Surface radiocarbon record from the northern Indian Ocean: understanding surface ocean processes

Due to nuclear bomb tests during mid 1950s and 1960s, enormous amount of bomb radiocarbon was introduced into the atmosphere and subsequently to the ocean. Corals growing in shallow oceanic region record the radiocarbon variations in ocean surface waters. The bomb radiocarbon signature embedded in coral can be useful in providing information about natural processes affecting the surface waters of the region. In this regard, coral based radiocarbon records from the Lakshadweep Islands and the Andaman Islands from the northern Indian Ocean has been analysed. The analysed coral ∆¹⁴C values of recent period show comparable or even higher than the atmospheric ∆¹⁴C values, suggesting that major fraction of bomb radiocarbon have transferred in to the ocean. The northern Andaman region show ∆¹⁴C decline rate of about 3.1 ‰ yr^-1 between 1978 to 2014. Whereas, the southern Bay of Bengal and the Lakshadweep records show relatively lower decline rate of 2.5 ‰ yr^-1 for the same period. Based on the coral and atmospheric radiocarbon values, air-sea CO₂ exchange rate over the Lakshadweep and Andaman region has been estimated. The bomb radiocarbon based estimate of air-sea CO₂ exchange rate over Lakshadweep is 13.4 ± 2.1 mol m^-2 yr^-1 and over northern Andaman is 8.8 ± 1.3 mol m^-2 yr^-1. The Lakshadweep region show net regional CO₂ flux of 2.5 ± 0.4 Tg C yr^-1, while the northern Andaman region shows value of -0.3 ± 0.04 Tg C yr^-1. This study discusses the spatial and temporal radiocarbon changes in the northern Indian Ocean and has implications to constraining the carbon flux over the region.