Marine Pollution Bulletin 56 (2008) 1461–1468

Contents lists available at ScienceDirect

Marine Pollution Bulletin

journal homepage: www.elsevier.com/ locate /marpolbul

Regional scale impacts of distinct CO2 additions in the North Sea

J.C. Blackford a,*, N. Jones a, R. Proctor b, J. Holt b

a Plym­outh Marine Lab­oratory, Prospect Place, Plym­outh PL1 3DH, UKb Proud­m­an Oceanographic Lab­oratory, 6 Brownlow Street, Liv­erpool L3 5DA, UK

a r t i c l e i n f o

Keyword­s:

CO2

Carbon capture and storage

Sequestration

North Sea

Leakage

Impacts

pH

0025-326X/$ - see front matter © 2008 Elsevier Lt

doi:10.1016/j.marpolbul.2008.04.048

* Corresponding author. Tel.: +44 (0)1752 63346

E-m­ail ad­d­ress: jcb@pml.ac.uk (J. Blackford).

a b s t r a c t

A marine system model applied to the North West European shelf seas is used to simulate the conse-

quences of distinct CO2 additions such as those that could arise from a failure of geological sequestration

schemes. The choice of leak scenario is guided by only a small number of available observations and

requires several assumptions; hence the simulations reported on are engineered to be worse case scenar-

ios. The simulations indicate that only the most ex­treme scenarios are capable of producing perturbations

that are likely to have environmental consequences beyond the locality of a leak event. Tidally driven mix­-

ing rather than air–sea ex­change is identifi­ed as the primary mechanism for dispersal of added CO2. We

show that, given the available evidence, the environmental impact of a sequestration leak is likely to be

insignifi­cant when compared to the ex­pected impact from continued non-mitigated atmospheric CO2

emissions and the subsequent acidifi­cation of the marine system. We also conclude that more research,

including both leak simulations and assessment of ecological impacts is necessary to fully understand the

impact of CO2 additions to the marine system.

© 2008 Elsevier Ltd. All rights reserved.

1. Intro­duc­tio­n

Emission of anthropogenically derived CO2 to the atmosphere

and the subsequent uptake by the oceans, leading to climate

change and ocean acidifi­cation, respectively, are both predicted

to cause severe environmental, ecological and resource impacts

(IPCC, 2001; Stern, 2006; Raven et al., 2005). Consequently there

is much interest in developing methods for reducing carbon emis-

sions, including carbon capture (from power stations) and its

subsequent storage in geological formations. An active sequestra-

tion programme has been in operation at the Sleipner fi­eld in the

North Sea since 1996 run by the Norwegian company Statoil. Here

carbon diox­ide is striped from natural gas by solvents and disposed

of in a saline formation with approx­imately one million tonnes of

CO2 sequestered each year. Further projects are planned for the

North Sea, ex­ploiting the large volumes of geological reservoirs in

the region. Injection of carbon diox­ide under high pressure into

depleted reservoirs is also of fi­nancial interest as it may lead to

enhanced oil recovery.

The delivery and geological storage of large volumes of highly

pressurised CO2 raises the concern of leakage and its potential envi-

ronmental consequences to the marine system. A number of mech-

anisms of leakage are possible, fast flow events such as a pipeline

failure, faulty injection well casings and transmissive faults or frac-

tures in the cap rock; and slow flow phenomena such as seepage

d. All rights reserved.

8; fax­: +44 (0)1752 633101.

through porous geological structures. Research is scarce but sug-

gests that in the long-term only a small fraction of sequestered CO2

might escape (DTI, 2003 and references therein). However, given

the possibility of leakage, it is prudent to assess the potential for

causing environmental impacts and to compare this with the pre-

dicted environmental impacts of ocean acidifi­cation.

2. Meth­o­do­l­o­gy

A marine system model (POLCOMS-ERSEM-HALTAFALL),

describing the North West European continental shelf, is used

to simulate the dynamics of added CO2 and it’s consequences in

terms of the resulting perturbation in pH. The model system is as

described in Blackford and Gilbert (2007) ex­cept for the ex­tension

to cover the whole of the North Western Shelf; salient details are

briefly reviewed here.

