope ratio between 239Pu and 240Pu suggest an additional contribution other than global fallout via ocean currents. Furthermore, single elevations in 240Pu/239Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.

Reconstruction of 100-year dynamics in Daphnia spawning activity revealed by sedimentary DNA.

Environmental DNA (eDNA) is currently developing as a powerful tool for assessing aquatic species dynamics. However, its utility as an assessment tool for quantification remain under debate as the sources of eDNA for different species is not always known. Therefore, accumulating information about eDNA sources from different species is urgently required. The objective of our study was to evaluate whether sedimentary DNA targeting two Daphnia species, D. galeata and D. pulicaria, could track Daphnia population dynamics and resting egg production. Applying a quantitative PCR targeting the mitochondrial 12S rRNA gene on sediment cores collected in Lake Biwa, Japan, we compared sedimentary DNA concentration of Daphnia with the abundance of remains and ephippia, reflecting their abundance and resting egg production, respectively. We found that the sedimentary DNA concentrations of Daphnia for the past century were inconsistent with their population abundance. However, the concentration was highly correlated with the resting egg production. Our results provide evidence that ephippia with resting eggs, released during spawning activities, was a significant source of Daphnia DNA archived in sediments. Our work provides critical insights for using sedimentary DNA as a monitoring tool for egg production dating back 100 years.

Historical record of polychlorinated biphenyls in a sediment core from Lake Biwa, Japan: Significance of unintentional emission and weathering signals revealed by full congener-specific analysis.

The occurrence of 209 PCB congeners was determined in a sediment core dated between 1930 and 2019 from Lake Biwa, a typical temperate monomictic lake in Japan. Concentrations of total PCBs ranged from 5.3 to 48 ng/g dry weight (dw), showing a highest peak at the 1960s to 1970s. The temporal trend of total PCBs in this sediment core generally matched with Japanese PCB production and emission pattern (i.e., increasing from the 1950s, peaking at 1970, and gradually decreasing since 1972). The vertical PCB profiles in our core were affected by physical mixing and bioturbation. By using a detailed and comprehensive analytical method, we have found elevated concentrations and special historical profiles of several congeners such as CB-7, -11, -47/48/75, -51, -68, and -209, which are still rarely included in routine PCB analysis. Some tetra-CB congeners like CB-47/48/75, -51, and -68 showed their concentration peaks at the early 2010s, which may be unintentionally produced during polymer manufacturing processes. PCB homolog- and congener-specific profiles in our sediment core samples have experienced weathering with higher proportions of penta- and hexa-CBs as compared to the Kanechlor usage pattern (i.e., dominated by tri- and tetra-CBs). Both intentional (i.e., technical mixtures) and unintentional (e.g., PCB-containing polymers and pigments) sources of PCBs were suggested from congener-specific analysis.

Determination of brominated flame retardants including polybrominated diphenyl ethers, pentabromoethylbenzene, hexabromobiphenyl, and decabromodiphenyl ethane in sediment samples: Validation of a rapid and efficient clean-up method and application to a sediment core from Lake Biwa, Japan.

The development of rapid and efficient analytical method for the determination of legacy and current-use brominated flame retardants (BFRs) has been performed due to environmental concern related to these pollutants. In the present study, we used an automated clean-up device equipped with pre-packed micro-column sets (containing sulfuric acid impregnated silica gel and silver-modified alumina) to develop an effective purification method for polybrominated diphenyl ethers (PBDEs), pentabromoethylbenzene, hexabromobiphenyl, and decabromodiphenyl ethane (DBDPE) in sediment extracts. Matrix-spiked sediments (n = 6) and the Standard Reference Material® 1944 samples (n = 6) were tested. Our method showed acceptable accuracy, repeatability, and sensitivity for almost all the target compounds with reduced processing time, labor requirement, and solvent amounts as compared to conventional clean-up method (e.g., sulfuric acid treatment and self-packed chromatographic columns). The validated method was applied to sediment core samples (n = 16) collected in 2019 from Lake Biwa, the largest lake in Japan. PBDEs were detected in sediment samples of 0-13 cm depth (dated between 1990 and 2019) at relatively low concentrations (median 5.7; range 2.6-9.4 ng/g dry weight). PBDE profiles were dominated by BDE-209, which accounted for 91 ± 10% of total PBDEs. Among other BFRs, only DBDPE was found in sediment layers of 0-9 cm depth (deposited between 2005 and 2019). DBDPE concentrations ranged from 0.43 to 1.6 (median 0.71) ng/g and showed increasing trend toward shallower depths.

Use of comprehensive target analysis for determination of contaminants of emerging concern in a sediment core collected from Beppu Bay, Japan.

