Download Global phosphorus cycle

Global phosphorus cycle
OCN 623 – Chemical Oceanography
11 April 2013
© 2013 Arisa Okazaki and Kathleen Ruttenberg
Outline
1.
2.
3.
4.
Introduction on global phosphorus (P) cycle
Terrestrial environment
Atmospheric environment
Marine environment
o
o
o
P in marine sediments
P in oceanic water column
Oceanic residence time of P
5. P biogeochemistry on long (geologic) time
scales
6. Summary
1. Introduction
The Global Phosphorus Cycle. Treatise on Geochemistry.
4 major components to the global P cycle
(1) Tectonic uplift and
exposure of P-bearing rocks
(4) Burial of mineral and
organic P in sediments
(2) Physical erosion and chemical
weathering of rocks
(3) Riverine transport of dissolved and
particulate P to lakes and ocean
The Global Phosphorus Cycle. Treatise on Geochemistry.
P Reservoirs and Fluxes mol P x 10 12
Atmosphere
0.1
0.0009
0.14
0.01
0.02-0.05
Land Biota
Ocean Biota
84-97
1.6 - 4.5
Minable
323-645
2-6.5
2-6.5
19.4-35
19.4-35
0.39-0.45
Land <60cm
Rivers .032 diss + 0.6 part
3100-6450
Ocean 0-300m
87.4
0.01 fisheries
0.64
0.3-0.6
Crustal rocks >60cm + marine
sediments
0.3 x 108 - 1.3 x 108
Deep Ocean
2,810
1.13-1.4 part
1.87 downwell
2. Terrestrial environment
Apatite
The most abundant primary P-bearing
mineral in crustal rocks
Naturally occurring acids drive
weathering reactions of minerals.
Ca10(PO4)6(OH, F, Cl)2
Dissolved inorganic P or DIP (simplest
form as PO43-) is directly taken up by
plants.
Returned to soil as organic P
P is also efficiently
sorbed by soil
constituents
Phosphate is particle reactive!
[DIP] in soil waters is maintained low
[𝑃]𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
)
[𝑃]𝑠𝑜𝑙𝑖𝑑
KD = (
is low
P is efficiently scavenged by:
Al(OH)3, Fe(OH)3, and other forms of Al- and Feoxyhydroxides in soils
e.g. Fe(OH)3
scavenges P
P cycling in rivers
• Rivers are the major source of P to the oceans.
• Most P in rivers is associated with particulate matter.
“PO43- buffer mechanism”
Thermodynamic equilibrium between DIP concentration and
suspended sediment
→ maintain constant level of bioavailable P (PO43-)
→ turbid rivers, e.g. Amazon, Congo, and Orinoco
Anthropogenic influence
e.g. fertilizer use, deforestation, waste water, etc.
→ Overall, 50 to 300% increase in riverine P flux to the ocean
P cycling in estuaries
P removal from water
column
Flocculation of Fe in
low-salinity region
Flocculation of humic
compounds
P addition to water
column
Remobilization of sorbed
P by displacement
reactions
Anoxic diagenesis in
sediments
Biological uptake of P
Groundwater seepage may be an important source of P to coastal
zone.
→ Not well understood.
3. Atmospheric environment
Atmospheric P reservoir and fluxes
are small
 No stable gaseous P
compounds
 Phosphine, PH3, (g): rare
Main atmospheric vector
 P containing dust
 Important for P-limited regions
e.g. Amazon, weathered HI
islands, oceanic gyres
4. Marine environment
In pelagic sediments
P deposition is
dominated by secondary
P minerals
 Authigenic carbonate
fluorapatite (CFA)
a.k.a. francolite
In coastal sediments
P deposition as detrital P
as well
The Global Phosphorus Cycle. Treatise on Geochemistry.
Coupled Fe-PO4 cycle in marine sediments
Fe-redox cycle
 Provides an
effective means
of trapping
phosphate in
sediments
 Promotes the
precipitation of
CFA – sink for P
Jarvis et al., 1994.
Authigenic carbonate fluorapatite (CFA)
 francolite
 Dominant P mineral in phosphorite deposits in the ocean
 Contain ca. 5 – 40 wt. % P2O5
 Compare with sedimentary rocks and seafloor sediments = less than
0.3 % wt. % P2O5
 Actively mined for production of fertilizer
 Why ‘carbonate’?
