Radium-226 in the global ocean as a tracer of thermohaline circulation: Synthesizing half a century of observations

• Published in: Earth-science reviews 2022-03, Vol.226, p.103956, Article 103956
• Main Authors: Xu, Bochao, Li, Sanzhong, Burnett, William C., Zhao, Shibin, Santos, Isaac R., Lian, Ergang, Chen, Xianyao, Yu, Zhigang
• Format: Article
• Language: English
• Subjects: 226Ra; Atlantic Ocean; concentration (composition); Deep ocean; Environmental Sciences; GEOSECS; GEOTRACES; Indian Ocean; Miljövetenskap; mixing; Oceanografi, hydrologi, vattenresurser; Oceanography, Hydrology, Water Resources; Pacific Ocean; radium isotope; residence time; Thermohaline circulation; tracer
• ISSN: 0012-8252; 1872-6828
• DOI: 10.1016/j.earscirev.2022.103956

Naturally occurring radium isotopes are powerful natural tracers of marine processes and support the quantification of elemental budgets. Ra-226 is the most abundant radium isotope in the ocean with the longest half-life (1600 years). Initially, 226Ra was used to understand global scale deep ocean circulation and later was applied as a tracer of submarine groundwater discharge and benthic fluxes. Here, we synthesize historical dissolved 226Ra data from multiple international research programs including GEOSECS, TTO and GEOTRACES to build a global picture of 226Ra distribution throughout the global ocean. The activities of 226Ra in the world's ocean increases along the thermohaline circulation pathways. Deep and bottom water masses are found with characteristic 226Ra properties confirming that 226Ra is a good thermohaline tracer. The vertical 226Ra distribution is consistent with the penetration of the Antarctic Bottom Water (AABW) into all three major oceans (Atlantic, Indian and Pacific) along their western boundaries. The North Atlantic Deep Water (NADW) flows southward and merge into the Antarctic Circumpolar current (ACC) mainly along the western boundary. The eastern flow branch of the NADW is relatively weak as it enters the Indian Ocean. North Pacific Deep Water (NPDW) is transported mostly southward along the eastern boundary. The East Pacific Rise splits the NPDW 226Ra signal and later merges into the ACC at intermediate depths (2000 m-3000 m) in the western South Pacific. A combination of the natural tracers 226Ra, Si and Ba can provide additional insights into water mixing as well as source/sink behaviors of these elements. Considering that 226Ra originates mainly from the bottom (sediments), it complements widely used top-down geochemical tracers such as 14C that penetrates the ocean from the atmosphere. With improved abilities to measure 226Ra in small volume samples, 226Ra can now be more widely applied as a geochemical tracer for oceanographic investigations to assess deep ocean mixing, transport, and residence times