The hydrodynamic model POLCOMS is a three-dimensional baro-

clinic system described by Holt and James (2001) and Proctor and

James (1996). It is a primitive equation fi­nite difference model; solv-

ing for velocity, surface elevation, potential temperature, salinity

and turbulent kinetic energy using spherical polar coordinates in

the horizontal and s-coordinates (Song and Haidvogel, 1994) in the

vertical. It employs a sophisticated advection scheme (the “Piece-

wise Parabolic Method”; James, 1996) to minimize numerical diffu-

sion and ensure the preservation of features even on coarse grids

under oscillatory flows. Turbulent viscosities and diffusivities are

calculated using a Mellor–Yamada level 2.5 turbulence closure, but

with an algebraically specifi­ed mix­ing length. The model is applied

to the Northwest European shelf on an approx­imately 7 km grid

1462 J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468

with 18 s-levels giving a vertical resolution between 0.5 and 15 m

depending on water depth. Holt et al. (2005) describes the detailed

evaluation of the model against observations concluding that the

model, with some ex­ceptions, generally accurately describes the

spatial and temporal variability in dynamic features of the region.

ERSEM is a complex­ functional type ecosystem model describ-

ing carbon and nutrient flows through both pelagic and benthic

lower trophic ecosystems (Blackford et al., 2004; Baretta et al.,

1995). However the ERSEM model dynamics do not impact on the

results presented here as there is no feedback between the altered

CO2, pH and the ecosystem processes included in the model at this

stage (see Section 3).

HALTAFALL (Ingri et al., 1967) is an iterative chemical specia-

tion model which, as applied here, uses calculated dissolved inor-

ganic carbon (DIC, the sum of the chemical species resulting when

CO2 dissolves in water) and parameterized total alkalinity (TA) to

derive pH and the partial pressure of CO2 in the water. The latter is

required to drive the air–sea flux­ calculation of CO2 which uses the

parameterization of Nightingale et al. (2000). Sensitivity to air–sea

flux­ parameterizations is discussed below. The model is forced by

an assumed invariant atmospheric CO2 concentration of 375 ppm,

riverine DIC inputs derived from Pätsch and Lenhart (2004) and

Thomas et al. (2005) and an assumption of zero flux­ divergence for

DIC at the lateral boundaries.

We choose to investigate three modes of CO2 release, relating to

the possible mechanisms of leakage. Parameterising the rate and

duration of a leak event is obviously speculative; apart from the

stochastic nature of such an event there is little information avail-

able to guide us towards realistic scenarios. We use two sources

to guide our choice of leak scenario. Klusman (2003a, b) reports

preliminary estimates of seepage from a terrestrial EOR – sequestra-

tion project in Colorado, USA of <3800 tonnes CO2 a¡1 over an area

of 78 km2 with 14C measurements indicating rates of <170 tonnes

CO2 a¡1. These estimates equate to 0.14–3.0 mmol m¡2 d¡1 which

are the unit relevant to the model system. The Colorado site has

accepted 23 £ 106 tonnes of CO2 since 1986. Secondly, we use the

typical capacity of the pipelines used to deliver CO2 to well systems,

100–250 mscfd (million standard cubic feet per day). This equates

to 1.34–3.15 £ 1011 mmol d¡1 or 1.60 £ 103–3.75 £ 103 tonnes C d¡1.

An important consideration, principally relating to fast-rate leak

events is the behaviour of the resulting high pressure CO2 plume;

it’s rate of travel to the sea surface and the balance between direct

gassing to the atmosphere and solution in the water column. There

is evidence from natural shallow (<20 m) high pressure gas seeps

that the majority of CO2 in bubble plumes can transfer to the water

column (Leifer et al., 2006). Hence we assume for simplicity all

CO2 from a leak is dissolved. For low pressure seepages we assume

all gas is dissolved in the bottom layer, for high pressure leaks we

assume an equal distribution of CO2 input through out the water

column.

