Nutrient supply from land

It has been proposed that fertilizer cocktails of macro- and micronutrients should be manufactured on land and transported by submarine pipe to a region significantly beyond the edge of the continental shelf. The nutrient ratios and the temporal supply rates could be controlled so that biological populations develop that optimize sequestration. Such environmental manipulation is today carried out in a sophisticated manner in terrestrial glasshouses where the physical conditions can be controlled, but, with close monitoring, there is no a priori reason why this should not also be possible in an environment such as the open ocean where control of the physical environment is unlikely to be possible.

Empirical support for this approach has been based largely on the observations that the substantial leakage of agricultural fertilizer to coastal seas increases production greatly (with the associated problems of excessive eutrophication; see Section 8.3.1). In the open oceans, observations have been limited to a purposeful release of phosphate and iron (Rees et al. 2006) and to various modelling studies that highlight the intimate and intricate relationships between the various essential elements and the forms in which they are present in the ocean (Dutkiewicz et al. 2005; Parekh et al. 2005).

On the face of it, macronutrient supply from land has much to recommend it. We believe that if properly implemented it is likely that such a scheme would lead to enhanced oceanic carbon sequestration. The drawback of the scheme is that the energetic costs of producing the cocktail and piping it from the land to regions of nutrient limitation are likely to be large with a carbon footprint that may be greater than the carbon sequestered. Nevertheless, it is worthwhile assessing the approximate costs of this proposal as there is a likelihood of successful sequestration.

As phosphorus is the ultimate limiting nutrient in the oceans, mining of phosphate-bearing rocks would have to be substantially increased. This is a demanding process and some sources currently used for fertilizer have a phosphate content as low as 4 per cent with many sources highly enriched in carbonate (Zapata & Roy 2004). One of the most common water-soluble fertilizers is diammonium phosphate (DAP), (NH4)2HPO4, which currently has a market price of approximately $420 tonne-1 FOB ( shared/Samples/SubPage181.asp) or $1700tonne-1 of phosphorus. The costs of purification and injection to nutrient-poor regions of the ocean are likely to be large but if one excludes these costs and uses the C :P Redfield ratio of 106: 1 one obtains a sequestration cost of the order of $45 tonne-1 carbon, a figure that is substantially less than the current trading price for carbon emissions. However, such a simple calculation may not be appropriate as a result of the details of timing. Although new production would increase rapidly in response to this nutrient supply (days), CO2 is absorbed by the ocean slowly (months) and depending on the local physics of the water column there is a possibility that this mismatch would prevent sequestration of atmospheric CO2.

The extraction of nitrogen gas from the atmosphere and conversion to urea has already been proposed and small-scale applications carried out (Ocean Nourishment Corporation; Initial studies of the cost of this process suggested that it was a viable proposition (Shoji & Jones 2001) with approximately 12 tonnes of CO2 captured per tonne of ammonia provided and a cost of approximately $25 tonne-1 of carbon claimed to be 'sequestered'. However, these calculations were based on the assumption that phosphate would always be present in unlimited quantities at the site of injection, an assumption that is incorrect in almost all regions of the oceans. As mentioned previously, relief of limitation by one nutrient will normally allow production to increase only to the point where it is limited by another. Thus, unless persuasive data are released demonstrating that this is likely to lead to sustained sequestration, we conclude that it will only provide a short-term and localized enhancement of the biological pump and possibly no effective sequestration of atmospheric CO2.

The issue of distance from the shelf edge is an important one and obviously affects the economic and engineering viability of this scheme substantially. From the modelling perspective, this raises particular problems that will need to be addressed. Shelf zones have rarely been included in ocean general circulation models and there are some very particular difficulties in accurately representing the physics in areas where the shelves meet the open ocean. Until recently, large-scale ocean circulation and shelf sea modelling have progressed in parallel with little interaction between the two. Recent increases in computational resources have, however, allowed refinement of finite difference grids and application of the finite-element approaches that allow an increase in the resolution towards the shelf and accurate representation of the shallow topography and the coast line (e.g. Davies & Xing 2005). Thus, although some of the ocean circulation models encompass the shelf edge, it will be a major challenge to embed in these models the appropriate ecosystem dynamics and carbon cycle.

8.2.2 Macronutrient supply from the deep ocean

An alternative method proposed to supply nutrients to the oceanic euphotic zone is the use of local wave power to pump deep nutrient-rich water from depths of several hundred metres to the surface ( htm; Lovelock & Rapley 2007). It is claimed that this would lead to enhanced production and sequestration of organic carbon via a direct stimulation of the biological carbon pump. This claim has been disputed on the grounds that deep waters also contain elevated concentrations of dissolved carbon dioxide that may be released to the atmosphere when these deep waters reach the surface (Shepherd et al. 2007).

