Global phosphorus cycle

Transcription

1 Global phosphorus cycle OCN 623 Chemical Oceanography 11 April Arisa Okazaki and Kathleen Ruttenberg

2 Outline 1. Introduction on global phosphorus (P) cycle 2. Terrestrial environment 3. Atmospheric environment 4. 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

3 The Global Phosphorus Cycle. Treatise on Geochemistry. 1. Introduction

4 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

5 The Global Phosphorus Cycle. Treatise on Geochemistry.

6 P Reservoirs and Fluxes mol P x Atmosphere Land Biota Ocean Biota Minable Land <60cm Rivers.032 diss part Ocean 0-300m part 1.87 downwell 0.01 fisheries Crustal rocks >60cm + marine sediments Deep Ocean 2, x x 10 8

7 2. Terrestrial environment Apatite The most abundant primary P-bearing mineral in crustal rocks Naturally occurring acids drive weathering reactions of minerals. Dissolved inorganic P or DIP (simplest form as PO 4 3- ) is directly taken up by plants. Returned to soil as organic P Ca 10 (PO 4 ) 6 (OH, F, Cl) 2 P is also efficiently sorbed by soil constituents

8 Phosphate is particle reactive! [DIP] in soil waters is maintained low K D = ( [P] solution ) is low [P] solid 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

9 P cycling in rivers Rivers are the major source of P to the oceans. Most P in rivers is associated with particulate matter. PO 4 3- buffer mechanism Thermodynamic equilibrium between DIP concentration and suspended sediment maintain constant level of bioavailable P (PO 4 3- ) 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

10 P cycling in estuaries P removal from water column Flocculation of Fe in low-salinity region Flocculation of humic compounds Biological uptake of P P addition to water column Remobilization of sorbed P by displacement reactions Anoxic diagenesis in sediments Groundwater seepage may be an important source of P to coastal zone. Not well understood.

11 3. Atmospheric environment Atmospheric P reservoir and fluxes are small No stable gaseous P compounds Phosphine, PH 3, (g): rare Main atmospheric vector P containing dust Important for P-limited regions e.g. Amazon, weathered HI islands, oceanic gyres

12 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.

13 Coupled Fe-PO 4 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.

14 Authigenic carbonate fluorapatite (CFA) francolite Dominant P mineral in phosphorite deposits in the ocean Contain ca wt. % P 2 O 5 Compare with sedimentary rocks and seafloor sediments = less than 0.3 % wt. % P 2 O 5 Actively mined for production of fertilizer Why carbonate? Fluorapatite (Ca 10 (PO 4 ) 6 F 2 ) incorporates the characteristics of the interstitial pore fluids. Disseminated authigenic carbonate fluorapatite CFA diluted with a high concentration of detrital sediment

15 The Global Phosphorus Cycle. Treatise on Geochemistry.

16 Another authigenic phosphate mineral Vivianite, Fe 3 (PO 4 ) 2 8H 2 O 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.

17 Dissolved inorganic P (DIP) 3 ionic species in seawater: HPO 2-4 (87%) PO 3-4 (12%) H 2 PO - 4 (1%) Dissolved organic P (DOP) =Total dissolved P (TDP) - DIP Atlas et al., 1976.

18 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.

19 Data from HOT site Plot (a) Depth m (circles); m (squares); m (triangles) Plot (b) Depth 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., diagnostic parameters for P- limitation (1) Dissolved inorganic N:P ratio (2) Presence of alkaline phosphatase (APase) activity

20 Oceanic P residence time Broecker and Peng (1980) and prior works have estimated T r (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 T r (P) = ca. 10,000 17,000 years Short enough for changes in P reservoirs to influence glacial-interglacial CO 2 cycles

21 Oceanic P-burial and P residence time fluctuate with sea level High sea level, interglacial period shelf Enhanced P burial slope Abyssal plain Low sea level, glacial period shelf Transport to open ocean slope Abyssal plain

22 5. P biogeochemistry on geologic time scales 1) Changes in oceanic P inventories can affect atmospheric CO 2 levels. Elevated biological productivity enhanced consumption of surface water CO 2 invasion of atmospheric CO 2 P as a limiting nutrient limits CO 2 draw-down 2) Assessing paleoceanographic P levels Cd:Ca ratio in benthic forams as a proxy for DIP [Cd] is linearly correlated to [PO 4 ] (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

23 Coupled P-Fe-O 2 cycles and oxygenation of the atmosphere If oceanic bottom waters are well-oxygenated Fe 2+ 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 O 2 in the atmosphere (young Earth) Little Fe oxyhydroxide precipitation Larger concentration of oceanic DIP Enhanced biological productivity maintain atmospheric O 2 reservoir

24 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

25 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 CO 2 cycles and atmospheric O 2 levels in geologic time scales.