Consequently, and after some sensitivity analysis we elected to

report on the following scenarios, summarised in Table 1.

i Long-term diffuse seepage: We assume a constant low level

seepage of CO2, spread homogeneously across the area of

one model box­ (49 km2), representing a movement of CO2

through permeable geological formations. We employ two

seepage rates, 3.85 £ 100 mmol m¡2 d¡1 similar to the upper

end of the Colorado observations (Klusman, 2003a, 2003b)

and a £100 treatment of 3.85 £ 102 mmol m¡2 d¡1, giving a

total input over one year of 3.02 £ 103 and 3.02 £ 105 tonnes

CO2, respectively.

ii Short-term leak: Analogous to a fracture in a pipeline that

persists for one day. We use two inputs, 6.93 £ 103 and

6.93 £ 104 mmol m¡2 d¡1 giving total inputs of 1.49 £ 104 and

1.49 £ 105 tonnes CO2, respectively, about 5 and 50 times a

typical pipeline capacity.

iii Long-term leak: Analogous to say, an immitigable fault in

the well casing, we assume a catastrophic out-gassing of

6.93 £ 103 mmol m¡2 d¡1 or 5.43 £ 106 tonnes CO2 over one

year, fi­ve times the input rate at Sleipner, or 5 years worth of

sequestered CO2.

Our fi­nal assumption is that the point source leaks (ii and iii)

disperse instantaneously into a single 7 £ 7 km model box­. Clearly

this is a weakness although the tidally driven horizontal mix­ing

processes in the region are strong (Holt et al., 2001) and would be

capable of achieving this mix­ing within a few days.

All modes of release were simulated at two sites, North (57.75N,

1.00E), approx­imating to the Forties oil fi­eld – and South (54N, 1E),

representative of the Viking group of oilfi­elds. The former site is

characterised by a water column depth of 138 m which is strongly

stratifi­ed during the summer. The latter site has a depth of 28.5 m

and is generally mix­ed throughout the year. The short-term leaks

(ii) were simulated at four times during the seasonal cycle on Julian

days 11, 101, 191 and 281, respectively, 11th January, 10th April, 8th

July and 8th October.

The scenarios used a four year spin-up simulation with annu-

ally repeating forcing conditions (weather and boundary forcing

and atmospheric CO2 values fi­x­ed at 375 ppm approx­imating the

Tabl­e 1

Simulated scenarios

Scenario Site Input

duration

days

Depth

(m)

Input con-

centration

(mmol m¡3 d¡1)

Daily input per metre square Daily input to model

environment

Total input

CO2

(mmol m¡2 d¡1)

Carbon

(g m¡2 d¡1)

CO2

(g m¡2 d¡1)

Carbon

(tonnes

box­¡1 d¡1)

CO2

(tonnes

box­¡1 d¡1)

Carbon

(tonnes)

CO2

(tonnes)

Seepage-

low

North 365 7.7 0.5 £ 100 3.85 £ 100 4.60 £ 10¡2 1.68 £ 10¡1 2.25 £ 100 8.23 £ 100 8.23 £ 102 3.02 £ 103

South 365 1.6 2.42 £ 100

Seepage-

high

North 365 7.7 5.0 £ 101 3.85 £ 102 4.60 £ 100 1.68 £ 101 2.25 £ 102 8.23 £ 102 8.23 £ 104 3.02 £ 105

South 365 1.6 2.42 £ 102

Short-term

leak-low

North 1 138.0 5.0 £ 101 6.93 £ 103 8.28 £ 101 3.04 £ 102 4.06 £ 103 1.49 £ 104 4.06 £ 103 1.49 £ 104

South 1 28.5 2.42 £ 102

Short-term

leak-high

North 1 138.0 5.0 £ 102 6.93 £ 104 8.28 £ 102 3.04 £ 103 4.06 £ 103 1.49 £ 105 4.06 £ 104 1.49 £ 105

South 1 28.5 2.42 £ 103

Long-term

leak

North 365 138.0 5.0 £ 101 6.93 £ 103 8.28 £ 101 3.04 £ 102 4.06 £ 102 1.49 £ 104 1.48 £ 106 5.43 £ 106

South 365 28.5 2.42 £ 102

Columns as follows: (3) the duration of the simulated input; (4) the water column depth receiving the added CO2 (for the seepage simulations the specifi­ed depths represent

the bottom layer of the model); (5) the input concentration per cubic metre; (6–8) the daily input per metre squared (column 4 multiplied by column 5); (9 and 10) the daily

input to the model environment (columns 7 and 8 multiplied by the area of input, 49.0 £ 106 m2); (11 and 12) the total input to the simulation (columns 9 and 10 multiplied

by the input duration in column 3).

J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468 1463

year 2000) to provide settled initial conditions. The only difference

in the one year simulation following the spin up period is the addi-

tion of CO2 as detailed in Table 1. Forcing and boundary conditions

are as described in Holt et al. (2005) and Blackford and Gilbert

(2007). In addition a control scenario with no CO2 input was simu-

lated to provide a baseline data set.

