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This study explores for the first time the possibilities that the 233U/236U atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, 233U originates from global fallout while 236U carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the 233U/236U ratio provides a tool to identify water masses with distinct U sources. In this work, 233U and 236U were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their 233U/236U signature; and (b) Deep and Bottom Waters show extremely low 233U and 236U concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high 233U/236U ratios are interpreted as a relative increase of naturally occurring 233U and 236U and thus for gradually reaching natural 233U/236U levels in the deep Arctic Ocean. Our results set the basis for future studies using the 233U/236U ratio to distinguish anthropogenic and pre‐anthropogenic U in the Arctic Ocean and beyond. Plain Language Summary: We study the presence of 233U and 236U in seawater samples taken in the Arctic Ocean from the GN01 GEOTRACES expedition in 2015. Both long‐lived manmade U isotopes were introduced worldwide during the open‐air testing of nuclear weapons (mainly in the 1960s). 236U is also linked to the civil uses of nuclear energy, particularly to the liquid effluents from Sellafield (United Kingdom) and La Hague (France) reprocessing plants (starting in the 1950s). Atlantic Waters flowing into the Arctic Ocean carry both the bomb‐tests and the reprocessing plants signals. Pacific Waters entering the Arctic Ocean carry the bomb‐tests signal only. Thus, they show different 233U and 236U compositions. We demonstrate that the 233U/236U ratio and the 236U composition can be used to identify water masses of Pacific and Atlantic origin in the upper waters of the western Arctic Ocean. Deep and Bottom Waters, where an anthropogenic influence is not expected, exhibit extremely low 233U and 236U concentrations but 233U/236U ratios above the ones observed in the upper Arctic Ocean. These features can be explained by the presence of naturally produced uranium isotopes of lithogenic origin. This study demonstrates the potential of the 233U/236U ratio to distinguish anthropogenic and natural U in the Arctic Ocean and beyond. Key Points: The 233U/236U ratio and the 236U composition unravel Pacific and Atlantic waters in Polar Surface Waters of the western Arctic OceanThe Atlantic Layer exhibit distinct 233U and 236U signals from Polar Surface WatersDeep and Bottom Waters show extremely low 233U and 236U concentrations with isotopic ratios distinct from known anthropogenic U sources [ABSTRACT FROM AUTHOR]

This paper presents the concentrations, vertical profiles and possible sources of selected major and trace elements in a sediment core covering ∼150 years of sedimentation in Lake Chaohu, eastern China. Element concentrations were measured by portable X-ray Fluorescence Spectroscopy (XRF) and were used to evaluate possible environmental consequences of the recent industrialization in China. Statistical analyses identify four groups: (1) organic carbon (OC), total nitrogen (TN), Pb, Zn, and As associated with the use of chemical fertilizers and pesticides; (2) Mn, Cr, Ni and Cu from industrial and mining activities; (3) Fe, Rb, K, Co, Ti and Ca influenced by post-depositional processes and land exploitation; and (4) Zr and Sr from the soil. The vertical profiles of elements placed in the first two groups show distinct increases in concentrations above depths of 20 cm (~1978), coincident with the timing of industrialization in China, and the anthropogenic-derived fluxes are higher than the lithogenic-derived fluxes over the last three decades. With the exception of Zr and Sr, association of the measured metals with organic carbon and nitrogen suggests that organic matter may act as a carrier phase. The geoaccumulation index (I(geo)) reveals increased contamination from elements in the first two groups in recent years. Element concentrations, compared with the Effects Range-Low (ERL) and Effects Range-Median (ERM) levels set by NOAA, suggest that adverse biological effects from Ni contamination are very likely.

Isotopic ratios of Pu and Np measured in sediment cores from 5 locations in the Ob River drainage basin show clear evidence of input from sources other than global fallout (non-fallout sources). Historical contaminant records obtained by combining isotopic ratio information with chronological information indicate that non-fallout inputs are from several sources that have varied significantly over the past 50 years. Unique isotopic signatures observed in sediments from tributaries that drain areas containing known or suspected sources of non-fallout contamination are used to identify the source of materials in sediments collected at downstream locations. These data can lead to a better understanding of the transport behavior, fate, and relative importance of particle reactive, weapons related contaminants originating from the nuclear facilities Mayak. Tomsk-7, and Semipalitinsk, which lie within the drainage basin. From our work to date, we draw the following conclusions: (1) Persistent non-fallout contamination is observed in the Ob River above its confluence with the Irtysh River, indicating contamination from the Tomsk-7 facility. (2) Non-fallout contamination in the Tobol River above its confluence with the Irtysh River indicates contamination from the Mayak facility. (3) Non-fallout contamination in the Irtysh River above its confluence with the Tobol River indicates contamination from the Semipalitinsk weapons test site. (4) The occurrence of isotopic ratios in Ob Delta sediments that are similar to those observed in source tributaries suggests that contamination from at least two sources has been transported along the length of the river system. (5) Global fallout, a result of high-yield atmospheric weapons tests conducted by the FSU and USA primarily, is the dominant source of Pu and Np to the region; however, there have been brief periods when inputs from non-fallout sources exceeded those from global fallout.