Temporal and spatial variations in bomb-produced radiocarbon along BEAGLE2003 lines—Revisits of WHP P06, A10, and I03/I04 in the Southern Hemisphere Oceans

Abstract

Radiocarbon ( 14C) in dissolved inorganic carbon was measured during revisit cruises along World Ocean Circulation Experiment-Hydrographic Programme (WHP) lines A10 in the South Atlantic, I03/I04 in the Indian, and P06 in the South Pacific Oceans from August 2003 to January 2004, during the Blue Earth Global Expedition 2003 (BEAGLE2003). Zonal means of the water-column inventory of bomb-produced 14C in 2003/2004 in the South Atlantic, Indian, and South Pacific Oceans were about 180, 128, and 159 × 10 12 atoms m −2, respectively. The smallest zonal inventory along the I03 line among the three lines was primarily due to a sampling bias, because the I03 line in the Indian Ocean was along 20°S, which is more equatorward than the other two lines along approximately 30°S in the South Atlantic and South Pacific Oceans. The I03 line in the Indian Ocean had the smallest zonal inventory of bomb-produced 14C and the largest of bomb-produced 137Cs, suggesting that the distribution of bomb 14C in the Indian Ocean is determined primarily by the thermocline ventilation within the Indian Ocean. The history of bomb 14C over time suggests that the bomb 14C inventory in the southern subtropical regions increased steadily up to the early 1990s. The rate of increase then slowed between the early 1990s and 2003/2004 because of a decrease in the bomb 14C influx from the atmosphere to the surface ocean. The highest bomb 14C inventory among the southern subtropical regions was in the subtropical Indian Ocean. However, the contribution of the Indonesian throughflow from the North Pacific and Indian Oceans to this large inventory in the Indian Ocean is not clear. The 14C data along the BEAGLE2003 lines in 2003/2004 were compared with those obtained during WHP in the 1990s and during the South Atlantic Ventilation Experiment in the late 1980s. The zonal averages of the decadal changes in 14C revealed that bomb 14C continued to increase between the late 1980s/1990s and 2003/2004 in the southern subtropical regions of the South Atlantic and South Pacific Oceans. In the Indian Ocean, an increase in the natural 14C as a result of circulation changes also contributed to the apparent decadal increase of the bomb 14C. All of our results suggest that the distribution of bomb 14C in the Southern Hemisphere is governed primarily by thermocline ventilation processes within the Southern Hemisphere.