The assumptions and scenarios chosen have been deliberately

done to address, given current knowledge, ex­treme worst case

scenarios of CO2 leakage, and thus set the upper boundaries of

potential environmental impacts.

3. Resul­ts and disc­ussio­n

We restrict the reporting of results to the pH anomaly. This

we take as the difference between pH computed for each leak

scenario compared with the ‘normal’ pH fi­eld resulting from the

control model run with no perturbation. Change in pH is not the

only effect resulting from increased DIC that has the potential to

modify ecosystem processes; however pH does provide an appro-

priate prox­y for the strength of the sum of ecosystem effects.

Much research is currently ongoing into the precise nature of

ecosystem response to high CO2 but, because of the complex­ity

of the processes involved and some pronounced inter species-dif-

ferences this is as yet unquantifi­able, although many individual

process responses have been identifi­ed (summarised in Raven et

al., 2005). In order to give some guidance we have elected to use

the following colour scheme to illustrate the potential detrimen-

tal effect on the ecosystem, it should be noted that this is qualita-

tive and subject to debate.

I White: Perturbation zero or below detection levels, of no ecolog-

ical signifi­cance.

I Green: No or minimal effect likely, perturbation less than natu-

ral variability.

I Yellow: Perturbation of the order of natural variability, poten-

tially small impacts.

I Orange: Some species and processes ex­periencing signifi­cant

impacts.

I Red: More wide ranging and signifi­cant to severe effects pre-

dicted.

The approx­imate average concentration of DIC in a typical

marine system is approx­imately 2.1 mol m¡3 (Takahashi et al.,

1981). In a 100 m water column this amounts to 2.1 £ 102 mol

or 2.4 kg carbon; in a 7 km £ 7 km £ 100 m depth model box­ this

equates to 1.2 £ 105 tonnes of carbon as DIC.

3.1. Long-term­ seepage

The input rate of 3.85 mmol CO2 m¡2 d¡1, equating to the mea-

sured loss rate at the Colorado site represents a perturbation of

0.002% in a 100 m water column. This is insignifi­cant and did

not cause a detectable signal in the model. The £100 treatment

(3.85 £ 102 mmol CO2 m¡2 d¡1) resulted in a small decrease of

pH, relative to the control, with a max­imum reduction of 0.12

pH unit in the vicinity of the sediment surface, with small per-

turbations propagating through the water column (Fig. 1). This

is signifi­cantly less than the natural range of variability and we

assume would not have any signifi­cant biogeochemical impact.

Natural variability of pH in the region is relatively larger than

for the general ocean given that it is driven by high rates of bio-

logical CO2 ex­change and riverine inputs as well as temperature

effects. Variability is typically between 0.2 and 0.4 pH units

over the annual cycle, although ex­treme phytoplankton blooms

can increase pH by over 0.5 units compared with background.

Like all of the simulations reported here the perturbation in the

shallower south site is larger than that in the north site because

the same amount of CO2 is injected into a smaller volume of

water. Also clearly visible, especially in the south site, are the

effects of the tidal mix­ing cycle with summer neap periods signif-

icantly restricting mix­ing resulting in the largest simulated pH

anomalies.

Fig. 1. Annual evolution of pH perturbation with depth of (£50) seep simulation. (a) north site and (b) south site.

1464 J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468

3.2. Short-term­ leak­

Fig. 2 details the perturbation arising from the smaller (£5

pipeline capacity) short-term leak scenario. For the north site

(Fig. 2a) the pH perturbation is no more than ¡0.1 units for at

most one day tailing off over a period of 3–8 days depending on

mix­ing rates. In the south site the initial perturbation is as much

as ¡0.2 pH units (as the same amount of CO2 is injected initially

into a smaller volume) and again short-lived, with the signal dis-

appearing over 5–9 days. Given current knowledge it is unlikely

that a CO2 perturbation of this magnitude would have an ecosys-

tem effect.

In contrast the £50 pipeline capacity leak scenario (Fig. 3) pro-

vokes pH perturbations that ex­ceed ¡0.5 pH units for about a day

at the north site and up to fi­ve days at the south site. The dura-

tion of disturbance is less than 10 days for the northern site and as

much as 20 days for the southern site. This scenario indicates the

approx­imate scale of leakage required to provoke what may be seri-

ous environmental consequences, although there is not yet enough

ex­perimental data to confi­rm or refute this.

