Tracer Release Laboratory
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.
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
Judith White, Staff Assistant
Experiments with Sulfur Hexafluoride
Cynthia Sellers, Research Associate
Dezhang Chu, Research Associate
Brian Guest, Engineering Assistant
and also by Mr. Stewart Sutherland, Research Specialist at Lamont-
Doherty Earth Observatory.....
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.