Efficient stripping of phosphate from surface waters
by photosynthesis combined with build-up at depth
due to respiration of biogenic particles results in the
classic oceanic dissolved nutrient profile. The progressive accumulation of respiration-derived phosphate at depth along the deep-water circulation
trajectory results in higher phosphate concentrations
in Pacific Ocean deep waters at the end of the trajectory than in the North Atlantic where deep water
originates (Figure 1).
The sole means of phosphorus removal from the
oceans is burial with marine sediments. The phosphorus flux to shelf and slope sediments is larger
than the phosphorus flux to the deep sea (Table 2)
for several reasons. Coastal waters receive continentally derived nutrients via rivers (including phosphorus, nitrogen, silicon, and iron), which stimulate
high rates of primary productivity relative to the
deep sea and result in a higher flux of organic matter
to continental margin sediments. Organic matter is
an important, perhaps primary, carrier of phosphorus to marine sediments. Owing to the shorter
water column in coastal waters, less respiration occurs prior to deposition. The larger flux of marine
organic phosphorus to margin sediments is accompanied by a larger direct terrigenous flux of particulate phosphorus (organic and inorganic), and
higher sedimentation rates overall. These factors
combine to enhance retention of sedimentary phosphorus. During high sea level stands, the sedimentary
phosphorus reservoir on continental margins expands, increasing the phosphorus removal flux and
therefore shortening the oceanic phosphorus residence time.
Terrigenous-dominated shelf and slope (hemipelagic) sediments and abyssal (pelagic) sediments
have distinct phosphorus distributions. While both are
dominated by authigenic Ca-P (mostly carbonate
fluorapatite), this reservoir is more important in
pelagic sediments. The remaining phosphorus in hemipelagic sediments is partitioned between ferric ironbound phosphorus (mostly oxyhydroxides), detrital
apatite, and organic phosphorus; in pelagic sediments
detrital apatite is unimportant. Certain coastal environments characterized by extremely high, upwelling-driven biological productivity and low
terrigenous input are enriched in authigenic apatite;
these are proto-phosphorite deposits. A unique process
contributing to the pelagic sedimentary Fe-P reservoir
is sorptive removal of phosphate onto ferric oxyhydroxides in mid-ocean ridge hydrothermal systems.
Mobilization of sedimentary phosphorus by microbial activity during diagenesis causes dissolved
phosphate build-up in sediment pore waters, promoting benthic efflux of phosphate to bottom waters
or incorporation in secondary authigenic minerals.
The combined benthic flux from coastal (sF cbf ) and
abyssal (sF abf ) sediments is estimated to exceed the
total riverine phosphorus flux (F 24(dþp) ) to the ocean.
Reprecipitation of diagenetically mobilized phosphorus in secondary phases significantly enhances
phosphorus burial efficiency, impeding return of
phosphate to the water column. Both processes impact the marine phosphorus cycle by affecting the
primary productivity potential of surface waters.
Topics of Special Interest in Marine
Phosphorus Research
Phosphate Oxygen Isotopes (d
18
O-PO 4 )
Use of the oxygen isotopic composition of phosphate
in biogenic hydroxyapatite (bones, teeth) as a paleotemperature and climate indicator was pioneered by
Longinelli in the late 1960s–early 1970s, and has since
been fairly widely and successfully applied. A novel
application of the oxygen isotope system in phosphates is its use as a tracer of biological turnover of
phosphorus during metabolic processes. Phosphorus
has only one stable isotope (
31
P) and occurs almost
exclusively as orthophosphate ðPO
3À
4 Þ under Earth
surface conditions. The phosphorus–oxygen bond in
phosphate is highly resistant to nonenzymatic oxygen
isotope exchange reactions, but when phosphate is
metabolized by living organisms, oxygen isotopic exchange is rapid and extensive. Such exchange results
in temperature-dependent fractionations between
phosphate and ambient water. This property renders
phosphate oxygen isotopes useful as indicators of
present or past metabolic activity of organisms, and
allows distinction of biotic from abiotic processes
operating in the cycling of phosphorus through the
environment.
Currently, the d
18
O-PO 4 system is being applied in a
number of studies of marine phosphorus cycling, including (i) application to dissolved sea water inorganic
phosphate as a tracer of phosphate source, water mass
mixing, and biological productivity; (ii) use in phosphates associated with ferric iron oxyhydroxide precipitates in submarine ocean ridge sediments, where
the d
18
O-PO 4 indicates microbial phosphate turnover
at elevated temperatures. This latter observation suggests that phosphate oxygen isotopes may be useful
biomarkers for fossil hydrothermal vent systems.