3.3. Long-term­ leak­

The results show, for both sites, a small area of high pertur-

bation centred over the release (Figs. 4 and 5). In the north this

perturbation does not ex­ceed ¡0.5 pH units; in the south a per-

turbation of ¡1.0 pH units is recorded. The area of max­imum

disturbance in both cases remains well constrained, although

a plume of acidifi­ed water is seen to spread from the release

driven by the regional circulation. This plume can be ex­tensive,

although the majority of the plume area is acidifi­ed by signifi­-

cantly less than 0.1 pH units. Ex­amination of the detailed model

output (not shown) indicates the possibility of retentive features

in circulation and mix­ing creating discrete small regions of low-

ered pH that move away from the release point and persist for a

week or so.

Ex­amination of the perturbation at the leak location (Fig. 5)

for both release sites clearly shows the influence of the tidal cycle

in determining the instantaneous perturbation strength. Pertur-

bation max­ima are associated with neap tides and minima with

springs and can differ by 0.4 or 0.8 pH units depending on site and

wind strength. This suggests that not only are the timing of leaks

an important consideration but ex­perimental efforts to investigate

ecosystem effects might need to consider using cyclically varying

pH treatments rather than a constant pH value.

3.4. Dispersion of ad­d­ed­ CO2

Fig. 6 details the increase in sea to air out-gassing of CO2 for

each scenario, at the point directly above each release site, along

with the control simulation air–sea ex­change rate that could

be considered ‘normal’. The increased flux­ due to the seepage

scenario is small but clearly apparent. The long-term leak

scenarios increase this flux­ by between two-fold and two-orders of

magnitude throughout the simulation. The short-term leak events

provoke similarly short-term but large flux­ increases. The instanta-

neous air–sea ex­change rate is driven by the episodic nature of the

wind forcing fi­elds.

Ex­amination of the air–sea CO2 flux­ model output (Table 2) indi-

cates that by the end of each scenario between 54% and 63% of

the CO2 addition has been lost to the atmosphere for the South

site leaks, whilst at the North site this drops to between 12% and

27%. This site disparity is driven by the difference in the initial CO2

concentration following from the difference in volume of the input

box­ between each site. In comparison with the short-term events,

the lower percentage out-gassing in the long-term scenarios the

can be attributed the zero time-lag between cessation of input and

the air–sea ex­change summation. However, in general out-gassing,

in the short to medium term, is signifi­cantly less than the simu-

lated CO2 inputs. This is indicative of the slowness of the air–sea

ex­change process in comparison with the speed of input events

and mix­ing processes. Thus a signifi­cant amount of carbon diox­ide

Fig. 2. Short-term leak scenario (£5 pipeline capacity). (a) north site and (b) south site. All perturbations below 0.01 pH unit have been masked for clarity.

J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468 1465

Fig. 4. Long-term leak evolution. (a) north site and (b) south site, times as stated. All perturbations below 0.01 pH unit have been masked for clarity.

Fig. 3. Short-term leak scenario (£50 pipeline capacity). (a) north site and (b) south site. All perturbations below 0.01 pH unit have been masked for clarity.

1466 J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468

is predicted to remain in the marine system for long periods after

a leak event; however mix­ing and diffusion ensure that the remain-

ing CO2 is highly diluted and does not raise the concentration of

CO2 signifi­cantly above normal levels. Hydrodynamic processes

are therefore the key mediating processes.

Given that the CO2 driven perturbations reported are primar-

ily controlled by carbonate chemistry (which is well constrained

and uncontroversial, e.g. Zeebe and Wolf-Gladrow, 2001) and

the physics of the system, the reliability of the results rests on

the ability of the POLCOMS model to represent realistic physics

for the region and the rate of out-gassing to the atmosphere.

The ability of POLCOMS is discussed in Holt et al. (2005) and

papers cited therein, and can be justifi­ably claimed to be fi­t for

purpose with the proviso issues relating to the model resolution

discussed above.

