Abstract
In order to determine the atmospheric input of 210Pb and seawater-dissolvable Pb in the East Sea (Japan Sea), we measured the concentrations of total 210Pb and dissolved Pb (<0.2 μm) in seawater and 210Pb and 226Ra in sinking particles. The East Sea is deep (∼3700 m) and enclosed by surrounding continents except for the shallow sills (<150 m). Since the East Sea is located off the East Asian continent under the westerlies, the concentrations of 210Pb and dissolved Pb in this sea are significantly affected by terrestrial sources through the atmosphere. The vertical profiles of total 210Pb and dissolved Pb generally showed a surface maximum and then decreased with depth. The concentrations of dissolved Pb in the surface water were 2 and 3 times higher than those in the North Pacific and North Atlantic Oceans, respectively. Using an independent box model (upper 1000 m or 2000 m), we estimate the atmospheric input of 210Pb to be 1.46 ± 0.25 dpm cm−2 y −1, which is within the range of published results from the land-based sites (0.44–4.40 dpm cm−2 y −1) in South Korea, China, and Japan. Based on this flux, the residence time of total 210Pb in the East Sea is calculated to be approximately 7.1 ± 1.6 years, which is twice lower than the previous estimation. Combining the residence time of 210Pb and the inventory of dissolved Pb, the atmospheric input of seawater-dissolvable Pb is estimated to be 0.98 ± 0.28 nmol cm−2 y −1. This flux is approximately 25% of the Pb flux through the wet deposition (acid-leachable fraction). Thus, our results suggest that the flux and fate of atmospheric Pb in the ocean can be successfully determined using an accurate mass balance model of naturally occurring 210Pb.
Highlights
The naturally occurring radionuclide 210Pb (t1/2 = 22.3 years), belonging to the 238U decay series, is produced from 226Ra (t1/2 = 1600 years), via 222Rn (t1/2 = 3.8 days) and other short-lived radionuclides (t1/2 < 30 min)
An excess of 210Pb over 226Ra (210Pbex: 210Pb-226Ra) is observed in the upper ocean, except for the polar regions, where the atmospheric input of 210Pb is small (Elsässer et al, 2011; Persson and Holm, 2014; Baskaran and Krupp, 2021). 210Pb is rapidly removed by particles in the surface water, Tracing Seawater Pb Using 210Pb so the residence time of 210Pb in the surface water is short: 1– 3 years in the North Pacific and North Atlantic Oceans (Nozaki and Tsunogai, 1973, 1976; Bacon et al, 1976; Nozaki et al, 1976; Rigaud et al, 2015)
The vertical profiles of total 210Pb in the East Sea showed the highest activities in the surface and decreased with depth, as observed in other major non-polar open oceans (Nozaki et al, 1980; Cochran et al, 1990; Kim, 2001; Rigaud et al, 2015; Tang et al, 2018; Horowitz et al, 2020; Figure 2A)
Summary
The decreasing trend of anthropogenic Pb flux to the Sargasso Sea in response to decline in the emission of United States leaded gasoline was revealed based on the reduced Pb/210Pb ratios in the surface water from 1979 to 1987 (Boyle et al, 1986; Shen and Boyle, 1988; Sherrell et al, 1992). Those studies combined the Pb/210Pb ratios with 3H-3He thermocline ventilation model (Jenkins, 1980) to reveal the importance of isopycnal transport on Pb distributions in that region. The models suffer from the lack of measured data (Nozaki et al, 1998), their results with the measured data can be more representative
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