Reevaluating the Role of Phosphorus as a Limiting
Nutrient in the Ocean
In terrestrial soils and in the euphotic zone of
lakes and the ocean, the concentration of dissolved
PHOSPHORUS CYCLE 573
by photosynthesis combined with build-up at depth
due to respiration of biogenic particles results in the
classic oceanic dissolved nutrient profile. The progressive accumulation of respiration-derived phosphate at depth along the deep-water circulation
trajectory results in higher phosphate concentrations
in Pacific Ocean deep waters at the end of the trajectory than in the North Atlantic where deep water
originates (Figure 1).
The sole means of phosphorus removal from the
oceans is burial with marine sediments. The phosphorus flux to shelf and slope sediments is larger
than the phosphorus flux to the deep sea (Table 2)
for several reasons. Coastal waters receive continentally derived nutrients via rivers (including phosphorus, nitrogen, silicon, and iron), which stimulate
high rates of primary productivity relative to the
deep sea and result in a higher flux of organic matter
to continental margin sediments. Organic matter is
an important, perhaps primary, carrier of phosphorus to marine sediments. Owing to the shorter
water column in coastal waters, less respiration occurs prior to deposition. The larger flux of marine
organic phosphorus to margin sediments is accompanied by a larger direct terrigenous flux of particulate phosphorus (organic and inorganic), and
higher sedimentation rates overall. These factors
combine to enhance retention of sedimentary phosphorus. During high sea level stands, the sedimentary
phosphorus reservoir on continental margins expands, increasing the phosphorus removal flux and
therefore shortening the oceanic phosphorus residence time.
Terrigenous-dominated shelf and slope (hemipelagic) sediments and abyssal (pelagic) sediments
have distinct phosphorus distributions. While both are
dominated by authigenic Ca-P (mostly carbonate
fluorapatite), this reservoir is more important in
pelagic sediments. The remaining phosphorus in hemipelagic sediments is partitioned between ferric ironbound phosphorus (mostly oxyhydroxides), detrital
apatite, and organic phosphorus; in pelagic sediments
detrital apatite is unimportant. Certain coastal environments characterized by extremely high, upwelling-driven biological productivity and low
terrigenous input are enriched in authigenic apatite;
these are proto-phosphorite deposits. A unique process
contributing to the pelagic sedimentary Fe-P reservoir
is sorptive removal of phosphate onto ferric oxyhydroxides in mid-ocean ridge hydrothermal systems.
Mobilization of sedimentary phosphorus by microbial activity during diagenesis causes dissolved
phosphate build-up in sediment pore waters, promoting benthic efflux of phosphate to bottom waters
or incorporation in secondary authigenic minerals.
The combined benthic flux from coastal (sF cbf ) and
abyssal (sF abf ) sediments is estimated to exceed the
total riverine phosphorus flux (F 24(dþp) ) to the ocean.
Reprecipitation of diagenetically mobilized phosphorus in secondary phases significantly enhances
phosphorus burial efficiency, impeding return of
phosphate to the water column. Both processes impact the marine phosphorus cycle by affecting the
primary productivity potential of surface waters.
Topics of Special Interest in Marine
Phosphorus Research
Phosphate Oxygen Isotopes (d
18
O-PO 4 )
Use of the oxygen isotopic composition of phosphate
in biogenic hydroxyapatite (bones, teeth) as a paleotemperature and climate indicator was pioneered by
Longinelli in the late 1960s–early 1970s, and has since
been fairly widely and successfully applied. A novel
application of the oxygen isotope system in phosphates is its use as a tracer of biological turnover of
phosphorus during metabolic processes. Phosphorus
has only one stable isotope (
31
P) and occurs almost
exclusively as orthophosphate ðPO
3À
4 Þ under Earth
surface conditions. The phosphorus–oxygen bond in
phosphate is highly resistant to nonenzymatic oxygen
isotope exchange reactions, but when phosphate is
metabolized by living organisms, oxygen isotopic exchange is rapid and extensive. Such exchange results
in temperature-dependent fractionations between
phosphate and ambient water. This property renders
phosphate oxygen isotopes useful as indicators of
present or past metabolic activity of organisms, and
allows distinction of biotic from abiotic processes
operating in the cycling of phosphorus through the
environment.
Currently, the d
18
O-PO 4 system is being applied in a
number of studies of marine phosphorus cycling, including (i) application to dissolved sea water inorganic
phosphate as a tracer of phosphate source, water mass
mixing, and biological productivity; (ii) use in phosphates associated with ferric iron oxyhydroxide precipitates in submarine ocean ridge sediments, where
the d
18
O-PO 4 indicates microbial phosphate turnover
at elevated temperatures. This latter observation suggests that phosphate oxygen isotopes may be useful
biomarkers for fossil hydrothermal vent systems.
Reevaluating the Role of Phosphorus as a Limiting
Nutrient in the Ocean
In terrestrial soils and in the euphotic zone of
lakes and the ocean, the concentration of dissolved
PHOSPHORUS CYCLE 573