Nutrient Distribution and the Consequences of Differing Supply Ratios
75
River (Mackey et al., 2002b). Although Wells, Vallis, and Silver (1999) suggested that
this enrichment might be sustained eastward in the EUC to induce plant growth when
upwelled in the eastern Pacific, the equatorial FeLine profiles at 140
W showed that this
was not the case: here, Fe profiles across the EUC are not anomalous, but nutrient-like
(Coale et al., 1996a).
In the Scotia Sea, the anomalously high iron content of near-surface Polar Front water
has been attributed to its passage along shoal topography in the Drake Passage (see later
discussion), and close to the Antarctic continent, Fe profiles may have surface maxima
caused by the presence of iron in melt water from icebergs and at the ice front. However,
recent observations made both across the Southern Ocean (Measures and Vink, 2001)
and in the eastern Pacific (Coale et al., 1996a) require no revision of the generalizations
discussed earlier.
Below the oceanic ferricline, the concentration of Fe td is maintained by the balance
between solution from sinking particles and scavenging by readsorption onto other
organic particles so that values cluster narrowly around 0.8 (0.4–1.4) nmol Fe td kg
−1 at
>500 m in each ocean. For this reason alone, we may expect that the remineralization
schedule of Fe from sinking organic particles will differ markedly from that of NO 3 ;
despite this difference, models of Fe dynamics continue to ignore this distinction (e.g.,
Fung et al., 2000). The JGOFS-NABE profiles at 47
and 56
N in the North Atlantic closely
resemble North Pacific VERTEX profiles, but those from the AMT in mid-latitudes differ,
because they lie either below the Saharan dust field or within the influence of a continental
borderland. In the former case, Fe td profiles have a shallow near-surface maximum
(1–5 nM), a subsurface minimum at 75–100 m, and then a deep profile resembling
the Moss Landing profiles from other oceans. Where the AMT stations are adjacent to
the continental slope, the Fe td maximum occurs at 80–100 m.
Johnson et al. remarked that the residence time of dissolved Fe in the ocean (100–200 y)
is much shorter than the thermohaline circulation time (1000 y) or than the residence
time of other biologically reactive elements, such as N or P (<10
−5 y). They conclude,
therefore, that the processes controlling Fe td concentration are unique: nutrient-like profiles and the relative constancy of deep-water concentrations are maintained because
the rate of removal of Fe from solution by organic ligands decreases substantially
below concentrations of about 06 nM kg
−1 . Further, remineralization of Fe from sinking
particles must be essentially continuous throughout the water column. This mechanism differentiates the dynamics of Fe in the deep ocean from those of the NO 3 /NH 4
system and explains the relative uniformity of the distribution of iron in the ocean
interior.
The distribution of nitrate in the oceans is much better described than that of Fe and
reflects the formation of the nitrate molecule within the deep ocean. The relatively very
long residence time of nitrogen means that interocean transport in the deep circulation
intervenes in the global distribution of the molecule; nitrate concentrations are twice as
high in the interior of the Pacific as they are in the Atlantic. Thus, the relatively young
Atlantic deep water at 1–3 km contains less nitrate than does older Pacific water at the
same depth, although concentrations are remarkably uniform within each ocean. However, shoaler than 500 m, the distribution of nitrate becomes progressively less uniform
and around the periphery of each subtropical gyre, nitrate concentrations are an order of
magnitude greater than in mid-gyre at the same depth (Fig. 5.1). These “textbook” highnitrate conditions exist as an equatorial feature in both Atlantic and Pacific Oceans, as a
zonal band across the high-latitude regions and as a meridional feature along the eastern
and western margins. The North Atlantic, open poleward so that the subarctic gyre is not
enclosed as it is in the Pacific, has a relatively very weak high-latitude subsurface zone of
high nitrate.
75
River (Mackey et al., 2002b). Although Wells, Vallis, and Silver (1999) suggested that
this enrichment might be sustained eastward in the EUC to induce plant growth when
upwelled in the eastern Pacific, the equatorial FeLine profiles at 140
W showed that this
was not the case: here, Fe profiles across the EUC are not anomalous, but nutrient-like
(Coale et al., 1996a).
In the Scotia Sea, the anomalously high iron content of near-surface Polar Front water
has been attributed to its passage along shoal topography in the Drake Passage (see later
discussion), and close to the Antarctic continent, Fe profiles may have surface maxima
caused by the presence of iron in melt water from icebergs and at the ice front. However,
recent observations made both across the Southern Ocean (Measures and Vink, 2001)
and in the eastern Pacific (Coale et al., 1996a) require no revision of the generalizations
discussed earlier.
Below the oceanic ferricline, the concentration of Fe td is maintained by the balance
between solution from sinking particles and scavenging by readsorption onto other
organic particles so that values cluster narrowly around 0.8 (0.4–1.4) nmol Fe td kg
−1 at
>500 m in each ocean. For this reason alone, we may expect that the remineralization
schedule of Fe from sinking organic particles will differ markedly from that of NO 3 ;
despite this difference, models of Fe dynamics continue to ignore this distinction (e.g.,
Fung et al., 2000). The JGOFS-NABE profiles at 47
and 56
N in the North Atlantic closely
resemble North Pacific VERTEX profiles, but those from the AMT in mid-latitudes differ,
because they lie either below the Saharan dust field or within the influence of a continental
borderland. In the former case, Fe td profiles have a shallow near-surface maximum
(1–5 nM), a subsurface minimum at 75–100 m, and then a deep profile resembling
the Moss Landing profiles from other oceans. Where the AMT stations are adjacent to
the continental slope, the Fe td maximum occurs at 80–100 m.
Johnson et al. remarked that the residence time of dissolved Fe in the ocean (100–200 y)
is much shorter than the thermohaline circulation time (1000 y) or than the residence
time of other biologically reactive elements, such as N or P (<10
−5 y). They conclude,
therefore, that the processes controlling Fe td concentration are unique: nutrient-like profiles and the relative constancy of deep-water concentrations are maintained because
the rate of removal of Fe from solution by organic ligands decreases substantially
below concentrations of about 06 nM kg
−1 . Further, remineralization of Fe from sinking
particles must be essentially continuous throughout the water column. This mechanism differentiates the dynamics of Fe in the deep ocean from those of the NO 3 /NH 4
system and explains the relative uniformity of the distribution of iron in the ocean
interior.
The distribution of nitrate in the oceans is much better described than that of Fe and
reflects the formation of the nitrate molecule within the deep ocean. The relatively very
long residence time of nitrogen means that interocean transport in the deep circulation
intervenes in the global distribution of the molecule; nitrate concentrations are twice as
high in the interior of the Pacific as they are in the Atlantic. Thus, the relatively young
Atlantic deep water at 1–3 km contains less nitrate than does older Pacific water at the
same depth, although concentrations are remarkably uniform within each ocean. However, shoaler than 500 m, the distribution of nitrate becomes progressively less uniform
and around the periphery of each subtropical gyre, nitrate concentrations are an order of
magnitude greater than in mid-gyre at the same depth (Fig. 5.1). These “textbook” highnitrate conditions exist as an equatorial feature in both Atlantic and Pacific Oceans, as a
zonal band across the high-latitude regions and as a meridional feature along the eastern
and western margins. The North Atlantic, open poleward so that the subarctic gyre is not
enclosed as it is in the Pacific, has a relatively very weak high-latitude subsurface zone of
high nitrate.