129I/127I and Δ14C records in a modern coral from Rowley Shoals off northwestern Australia reflect the 20th-century human nuclear activities and ocean/atmosphere circulations.

Radionuclides produced by 20th-century human nuclear activities from 1945 (e.g., atmospheric nuclear explosions and nuclear-fuel reprocessing) made significant impacts on earth's surface environments. Long-lived shallow-water corals living in tropical/subtropical seas incorporate the anthropogenically-produced radionuclides, including 129I and 14C, into their skeletons, and provide time series records of the impacts of nuclear activities. Here, we present 129I/127I and Δ14C time series records of an annually-banded modern coral skeleton from Rowley Shoals, off the northwestern coast of Australia, in the far eastern Indian Ocean. The 129I/127I and Δ14C records, covering the period 1930s-1990s, exhibit distinct increases caused by the nuclear activities, and their increasing profiles are clearly different from each other. The first distinct 129I/127I increase occurs from 1955 to 1959, followed by a decrease in 1960-1963. The increase is probably due to US atmospheric nuclear explosions in Bikini and Eniwetok Atolls in 1954, 1956 and 1958. The 129I produced in those nuclear tests would be transported by the North Equatorial Current, a portion of which passes through the Indonesian Throughflow and then reaches Rowley Shoals. This initial increase from 1955 is, however, absent in the Δ14C record, which shows a distinct increase from 1959 and its peak around the mid-1970s, followed by a gradual decrease. This absence and the 4-year-delayed Δ14C increase are likely due to dilution of explosion-produced 14C with natural carbon (by seawater mixing and air-sea gas exchange) being much more intense than that of explosion-produced 129I with natural iodine (by the same processes), suggesting that the 129I/127I ratio is a more conservative anthropogenic tracer in surface ocean waters, as compared to Δ14C. The second 129I/127I increase is contemporaneous with a rapid Δ14C increase during 1964-1967, followed by a rapid 129I/127I decrease in 1968-1969; the increases can be ascribed to very large atmospheric nuclear explosions conducted in the former Soviet Union in 1961-1962. The third 129I/127I increase appears between 1969/1970 and 1992, which can be attributed to airborne 129I released from nuclear-fuel reprocessing facilities in Europe, the former Soviet Union and the US. The coral 129I/127I and Δ14C time series records, combined with previous studies, enhance our understanding of the behavior of anthropogenic 129I and 14C in the global ocean and atmosphere.

Nagasaki sediments reveal that long-term fate of plutonium is controlled by select organic matter moieties.

Forecasting the long-term fate of plutonium (Pu) is becoming increasingly important as more worldwide military and nuclear-power waste is being generated. Nagasaki sediments containing bomb-derived Pu that was deposited in 1945 provided a unique opportunity to explore the long-term geochemical behavior of Pu. Through a combination of selective extractions and molecular characterization via electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS), we determined that 55 ±â€¯3% of the bomb-derived 239,240Pu was preferentially associated with more persistent organic matter compounds in Nagasaki sediments, particularly those natural organic matter (NOM) stabilized by Fe oxides (NOMFe-oxide). Other organic matter compounds served as a secondary sink of these bomb-derived 239,240Pu (31 ±â€¯2% on average), and <20% of the 239,240Pu was immobilized by inorganic mineral particles. In a narrow, 239,240Pu-enriched layer of only 9-cm depth (total core depth was 600 cm), N-containing carboxyl aliphatic and/or alicyclic molecules (CCAM) in NOMFe-oxide and other NOM fractions immobilized the majority of 239,240Pu. Among the cluster of N-containing CCAM moieties, hydroxamate siderophores, the strongest known Pu chelators in nature, were further detected in these "aged" Nagasaki bomb residue-containing sediments. While present long-term disposal and environmental remediation modeling assume that solubility limits and sorption to mineral surfaces control Pu subsurface mobility, our observations suggest that NOM, which is present in essentially all subsurface systems, undoubtedly plays an important role in sequestrering Pu. Ignoring the role of NOM in controlling Pu fate and transport is not justified in most environmental systems.

Seasonal and snowmelt-driven changes in the water-extractable organic carbon dynamics in a cool-temperate Japanese forest soil, estimated using the bomb-(14)C tracer.

Water-extractable organic carbon (WEOC) in soil consists of a mobile and bioavailable portion of the dissolved organic carbon (DOC) pool. WEOC plays an important role in dynamics of soil organic carbon (SOC) and transport of radionuclides in forest soils. Although considerable research has been conducted on the importance of recent litter versus older soil organic matter as WEOC sources in forest soil, a more thorough evaluation of the temporal pattern of WEOC is necessary. We investigated the seasonal variation in WEOC in a Japanese cool-temperate beech forest soil by using the carbon isotopic composition ((14)C and (13)C) of WEOC as a tracer for the carbon sources. Our observations demonstrated that fresh leaf litter DOC significantly contributed to WEOC in May (35-52%) when the spring snowmelt occurred because of the high water flux and low temperature. In the rainy season, increases in the concentration of WEOC and the proportion of hydrophobic compounds were caused by high microbial activity under wetter conditions. From summer to autumn, the WEOC in the mineral soil horizons was also dominated by microbial release from SOC (>90%). These results indicate that the origin and dynamics of WEOC are strongly controlled by seasonal events such as the spring snowmelt and the rainy season's intense rainfall.