Temporal variation of 137Cs distribution and inventory along 165 deg. E in the North Pacific from the 1960s to the present

An enhanced observation of 137Cs along 165 deg. E in the North Pacific Ocean was conducted in 2002. The primary objective of this enhanced observation is to develop an understanding of the present open-ocean distribution of radionuclides in the water column in this region. Relatively large numbers of 137Cs concentration data are available in this interested area since 1960s, then we are able to do a study on the temporal variation of 137Cs distribution and inventory. Recent progress of 137Cs measurements in seawater at an underground laboratory at Ogoya made it possible to determine the 137Cs concentration using small volume seawater samples. The samplings in 2002 were carried out during two cruises by R/V Ryofu-maru operated by Japan Meteorological Agency. Among the stations between 49 deg. N and 6 deg. S along 165 deg. E, seawater samples shallower than 1000 m were collected at 20 stations. An average horizontal spacing was 350 km and vertical spacings were 100 m or less at each station. Sample volumes ranged from 10 to 20 liters. Deeper water samples were collected at 8 stations. Vertical spacings varied from 200 to 500 m and sample volumes varied from 20 liter to 100 liter for deeper seawater samples. A preliminary result of 137Cs concentration along 165 deg. E for shallower layers is shown. 137Cs rich water was seen between 40 deg. N and 15 deg. N as shown in Fig. 1. The highest 137Cs inventories were observed at latitudes between 30 deg. N and 40 deg. N in the North Pacific in the 1960s corresponding to the pattern of global fallout in this region. The body of global fallout of 137Cs in 2002 is, however, within the subtropical gyre where a maximum 137Cs concentration was observed around 20 deg. N, the southern edge of the subtropical gyre, and in the 200-500 m layer. This finding supports the idea that the body of global fallout has been captured within the subtropical gyre during these four decades. Southward transport also can be seen around 200 m depth between 20 deg. N and equator. Aoyama et al. already showed that southward transport of 137Cs had been caused an increase of 137Cs inventory at the lower latitude of 10-20 deg. N in the 1970s and 1980s. Hirose and Aoyama also reported that shorter apparent half-residence time of 137Cs concentration in surface seawater occurred at the higher latitude in the North Pacific while longer apparent half-residence time of 137Cs concentration occurred in the Equatorial Pacific. These suggest that the southward transport of 137Cs is an important process to regulate the temporal variation of inventory of 137Cs and concentration of 137Cs in surface water in the Equatorial Pacific