Cross equator transport of 137Cs from North Pacific Ocean to South Pacific Ocean (BEAGLE2003 cruises)

The anthropogenic radionuclides such as 137Cs, 90Sr, 99Tc, 129I and some transuranics are important tracers of transport and biogeochemical processes in the ocean. 137Cs, with a half-life of 30 years, a major fission product present in a dissolved form in seawater, is a good tracer of oceanic circulation at a time scale of several decades. E P6 line along 30°S during the BEAGLE cruise in 2003, surface seawater (around 80 L) was collected a few meters below the ocean surface by a pumping system. Water column samples (from 5 to 20 L) were collected using a Rosette multisampling system and Niskin bottles. 137Cs was separated from seawater samples using ammonium phosphomolybdate (AMP) and analysed for 137Cs in low-level HPGe gamma-ray spectrometers. Results allowed to draw a detailed picture of the distribution of 137Cs in the South Pacific Ocean along P6 line. s depth section was depicted from about 160 samples. 137Cs concentrations in the subsurface layers ranged from 0.07 ± 0.04 Bq m−3 to 1.85 ± 0.145 Bq m−3, high in the Tasman Sea and very low in the eastern region where upwelling occurs. Water column inventories of 137Cs from surface to 1000 dbar depth ranged from 270 ± 104 to 1048 ± 127 Bq m−2. It was concluded that the source of higher 137Cs concentration and inventories in the Tasman Sea was 137Cs deposited in the mid latitude of the North Pacific Ocean and transported across the equator during four decades.