Phosphorus removal from oceans

The sole means of phosphorus removal from the oceans is burial with marine sediments... 

Implications of these results are that phosphorus removed from the surface waters as biological flux is 30-65 times more hkely to come from upwelling than from rivers (1.3-3.0 x 10 /4.6 x 10 ), indicating that ocean circulation is far more important in regulating biological productivity than river inflow. Also, only 1 in 30-65 atoms of P that rains to the deep ocean is actually buried the rest are degraded in the deep and recycled back to surface waters. This results in a residence time for phosphorus with respect to burial of 30 000-65 000 y 30-65 times the ocean circulation rate. 

The problem is to calculate the steady-state concentration of dissolved phosphate in the five oceanic reservoirs, assuming that 95 percent of all the phosphate carried into each surface reservoir is consumed by plankton and carried downward in particulate form into the underlying deep reservoir (Figure 3-2). The remaining 5 percent of the incoming phosphate is carried out of the surface reservoir still in solution. Nearly all of the phosphorus carried into the deep sea in particles is restored to dissolved form by consumer organisms. A small fraction—equal to 1 percent of the original flux of dissolved phosphate into the surface reservoir—escapes dissolution and is removed from the ocean into seafloor sediments. This permanent removal of phosphorus is balanced by a flux of dissolved phosphate in river water, with a concentration of 10 3 mole P/m3. 

Figure 9 is important with respect to removal of iron from the surface ocean. Nitrogen and phosphorus are assimilated and removed from the surface ocean at a ratio within about a factor of 2 of the Redfield ratio (Falkowski, 2000 Karl,... 

There is no particular a priori reason to suspect that oxygen production was any different from that in the modern ocean, except that there may have been constraints imposed by different availabilities of key nutrients such as phosphorus (Bjerrum Canfield 2002). Today, the availability of fixed nitrogen may constrain productivity on geological time scales (Falkowski 1997), but in the Archaean phosphorus removal by adsorption on iron oxides could have reduced P availability, significantly reducing productivity compared with today. 

The iron-based redox cycle depicted in Figure 18.9 provides an effective preconcentrating step for phosphorus by trapping remineralized phosphate in oxic sediments. The conversion of phosphorus from POM to Fe(lll)OOH to CFA is referred to as sink switching. Overall this process acts to convert phosphorus from unstable particulate phases (POM to Fe(lll)OOH) into a stable particulate phase (CFA) that acts to permanently remove bioavailable phosphorus from the ocean. This is pretty important because most of the particulate phosphate delivered to the seafloor is reminer-alized. Without a trapping mechanism, the remineralized phosphate would diffuse back into the bottom waters of the ocean, greatly reducing the burial efficiency of phosphorus. 

Heterotrophic respiration fueled by the rain of organic matter from the surface ocean is ubiquitous in marine sediments. Its rate determines one of the important characteristics of the sedimentary environment the depth of redox horizons below the sediment-water interface. Heterotrophic respiration is the process by which carbon and nutrients are returned to the water column it is important in the marine fixed nitrogen and sulfur cycles and the accumulation of metabolic products sets the conditions for the removal of phosphorus from the oceans in authigenic minerals. A great deal of effort has been directed toward quantifying the rates, pathways, and effects of metabolism in sediments. 

As noted in Table 2, between 40% and 75% of phosphorus buried in continental margin sediments is potentially reactive, and 90% to 100% of phosphorus buried in abyssal sediments is potentially reactive. The reactive-P fraction of the total sedimentary P-reservoir represents that which may have passed through the dissolved state in oceanic waters, and thus represents a true P-sink from the ocean. The minimum reactive-P burial flux was calculated as the sum of 0.4(sFcs) + 0.9 (sFas) the maximum reactive-P burial flux was calculated as the sum of 0.75(sFcs) + l(sFas). Both the flux estimates and the percent reactive-P estimates have associated with them large uncertainties. Residence time estimates are calculated as the oceanic phosphorus inventory (reservoirs 4 and 5 (Table 1) = 3 X 10 moles P) divided by the minimum and maximum input and removal fluxes. 

The revised, larger burial flux cannot be balanced by the dissolved riverine input alone. However, when the fraction of riverine particulate phosphorus that is believed to be released upon entering the marine realm is taken into account, the possibility of a balance between inputs and outputs becomes more feasible. Residence times estimated on the basis of phosphorus inputs that include this releasable riverine particulate phosphorus fall within the range of residence time estimates derived from phosphorus burial fluxes (Table 5). Despite the large uncertainties associated with these numbers, as evidenced by the maximum and minimum values derived from both input and removal fluxes, these updated residence times are all significantly shorter than the canonical value of 100 000 years. Revised residence times on the order of 10 000-17 000 y make phosphorus-perturbations of the ocean-atmosphere CO2... 

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