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Actinium‐227 (227Ac) has been used as a powerful tracer of diapycnal mixing in the ocean, assuming that it is conservative and originates mainly from deep‐sea sediments. However, here we show an unexpectedly large source (continental margin) and sink (scavenging) of 227Ac in the ocean, based on high‐resolution 227Ac distributions obtained for the first time by mooring Mn‐fibers in the East Sea (Japan Sea). Although we expected a decrease in radium‐228 (228Ra) to 227Ac ratios with depth owing to their different half‐lives, the ratios increased with depth in the upper layer, indicating efficient removal of 227Ac by particle scavenging. In addition, unusually high 227Ac activities (∼15 dpm m−3) were observed in the surface layer, likely due to the horizontal transport of 227Ac‐enriched shelf water. Thus, our results suggest refining our understanding of the geochemical cycle and application of 227Ac in the ocean. Plain Language Summary: Distributions of 227Ac provide crucial information for the vertical mixing of the deep ocean on timescales of up to 100 years. However, behaviors of 227Ac in the ocean have not been well understood to date because of its extremely low concentration. In this study, we for the first time determined high‐resolution 227Ac profiles by mooring Mn‐fibers in a marginal sea of the northwestern Pacific Ocean. Our results display that the shelf source inputs as well as efficient removal by particle scavenging have been overlooked so far. In particular, we emphasize that the removal of 227Ac by particle scavenging revealed in this study should be considered when using 227Ac as a tracer of mixing rates in the ocean. Key Points: The high‐resolution measurement of 227Ac with our Mn‐fiber mooring method agrees very well with the onboard Mn‐fiber filtration methodSignificantly high 227Ac activities, which might originate from the 227Ac‐enriched shelf water, are observed in the surface layer (0–100 m)High 227Ac scavenging rates are calculated based on the increasing trend of 228Ra to 227Ac ratio with depth in the upper layer (0–1,000 m) [ABSTRACT FROM AUTHOR]

Thorium-234 (234Th; t1/2 = 24.1 days) has been widely used as a tracer of particle settling and organic carbon export in the ocean. However, the use of 234Th in the ocean has been hampered by labor-intensive procedures, low throughputs, and large uncertainties related to source impurities. Here, we demonstrate a more efficient technique developed to analyze 234Th in seawater by modifying the traditional Fe(OH)3 co-precipitation method. The advantages and shortcomings of this method were compared with the 234Th analytical method using MnO2 precipitation. In the new method, following the equilibration of the 230Th spike with 234Th, Th was co-precipitated with Fe. Although the precipitates include a fraction of U, Fe precipitation was fast, and the supernatant can be easily siphoned off, allowing simple handling of several samples simultaneously. The Th adsorbed onto particles, collected by Fe precipitation, was desorbed in ~ 4 M HNO3 solution by heating at 230 °C for 30 min in a sealed Teflon bottle. Then, Th was separated from U using UTEVA resin. The counting source of Th was prepared by micro-precipitation of Ce. We were able to process ~ 60 samples for 5 days onboard, and the first counting was completed within a week upon returning to the laboratory. The main advantages of this method include (1) easy increase in the sample volume, (2) rapid Fe precipitation, (3) high-purity Th sources for beta and alpha counting, (4) no need for mass spectrometry, and (5) the high throughput. [ABSTRACT FROM AUTHOR]