To first order, assuming Redfield stoichiometry, the net supply of DIC in the upwelled water will be just sufficient to supply the carbon required for the additional photosynthesis generated by the upwelled nutrients, without requiring drawdown of CO2 from the atmosphere. However, there are second-order effects to be considered, the most significant of which is the way in which the composition of the water pumped from depth (C : N: P) differs from that of the settling particles. It is widely accepted that nitrogen is preferentially remineralized relative to carbon from sinking organic material (e.g. Anderson & Sarmiento 1994; Christian et al. 1997). Consequently, upward flux of this relatively nitrate-rich water will allow a sinking flux of carbon larger than that contained in the upwelled water, thus potentially allowing a net air-sea flux of CO2 to occur. Whether the offset between the ratio of these two elements, C and N, in the upwelled water and in the sinking particles could be sufficiently large for this strategy to become a plausible means to sequester CO2 from the atmosphere is unclear at present.

As far as we are aware, no comprehensive studies have examined the effects of these pipes at the time and space scales most pertinent to the anticipated effects. Preliminary calculations (A. Yool et al. unpubl. data) using upper limit assumptions for the effective translocation of nutrients indicate that the efficiency of sequestration is low. A very large number of pipes (approaching 1000 km2 in total area) would therefore be needed to achieve sequestration of 1 GtC yr-1.

An inherent part of this scheme's design is that the pipes will supply not only nutrients but also denser water to the surface. This leads to a statically unstable situation: denser fluid overlying lighter fluid. Depending on the upward flow rate and the rate of lateral surface mixing, this will increase mixing and deepening of the upper mixed layer, with consequences for the light field experienced by phytoplankton. These detailed concerns may best be assessed by high-resolution non-hydrostatic physical models or finite-element models with adaptive mesh.

Although early results suggest that the usefulness of the pipes scheme may be limited by its efficiency, large uncertainties still exist in their precise operation at the local scale, and how this translates to their ability to enhance oceanic uptake of CO2. We believe that this scheme cannot be dismissed as a potential solution yet and that further research is warranted.

8.2.3 Iron supply to HNLC regions (enhance macronutrient uptake)

Ice-core records indicated that during past glacial periods, naturally occurring iron fertilization had repeatedly drawn massive amounts of carbon from the atmosphere. Several observational programmes have been carried out in present-day HNLC regions where there are regionally restricted natural supplies of iron and it has been suggested that this supply of iron is sufficient to relieve macronutrient limitation and hence enhance local productivity (Figure 8.4, red squares). The two most recent observational programmes, both in the Southern Ocean, examined the region around the Crozet Islands, and that associated with a shallow plateau near Kerguelen Island (Blain et al. 2007; Pollard et al. 2007). These studies have shown that not only is there enhanced surface production and nitrate reduction as a result of the local iron supply, but that this enhancement leads to increased fluxes of organic carbon below the euphotic zone, some of which reaches the sediments. The conclusion has been that natural iron fertilization in such HNLC regions promotes carbon export and sequestration by measurable amounts.

The amount of carbon sequestered per unit addition of iron is of considerable interest and is termed the iron fertilization efficiency (IFE). Results from the field programmes indicate that the value of IFE at Crozet is four times lower than that calculated from Kerguelen, although the uncertainties at both locations are large and the difference between them probably not statistically significant. A key goal of future observational programmes must be to refine this value.

Twelve artificial iron fertilization experiments have been carried out since 1993 to examine the effects of in situ addition of this micronutrient on upper ocean biogeochemistry (summarized in de Baar et al. 2005; Boyd et al. 2007). These experiments have shown that supplementing these areas with iron has a significant effect on biological processes in these regions and on the cycles of the major

Figure 8.4 Annual average surface nitrate showing the locations of iron experiments referred to in Boyd et al. (2007); red, natural Fe studies; white, Fe addition experiments; green, Fe+P addition experiments (see also colour plate).

elements such as carbon, nitrogen, silicon and sulphur. Although all experiments enhanced the growth of phytoplankton, they were not all designed to measure export from the upper ocean and none was designed to measure sequestration. There was nevertheless evidence of enhanced export flux in several of the experiments, and one may expect this led to enhanced sequestration though to an unknown extent.

