New appraisal of radium 226 as a large‐scale oceanic mixing tracer

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Abstract
Radium 226 and Ba data in the eastern and central Indian Ocean collected during the Geochemical Ocean Sections Study (GEOSECS) expeditions are reviewed and used to delineate the geochemical cycle of both species. The data show that although particulate uptake of 226Ra and Ba occurs in the surface ocean, their regeneration may largely take place near or below the sediment‐water interface. Therefore, over much of the deep ocean save close to the bottom, 226Ra can be regarded as being chemically conservative. Taking advantage of this geochemical aspect of 226Ra enables us to compute the vertical (z) and horizontal (meridional, x) eddy diffusivities (Kz and Kx) and advective velocities (Vz and Vx) and their variability in the eastern and central parts of the Indian Ocean. The absolute magnitudes of these mixing parameters are calibrated against the decay rate of 226Ra. The computation is done by numerically solving the mass continuity equations for three properties: 226Ra, total salt, and mass of seawater. The grid size used in the calculations, in which the advective transport is taken to be along the isopycnal surface, has dimensions of Δx ≈ 700 km and Δz ≈ 400 m. The model calculation shows that in the eastern and central Indian Ocean along the GEOSECS traverses, values of diapycnal diffusivity (Kz) of 1–10 cm2 s−1 are found near the surface and the bottom. Minima occur in the thermocline region where Kz falls to about 10−2 cm2 s−1. Below, Kz has values largely in the range 0.1–1 cm2 s−1 and increases downward. The horizontal (isopycnal) diffusivity varies between 104 and 108 cm2 s−1, mainly in the range 106–107 cm2 s−1. Upwelling velocities are mostly in the lower end of the range 1–10 m yr−2, whereas downwelling, unlike upwelling which tends to be localized, occurs with velocities of 0.1–1 m yr−2. The meridional component of subsurface currents is in the range of 0.01–0.1 cm s−1, mostly toward the south, and not restricted to a particular depth interval. The current field deduced in the study area provides a broad picture of the strength and pattern of return flows of the deep water introduced from the south and thought to be principally transported northward via the relatively narrow, intense currents along the western boundaries of the basins. The present study as well as recent developments in the high‐precision and high‐sensitivity mass spectrometric analysis of 226Ra suggests that this naturally occurring isotope warrants further exploitation as a rate tracer for large‐scale ocean circulation and mixing processes.