NEW APPRAISAL OF RA-226 AS A LARGE-SCALE OCEANIC MIXING TRACER

Radium 226 and Ba data in the eastern and central Indian Ocean collect ed during the Geochemical Ocean Sections Study (GEOSECS) expeditions a re reviewed and used to delineate the geochemical cycle of both specie s. The data show that although particulate uptake of Ra-226 and Ba occ urs in the surface ocean, their regeneration may largely take place ne ar or below the sediment-water interface. Therefore, over much of the deep ocean save close to the bottom, Ra-226 can be regarded as being c hemically conservative. Taking advantage of this geochemical aspect of Ra-226 enables us to compute the vertical (z) and horizontal (meridio nal, x) eddy diffusivities (K(z) and K(x)) and advective velocities (V (z) and V(x)) and their variability in the eastern and central parts o f the Indian Ocean. The absolute magnitudes of these mixing parameters are calibrated against the decay rate of Ra-226. The computation is d one by numerically solving the mass continuity equations for three pro perties: Ra-226, total salt, and mass of seawater. The grid size used in the calculations, in which the advective transport is taken to be a long the isopycnal surface, has dimensions of DELTAx almost-equal-to 7 00 km and DELTAz almost-equal-to 400 m. The model calculation shows th at in the eastern and central Indian Ocean along the GEOSECS traverses , values of diapycnal diffusivity (K(z)) of 1-10 cm2 s-1 are found nea r the surface and the bottom. Minima occur in the thermocline region w here K(z) falls to about 10(-2) cm2 s-1. Below, K(z) has values largel y in the range 0.1-1 cm2 s-1 and increases downward. The horizontal (i sopycnal) diffusivity varies between 10(4) and log cm2 s-1. Mainly in the range 10(6)-10(7) cm2 s-1. Upwelling velocities are mostly in the lower end of the range 1-10 m yr-1, whereas downwelling, unlike upwell ing which tends to be localized, occurs with velocities of 0.1-1 m yr- 1. 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 part icular depth interval. The current field deduced in the study area pro vides a broad picture of the strength and pattern of return flows of t he deep water introduced from the south and thought to be principally transported northward via the relatively narrow, intense currents alon g the western boundaries of the basins. The present study as well as r ecent developments in the high-precision and high-sensitivity mass spe ctrometric analysis of Ra-226 suggests that this naturally occurring i sotope warrants further exploitation as a rate tracer for large-scale ocean circulation and mixing processes.