 Fluorapatite (Ca10(PO4)6F2) incorporates the characteristics of the
interstitial pore fluids.
“Disseminated” authigenic carbonate fluorapatite
 CFA diluted with a high concentration of detrital
sediment
The Global Phosphorus Cycle. Treatise on Geochemistry.
Another authigenic phosphate mineral
Vivianite, Fe3(PO4)2∙8H2O
Formation is restricted to anoxic
environments with excess
reactive Fe oxyhydroxides.
• Leftovers after iron sulfide
formation
• e.g. deltaic marine
environments
The Global Phosphorus Cycle. Treatise on Geochemistry.
Dissolved inorganic P (DIP)
3 ionic species in seawater:
HPO42- (87%)
PO43- (12%)
H2PO4- (1%)
Dissolved organic P (DOP)
=Total dissolved P (TDP) - DIP
Atlas et al., 1976.
Net primary production
oceancolor.gsfc.nasa.g
ov/FEATURE/gallery.ht
ml
Estimates of total
marine primary
productivity
Schlesinger, W.H. (1997) Biogeochemistry:
An analysis of global change. Academic
Press, San Diego.
Data from HOT site
• Plot (a)
Depth 0-100 m (circles); 100-200 m
(squares); 200-500 m (triangles)
• Plot (b)
Depth 0-100 m
• Plot (c)
Sediment trap-collected
particulate matter at 150 m
• Redfield ratio = dashed lines
Shift in the N:P ratio: >16
Karl et al., 1997.
2 diagnostic parameters for Plimitation
(1) Dissolved inorganic N:P ratio
(2) Presence of alkaline
phosphatase (APase) activity
Oceanic P residence time
Broecker and Peng (1980) and prior works have estimated
Tr (P) = ca. 100,000 years
 Recent studies have identified new P-sinks (e.g. CFA and
other authigenic minerals)
 Recognition of high burial rates of P in oceanic margins
Updated Tr (P) = ca. 10,000 – 17,000 years
 Short enough for changes in P reservoirs to influence
glacial-interglacial CO2 cycles
Oceanic P-burial and P residence time fluctuate
with sea level
Enhanced P burial
High sea level,
interglacial period
shelf
slope
Abyssal plain
Transport to open ocean
Low sea level,
glacial period
shelf
slope
Abyssal plain
5. P biogeochemistry on geologic time scales
1) Changes in oceanic P inventories can affect atmospheric
CO2 levels.
 Elevated biological productivity → enhanced consumption
of surface water CO2 → invasion of atmospheric CO2
 P as a limiting nutrient limits CO2 draw-down
2) Assessing paleoceanographic P levels
 Cd:Ca ratio in benthic forams as a proxy for DIP
 [Cd] is linearly correlated to [PO4] (DIP) in modern oceans.
 DOP can be an important, if not the primary, source of P to
phytoplankton.
 May be better to look at the relationship between Cd and TDP
Coupled P-Fe-O2 cycles and oxygenation of the
atmosphere
If oceanic bottom waters are well-oxygenated…
 Fe2+ oxidizes to form Fe oxyhydroxide precipitates
 Efficiently scavenge DIP resupplied at the surface water
 Reduced biological productivity
If deep ocean was anoxic and there was little O2 in the
atmosphere… (young Earth)
 Little Fe oxyhydroxide precipitation
 Larger concentration of oceanic DIP
 Enhanced biological productivity → maintain atmospheric
O2 reservoir
4 major components to the global P cycle
(1) Tectonic uplift and
exposure of P-bearing rocks
(4) Burial of mineral and
organic P in sediments
(2) Physical erosion and chemical
weathering of rocks
(3) Riverine transport of dissolved and
particulate P to lakes and ocean
6. Summary
• Terrigenous (and also aeolian) input of P to ocean
• P is efficiently scavenged by Fe oxyhydroxides.
• P may be removed from sediments by authigenic mineral
formation.
• P can be re-mobilized by microbial respiration of organic
matter or reductive dissolution of Fe oxyhydroxides.
• Shift to P-limitation in oligotrophic open ocean
• Changes in P reservoirs can influence glacial-interglacial CO2
cycles and atmospheric O2 levels in geologic time scales.