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

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, Ra-226 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 Ra-226 data from multiple international research programs including GEOSECS, TTO and GEOTRACES to build a global picture of Ra-226 distribution throughout the global ocean. The activities of Ra-226 in the world's ocean increases along the thermohaline circulation pathways. Deep and bottom water masses are found with characteristic Ra-226 properties confirming that Ra-226 is a good thermohaline tracer. The vertical Ra-226 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 Ra-226 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 Ra-226, Si and Ba can provide additional insights into water mixing as well as source/sink behaviors of these elements. Considering that Ra-226 originates mainly from the bottom (sediments), it complements widely used top-down geochemical tracers such as C-14 that penetrates the ocean from the atmosphere. With improved abilities to measure Ra-226 in small volume samples, Ra-226 can now be more widely applied as a geochemical tracer for oceanographic investigations to assess deep ocean mixing, transport, and residence times