In recent years, concern about the release of anthropogenic organic micropollutants referred to as contaminants of emerging concern (CECs) has been growing. The objective of this study was to find potential CECs by means of an analytical screening method referred to as comprehensive target analysis with an automated identification and quantification system (CTA-AIQS), which uses gas and liquid chromatography combined with mass spectrometry (GC-MS and LC-QTOF-MS). We used CTA-AIQS to analyze samples from a sediment core collected in Beppu Bay, Japan. With this method, we detected 80 compounds in the samples and CTA-AIQA could work to useful tool to find CECs in environmental media. Among the detected chemicals, three PAHs (anthracene, chrysene, and fluoranthene) and tris(isopropylphenyl)phosphate (TIPPP) isomers were found to increase in concentration with decreasing sediment depth. We quantified TIPPP isomers in the samples by means of targeted analysis using LC-MS/MS for confirmation. The concentration profiles, combined with previous reports indicating persistent, bioaccumulative, and toxic properties, suggest that these chemicals can be categorized as potential CECs in marine environments.

Contamination levels and temporal trends of legacy and current-use brominated flame retardants in a dated sediment core from Beppu Bay, southwestern Japan.

Contamination levels and temporal trends of polybrominated diphenyl ethers (PBDEs) and some alternative brominated flame retardants (BFRs) were examined in a dated sediment core from the deepest part of the Beppu Bay, southwestern Japan. PBDEs were found in the upper layers of 0-15 cm depth at concentrations ranging from 5200 to 32,600 pg g-1 with the peak estimated at 1995. Decabromodiphenyl ether (BDE-209) was the most abundant congener, accounting for 96% in average of total PBDEs. The vertical profile of BDE-209 observed in our sediment core generally agreed with the historical pattern of domestic demand of commercial deca-BDE mixtures in Japan, and perfectly matched with maximum stock of these products (i.e., 42,000 tons in 1995). Among alternative BFRs, only decabromodiphenyl ethane (DBDPE), a replacement of deca-BDE, was found at significant levels with concentrations of 69-850 pg g-1 in sediment layers dated between 1991 and 2011. Ratios of DBDPE to BDE-209 gradually increased during this period, implying opposite trends of these two compounds and the role of DBDPE as a deca-BDE's alternative. The occurrence of deca-BDE components in sediments may pose medium risk to benthic aquatic life, while the ecological risk of other PBDE homologs and DBDPE was negligible.

Sedimentary DNA tracks decadal-centennial changes in fish abundance.

Far too little is known about the long-term dynamics of populations for almost all macro-organisms. Here, we examined the utility of sedimentary DNA techniques to reconstruct the dynamics in the "abundance" of a species, which has not been previously defined. We used fish DNA in marine sediments and examined whether it could be used to track the past dynamics of pelagic fish abundance in marine waters. Quantitative PCR for sedimentary DNA was applied on sediment-core samples collected from anoxic bottom sediments in Beppu Bay, Japan. The DNA of three dominant fish species (anchovy, sardine, and jack mackerel) were quantified in sediment sequences spanning the last 300 years. Temporal changes in fish DNA concentrations are consistent with those of landings in Japan for all three species and with those of sardine fish scale concentrations. Thus, sedimentary DNA could be used to track decadal-centennial dynamics of fish abundance in marine waters.

Characterization of mono- to deca-chlorinated biphenyls in a well-preserved sediment core from Beppu Bay, Southwestern Japan: Historical profiles, emission sources, and inventory.

Contamination levels and profiles of mono- to deca-chlorinated biphenyls (PCBs) were characterized in a sediment core dated in 1954-2011 from Beppu Bay, southwestern Japan, providing a comprehensive and detailed picture on the environmental occurrence, temporal trends, and emission sources of these pollutants in the study area. Concentrations of total PCBs in the core ranged from 3.5 to 150 (median 15) ng g-1 dry weight and exhibited depth profile matching with Japanese PCB production and emission patterns (i.e., drastically increasing from the early 1960s, peaking in 1970, and then rapidly decreasing). Origin of PCBs in the studied samples largely associated with Kanechlor mixtures (e.g., KC-300 and KC-400), especially for sediment layers dated between the mid-1960s and early 1970s (i.e., the intensive PCB production period in Japan). In addition, dechlorination and weathering signals and emerging inputs of PCBs were also observed in deeper and shallower sediment segments with notable proportions of some unique congeners such as CB-47/48/51 and CB-11, respectively. Historical fluxes of PCBs in our samples showed quite similar vertical shape as concentrations. In the context of national implementation for complete treatment of PCB-containing waste until 2024, further investigations on spatiotemporal trends and environmental loads of PCBs in Japan are necessary.

Lead isotope ratios in six lake sediment cores from Japan Archipelago: Historical record of trans-boundary pollution sources.