There is not yet a complete consensus on the parameteriza-

tion of air–sea ex­change rates and this is a rapidly developing

research area, (e.g. Borges and Wanninkhof, 2007). The chosen

air–sea parameterization for this study is the Nightingale and Liss

function derived from a study of CO2 flux­es in the North Sea, i.e.

the appropriate gas in the region of interest. Sensitivity analysis

of alternative parameterizations have identifi­ed little impact on

air–sea flux­ dynamics in this model (not published) which con-

curs with other studies (e.g. Merico et al., 2006) who also found

no signifi­cant sensitivity. This arises from the feedback between

ex­change and concentration in that a low ex­change rate increases

the differential in partial pressure between air and sea, increas-

ing the net instantaneous ex­change. With suf­fi­ciently resolved

time-steps (200 s in this model) this feedback is tightly coupled.

Given however that we are ex­amining short-term perturbations

signifi­cantly greater than natural variability we performed a

sensitivity analysis on the air–sea flux­ parameterization. We

repeated selected simulations using the Wanninkof 1992 parame-

terization, with the results differing by less than 1% (not shown).

The lack of sensitivity to air–sea ex­change also indicates that the

primary driver for CO2 dispersion is mix­ing and dilution within

the water mass.

3.5. Acid­ifi­cation pred­icted­ from­ oceanic uptak­e of atm­ospheric

anthropogenic CO2

Acidifi­cation rates for the UK shelf region arising from the uptake

of atmospheric emissions (Blackford and Gilbert, 2007) depend on

CO2 emission scenarios but are generally consistent with rates cal-

culated for the surface ocean system (Caldeira and Wickett 2003).

Table 3 details predicted acidifi­cation for given atmospheric CO2

concentration and suggests approx­imate dates based on contin-

uing non-mitigated emissions. The key fi­nding is that acidifi­cation

of 0.7 pH units below pre-industrial levels will be the global conti-

nental shelf and surface ocean result if known fossil fuel reserves

are combusted. The implications of such a reduction in ocean pH

are hypothesised to be severely damaging to the marine ecosystem

and the biogeochemical cycles it mediates.

3.6. Future work­ inv­estigating ecological feed­b­ack­s

The ERSEM model computes the uptake and production of DIC

as controlled by biological activity, which contributes to the natu-

ral variability of DIC (and pH) in space and time. An analysis to test

the sensitivity of the results to the background or control (DIC), pH

and air–sea ex­change was performed by repeating a selection of

simulations with the ecological model processes turned off, giving

a contrasting annual cycle. The results were identical demonstrat-

ing that the results are not sensitive to the background carbonate

system as long as the system is spun up to equilibrium. This indi-

cates that the results presented would be broadly applicable to con-

trasting marine regions.

There is (as yet) no feedback between the model’s biological pro-

cesses and (DIC) or pH. Inclusion of such sensitivities may serve to

ex­acerbate or moderate leak driven perturbations, depending on

whether feedbacks are positive or negative. Thus the justifi­cation

for using the fully coupled POLCOMS-ERSEM system in this con-

tex­t are the future plans to couple the simulated pH perturbation

to ecosystem processes such as the effect on nitrifi­cation (Black-

ford and Gilbert, 2007). There is an emerging body of evidence on

Fig. 5. Evolution of pH perturbation with depth at the release site. (a) north site and (b) south site.

J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468 1467

Tabl­e 3

Predicted marine pH under a business as usual emissions scenario

Approx­.

datea

Atmospheric

pCO2

Marine pH pH change

1800 260 8.20

1900 285 8.17 0.03

1950 315 8.14 0.06

2000 375 8.08 0.12

2050 500 7.97 0.23

2100 700 7.84 0.36

2150 1000 7.70 0.50

2250 1650 7.50 0.70

a The dates follow the IPCC IS92a scenario until 2100 then assuming a logistic

function for the burning of remaining fossil fuel reserves as used by Caldeira and

Wickett (2003).

Tabl­e 2

The percentage of input CO2 out-gassed over each scenario

(%) of input lost to atmosphere

North South

Seep 12 54

Large short-term leak 27 63

Long-term leak 20 60

ecosystem response to short-term elevations of CO2 (e.g. Ohsumi,

2004 and other papers in this special issue; Widdicombe and Need-

ham, 2007) and this will be a key (modelling) research area for

the nex­t several years. However many of the emerging ecological

responses are ambiguous in that there is high species specifi­city

and little research has been done on recovery. There is also an

intriguing trade off between the effects of high CO2 on physiologi-

cal processes and short-term physiological stress responses which

in some instances have opposite effects. In summary encoding eco-

system effects is currently at best speculative. Therefore we have

taken the approach to address only the direct chemical perturba-

tion in this study as the fi­rst of many necessary steps to evaluat-

ing ecosystem response. Hence in the work submitted there is no

feed back between the ecosystem model and the pH perturbation,

which could potentially modify the biological component of the

DIC dynamic for the leakage simulations.

