Strongly strati ed water structure and densely populated catchment make the Baltic Sea one of the most polluted seas. Understanding its circulation pattern and time scale is essential to predict the dynamics of hypoxia, eutrophication, and pollutants. Anthropogenic 236 U and 233 U have been demonstrated as excellent transient tracers in oceanic studies, but unclear input history and inadequate long-term monitoring records limit their application in the Baltic Sea. From two dated Baltic sediment cores, we obtained high-resolution records of anthropogenic uranium imprints originated from three major human nuclear activities throughout the Atomic Era. Using the novel 233 U/ 236 U signature, we distinguished and quanti ed 236 U inputs from global fallout (43.3%-50.5%), Chernobyl accident (< 0.9%), and discharges of civil nuclear industry (48.6%-56.7%) to the Baltic Sea. We estimated the total release of 233 U (7-15kg) from the atmospheric nuclear weapons testing, and pinpointed 233 U peak signal in the mid-to-late 1950s as a potential time marker for the onset of the Anthropocene Epoch. This work also provides fundamental 236 U data for Chernobyl accident and early discharges from civil nuclear facilities, prompting worldwide 233 U-236 U tracer studies. We anticipate our data to be a broader application in model-observation interdisciplinary research on water circulation and pollutant dynamics in the Baltic Sea.

An analytical method was developed for the determination of ultralow level 135 Cs in environmental samples by chromatographic separation of cesium with AMP-PAN and AG50W-X8 columns and sensitive measurement of cesium isotopes with triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was simply released by acid leaching using aqua regia from environmental solid samples and preconcentrated on AMP-PAN column. The cesium adsorbed on the column was effectively eluted with NH 4 Cl solution without dissolving the AMP. The excessive amount of NH 4 Cl in the eluate was removed by sublimation in the presence of small amount of LiCl. The remaining barium and other interfering elements such as Mo, Sn, Sb, and Li were efficiently removed using cation exchange chromatography (AG50W-X8). The decontamination factors of this procedure are above 4 × 10 7 for barium and 4 × 10 5 for molybdenum; the chemical yields of cesium are more than 85% for samples of less than 10 g. This method enables to separate cesium from large size of samples for the determination of ultralow level 135 Cs, avoiding the problem of removal of a huge amount of Mo in the dissolved AMP. Intrinsic 137 Cs in the environmental samples measured by gamma spectrometry before and after separation was used as internal isotope dilution standard for quantitative determination of 135 Cs without complete release and recover of radiocesium. The interference of barium ( 135 Ba and 137 Ba) to the ICP-MS measurement of 135 Cs and 137 Cs was further suppressed to 8 × 10 −5 by using N 2 O as the reaction gas in ICP-MS/MS at a flow rate of 0.7 mL/min, so a total suppression of 2 × 10 −12 for Ba was achieved, making the isobaric interference of Ba isotopes to the measurement of 135 Cs and 137 Cs in environmental samples negligible. A detection limit of 9.1 × 10 −17 g/g for 135 Cs and 137 Cs was achieved for 60 g samples. The developed method was validated by analysis of standard reference materials and successfully applied for the determination of 135 Cs concentrations and 135 Cs/ 137 Cs ratios in soil samples collected from Denmark, Sweden, and Ukraine. The 135 Cs/ 137 Cs isotopic ratios in Danish soil (2.08−2.68) were significantly higher than that from Sweden and Ukraine (0.65−0.71), indicating different sources of radiocesium. This work demonstrated the application of 135 Cs/ 137 Cs as a unique fingerprint for discriminating the sources of radioactive contamination and estimating their contribution to the total inventory, which will be useful for nuclear forensics and environmental tracer studies.

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