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

Decadal changes of radiocarbon (14C) 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-14C inventory in the whole water column increased by ~56% to a level of 7.7 × 109 atoms /cm2 from 1978 to 2014, and the bomb-14C mean penetration depth reached ~550 m, which is more than triple of the depth in 1978. This is accompanied by a significant decrease of Δ14C in the upper layer, with an average decline rate of 2.5‰ per year for the surface water. The Δ14C in the range of ~100 m – ~750 m depth is found to increase with time, revealing an active downward transfer of bomb-14C signature from the upper water to the intermediate layer. The enhanced bomb-14C 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 14C-enriched waters from the southern subtropical gyre. In addition, the Southwest Monsoon Current is inferred to have contributed to the observed small increase of 14C at the site within the Bay of Bengal. This study therefore highlights the dynamic redistribution of anthropogenic bomb-14C 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.