A Study of Oceanic Subduction Using Tritium-Helium Dating.
Final rept. 1 Jan 90-30 Sep 96,
WOODS HOLE OCEANOGRAPHIC INST MA DEPT OF MARINE CHEMISTRY AND GEOCHEMISTRY
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Tritium and He3 measurements were made on samples taken at over 200 stations during the period 1991-1993 in the Subduction Area. Combining these data with salinity, oxygen and geostrophy we can compute isopycnal diffusivities, oxygen consumption rates, and absolute velocities. The reference level velocities are determined to an accuracy of 1mms. A classic absolute velocity spiral is seen. The depth variation of computed vertical velocities is consistent with vorticity conservation, and extrapolates to within errors of surface ekman pumping and subduction rates. Isopycnal diffusivities are approximately 1200 m squareds at a depth of 300m, and decrease gradually downward. The oxygen utilization rates OURs determined by the oxygen equations show an exponentially decreasing magnitude with depth. On the shallowest horizons, vertical integration of OUR yields a net water column oxygen demand of 4 molO2m squaredy, which corresponds to an export production of 2.5 molm squaredy carbon. I present a simple scheme to show how to relate the H3-He3 age to the advective age, and after correcting for vortex stretching, compute the subduction rate as a function of depth. The shallowest horizon has a subduction rate indistinguishable from climatological ekman pumping rates, but gradually exceeds the projected rates with depth. The difference may be attributed to the increasing importance of buoyancy forced subduction at higher latitudes.
- Physical and Dynamic Oceanography
- Radioactivity, Radioactive Wastes and Fission Products