INVERSE MODELING OF TRACERS AND
NUTRIENTS
R. Schlitzer, Alfred Wegener Institute for Polar and
Marine Research, Bremerhaven, Germany
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Seawater contains a large variety of dissolved and
particulate constituents, commonly referred to as tracers. Many of these tracers (such as the dissolved
nutrients, phosphate, nitrate, and silicate, or the
radioactive carbon isotope
14
C (radiocarbon)) are
natural and are part of the ocean system since
geological times. Other tracers, such as the chlorofluorocarbons (CFCs) and various radioactive isotopes
from nuclear bomb testing, are of anthropogenic origin, and have been introduced into the ocean only
during the last decades. The distributions of anthropogenic tracers in the ocean change markedly on
decadal or annual timescales. Naturally occurring
tracers, like the nutrients, are believed to be near
steady state. However, recent data indicate decadal
and inter-annual changes in ocean oxygen and nutrient
distributions, likely caused by changes in circulation
and biogeochemical processes associated with global
climate change.
Some ocean tracers, such as radiocarbon and the
CFCs, act as passive tracers. They are chemically
inert and are being transported and redistributed in
the ocean by currents after entering the ocean from
the atmosphere. When dense surface waters sink and
spread in the ocean interior, they carry the tracer
signal with them and produce tongues of high tracer
concentrations along their spreading paths. An example of such tracer tongues and concentration
maxima in the ocean interior can be seen in the zonal
CFC section in Figure 1. Note the pronounced CFC
maxima at about 1500- and 3500-m depth at the
western boundary, revealing the two cores of the
deep western boundary current (DWBC) carrying
tracer laden waters southward at the western side of
the basin. The spatial extent of the tongues and the
concentration gradients along the tongues can, in
principle, be used to determine the pathways and
velocities of the currents. Such inferences require
advanced mathematical methods, some of which are
described below.
Other tracers (like the macronutrients phosphate,
nitrate, and silicate, or the micronutrients Fe and
Zn) are chemically and biologically active and are a
prerequisite for biological production in the ocean’s
surface layer. Surface nutrient concentrations are
very low in most regions of the world ocean due to
70° W
6000
0 .0 5
.05
5000
1
0 .2
0 .2 5
2.25
0 .2 5
0 .2 5
0 .2
0.5
0.05
1.5
0 .1
0
.
1
0.2
0.25
2 . 2 5
0.5
0. 1
1.5
0.1
0.2
WOCE A5
Jul./Aug. 1992
CFC-11 (pmol kg
−1 )
2
.2
5
2.25
1.5
0.5
0.0 5
0.05
0. 1
1
0.1
0.1
0.25
1
2
0 . 0 5
0 .0 5
2.25
2.25
2
0.5
1
2
0 . 2 5
0. 1
4000
3000
Depth (m)
2000
1000
0
60° W
5 0 ° W
40° N
EQ
Ocean Data View
Ocean Data View
80° W
40° W
0°
40° W
30° W
20° W
2.5
2
1.5
1
0.5
0
Figure 1 Zonal section of chlorofluorocarbon 11 at about 241 N in the Atlantic, clearly showing the traces of the upper and lower
branches of the deep western boundary current (DWBC). Chlorofluorocarbons (CFCs) are man-made gases that invade the ocean
from the atmosphere in increasing amounts since the 1940s. Before that time the ocean was CFC-free. High CFC concentrations in
the ocean interior are clear indications of vigorous subsurface currents.
188
NUTRIENTS
R. Schlitzer, Alfred Wegener Institute for Polar and
Marine Research, Bremerhaven, Germany
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Seawater contains a large variety of dissolved and
particulate constituents, commonly referred to as tracers. Many of these tracers (such as the dissolved
nutrients, phosphate, nitrate, and silicate, or the
radioactive carbon isotope
14
C (radiocarbon)) are
natural and are part of the ocean system since
geological times. Other tracers, such as the chlorofluorocarbons (CFCs) and various radioactive isotopes
from nuclear bomb testing, are of anthropogenic origin, and have been introduced into the ocean only
during the last decades. The distributions of anthropogenic tracers in the ocean change markedly on
decadal or annual timescales. Naturally occurring
tracers, like the nutrients, are believed to be near
steady state. However, recent data indicate decadal
and inter-annual changes in ocean oxygen and nutrient
distributions, likely caused by changes in circulation
and biogeochemical processes associated with global
climate change.
Some ocean tracers, such as radiocarbon and the
CFCs, act as passive tracers. They are chemically
inert and are being transported and redistributed in
the ocean by currents after entering the ocean from
the atmosphere. When dense surface waters sink and
spread in the ocean interior, they carry the tracer
signal with them and produce tongues of high tracer
concentrations along their spreading paths. An example of such tracer tongues and concentration
maxima in the ocean interior can be seen in the zonal
CFC section in Figure 1. Note the pronounced CFC
maxima at about 1500- and 3500-m depth at the
western boundary, revealing the two cores of the
deep western boundary current (DWBC) carrying
tracer laden waters southward at the western side of
the basin. The spatial extent of the tongues and the
concentration gradients along the tongues can, in
principle, be used to determine the pathways and
velocities of the currents. Such inferences require
advanced mathematical methods, some of which are
described below.
Other tracers (like the macronutrients phosphate,
nitrate, and silicate, or the micronutrients Fe and
Zn) are chemically and biologically active and are a
prerequisite for biological production in the ocean’s
surface layer. Surface nutrient concentrations are
very low in most regions of the world ocean due to
70° W
6000
0 .0 5
.05
5000
1
0 .2
0 .2 5
2.25
0 .2 5
0 .2 5
0 .2
0.5
0.05
1.5
0 .1
0
.
1
0.2
0.25
2 . 2 5
0.5
0. 1
1.5
0.1
0.2
WOCE A5
Jul./Aug. 1992
CFC-11 (pmol kg
−1 )
2
.2
5
2.25
1.5
0.5
0.0 5
0.05
0. 1
1
0.1
0.1
0.25
1
2
0 . 0 5
0 .0 5
2.25
2.25
2
0.5
1
2
0 . 2 5
0. 1
4000
3000
Depth (m)
2000
1000
0
60° W
5 0 ° W
40° N
EQ
Ocean Data View
Ocean Data View
80° W
40° W
0°
40° W
30° W
20° W
2.5
2
1.5
1
0.5
0
Figure 1 Zonal section of chlorofluorocarbon 11 at about 241 N in the Atlantic, clearly showing the traces of the upper and lower
branches of the deep western boundary current (DWBC). Chlorofluorocarbons (CFCs) are man-made gases that invade the ocean
from the atmosphere in increasing amounts since the 1940s. Before that time the ocean was CFC-free. High CFC concentrations in
the ocean interior are clear indications of vigorous subsurface currents.
188