Abstract 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.

中文翻译：

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热带印度洋东部海水放射性碳年代际变化及其海洋学意义

摘要 利用地球化学海洋剖面研究（GEOSECS）和世界海洋环流实验（WOCE）考察队在 2014 年春季测量的三个台站的重新使用，评估了热带印度洋东部海水放射性碳（14C）的年代际变化1978年至2014年整个水体中的bomb-14C存量增加了~56%至7.7×109原子/cm2的水平，bomb-14C平均穿透深度达到~550 m，是其三倍多1978 年的深度。伴随着上层 Δ14C 的显着下降，地表水平均每年下降 2.5‰。发现在 ~100 m - ~750 m 深度范围内的 Δ14C 随时间增加，揭示了 Bomb-14C 特征从上层水到中间层的积极向下转移。通过该地区水柱增加的14C炸弹库存归因于太平洋水通过印度尼西亚通流侵入印度洋以及来自南亚热带环流的富含14C的水域的平流。此外，据推测，在孟加拉湾内观测到的 14C 小幅增加是西南季风流造成的。因此，这项研究强调了人为炸弹 14C 通过从浅到深的垂直混合过程以及通过东热带印度洋及其邻近地区之间的盆地间和盆地内环流的动态重新分布。