4. Co­nc­l­usio­ns

The assumptions and the choices of leakage scenario used in

this work were chosen to signifi­cantly over-estimate the amount

of CO2 injection into the marine system that might arise from a fail-

ure of sequestration, based on current knowledge. Throughout, the

Seep - North

0

0.05

0.1

0.15

0.2

0.25

0 61 122 182 243 304 365

flux

incr

ease

Short-term leak - North

0

2

4

6

8

10

0 61 122 182 243 304 365

flux

incr

ease

Long-term leak - North

01234567

0 61 122 182 243 304 365

flux

incr

ease

Seep - South

0

0.05

0.1

0.15

0.2

0.25

0 61 122 182 243 304 365

Short-term leak - South

0

5

10

15

20

25

0 61 122 182 243 304 365

Long-term leak - South

0

5

10

15

0 61 122 182 243 304 365

No input - North

-0.5

0

0.5

1

1.5

0 61 122 182 243 304 365julian days

air-

sea

flux

No input - South

-0.2-0.1

00.1

0.20.30.4

0 61 122 182 243 304 365julian days

Fig. 6. Sea to air CO2 ex­change rate increase for each scenario (g C m¡2 d¡1), top three rows. The data represents the increase in out-gassing for each scenario compared with

the control run. (a) North site, (b) South site, scenarios as labelled. The bottom row shows the air–sea flux­ (g C m¡2 d¡1) for each site for the control scenario. Out-gassing is

positive.

1468 J. Black­ford­ et al. / Marine Pollution Bulletin 56 (2008) 1461–1468

model suggests that any leak driven perturbation approaching the

magnitude predicted from non-mitigated atmospheric acidifi­cat-

ion will be restricted spatially and temporally and perturbations

on large temporal (greater than a few days) or spatial (more than

a few kilometres) scales will be signifi­cantly less than natural vari-

ability. This work does not address the local scale (<1 km) impacts

of massive point source CO2 injections, and it is likely that on some

localised scale, such events would have a catastrophic effect on the

environment. Whilst such an event may be damaging politically or

socially; it should be seen in the contex­t of the 7.46 £ 105 km2 area

of the North Sea and the multitude of other ongoing anthropogenic

disturbances. Whilst recovery in the water column would likely be

a function of the strong horizontal mix­ing processes, we lack even

initial research on the recovery rates of the relatively immobile

benthic communities and this may be a key area for research. This

work also does not address the detailed modifi­cation of sediment

chemistry profi­les in response to a wide area seepage event, which

may or may not have implications for the health of the sediment,

although the relatively small seepage rates observed would sug-

gest less, rather than more, effect.

More work remains to be done to look at local and fi­ne

scale responses and more scenarios covering a wider range of

locations, input levels, timings and durations. For ex­ample, the

model indicates that the tidal and wind state can signifi­cantly

modify the in situ perturbation; it would be valuable to consider

the potentially cyclical nature of the perturbation when design-

ing ex­posure ex­periments. There is also a need to quantify the

response of the ecosystem and its components and ultimately

address impacts on bio-resources. However, on the basis of this

limited, coarse scale, fi­rst look we conclude that: fi­rstly, seepage

of CO2 from geological formations would not have a signifi­cant

impact on the overlying ecosystem and secondly, that even mas-

sive injections of CO2 from sequestration delivery systems would

have minimal effect on the regional scale and an insignifi­cant

effect when compared with that ex­pected to result from surface

ocean acidifi­cation driven by continued uncontrolled CO2 emis-

sions to the atmosphere.

Ac­kno­wl­edgements

This work was part funded by a NERC/ESRC Grant (UKCCSC

NE/C5165X/1), a Grant from DEFRA/DTI (IMCO2, ME2107) and the

NERC funded core program of Plymouth Marine Laboratory. The

authors would also like to thank Dr. Mark Wilkinson (University of

Edinburgh) for guidance with regard to observations of CO2 seep-

age and an anonymous referee for comments which enabled us to

improve on the initial version of the manuscript.

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