Inomata Yayoi, Aoyama Michio, Hirose Katsumi
Geochemical Research Department, Meteorological Research Institute, Tsukuba, Ibaraki, 305-0052, Japan.
J Environ Monit. 2009 Jan;11(1):116-25. doi: 10.1039/b811421h. Epub 2008 Nov 19.
We investigated spatial and temporal variations in (137)Cs concentrations in the surface waters of the global ocean for the period from 1957 to 2005 using the "HAM database - a global version". Based on the 0.5-y average value of (137)Cs concentrations in the surface water in each sea area, we classified the temporal variations into four types. (1) In the North Pacific Ocean where there was high fallout from atmospheric nuclear weapons tests, the rates of decrease in the (137)Cs concentrations changed over the five decades: the rate of decrease from the 1950s to the 1970s was much faster than that after the 1970s, and the (137)Cs concentrations were almost constant after the 1990s. Latitudinal differences in (137)Cs concentrations in the North Pacific Ocean became small with time. (2) In the equatorial Pacific and Indian Oceans, the (137)Cs concentrations varied within a constant range in the 1970s and 1980s, suggesting the advection of (137)Cs from areas of high global fallout in the mid-latitudes of the North Pacific Ocean. (3) In the eastern South Pacific and Atlantic Oceans (south of 40 degrees S), the concentrations decreased exponentially over the five decades. (4) In the Arctic and North Atlantic Oceans, including marginal seas, (137)Cs concentrations were strongly controlled by discharge from nuclear reprocessing plants after the late 1970s. The apparent half-residence times of (137)Cs in the surface waters of the global ocean from 1970 to 2005 ranged from 4.5 to 36.8 years. The apparent half-residence times were longer in the equatorial region and shorter in the higher latitudes. There was no notable difference between the latitudinal distributions of the apparent half-residence times in the Pacific and Indian Oceans. These results suggest that (137)Cs in the North Pacific Ocean is transported to the equatorial, South Pacific, and Indian Oceans by the oceanic circulation.
我们利用“HAM数据库 - 全球版”，研究了1957年至2005年期间全球海洋表层水中（137）铯浓度的时空变化。根据每个海域表层水中（137）铯浓度的0.5年平均值，我们将时间变化分为四种类型。（1）在因大气核武器试验而沉降物较多的北太平洋，（137）铯浓度在五十年间的下降速率发生了变化：20世纪50年代至70年代的下降速率比70年代以后快得多，90年代以后（137）铯浓度几乎保持不变。北太平洋（137）铯浓度的纬度差异随时间变小。（2）在赤道太平洋和印度洋，（137）铯浓度在20世纪70年代和80年代在一个恒定范围内变化，这表明（137）铯是从北太平洋中纬度全球沉降物高值区域平流过来的。（3）在南太平洋东部和大西洋（南纬40度以南），浓度在五十年间呈指数下降。（4）在北极和北大西洋，包括边缘海，20世纪70年代后期以后，（137）铯浓度受到核后处理厂排放的强烈控制。1970年至2005年全球海洋表层水中（137）铯的表观半停留时间为4.5至36.8年。表观半停留时间在赤道地区较长，在高纬度地区较短。太平洋和印度洋表观半停留时间的纬度分布之间没有显著差异。这些结果表明，北太平洋中的（137）铯通过海洋环流被输送到赤道、南太平洋和印度洋。
