The Tracer Release Laboratory in the Department of Applied Ocean Physics and Engineering at the Woods Hole Oceanographic Institution has as its purpose the investigation of circulation and dispersion in the ocean through the use of controlled releases of sensitive tracers into the system under study.

Most of the experiments performed thus far by the laboratory have had as their focus the measurement of the vertical diffusivity in the density- stratified ocean. In these experiments a tracer is released in a patch as close as possible to a target surface of constant density, and the subsequent dispersion across density surfaces (diapycnal diffusion) is measured. Details of the dispersion along the density surfaces (isopycnal dispersion) are inevitably observed during the experiments as well as the cross-density dispersion (diapycnal dispersion), however.
 

Personnel

The laboratory is led by Dr. James R. Ledwell, who is also a member of the Coastal and Ocean Fluid Dynamics Laboratory. Terence Donoghue is a permanent engineer in the laboratory. These two are assisted part time by

Experiments with Sulfur Hexafluoride

Experiments lasting several years in the open ocean have been performed with sulfur hexafluoride (SF6), detectable in quantities as low as 10-17 moles in a seawater sample by gas chromotography with an electron capture detector [Wanninkhof et al, 1991, Law et al., 1993]. SF6 is virtually inert in the marine environment, i.e., it is a chemically conserved and it is harmless to marine organisms. The only known environmental impact of the substance is that it is a minor greenhouse gas [Ko et al., 1993]. A few hundred kilograms of SF6 is sufficient for multiyear oceanic experiments, a tiny amount compared with a years global production of more than a million kilograms [Maiss and Levin, 1994].

Pilot experiments with small quantities of SF6 were performed during the period 1985-1990 in Santa Monica Basin and Santa Cruz Basin, off the coast of Southern California [Ledwell and Watson, 1991; Ledwell and Hickey, 1995, Ledwell and Bratkovich, 1995]. These experiments yielded accurate measurements of the diapycnal diffusivity in the interior of the basins, but also showed the diapycnal diffusion in the basins, which have areas of approximatley 2000 km at sill depth, to be dominated by processes near the boundaries.

The first open ocean experiment was the North Atlantic Tracer Release Experiment (NATRE), performed in the pycnocline of the subtropical North Atlantic from 1992 to 1994 [Ledwell et al., 1993; Ledwell et al., 1997]. This experiment successfully measured the diapycnal diffusivity in the pycnocline and revealed the processes of lateral dispersion and homogenization of a tracer at scales from 300 meters to 1000 km. Estimates of the diapycnal diffusivity from measurements of turbulent dissipation rates by other groups during the experiment are consistent with the result given by the tracer, within a factor of 2 or so.

A second large-scale experiment was started in 1996 at 4000 meters depth in the Brazil Basin, on the western flank of the Mid Atlantic Ridge, the Brazil Basin Tracer Release Experiment (BBTRE). Extensive surveys of dissipation rates and finestructure were made during the release cruise in 1996 and the first tracer survey cruise in 1997, by the turbulence and microstructure group at Woods Hole Oceanographic Institution. Both the tracer and the microstructure data have revealed enhanced mixing over the flanks of the ridge, with important implications for the heat and salt budgets and the circulation of abyssal basins [Polzin et al., 1997].
 

Experiments with Fluorescent Dyes

The effective sampling rate for SF6 with a shipboard gas chromatograph is on the order of 200 samples/day. This sampling rate is adequate for experiments which last many months, but is a severe limitation for studies of processes which unfold in a matter of hours or days. Fluorescent dyes, can be sampled at 5 hz with submersible instruments, although their minimum detectable level in this mode is nearly a million times worse than for sulfur hexafluoride. The dyes are superior tracers for experiments in which the final volume of the tracer patch is on the order of 1 cubic kilometer. In the stratified open ocean, this implies experimental durations of 5 days or so.

Two dyes have been used so far by our laboratory, Rhodamine-WT and fluorescein. Rhodamine-B has been used by us in the past, but is a suspected carcinogen and is much more particle reactive than Rhodamine-WT. WT and fluorescein are both nontoxic, and are accepted for use in tracer work in drinking water supplies. Rhodamine is stable in sunlight, but it is expensive and it suffers interference from background fluorescence in natural waters. Fluorescein is inexpensive, suffers much less from background, but decays rapidly in sunlight. Neither dye suffers significant adsorption onto particulates during a 5-day experiment, although fluorescein is reputed to be superior in this respect. Fluorescein is clearly the tracer to choose for an experiment in conditions of low light.

The goal of most of the dye experiments performed thus far has been similar to that of the sulfur hexafluoride experiments, namely to measure the diapycnal diffusivity and the lateral dispersion in the stratified ocean, and to place these in a dynamical context. The first such experiment was performed in July 1993 in Massachusetts Bay at the outfall site of sewage treatment plant that is just being completed for the metropolitan Boston area. This was followed in 1995 by an experiment near the south shore of Massachusetts, in the far field of the outfall site. These experiments were jointly funded by the Massachusetts Water Resources Authority and the WHOI Sea Grant Program of the National Oceanographic and Atmospheric Administration.

A series of dye releases has been performed as a component of the Coastal Mixing and Optics Experiment, funded by the Office of Naval Research, in 1995, 1996 and 1997. In these experiments, dye was released at various levels over the mid continental shelf south of Martha's Vineyard to measure diapycnal diffusivity. Interspersed with the dye surveys were intense surveys with microstructure profilers performed by Dr. Neil Oakey and his colleagues from the Bedford Institute of Oceanography of Canada.