Decadal changes of seawater radiocarbon in the eastern tropical Indian Ocean and their oceanographic implications

Abstract

Decadal changes of radiocarbon ( 14 C) of seawater in the eastern tropical Indian Ocean are evaluated by the reoccupation of three stations previously measured by the Geochemical Ocean Sections Study (GEOSECS) and the World Ocean Circulation Experiment (WOCE) expedition in the spring of 2014. The bomb- 14 C inventory in the whole water column increased by ~56% to a level of 7.7 × 10 9 atoms /cm 2 from 1978 to 2014, and the bomb- 14 C mean penetration depth reached ~550 m, which is more than triple of the depth in 1978. This is accompanied by a significant decrease of Δ 14 C in the upper layer, with an average decline rate of 2.5‰ per year for the surface water. The Δ 14 C in the range of ~100 m – ~750 m depth is found to increase with time, revealing an active downward transfer of bomb- 14 C signature from the upper water to the intermediate layer. The enhanced bomb- 14 C inventory through the water column in the region is attributed to the incursion of Pacific water into the Indian Ocean through the Indonesian Throughflow and the advection of 14 C-enriched waters from the southern subtropical gyre. In addition, the Southwest Monsoon Current is inferred to have contributed to the observed small increase of 14 C at the site within the Bay of Bengal. This study therefore highlights the dynamic redistribution of anthropogenic bomb- 14 C through shallow-to-deep vertical mixing processes, as well as through the inter- and intra- basin circulations between the eastern tropical Indian Ocean and its adjacent areas. • Bomb- 14 C inventory increased in the eastern tropical Indian Ocean from 1978 to 2014 • Downward transfer of bomb- 14 C to the intermediate layer is increasingly important • Ocean redistribution of anthropogenic CO 2 is controlled by local circulation process