Sediment cores from six lakes situated from north to south on the Japanese Archipelago were collected during 2009-2010 to investigate the hypothesis that deposition of lead (Pb) was coming from East Asia (including China, South Korea and eastern part of Russia). Accumulation rates and ages of the lake sediment were estimated by the (210)Pb constant rate of supply model and (137)Cs inputs to reconstruct the historical trends of Pb accumulation. Cores from four lakes located in the north and central Japan, showed clear evidence of Pb pollution with a change in the (206)Pb/(207)Pb and (208)Pb/(207)Pb ratios in the recent sediment as compared to the deeper sediment. Among the six studied lakes, significant inputs of anthropogenic lead emissions were observed at Lake Mikazuki (north Hokkaido in north Japan), Lake Chokai (north of Honshu), and Lake Mikuriga (central part of Honshu). Pb isotopic comparison of collected core sediment and previously reported data for wet precipitation and aerosols from different Asian regions indicate that, before 1900, Pb accumulated in these three lakes was not affected by trans-boundary sources. Lake Mikazuki started to receive Pb emissions from Russia in early 1900s, and during the last two decades, this lake has been affected by trans-boundary Pb pollution from northern China. Lake Chokai has received Pb pollutant from northern China since early 1900s until 2009, whereas for the Lake Mikuriga the major Pb contaminant was transported from southern China during the past 100years. The results of our study demonstrate that Japan Archipelago has received trans-boundary Pb emissions from different parts of East Asian region depending on location, and the major source region has changed historically.

Phospholipid-derived fatty acids and quinones as markers for bacterial biomass and community structure in marine sediments.

Phospholipid-derived fatty acids (PLFA) and respiratory quinones (RQ) are microbial compounds that have been utilized as biomarkers to quantify bacterial biomass and to characterize microbial community structure in sediments, waters, and soils. While PLFAs have been widely used as quantitative bacterial biomarkers in marine sediments, applications of quinone analysis in marine sediments are very limited. In this study, we investigated the relation between both groups of bacterial biomarkers in a broad range of marine sediments from the intertidal zone to the deep sea. We found a good log-log correlation between concentrations of bacterial PLFA and RQ over several orders of magnitude. This relationship is probably due to metabolic variation in quinone concentrations in bacterial cells in different environments, whereas PLFA concentrations are relatively stable under different conditions. We also found a good agreement in the community structure classifications based on the bacterial PLFAs and RQs. These results strengthen the application of both compounds as quantitative bacterial biomarkers. Moreover, the bacterial PLFA- and RQ profiles revealed a comparable dissimilarity pattern of the sampled sediments, but with a higher level of dissimilarity for the RQs. This means that the quinone method has a higher resolution for resolving differences in bacterial community composition. Combining PLFA and quinone analysis as a complementary method is a good strategy to yield higher resolving power in bacterial community structure.

Sedimentary records of metal deposition in Japanese alpine lakes for the last 250 years: recent enrichment of airborne Sb and In in East Asia.

Concentrations of 18 elements, including Sb, In, Sn, and Bi, were measured in sediment cores from two pristine alpine lakes on Mount Hachimantai, northern Japan, representing the past 250 years. Vertical variations in concentrations are better explained by atmospheric metal deposition than by diagenetic redistribution of Fe and Mn hydroxide and organic matter. Anthropogenic metal fluxes were estimated from (210)Pb-derived accumulation rates and metal concentrations in excess of the Al-normalized mean background concentration before 1850. Anthropogenic fluxes of Sb and In showed gradual increases starting around 1900 in both lakes, and marked increases after 1980. Comparison of Sb/Pb and Pb stable isotope ratios in sediments with those in aerosols of China or northern Japan and Japanese source materials (recent traffic- and incinerator-derived dust) suggest that the markedly elevated Sb flux after 1980 resulted primarily from enhanced long-range transport in aerosols containing Sb and Pb from coal combustion on the Asian continent. The fluxes of In, Sn, and Bi which are present in Chinese coal showed increasing trends similar to Sb for both study lakes. This suggests that the same source although incinerators in Japan may not be ruled out as sources of In. The sedimentary records for the last 250 years indicate that atmospheric pollution of Sb and In in East Asia have intensified during recent decades.

Spatial distribution and corresponding determining factors of metal concentrations in surface sediments of Beppu Bay, southwest Japan.

This study determined the factors contributing to the spatial distribution of 14 metal concentrations in the surface sediments of Beppu Bay on the basis of comparisons of the organic geochemical properties and environmental parameters through principal component analysis (PCA) and redundancy analysis (RDA). The results of PCA and RDA showed that the concentrations of V, Cr, Co, and As were closely related to the distances between the sampling sites and the Oita River. This indicated that these metals originated from the river's drainage area. The Mn, Cu, Mo, and Cd concentrations were related to the water depth. These results indicated that the Mo, Cd, and Cu deposition processes were controlled by oxygen depletion, and that these elements accumulated in the deeper parts of the bay under anoxic conditions.