TRACER RELEASE EXPERIMENTS
A. J. Watson, University of East Anglia, Norwich, UK
J. R. Ledwell, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Since the mid 1980s, analytical and engineering
techniques have been developed to enable the compound sulfur hexafluoride (SF 6 ) to be used as a tracer
for oceanographic experiments. SF 6 is a stable and
inert substance with an exceptionally low level of
detection, and its use enables large bodies of water to
be unambiguously marked, allowing the investigator
to keep track of a particular parcel of water. Three
kinds of experiment have thus far made use of this
technique: (1) measurement of mixing and transport
integrated over large regions; (2) estimates of gas
transfer velocities at the surface of the sea; (3) open
ocean iron enrichment experiments. This article
briefly describes the techniques used, and the major
results from each of these types of process study.
The Tracer
Sulfur hexafluoride is an inert perfluorine, routinely
detectable in sea water at B0.01 fmol kg
À1 by electron-capture gas chromatography (1 fmol ¼ 10
À15
mol). At room temperature and pressure SF 6 is a gas,
but it forms a dense (r ¼ 1880 kg m
À3 ) liquid at
pressures exceeding 20 bar. It is extremely stable in
the environment and, other than being an asphyxiant, the pure compound has no known toxic effects.
It is produced commercially largely (B80%) for use
as a gaseous insulator in high-voltage installations.
Much of this industrial production eventually finds
its way into the atmosphere. The atmospheric mixing
ratio was about 4 Â 10
À12 in 1999, and is growing at
about 7% per year. Its solubility is very low, so that
the surface concentrations in equilibrium with the
atmospheric concentration are on the order of
1 fmol kg
À1
. The combination of very low detection
limit, nontoxicity, low marine background concentration, ease of analysis and inertness make SF 6 a
nearly ideal tracer.
SF 6 is included in the Kyoto Protocol because,
molecule-for-molecule, it is a powerful greenhouse
gas with a long (41000 years) lifetime in the
atmosphere. The signatory nations are thus committed to controlling the rate of its production.
However, for any realistic future emission scenario,
SF 6 will remain insignificant (o1%) as a contributor
to the anthropogenic greenhouse effect for the foreseeable future.
Mixing Experiments in the Deep
Ocean
To measure diapycnal mixing (i.e. mixing acrossdensity surfaces) by tracer release, the tracer is released, as near as possible, onto a single, well-defined
density surface, and its subsequent spread onto
neighboring surfaces is monitored. If the mixing occurs in accordance with Fick’s law, the square of the
mean width of the concentration distribution increases linearly with time, the rate of increase being a
direct measure of the diffusivity. The advantage of
this strategy compared to the documentation of
temperature or velocity microstructure, is that it
gives an unambiguous measurement integrated over
a substantial time and space scale. In practice, in the
open ocean these scales are of order months or years,
and hundreds or thousands of kilometers – hence
also the method’s main disadvantage, which is that it
must be done on a large scale.
At the time of writing, five experiments of this
kind have been initiated in the open ocean. The first
two, relatively small-scale releases, were made in the
ocean-floor basins off the coast of Southern California and the remaining three in the thermocline of
the North Atlantic, the deep Brazil Basin and the
central Greenland Sea. Below we describe the release
method used in most of these experiments, and the
results of the North Atlantic experiment in more
detail. Mixing rates from all five experiments are
then compared.
Release Method
Sulfur hexafluoride is very insoluble; for small-scale
experiments it can be dissolved by presaturating
drums or tanks of water with the gas. However, the
practical limit for the amount which can be injected
in this way is a few moles, sufficient for tracer experiments on the 10–100 km scale only. For large
open ocean releases, we designed an injection package which releases liquid SF 6 into water by pumping
it through fine orifices at high pressure, so that it
breaks into an emulsion of fine droplets on contact
175
A. J. Watson, University of East Anglia, Norwich, UK
J. R. Ledwell, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Since the mid 1980s, analytical and engineering
techniques have been developed to enable the compound sulfur hexafluoride (SF 6 ) to be used as a tracer
for oceanographic experiments. SF 6 is a stable and
inert substance with an exceptionally low level of
detection, and its use enables large bodies of water to
be unambiguously marked, allowing the investigator
to keep track of a particular parcel of water. Three
kinds of experiment have thus far made use of this
technique: (1) measurement of mixing and transport
integrated over large regions; (2) estimates of gas
transfer velocities at the surface of the sea; (3) open
ocean iron enrichment experiments. This article
briefly describes the techniques used, and the major
results from each of these types of process study.
The Tracer
Sulfur hexafluoride is an inert perfluorine, routinely
detectable in sea water at B0.01 fmol kg
À1 by electron-capture gas chromatography (1 fmol ¼ 10
À15
mol). At room temperature and pressure SF 6 is a gas,
but it forms a dense (r ¼ 1880 kg m
À3 ) liquid at
pressures exceeding 20 bar. It is extremely stable in
the environment and, other than being an asphyxiant, the pure compound has no known toxic effects.
It is produced commercially largely (B80%) for use
as a gaseous insulator in high-voltage installations.
Much of this industrial production eventually finds
its way into the atmosphere. The atmospheric mixing
ratio was about 4 Â 10
À12 in 1999, and is growing at
about 7% per year. Its solubility is very low, so that
the surface concentrations in equilibrium with the
atmospheric concentration are on the order of
1 fmol kg
À1
. The combination of very low detection
limit, nontoxicity, low marine background concentration, ease of analysis and inertness make SF 6 a
nearly ideal tracer.
SF 6 is included in the Kyoto Protocol because,
molecule-for-molecule, it is a powerful greenhouse
gas with a long (41000 years) lifetime in the
atmosphere. The signatory nations are thus committed to controlling the rate of its production.
However, for any realistic future emission scenario,
SF 6 will remain insignificant (o1%) as a contributor
to the anthropogenic greenhouse effect for the foreseeable future.
Mixing Experiments in the Deep
Ocean
To measure diapycnal mixing (i.e. mixing acrossdensity surfaces) by tracer release, the tracer is released, as near as possible, onto a single, well-defined
density surface, and its subsequent spread onto
neighboring surfaces is monitored. If the mixing occurs in accordance with Fick’s law, the square of the
mean width of the concentration distribution increases linearly with time, the rate of increase being a
direct measure of the diffusivity. The advantage of
this strategy compared to the documentation of
temperature or velocity microstructure, is that it
gives an unambiguous measurement integrated over
a substantial time and space scale. In practice, in the
open ocean these scales are of order months or years,
and hundreds or thousands of kilometers – hence
also the method’s main disadvantage, which is that it
must be done on a large scale.
At the time of writing, five experiments of this
kind have been initiated in the open ocean. The first
two, relatively small-scale releases, were made in the
ocean-floor basins off the coast of Southern California and the remaining three in the thermocline of
the North Atlantic, the deep Brazil Basin and the
central Greenland Sea. Below we describe the release
method used in most of these experiments, and the
results of the North Atlantic experiment in more
detail. Mixing rates from all five experiments are
then compared.
Release Method
Sulfur hexafluoride is very insoluble; for small-scale
experiments it can be dissolved by presaturating
drums or tanks of water with the gas. However, the
practical limit for the amount which can be injected
in this way is a few moles, sufficient for tracer experiments on the 10–100 km scale only. For large
open ocean releases, we designed an injection package which releases liquid SF 6 into water by pumping
it through fine orifices at high pressure, so that it
breaks into an emulsion of fine droplets on contact
175