This fertilization method has been the focus of more publicity than other methods, largely stemming from an informal sound bite by John Martin in 1988 that an ice age could be initiated with 'half a tanker full of iron'. The laboratory experiments that formed the basis for Martin's comments indicated that every ton of iron added to HNLC regions could sequester 30000-100000 tonnes of carbon.

Models of progressively increasing resolution and realism have been used during the last 20 years in order to evaluate the potential for iron fertilization of HNLC regions as a means of consuming nutrients and sequestering carbon. Early simplistic models (e.g. Peng & Broecker 1985) indicated a possible reduction in atmospheric CO2 of 50-100 ppm; however, recent studies with higher resolution three-dimensional models coupled to ecosystem dynamics including iron have suggested that addition of iron is much less efficient (order of 10 ppm) because the other limiting factors of light and grazing become dominant (e.g. Dutkiewicz et al. 2005; Aumont & Bopp 2006).

The link between nutrient supply and ecosystem dynamics is complex, especially for micronutrients. Formulations suitable for global ecosystem models are only now becoming available with detailed physiological models of iron cycling (e.g. Flynn 2001) being implemented in ecosystem models (e.g. Fasham et al. 2006). However, modellers still face many problems in representing aspects of iron cycling such as the complex speciation of iron in the marine environment, bioavailability (e.g. binding by organic ligands), photochemical processes and interactions with colloids (Weber et al. 2005).

Global biogeochemical models are not yet capable of accurately predicting both upper ocean production and consequent export of organic matter to deep waters (e.g. Gehlen et al. 2006), let alone the impact of a perturbation due to iron fertilization on the system. Our understanding of the mechanisms contributing to export remains incomplete, compromising the ability to successfully predict the ecosystem response to perturbations in iron supply. The data from the iron fertilization experiments are in themselves inconclusive, further contributing to the difficulties in reducing uncertainties in IFE through modelling.

The final conclusion from Aumont & Bopp (2006) was that 'the tool used in this study is a simplified (and simplistic) representation of reality. Thus, large uncertainties remain concerning the efficiency of iron fertilization that should be explored using more observations and/or other models.' We concur entirely with this conclusion and until these other studies are carried out it will be impossible to state with confidence whether iron fertilization in HNLC regions is likely to be effective in sequestering anthropogenic carbon. Only after these studies are completed will it be possible to determine the net benefit of the activity after taking into account the carbon costs.

8.2.4 Iron supply to LNLC regions (enhance nitrogen fixation)

In areas of the ocean where surface waters contain residual phosphate but are deficient in nitrate, nitrogen fixation (which has an especially high dependency on iron) is limited by this micronutrient (e.g. Falkowski 1997). The supply of iron could, if supported by sufficient local supplies of phosphorus, facilitate nitrogen fixation leading to enhanced productivity and thus possibly also carbon sequestration. As for HNLC areas, an important question regarding the efficacy of iron fertilization in LNLC regions is the extent to which other limiting factors, notably phosphorus, become limiting. The problem is exacerbated by our relatively poor understanding of the mechanisms of nutrient supply, including P, to the oligotrophic gyres. The addition of more 'plankton functional types' such as N2 fixers in marine ecosystem models is fraught with difficulty given our limited understanding of plankton physiology (Anderson 2005) but this is clearly a crucial task in the context of iron fertilization of LNLC regions.

The good correlation between the diazotroph Trichodesmium sp. abundance and estimated dust deposition (Tyrrell et al. 2003) in the subtropical North Atlantic Ocean gives further support to this notion. Similarly, the South Atlantic oligotrophic gyre has low nitrate and iron concentrations but with residual phosphate. The effect of this on carbon sequestration has yet to be determined through large-scale field experiments although the evidence is strong that iron and phosphorus provide pivotal co-limitation of nitrogen fixation (Mills et al. 2004).

8.2.5 Conclusions

None of these four methods has yet been fully explored either by adequate field experimentation or by appropriate computational modelling of the system. Both are required to determine the likelihood that sequestration can be enhanced and by how much but there is definite potential that some or all of the proposed methods could enhance sequestration. However, no serious and detailed assessments have been published on the full economic and/or energetic costs required in order to implement any of the methods. At present, the carbon trading market is developing at great speed ($10.9 billion in 2005 and $30.2 billion in 2006) but not all industrial sectors are involved. The consequence of this partial involvement of industry is that a direct comparison of costs between the various fertilization methods is much more difficult and will require detailed and thorough analyses. Nevertheless, it seems likely that iron fertilization would be the most cost-effective, simply because the quantity and cost of the fertilizing material required are both small.

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